09.11 - 평가문제(Quiz: Our Eyes on the Sky)

09.11 - 평가문제(Quiz: Our Eyes on the Sky)

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[처음][이전][다음]

09.10 - 요약: 블랙홀 찾기(Summary: Our Eyes in the Sky)

09.10 - 요약: 블랙홀 찾기(Summary: Our Eyes in the Sky) [커세라 강의 페이지]



[MUSIC] Black holes don't emit light, but that doesn't mean that we can't detect them. Just one of the ways is through their interactions with background light, gravitational lensing.

Play video starting at 18 seconds and follow transcript0:18

When a black hole is in a binary system, the companion star can be seen. In some cases, mass flows from the companion star into an accretion disk surrounding the black hole, which then emits light.

Play video starting at 30 seconds and follow transcript0:30

The electromagnetic radiation emitted by the disk, jet and corona varies in time as the rate of in-falling gas changes. Super massive black holes, in the centers of galaxies, also give away their presence through the energy released by accretion. Jets of gas, energized by gravitational potential energy, interact with the black hole's host galaxy, affecting the formation of nearby stars in the galaxy.

Play video starting at 58 seconds and follow transcript0:58

Supermassive black hole jets are thought to accelerate particles such as protons and neutrinos. Perhaps this is the origin of cosmic rays.

Play video starting at 1 minute 8 seconds and follow transcript1:08

Isolated black holes most certainly exist in our galaxy but are hard to detect. They can give away their location when they travel between a distant star and the Earth. The gravitational micro-lensing effect briefly causes the light from the distant star to appear brighter.

Play video starting at 1 minute 27 seconds and follow transcript1:27

Scientists have learnt all these things through the observation of light or electromagnetic radiation, but this isn't the only tool they have at their disposal. Recent projects like LIGO and Virgo are built to be incredibly sensitive gravitational wave observatories. What are they looking for? They're looking for evidence of merging black holes which generate gravitational radiation. That's right, waves in the fabric of space-time.

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[처음][이전][다음]

09.09 - Seeking Out the Elusive

09.09 - Seeking Out the Elusive [커세라 강의 페이지]



Stellar mass black holes have been studied for approximately the last 50 years. Supermassive black holes have been observed for almost as long. However, the identification and study of intermediate mass black holes is still in it the infancy.

The reason for the lack of observations of intermediate mass black holes is not due to the lack of interest. On the contrary, intermediate mass black holes are thought to be the seeds of supermassive black holes and so investigations into their nature could help unlock the mysteries of the formation of supermassive black holes.

The study of intermediate mass black holes is such a new field because members of this class of black hole have been incredibly elusive. In the rare instances when intermediate-mass black holes were identified, their classification was contentious and later many were disproved.

At present, there are only a handful of strong candidates for the intermediate-mass black hole class. ESO243-49HLX-1 is the first candidate intermediate mass black hole we will discuss. This source first hit the headlines in 2009, when its discovery was announced in the journal "Nature". This black hole resides in a star cluster that is in orbit around the galaxy ESO243-49, a galaxy that is 320 million light years away from us.

The designation HLX means hyper luminous X-ray source. While the 1 following is the same convention as of our black holes we have discussed in the course. This is the brightest X-ray object in its host galaxy outside the galaxy's core.

Astronomers were quick to follow up on a source. Making observations in multiple wavelengths from radio, all the way through to X-rays and gamma rays. The temperature of an accretion disc around a black hole can give us an indication of the black hole's mass. The more massive the black hole, the cooler the disc appears to be. We have also found that more massive black holes tend to be brighter or more luminous than the lower mass cousins when they are actively feeding. ESO243-49HLX-1 is approximately a thousand times brighter than one stellar-mass black hole at the same distance would be.

The main difference observed between ESO243-49HLX-1, and say our old friend Cygnus X-1, is that appears brighter and then its spectrum has been shifted towards the redder end of the spectrum. As astronomers watched over the next few years, we saw repeated brightening of this source. ESO243-49HLX-1 appeared to increase in brightness in a period of just over a year. The spectrum of this source was seen to change in shape with brightness.

The time dependent emission of ESO243-49HLX-1 is very similar to the behavior of X-ray binaries, leading astronomers to believe that it is also a binary system. This means that the black hole ESO243-49HLX-1 may be actively consuming a star. However, the cyclical nature of the outburst lead some to suggest that this black hole may not be slowly sipping on the surface of a star but may instead be ripping off larger chunks before gobbling them up in feeding frenzies.

This would trigger outbursting behavior observed that mirrors that of outbursts of X-ray binaries. What could cause these periods of gloat and fasting?

The periodic feeding frenzies could be explained if the binary system's orbit is elliptical instead of circular. This means that the distance between the star and the black hole changes periodically during the orbit. When the star is at its furthest point from the black hole, no mass will be transferred from the star to the black hole. However, when the star is close to the black hole, the gravitational pull of the black hole will increase, allowing the black hole to attract large amounts of material from the star to the accretion disc. This famine and bloat cycle has been suggested as a way to create the regular outburst witnessed from ESO243-49HXL-1.

쌍성계의 동반성이 블랙홀을 타원궤도로 도는 경우, 거리가 가까울때 많은 물질이 이동하고 멀때는 상대적으로 적은양의 물질이 이동한다. 블랙홀의 이런 과식과 굶주림의 주기적으로 반복되고 있다.

Further studies have indicated that this black hole weighs in at about 10,000 times the mass of our Sun and that resides in the center of a dwarf galaxy that has been stripped and squashed during its interactions with ESO243-49. These interactions may have triggered the formation of the star now feeding the black hole and may have pushed it into an elliptical orbit that now feeds the beast.

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The cigar shaped galaxy, M82, is the home of another candidate intermediate mass black hole. M82 is a starburst galaxy where stars formed at a rate much higher than our own galaxy.

The X-ray inset shows us a bright X-ray source named X-1, which is possibly an intermediate-mass black hole. M82 X-1 lies outside the galaxy's core. It was first detected in 1978 when it piqued the interest of astronomers due to its strange brightness. The reason that this thought source was thought to be so strangely bright, is that it emits light at a much greater rate than we would expect to see if the source was a stellar mass black hole.

The luminosity of M82 X-1 could have been explained by a supermassive black hole and yet supermassive black holes are found in galaxy centers, while M82 X-1 does not lie in the center of its host galaxy. This left astronomers with two options. The first option is that this black hole lies in the intermediate mass black hole range. If this was the case, it could explain the luminosity of M82 X-1. However, an alternative theory emerged. The idea that this may be a stellar mass black hole undergoing a feeding frenzy. If it is a stellar mass black hole then it is creating a previously on observed gigantic rates.

As more observations have been made of this source, the mass estimate of M82 X-1 has bounced between stellar mass and intermediate mass over the last 40 years. Today, evidence seems to be mounting in support of an intermediate mass black hole with a mass estimated in the range of 400 to 100 thousand solar masses. It's interesting to note however, that M82 X-1's close neighbor, M82 X-2, has recently shown just how extreme accreters can get. In 2014, a team working with data from New Star Chandra and Swift Satellites, discovered that a previously observed X-ray source, M82 X-2, is in fact a class of neutron stars known as a pulsar.

A pulsar is a rotating neutron star that shoots jets of emission from its poles. These jets sweep across all field of view as the star rotates like a lighthouse beam, sweeps over the rocks and out to sea. The striking thing about this neutron star is that it is 100 times brighter than theory predicts it could be. This means that M82 X-2 is one of the most extreme objects in our galaxy, and it can be used to help us understand how supermassive black holes could grow so quickly in the early universe.

Enough about neutron stars though. Let's get back to intermediate-mass black holes. The next candidate intermediate-mass black hole is XJ1417+52. This black hole has a mass that lies at the upper boundary for intermediate-mass black holes approaching the range of the supermassive black holes.

Omega Centauri is a globular cluster found in the constellation of Centaurus. It is located almost 16,000 light years away and it's the largest globular cluster in the Milky Way. This star cluster may be home to our final example of a candidate intermediate-mass black hole.

Unlike the other candidate intermediate-mass black holes, this black hole was not identified through its X-ray emission. Most likely, this black hole is not actively feeding since there is no substantial food source nearby. The candidate intermediate mass black hole in Omega Centauri within feed, I observing the motions of stars using optical telescopes. These stars in the globular cluster move around randomly like a swarm of bees. The speeds of these stars are related to the total mass of the star cluster. From the observations of the star's motion, the mass, the center of the cluster can be inferred.

Different observations suggest that black hole with a mass in the range of 10,000 to 15,000 solar masses lies in the center of Omega Centauri.

While there are a few other candidate intermediate mass black holes, their numbers are limited and many are still contentious. Many candidate intermediate mass black holes have been ruled out upon further study. The search for the elusive intermediate mass black hole is an active area of research, a field that is definitely ongoing.

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[인터뷰] [커세라 페이지]

Can intermediate mass black holes be found in globular clusters?

중간급 블랙홀은 구상성단에서 발견될 것인가?

Interview with Dr. Craig Heinke

If you smash several black holes together in the center of a globular cluster, they end up with an unusually high mass black hole. So when I say unusually, so black holes are thought to be born initially from stars at masses somewhere between three and a few dozens of solar masses, say 3-50 solar masses, give it a random number. Well, if you smash a bunch of these together, you might end up with something that's hundreds or thousands of solar masses, and so that we call an intermediate mass black holes because they're also the giant black holes at the centers of galaxies, the supermassive ones. So these intermediate mass black holes, people have long thought that there might be some of these at the centers a globular clusters. But we haven't found convincing evidence for them yet. There's some very suggestive evidence in some globular clusters, but so far none of it has been confirmed by multiple lines of evidence. So that's an area that future very large telescopes, such as the 30-meter telescope or the European Extremely Large Telescope. These might be able to give us stronger evidence as to whether these large black holes lurk at the centers. globular clusters.

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[처음][이전][다음]

09.08 - 블랙홀이 은하에 미치는 영향(Impact of Black Holes on Galaxies)

09.08 - 블랙홀이 은하에 미치는 영향(Impact of Black Holes on Galaxies) [커세라 강의 페이지]



Supermassive black holes with masses larger than one million solar masses are found at the centers of most galaxies. But what effect do these black holes have on the galaxies themselves?

Do the properties of the host galaxy affected central black hole? These questions are important areas of an active study in the field of black hole feedback, a forefront area for black hole physics today.

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Let's look again at Sagittarius A*, the black hole at the center of our own galaxy, the milky way. Sagittarius star has a mass of four million times the mass of our sun. Although this seems impressively large, it is small when compared to the total mass of the entire milky way galaxy. Sag A* weighs four million times the mass of our sun, but the milky way weighs close to one trillion solar masses. Therefore, our galaxy is one million times heavier than the black hole at its center. Sag A* contributes only a tiny amount of the total mass of the galaxy.

Similarly, the size of a black hole, Sag A*, is also tiny compared to the size of the galaxy. The radius of Sag A* event horizon extends about 12 million kilometers which itself is almost 30 times larger than the distance between the Moon and the Earth, a reasonable size. However, the event horizon is much smaller than the distance between the Earth and the Sun which is around 150 million kilometers. Black holes accretion disk is larger but it's still less than a light year across. In contrast, the milky way galaxy is huge, with a diameter of more than 100,000 light years. To give you a sense of the astronomical size scale.

If Sag A*'s accretion disk will shrink down to the size of a penny, our galaxy would still be as big as the Earth. Supermassive black holes living in the centers of other galaxies also have relatively tiny masses and sizes compared to their host galaxies.

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For instance, the galaxy M87 has a supermassive black hole with a mass of three billion solar masses. The massive its host galaxy M87, is more than two trillion solar masses, which is more than a 1,000 times larger than the central black holes mass. This suggests that the overall impact of a supermassive black hole on its host galaxy should be quite small.

The funny thing is, this doesn't seem to be the case. In the 1990s, astronomers measured the masses of many supermassive black holes along with the mass of their host galaxies. They found a strong correlation between the black holes mass and the galaxy's mass. Essentially, they found that larger mass galaxies have larger mass black holes at their centers.

They suggests that the galaxies and their central black holes have a symbiotic relationship, meaning that as one grows, so does the other. This opened up an interesting new area of study that is still under investigation today. How can they impact each other so much?

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The exchange of mass and energy between galaxies and their central black holes is called feedback. The galaxy supplies some gas and dust that accretes onto the central black hole causing the black hole to slowly grow. As the gas accretes, thermal heating causes some of the gas to be ejected from a region near the black hole in the form of high speed jets. These jets can extend thousands of light years which heats up the gas and dust in the central parts of the galaxy. So, as long as the galaxy is feeding the black hole, the black hole throws some of the gas back out at high speeds that feeds back into the galaxy.

The outflowing gas can affect the galaxy in two ways:

Firstly, the outflowing gas can push the interstellar gas outwards clearing large regions of gas and dust. By clearing the areas of gas and dust, there is a lack of material required to form new stars. So, this can slow down the birth rate of stars in this region. This may happen in the brightest active galactic nuclei helping regulate star formation.

The second effect of outflowing gas can strangely have the opposite effect. If the black holes jets interact with large gas clouds, they can compress the clouds, triggering formation of new stars. When this happens the black hole actually helps galaxies form new stars.

So, the central black hole of galaxy can suppress or promote the formation of stars that could potentially have planets and maybe even life. In other words, black holes aren't just doom, and gloom, or death through spaghettification.

은하 중심부의 블랙홀은 대재앙이 될수도 있고 새로운 생명력을 불어넣을 수도 있다.

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We think of astronomy as a science where cosmic objects, like stars or galaxies, are studied by observing the light that they emit. But there's more to the universe than just light. The universe consists of uncountable elementary particles; like electrons, protons, and neutrons that make up all the elements such as; hydrogen, helium, and so on. These in turn makeup all the molecules and matter.

Amazingly here on the Earth we often detect particles that originate from cosmic sources, particles that come from distant parts of the universe. These particles are called cosmic rays and carry tremendous amounts of energy.

The highest energy cosmic ray ever measured affectionately named the OMG particle carried 48 joules of energy. To put that into context, that's about the same as a baseball pitched at a speed of 28 meters per second. But since all our energy was carried in atomic nucleus, it was traveling at only a whisper slower than the speed of light. When astronauts travel beyond the protection of Earth's atmosphere, they report strange flashes of light visible even when their eyes are closed. Although this phenomenon hasn't been studied in detail, scientists think that astronauts are observing flashes of light when a cosmic ray travels through their eyes.

These high-energy particles emit Cherenkov radiation as they pass through. This phenomena is known as the cosmic ray visual phenomena.

In order to understand the nature of cosmic rays which can range from a single electron to nuclei of heavy atoms, we must identify the energy source capable of accelerating them. Lower energy cosmic rays like the ones we detect from ozone are accelerated by the sun's rapidly changing energetic magnetic fields.

High-energy cosmic rays originate outside our solar system and a common occurrence. So are accelerated during supernova explosions taking place in our own galaxy, but where else can they originate. Most cosmic rays are charged particles like protons or occasionally a heavy atomic nucleus like an atom of iron. But we also know about even smaller fundamental particles, like the neutrino.

We've already seen that neutrinos which are uncharged particles with tiny masses can travel incredible distances without being stopped by interactions with other types of matter. This makes neutrinos almost impossible to detect since it's extremely rare for them to participate in particle interactions.

This is why neutrino observatories are some of the most specialized detectors on the planet. Neutrino detectors regularly see neutrinos created in the sun's core. In 1987 the Kamiokande chain detector in Japan detected neutrinos from a supernova explosion called SN 1987A. This detection led to the award of the Nobel Prize in physics to Masatoshi Koshiba, the leader of the Kamiokande experiment in 2002.

More recently, the Ice Cube detector located in Antarctica detected some very high energy neutrinos that are unlikely to be emitted by supernova. The source of the ice cubes highest energy neutrinos is still a mystery, but one possibility is that the neutrinos may have come from the jets of a supermassive black hole.

Occasionally, detectors such as the Pierre Auger cosmic ray observatory in Argentina detect ultra-high energy particles, like the OMG particle The term ultra-high energy refers to particles whose energies are millions of times more energetic than anything humans can create. What we mean by this is that the highest energy particles that humans have created are 40 million times less energetic than the highest ultra-high cosmic rays ever seen.

Ultra-high energy cosmic rays can't originate from supernova explosions. Since scientists have determined through simulations that a supernova can't accelerate particle to ultra-high energies. The origin of ultra-high energy cosmic rays is still a deep mystery in astrophysics.

We do know that the highest energy cosmic rays almost certainly originate from sources vast distances beyond our galaxy. Again, one plausible explanation is that the jets of supermassive black holes at the centers of distant galaxies are capable of accelerating particles to incredibly high speeds. If this is true, then our detectors here on the Earth are occasionally registering matter which escaped from the neighborhood of a black hole.

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[인터뷰][커세라 페이지]

What is the impact of a supermassive black hole on a galaxy?

Interview with Dr. Sarah Gallagher, Professor at Western University

When supermassive black holes are growing and they have the accretion disk, you have material that's swirling in and all that light basically can drive a wind, and the way that works is the actual individual photons. The individual light particles have momentum that carry a punch, and so when they encounter gas they can blow these really energetic winds. The way they might impact their host galaxy is that depending on how much mass is in the wind so the mass could easily be a few times the mass of our sun, which is comparable to the amount of gas that's falling into the black hole per year. So few times the mass of the Sun per year could be coming out as winds and a few times the mass of the Sun could be falling into the black hole causing the mass to grow. So, if you have that much gas that's being blown out from the central part of the black hole system in the center of a galaxy and its going out at tens of thousands of kilometers per second that gas has a lot of energy and what it can do is it can impact the other gas in the galaxy and it can do. We're not quite sure what it could do, but what it could do there's sort of two things that it could do. One is that you have gas that's coming out at very high velocity from the center of the galaxy near the black hole and it can basically plow into other gas in the galaxy. If it has enough energy it can actually plow it out of the entire galaxy, and what happens is if it blows all the gas out of the galaxy, it can completely shut off the star formation because that gas is the fuel of star formation. What it could also do if it's not going as fast as it could plow into the gas in the galaxy and it can compress it, and if it can compress it it might actually trigger star formation and cause star formation to occur. So, either of those two is our plausible explanations, but what we really need to do is we need to learn more about the properties of the actual winds. What are the velocities? What are the geometries? How much mass is in them? Once we have a better handle of that then we'll have a better idea of how it actually could ease impacting the galaxies that it lives in. But those are sort of the most extreme scenarios that could enhance star formation by causing the gas clouds to compress or it could just blow out all the gas and shut off star formation, and that's why the event of being a quasar which is something that all massive galaxies go through, it's kind of like being a teenager. It's just like this phase of life. It's kind of dramatic. It's potentially unpleasant and you're definitely different afterwards, but it's not clear what the change actually will be. So that's something that I think is really fascinating and one reason why if you really want to understand how galaxies evolve, you have to understand how their black holes grow because this quasar phase could be really fundamentally important to shaping them over time.

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[처음][이전][다음]

09.07 - It's On... Now What?

09.07 - It's On... Now What?



Black holes will eat whenever there is something for them to grasp in their gravitational clutches. The rate they eat depends on how much material can be captured. Which means that just like people, black holes will eat at different rates and at different times.

블랙홀은 중력으로 끌어당길 수 있는 것이 있을때 먹는다. 얼마나 많이 잡았는지에 따라 먹는 양도 결정된다. 사람의 식사와 마찬가지로 블랙홀이 먹는 양과 시간은 형편에 따라 다르다.

Sometimes you're having an off day, you don't really want anything to eat or you may be fasting. Yet on high days and holidays you may overindulge, eating way more than normal. Although black holes don't really choose their food in the same way we do, they can go through cycles of feast or famine or anything in between. This change in food intake can result in change in the strength of the different components of the spectrum of the black hole. So what are these changes and what do they look like?

블랙홀은 인간처럼 입맛따라 먹을 것을 고를 수는 없지만 주기적으로 풍부하게 먹을때도 있고 굶기도 하며 그 중간일 때도 있다. 블랙홀이 흡입하는 음식에 따라 스펙트럼의 모습[최대 밝기의 파장]도 달라진다. 어떤 변화가 있는지 그리고 어떤 모습을 하는지 살펴보기로 하자.

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[스펙트럼분석]

If we take another look at our spectrum of an accreting stellar mass black hole, we can see the different components that we've learned about. We have an accretion disc, a corona, and jet.

If we zoom in on this plot to take a look at just the X-ray band, we can see the two X-ray components more clearly. Here, we see that the disk is dominating the emission from the black hole while the corona slopes gently upwards as we move to shorter wavelengths.

Now let's look at some real data taken from an old friend Cygnus X-1 and see how this compares. If you recall, Cygnus X-1 is a black hole binary that contains a stellar mass black hole weighing in at about 15 times the mass of our sun and a hot blue companion star.

우리에게 익숙한 시그너스 X-1의 실제 관측자료를 보자. 시그너스 X-1은 항성급 쌍성계 블랙홀로 블랙홀의 질량은 태양의 15배 가량이고 뜨거운 청색 동반성을 가지고 있다.

If we look at the X-ray spectrum of this source, we can see a large bump at lower X-ray energies which is explained by the thermal emission from the accretion disk. The bump's long tail, as it is sometimes referred to, extends to higher photon frequencies or shorter wavelengths. The spectrum looks pretty similar to what we were expecting to see. So what have we seen during a different observation of Cygnus X-1? Well, something quite different.

The blue line has a steeper incline increasing towards higher energies and seems to peak in the same area as the plot that we would expect to see the corona. So what's happening here? The truth is scientists haven't collected enough information to know for sure. This is the type of change that drives astronomers to continue investigating these sources and for them to ask the questions about what could be causing these changes.

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Our view of black holes changes over time. Sometimes we can get more emission from the disk, while at other times we can receive more photons from the corona. What physical mechanism could be driving this?

블랙홀 관측자료가 시간에 따라 다르게 나온다. 어떤 때는 강착 원반 방출 특성을 보이고 또 다른 때에는 코로나에서 방출 특성을 보인다. 이런변화를 일으키는 물리적인 장치는 무엇일까?

One of the leading theories is that the change in the amount of food available to a black hole changes the portion sizes of each component. As the amount of food from say, a companion star changes, the serving size of a disk could decrease while the corona increases.

유력한 이론은 블랙홀에 주어지는 음식의 양이  각 구성요소에 변화를 준다는 것이다. 일테면 동반성이 공급하는 물질이 원반의 크기를 줄이는 역활을 하면 코로나는 증가한다는 것이다.

When we look back at the image we built up during module eight, we saw the disk extending towards the black hole with a corona that could either be explained by the lamp post model or the sandwich model with light coming from both the corona and the disk as material moves in towards the black hole.

If our portion sizes are changing, how does this affect the view of the system? Assuming we could go for a visit. Well, let's simplify things to start off with. We're going to stick with sandwich model for the rest of this video, given all of the food.

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Here, the filling, or a disk, takes up a lot of our view. So we could say it's dominating the picture. In fact, it's stretching all the way down to the inner most stable circular orbit or ISCO. This is when the disc is at its brightest. It can be so bright in fact that the emission from the accretion disk can be the brightest component in the optical band of the spectrum. When this is the case, we would not be able to see what kind of star the black hole is consuming. The bread of our sandwich is almost missing. The corona is so thin and wispy that we don't really receive too many photons from it.

The sketch we see here of our spectrum matches quite closely to the spectrum of Cygnus X-1 shown in red. Astronomers call this the high state. This name is historical as it comes from the early days of X-ray astronomy. It's known as the high state because it's a higher luminosity. It was the brighter option. Have we mentioned astronomers like simple names?

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Following that line of thought, the next state I would like to mention is the low state. So named because it's the fainter one. When black holes are in the low state the sandwich is switched. Where the disk may feel thin sort of stretched like too little butter scraped over too much bread. In the low state, the innermost part of the disk is not at the ISCO, it is found some distance away. We learned earlier that we can think of the disk as being made up of a series of many rings.

As we progress inwards through the disk, the temperature of each ring increases. This means that the highest temperature we detect from the disk, also known as the peak temperature, which come from the innermost ring of the disk.

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In the low state, the inner disc is further away from the black hole. This means that the disk spectrum is cooler and so shifted to the left of our plot. We also find the disc is fainter. So faint in fact that this is a great time to check out the companion stars of the black hole. Here, we have a lot more bread for our sandwich with the corona dominating the innermost region around the black hole. With this increasing corona, we receive more photons as it begins to dominate the X-ray spectrum. This is more akin to the spectrum of Cygnus X-1 shown here in blue. These two strikingly different views of the same source belt with the same components.

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But what about the jet? It turns out, although the disc and corona appear to be permanent features, the jet is not. The jet is strongly associated with the low state. Although astronomers don't fully understand how jets are launched, they have found strong ties between the emission seen in X-rays and the radio emission.

By combining information relating to the brightness of these electromagnetic bands, you can obtain estimates on the mass of the central black hole.

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In order to investigate the high state and low state that had been seen in the emission from stellar-mass black holes in binary systems, astronomers continue to make observations of these sources. Over time, they found that black holes could get even brighter, which seem to correlate with the change in the shape of the spectrum. This called for a new accretion state known as the very high state.

If we break this model apart to build a picture of what we would find there there is both lots of disc possibly extending to the ISCO and a lot of corona. In this case, we seem to have a sandwich that is more balanced with both a good amount of filling and bread, tasty. During the very high state, it's possible to see the jet although it's not always present.

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We have explored three black hole brightness states. In addition, some astronomers are lobbying for a fourth intermediate state that seems to live somewhere between the high and low states we've already discussed. But how do these states relate to one another? By looking many times at multiple sources, astronomers have seen cycles emerging within many systems. Cycles start with black holes that are either off or in the low state. When a feeding frenzy occurs, the black hole will rapidly brighten. This can take as little as hours to occur. Given the rapid rise, many times this can be missed by observers.

After the low state, the black hole transitions to the high state and possibly even the very high state or beyond. Black holes can hang out in the high state for a while depending on their food source. With some sources like GRS 1915+105 seeming to stay in this state for decades. Towards the end of their dinner sitting, they slowly return back to the low state before feeding away. These cycles are called outbursts.

These outbursting cycles can also take place in supermassive black holes but over much longer time scales, with outbursts lasting centuries, too long for an astronomer to observe in their lifetime.

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[처음][이전][다음]

09.06 - 은둔자 블랙홀(The Hermits of the Black Hole Family)

09.06 - 은둔자 블랙홀(The Hermits of the Black Hole Family) [커세라 강의 페이지]

Until now, we have only discussed black holes that are either in a binary star system, or located at the center of a galaxy. Surely, those are the only ways to find black holes. Indeed, scientists think that there are isolated black holes lurking within our galaxy. So, how do we detect them?

지금까지 쌍성계의 블랙홀, 은하중심의 블랙홀에 대해 이야기 했다. 이런 블랙홀들은 [직접적이진 않지만] 관측 할 수 있었다. 과학자들은 우리 은하 여기저기에 잠복한 독립 블랙홀이 있다는데 동의한다. 그럼 이 블랙홀을 관측할 방법은 무엇인가?

We need to remember the presence of gas between stars called the interstellar medium. If an isolated black hole travels through a gas cloud, we would expect to see some of the gas to accrete onto the black hole. The accreting gas should emit x-rays, which could potentially be detected.

성간물질이 블랙홀로 강하게 빨려들어가면 X선이 방출될 것이다.

Although astronomers detect lots of x-rays emitted from gas clouds, conclusive evidence for isolated black holes using this method hasn't been detected.

비록 천문학자들은 가스 구름에서 X선이 방출되는 것을 관측하긴 했지만 이것이 블랙홀의 증거라고 결론지을 만한 것을 찾아내진 못했다.

If there isn't light being generated by an isolated black hole, is it still possible to detect them? Of course. Black holes change the gravitational field in their local environment. So, light passing by is influenced by the gravitational field. Since black holes can strongly warp space-time, they cause light to travel on curved paths. We see the light from behind the black hole as being warped by gravity. This is called gravitational lensing.

독립 블랙홀이 빛을 방출하지 않는다면 블랙홀을 발견할 다른 방법은 없을까? 물론 있다. 무거운 블랙홀이 주변의 시공간을 왜곡하여 뒷편의 별빛을 휘게 만든다. 바로 중력 렌즈 라는 것이다. 

 

Here's a computer-generated starscape showing the constellation of Orion along with the bright stars Procyon and Sirius. Brighter stars are represented by larger circles, and dimmer stars by smaller circles. Light can't escape if it enters the black hole's event horizon. So, if a black hole were to get in the way of our view of Orion, we might see something like this computer generated image, with a dark circular region where we see no stars.

The black circle corresponds to the event horizon of the black hole. The black hole is blocking our view in the same way a cat blocks your view when it decides it's time for attention. But if we look carefully at the area around the dark region, it looks as though many more stars have appeared around its outer edge. The strange appearance of these additional stars is an optical illusion. What we're actually seeing is multiple images of the stars that reside in the background. For instance, you should be able to see two images of the group of three belt stars on either side of the black hole. Similarly, you can see two images of the very bright star called Sirius. What we would see is more complicated than just a black circle in the sky. Instead, light from the faraway stars curves around the black hole, and arrives in our eyes as though the star is located at many locations. The black hole's gravity distorts the image of the background stars, giving away its presence. In reality, there is no black hole close enough to give us a view like this distorted picture of Orion, but we can use the concept of gravitational lensing to identify some isolated black holes.

If this lensing effect is difficult to wrap your head around, you probably feel like a photon that's been bent in the space-time around a black hole. Let's have a look at a real-world example of image distortion by your dining room glasses. Stemmed glassware are simple vessels that can be filled with a liquid. Since an empty glass has curved edges, it bends and warps light just like a lens. When the glass passes in front of a picture, like this star map, a distorted view of the background becomes visible. Since the curvature of the glass allows multiple images of the same object, we see multiple images of some stars. Depending on where the stars are, they can also be distorted into rings.

When gravity is weak, light travels on paths that we consider straight. Since gravity can distort space-time, photons are forced to travel along geodesics, which are curved paths that bend around the black hole. To us, the resulting images have multiple views of the same object along with great arcs of refracted lights similar to what we saw in the glasses. For this reason, when the gravity of a massive object curves the path of light, we call the massive object a gravitational lens. A gravitational lens can be a star like our sun, a galaxy, or a black hole. The more massive, the better.

When the faraway object, the nearby mass, and the earth have perfect alignment, the image that we see is called an Einstein ring. This diagram demonstrates the perfect alignment between the Earth and nearby galaxy at a faraway star. Light from the star can be emitted on paths that go around the galaxy and reach the earth in many different ways. The light can go over or under or beside the galaxy. The result is that the light in our telescope from the distant star looks like it comes from a ring in the sky that surrounds the nearby galaxy. Here is an example of an Einstein ring captured in a photo taken by the Hubble Space Telescope. The fuzzy orange blob in the center of the picture is a nearby galaxy. The blue circular yellow is a distant galaxy lying behind the orange galaxy. The mass of the nearby orange galaxy warps space time, and the light from the faraway blue galaxy appears like a ring due to the gravitational lensing effect. The ring is not a perfect circle due to the fact that the orange galaxy's mass is not located at one point.

If the source of light and the lens' mass do not line up perfectly, we see multiple copies of the same faraway galaxy. In this picture, we see four lights that are all images of the same faraway quasar that is behind the nearby galaxy. The nearby galaxy is the fuzzy light in the center of the four quasars. This is called an Einstein Cross since it resembles a cross.

When astronomers view images of gravitational lensing, they can compute the mass contained in the nearby galaxy. In many cases, the mass calculated from gravitational lensing is larger than the mass inferred from looking at the bright stars. This is one method used to show the existence of dark matter in galaxies. Although black holes could be a type of dark matter, most dark matter is not made of black holes.

The size of an Einstein ring is related to the mass of the nearby object. For the images that we've shown, the nearby mass is a galaxy and galaxies have gigantic masses larger than 100 billion suns. The large mass gives a large deflection and a bigger looking Einstein ring. If the lens mass is small, where small means similar to the sun's mass and the distance is far from us, then the size of the ring will be too small for a telescope to resolve. Instead of seeing a ring, the light from the faraway star will appear brighter. This situation is called gravitational microlensing. The mass causing the lensing could be a dim star like a brown dwarf or a black hole.

Astronomers have been monitoring many stars in a nearby galaxy to look for the microlensing brightening effect due to an isolated black hole in our galaxy. If the black hole is traveling between the Earth and the faraway stars, then they will appear brighter while the black hole is in front of them. This gives us a view of a star that appears brighter for a short time period.

Microlensing by black holes is very rare, but it is seen occasionally. This image shows a gravitational lensing event that occurred in 1996. The top panel shows a dim star on April 28th and the bottom panel shows the same star on November 15th. The image on November 15th is brighter and a further analysis show that the star appears brighter because a black hole with a mass around ten times larger than the sun passed by. Only a handful of black holes have been found this way since this is a really rare event.

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[인터뷰] [커세라 페이지]

What does microlensing have in common with exoplanet searches?

Interview with Dr. Kelsey Hoffman, Researcher at the SETI Institute

So there is two different main aspects of it. One is looking for objects, whether they be planets or compact objects. The other part is understanding the structure inside these objects. So first of all, the looking for them is through the transit method, where you have your source, like your sun-like star and a planet that orbits in between you and the sun, or the star. When a planet does this though, you'll see the light from the source decrease. Now when it comes to compact objects, and you may be replaced like an Earth-like planet with a white dwarf which has the same radius. When that is going in front because of the mass of that white dwarf, it actually acts as a lens and magnifies what you see. Then in this case, you get something instead of the light going down, it increases in what we call microlensing. So in the gravitational lensing, the mass of the object allows the light will now bend around the object instead of being blocked by it. In these cases you get an amplification because the light bending will create different images of this and so then it increases the light that you see.

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[처음][이전][다음]

09.05 - 궁수자리 A*에는 특별한 것이 있다(The Special Case of SGR A*)

09.05 - 궁수자리 A*에는 특별한 것이 있다(The Special Case of SGR A*) [커세라 강의 페이지]



Our sun resides in a little corner of the universe known as the Milky Way Galaxy, so named because of the Milky path that leads across our night sky. The Milky path that we see is made up of about 100 billion stars, each in a lazy orbit around the center of the galaxy that takes around 200 million years.

An observer on earth, looking towards the center of the galaxy would see the constellations of the Scorpius, the scorpion, and Sagittarius the archer. I think Sagittarius looks more like a teapot. Sagittarius is where our central supermassive black hole gets its name. Sagittarius a star, which is sometimes shortened to SGR A* as nickname.

This constellation is easiest to observe in July and August, when it's visible in the evening sky. Looking at the constellation of Sagittarius, we see that the bulge of the Milky Way is located at the westernmost end of the teapot. Since Earth hangs out in one of the outermost arms of the Milky Way galaxy, tilted with respect to the plane of the galaxy, it's actually easier for observers in the southern hemisphere to enjoy the view of the galactic bulge. Notice that the more we zoom into the center of the galaxy, the more crowded the stellar environment becomes.

Even though we call Sagittarius A* a supermassive black hole, it's a lightweight contender in the supermassive category. One of the reasons SGR A* isn't bigger is that it isn't presently eating very much at all. It sits in a region with many stars but none of them have strayed within gravitational reach for SGR A* to grab.

[궁수자리 A*]는 초거대 블랙홀이긴 하지만 그 급내에서는 작은 편이다. SGR A*가 크지 못한 이유는 주변에 흡입할 별이 많지 않아서 크게 성장하지 못했다.

So, what's so special about Sagittarius A*? Well, it's probably the only supermassive black hole within about 2.5 million light years. The next nearest known supermassive black hole is at the core of the Andromeda galaxy.

SGR A*이 특별하다는데 왜 때문이죠? 2백 5십만 광년 이내에서 발견할 수 있는 초거대 블랙홀로서는 유일하다는 점이다[우리은하 중심에 있다. 지구에서 약 2만 5천 9백 광년 떨어져 있다]. 그 다음으로 가까운 초거대 블랙홀은 안드로메다 은하 중심에 있다[2백 55만 광년].  

Due to the proximity to Earth, SGR A* is the most studied supermassive black hole, and we are fortunate that it isn't currently eating. This gives us an excellent opportunity to see the environment around it without being blinded by the glare from the accretion disc.

지구에서 가까운 만큼 많이 관측되고 연구된 초거대 블랙홀이다. 그리고 아주 다행스럽게도 현재 흡입중이 아니다. 이는 강착원반으로부터 나오는 방출광이 없으므로 관측을 방해하는 요소가 없다는 뜻이다.

  

If SGR A* isn't currently feeding, how do we even know it's there? While scientists using ESO's very large telescope, have actually image the core of the galaxy and revealed the motion of the stars and dust clouds surrounding the central black hole.

From 2000 to 2011, this video shows the stars orbiting Sagittarius A*. Which reveal to astronomers just how massive the black hole is.

 

2000년에서 2011년사이에 궁수자리 A*을 관측한 사진을 이어붙인 영상이다. 블랙홀의 질량을 짐작케 하는 사진이다.

In fact, one of the objects called G2, isn't a star at all, but a cloud of molecular dust. Researchers predicted that G2 would be captured by the gravity of the supermassive black hole, with a collision around 2014. They predicted it would be consumed by Sagittarius A*, causing it to light up in the X-ray spectrum. However, when G2 passed the central object not much happened. It's still a mystery why G2 wasn't eaten by Sagittarius A*.

[블랙홀 인근에] G2라고 불리는 천체가 있는데  별이라기 보다 분자먼지 구름이다. 연구자들은 G2가 초거대 블랙홀에 의해 붙들려 들었다가 2014년경 충돌하리라고 예상했다. 궁수자리 A*에 빨려들어 가면서 X선 방출이 있을 거라고 기대 했다. 하지만 G2는 [블랙홀이 있을 것이라고 짐작되는] 중심부를 비켜가면서 큰 사건은 없었다. 지금도 왜 G2가 초거대 블랙홀에 의해 먹히지 않았는지 미스테리로 남아있다.

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[인터뷰] [커세라 페이지]

Why is Sagittarius A* important?

Interview with Dr. Fiona Harrison, Professor at Caltech University

Sagittarius A star is the very heart of the Milky Way. And at the very heart of the Milky Way, there's a supermassive black hole that's 4 million times the mass of the sun. But what's mysterious about this black hole is unlike many others that are quite active, there's a lot of dust and gas around it, but it's very quiet. It doesn't emit much radiation. You can see it in the radio. And about once a day, you can see it flare in the X-ray. And what these flares can teach us is what's going on in the region very close to this supermassive black hole. And eventually, we're going to have a huge array of radio telescopes that may even be able to image the event horizon of this supermassive black hole. It will be our first real view of the event horizon. We could tell, for example, if this black hole is spinning or not, which would tell us something about how it formed. And so by studying Sagittarius A star across the entire spectrum, because it's so close, we can learn what's going on with these dormant black holes. What is the center? It's the closest supermassive black hole that we have to study.

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[처음][이전][다음]

09.04 - 대식가 초거대 블랙홀(The Alternative Diets of Supermassive Black Holes)

09.04 - 대식가 초거대 블랙홀(The Alternative Diets of Supermassive Black Holes)

[커세라 강의 페이지]



Much of our discussion around the feeding of black holes has revolved around stars, the snack of choice for Stellar-mass black holes. Gourmet!

블랙홀은 주변의 별을 흡수하는 항성급 블랙홀에 대해 논의했었다. 이 항성급 블랙홀은 입맛이 까다로워 주변을 도는 동반성을 골라 먹는다. 미식가인 셈이다.

Since stellar-mass black holes make up the majority of binary pairs we associate with stellar companions, what do you think a supermassive black hole likes to snack on?

초거대 블랙홀도 항성급 블랙홀이 쌍성계를 이루는 동반성을 주로 흡입하듯이 초거대 블랙홀도 그럴까?

Well, if you were thinking surely they also eat stars, you'd be correct, but they are also voracious consumers of the gas and dust that happened to fall in too close. Some of the most spectacular phenomenon theorized to occur in the environment around a supermassive black hole are called tidal disruption events.

물론 초거대 블랙홀도 별을 잡아먹지만 근처에 떨어지는 것은 뭐든 먹는다. 초거대 블랙홀 주변에서 일어나는 굉장한 광경은 이론적인 '조석 파멸 사건'이다.

These feeding frenzies are so violent that the stars being consumed are completely torn apart. During a tidal disruption event, stars passing close to a supermassive black hole are disrupted by the strong tidal forces of the black hole's gravitational field.

(초거대 블랙홀 주변의) 조석력이 얼마나 센지 빨려들어가는 별이 온전히 제모습을 가누지 못하고 부숴진채 빨려 들어간다.

The tidal forces deform the star into a long string of hot glowing gas, just like this artist's impression of what a disrupted star might look like.

Astronomers can detect these tidal disruption events as a sudden increase in the brightness of light around the black hole.

However, direct imaging of these disruptions is not yet possible, which is why we only show an artist drawing here. The gas from a destroyed star then accretes onto the disk of the supermassive black hole feeding its insatiable appetite.

This effect was used in the Doctor Who episode, The Impossible Planet, when the Scarlet system which was home to the Baluchi was drawn out into a red cloud before being accreted onto a black hole.

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Supermassive black holes can feed on stars, but the gas that the black holes devour doesn't have to come from a disrupted star. Are there other options? What about clouds of dust and gas?

초거대 블랙혼은 별을 흡입한다. 하지만 이 블랙홀이 게걸스레 흡입하는 가스는 붕괴된 별에서 나온 것들 만은 아니다. 그렇다면 빨아들이는 다른게 있을까? 먼지나 가스 구름 일까?

The centers of galaxies can be pretty messy after all. Astronomers are puzzled by the sheer size of supermassive black holes. The mass of their interiors came from somewhere, and until recently, scientists thought that supermassive black holes feed on a steady diet of hot ionized gas from the halo of the galaxy.

은하의 중심은 매우 밀집되어 있을 것이다. 천문학자들은 초거대 블랙홀의 기형적인 크기에 놀라고 있고 최근까지도 초거대 블랙홀은 은하 중심의 밀집된 물질들(halo)에서 뜨거운 이온화된 가스들을 안적적으로 흡입한다고 생각했다.

Similarly, supermassive black holes can feed on cold molecular gas clouds, sometimes the remnants of ancient supernova explosions. In this case, the food is more like a soup with lumpy noodles and vegetables in it.

이에 덧붙여 차가운 가스 구름들 때때로 고대 초신성 폭발 잔해들도 흡입한다. 이것들은 뜨거운 국물에 겯들인 국수조각이나 야채라고 보면 되겠다.

In this image, the supermassive black hole is hidden away in the center of the galaxy in the cluster ABEL 2597, which is shown as blue light in this image. The red blobs show the location of cold clouds of molecular carbon monoxide gas.

[A Galaxy-Scale Fountain of Cold Molecular Gas Pumped by a Black Hole]

When a lumpy carbon monoxide chunk is consumed by a black hole, it temporarily blocks the light emitted from the accretion disk and jets like the black blob in the inset of this image.

All of this food talk is giving me indigestion, I hope I don't, excuse me.

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Supermassive black holes are not the most eloquent of dinner guests either. Not only do they demand more, they don't care whether you serve them a delicious apple pie made from scratch or a pile of rocks. While they do clean up after themselves, we can all agree that they need to work on their manners. No one is going to sit at a table with a supermassive black hole.

초거대 블랙홀은 뭐든 안가리고 먹는다. 그 누구든 옆에 앉아 같이 밥먹고 싶지 않을 것이다. [초거대 블랙홀이 가진 엄청난 중력을 고려해 보라.]

We've also learned that the jets of a black hole are hot gases being accelerated into interstellar space. This is kind of like a black hole burping. While gas flows inwards towards the black hole, the material and energy can be thrown out in the form of powerful relativistic jets.

On closer inspection, astronomers have also found that supermassive black holes can also eject gas from the region even closer to the event horizon.

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Here, we see the Whirlpool Galaxy, a beautiful example of two colliding galaxies. The smaller galaxy visible in the inset has a bright X-ray source visible in blue, which indicates the presence of a supermassive black hole. The blue arcs and bands are hot glowing gas being ejected from the region around the black hole, this is evidence of black holes burping. Excuse me again. How rude?

멋진 월풀 은하를 보자. 두개의 은하가 충돌하는 예다. 작은 은하에서 강력한 X선이 방출되고 있다. 이는 초거대 블랙홀의 증거라고 볼 수 있다. 청색 원호모양은 블랙홀 주변에서 뜨거운 가스들이 빛을 내고 있다. 블랙홀이 트림을 하고 있다.

We have a supermassive black hole at the center of our own galaxy, Sagittarius A*. I wonder what its favorite foods are.

우리은하도 중심에 초거대 블랙홀을 가지고 있다. 궁수 A* 다. 이 블랙홀은 무엇을 먹고 있을까?

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[처음][이전][다음]

09.03 - 블랙홀 쌍성계의 동반성(Companion Stars in Black Hole Binaries)

09.03 - 블랙홀 쌍성계의 동반성(Companion Stars in Black Hole Binaries)

[커세라 강의 페이지]

Stellar mass black holes are most easily identified when they are accompanied by a companion. The gravitational effect of a black hole on it's companion star can help give us a location where the black hole might be hiding.

항성질량급 블랙홀은 동반성을 관찰하여 수월하게 찾아낼 수 있다. 블랙홀이 동반성에 미치는 중력효과는 숨어있는 블랙홀을 찾는데 도움이된다.

If the black hole is actively feeding on the companion star, we will be able to see this clearly in the X-ray portion of the spectrum. In fact, because these systems are so bright in X-rays, they are often referred to as X-ray binaries. We should note however, that this term refers to systems containing a star and a compact object.

만일 블랙홀이 동반성에서 물질을 활발히 흡입하고 있다면 [강착원반이 고온이므로] 스펙트럼의 X선 영역에서 강한 빛이 관측된다. 실제로 이런 쌍성계는 X 선 영역에서 매우 밝으므로 X선 쌍성계라 불린다. 단, 이 X선 쌍성계가 별과 밀집천체의 쌍을 지칭한다는 점에 주목하자.

As we're aware, compact objects can be either neutron stars or black holes. As such, it is important that when we observe these systems we try and find the mass of the compact object. If the mass of the compact object is more than three solar masses, it must be a black hole. If it's lighter than that, it's likely a neutron star.

이 밀집천체란 블랙홀일 수도 있고 중성자 별일 수도 있다. 이런 쌍성계의 관측에서 밀집천체의 질량을 파악하는 것이 중요하다. 밀집천체가 태양질량의 세배를 넘으면 블랙홀이고 넘지 않을 경우 중성자별일 가능성이 높다.

In some cases though, it can be very hard to tell the mass of the compact object. When astronomers are unsure of the characteristics of systems like this, they list them as a black hole candidate.

하지만 밀집천체의 질량을 알아내기는 어려운 경우도 있다. 그래서 천문학자들은 블랙홀 후보군이라고 분류한다.

There are two types of X-ray binaries: high-mass X-ray binaries and low-mass X-ray binaries. But this classification is not based on the mass of the compact object. It may seem strange to you at first, but X-ray binaries are classified by the mass of the companion star, not the compact object.

X선 쌍성계는 두가지 종류로 분류된다. 고질량 X선 쌍성계와 저질량 X선 쌍성계다. 하지만 이런 분류는 밀집천체의 질량에 따른 분류가 아니다. 동반성의 질량에 따른 분류다.

The companion stars in low-mass X-ray binaries have masses that are the mass of the Sun or smaller. High-mass X-ray binaries have companion stars that are at least 10 times more massive than the Sun.

저질량 X선 쌍성계의 동반성은 태양 혹은 그이하의 질량을 갖는다. 고질량 X선 쌍성계의 동반성은 최소한 태양질량의 10배 이상이다.

Any time astronomers come up with a classification like this, you'll find that some objects don't quite fit. So, we also have an in-between group that is sometimes called intermediate-mass X-ray binaries. But their properties are usually pretty similar to the low-mass X-ray binary group.

이런 분류에 맞지 않은 경우도 있다. 두 종류 사이에 낀 경우 중간급 X선 쌍성계라고 한다. 이 급의 특성은 저질량 X선 쌍성계에 가깝다.

Why would astronomers choose to classify binary systems based on the type of companion star, rather than the type of compact object?

쌍성계를 밀집천체 대신 동반성으로 분류하는 이유가 뭘까?

The reason for this classification is that the properties of the system depend more on the type of donor star than the type of compact object. What this means is that observations of these systems vary more dramatically if you compare high-mass and low-mass X-ray binaries than if you were to compare stellar-mass black holes and neutron stars that are both feeding on, say, a low-mass star.

쌍성계의 특성이 동방성의 형에 더 명확하기 때문이다. [블랙홀은 판별할 어떤 것도 보여주지 않는다.] 고질량 X선 이중성과 저질량 X선 이중성의 관측한 자료를 비교해보면 저질량 별로부터 물질을 공급받는 항성급 블랙홀과 중성자별의 차이보다 매우 극명한 차이를 보여주고 있다.

저질량 동반성을 가진 블랙홀과 중성자별은 관측자료상 구분이 어렵다. 이에 비해 동반성의 크기에 따른 쌍성계의 밀집천체에서 방출되는 X선의 차이는 매우 크다.

When their companion is a low-mass star such as in a low-mass X-ray binary, the gas from the companion star flows to the black hole via Roche lobe overflow that we studied in an earlier module.

X선 쌍성계의 동반성의 질량이 작은 경우 동반성에서  블랙홀로 흡수되는 가스 흐름은 로슈로브 넘침을 통해 이뤄진다.

Also, recall high-mass stars tend to have larger outflows of material in the form of powerful stellar winds. In high-mass X-ray binaries the mass loss through wind ends up being accreted onto the black hole. This is called wind fed accretion. However, we should note that high-mass stars can also feed black holes via Roche lobe overflow. Typically, low-mass companions are small in size, while high-mass companions are large.

고질량 동반성의 경우 항성풍을 통해 대량의 물질 흐름이 일어난다. 물론 고질량 별에서도 로슈로브 넘침은 일어나며, 저질량 동반성보다 규모도 훨씬 크다.

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Small stars can orbit closer to the black hole than large stars can. Kepler's laws of motion tells us that stars with small orbital separations orbit with faster speeds and take shorter amount of time to orbit. Low-mass X-ray binaries typically have short orbital periods that can range from less than an hour to many hours.

블랙홀 쌍성계에도 케플러 행성궤도 법칙이 적용된다.  저질량 X선 쌍성계의 경우 공전 주기는 수시간 가량이다.

Meanwhile, the larger companions in high-mass X-ray binaries orbit further away from the center of mass of the system and can take a few days to complete one orbit. This means that the feeding or mass transfer mechanism, and so the rate of mass transfer along with the orbital period, can be greatly impacted by the type of companion star.

이에반해 큰 동반성을 가진 X선 쌍성계의 경우 궤도 반경이 길어 공전주기는 몇일 가량된다. 따라서 블랙홀이 덩치를 불리는 물질 전달 비율은 동반성의 종류에 따라 크게 영향을 받는다.

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Stars are usually classified by observing their color in visible light, since this is the portion of the spectrum where they are usually the brightest. We've already learned that accretion disks around black holes will also emit some visible light. This means that if we want to view the companion star of the black hole, we'll have to wait until a black hole was finished eating a major meal so that the disk isn't emitting light which would otherwise pollute our image.

별이 내는 빛 중 가시영역 스펙트럼에서 가장 강한 파장[스펙트럼 형]에 따라 별을 분류해왔다. 강착원반 역시 가시광 영역의 빛을 낸다. 따라서 동반성을 관측 하려면 블랙홀이 물질을 다 빨아들여 더이상 강착원반이 관측 영상에 방해를 주지 않을 때까지 기다려야 한다.

* 모든걸 빨아들이는 블랙홀 대신 동반성을 관측해야 한다. 멀리있는 쌍성계 블랙홀을 관측할때 동반성과 함께 블랙홀의 강착원반에서 방출되는 빛이 포함 되므로 동반성 만을 정확하게 관측하여 판단하기 어렵다.

When astronomers want to classify a star, they look at it using different filters to determine the star's properties. Low-mass stars with masses less than the Sun's mass are dim and have colors that range from yellow to orange to red. High-mass stars are bright and are blue in color.

천문학자들이 별을 분류하기 위해 다양한 종류의 필터를 사용하여 별의 특성을 알아낸다.

 태양보다 작은 질량의 별은 어둡기도 하고 황색에서 적색계통의 빛을 낸다. 높은 질량의 별은 밝고 청색빛을 낸다.

Since low-mass stars are dim, they can be difficult to detect. So, sometimes we have trouble detecting the companion in a low-mass X-ray binary, and the binary is classified based on its X-ray emission instead. The companion stars in high-mass X-ray binary systems are usually easier to see since they are so bright, meaning that in many cases we can also obtain a detailed spectrum of the star.

어두운 저질량 별은 발견하기 어렵다. 따라서 저질량 X선 이중성에서 동반성을 찾기는 곤란하다. 그대신 X선 방출로 분류한다. 고질량 X선 이중성계에서 동반성은 아주 밝기 때문에 비교적 찾기 쉽다. 그리고 대부분 해당 별의 상세한 스펙트럼을 얻을 수 있다.

So, it isn't at all surprising that the first confirmed black hole, Cygnus X-1 has a bright blue high-mass companion star. However, accretion disks can also look very blue, bluer in fact than hot blue stars. This means that when the disk is bright, it can be incredibly hard to work out what kind of star is feeding the compact object.

청색 고질량 동반성을 가진 시그너스 X-1이 블랙홀로 확인된 것은 놀랄일도 아니다. 실제로 강착원반은 보통의 별보다 더 청색을 띄었다. 원반이 너무 밝아서 어떤 종류의 별이 밀집천체로 공급되는지 밝혀내기는 굉장히 어렵다. 

X-ray images of black holes are not quite as impressive to look at as some of the other types of images we've seen in this course. They can be fairly featureless with just a series of dots scattered in a black section of the sky, except, of course, when they suddenly change.

블랙홀의 X선 영상은 이 강좌중에 봐왔던 다른 영상들보다 더 인상적이진 않다. 급격한 변화를 보였던 점을 제외하면 검은 하늘에 그저 특징없는 몇개의 흩어진 점일 뿐이다.

The left-hand image shows an X-ray image of the sky near our old friend, Cygnus X-1. In the left image taken before June 2015, we see full bright, X-ray point sources. Cygnus X-1 is the brightest X-ray source in Cygnus and a high-mass X-ray binary.

왼편의 사진은 시그너스 X-1 인근의 X선 관측상이다. 2015년 6월 이전에 찍은 사진으로 X선 광원들이 밝게 보인다. 시그너스 X-1은 백조자리에서 밝은 가장 밝은 X선 광원으로 고질량 X선 이중성 이다.

Cygnus X-3 was the third X-ray source discovered in Cygnus and is a low-mass X-ray binary. At this moment, it is unknown whether there is a neutron star or a black hole in Cygnus X-3. 3A 1954+319 is also a low-mass X-ray binary, most likely harboring a neutron star. Cygnus A is a supermassive black hole, but it looks dim because it's in a galaxy far, far away, while the other sources are in our own galaxy.

시그너스 X-3는 백조자리에서 세번째 밝은 X선 광원으로 저질량 X선 이중성이다. 현재까지 블랙홀인지 중성자 별인지 확인되지 않았다. 3A 1954+319 역시 저질량 X선 이중성인데 중성자별로 추정된다. 시그너스 A는 초거대 블랙홀로 아주 멀리 떨어져 있는 은하여서 어둡게 보인다.

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A small X marks the spot where the low-mass X-ray binary, V404 Cyg, suddenly became as bright as Cygnus X-1 and Cygnus X-2 in June 2015. V404 Cyg is close to 8,000 light-years away from us. The companion is a type K star, which means that it's orange in color and has a mass that is just 40 percent of our Sun's mass. The black hole has a mass that is seven times our Sun's mass. So, there is no danger that this could be a neutron star masquerading as a black hole.

작은 십자표시는 저질량 X선 이중성으로 V404 Cyg다. 2015년 6월에 갑자기 시그너스 X-1과 X-2보다 밝게 관측되었다. V404 Cyg는 지구로부터 약 8천광년 떨어져 있다. 동반성은 K형 별로 태양질량의 40% 로 오렌지색을 띈다. 이 쌍성계의 블랙홀은 태양질량의 7배다. 따라서 블랙홀을 가장하지만 여지없이 중성자별이다.

In this movie, the black hole and it's accretion disk are the bluish white light at the center of the image. The accretion disk suddenly erupted on June 26th, 2015, emitting X-rays in all directions. These X-rays form a spherical shell front that expands and collides with dust clouds far away from the black hole. The red rings are X-rays that are reflected off the dust that lies between the black hole and the Earth. Although the wave front is a sphere, we see circles since the dust clouds are a series of surfaces between the black hole and the Earth.

관측 동영상에서 보면 중심의 청백색이 블랙홀과 강착원반이다. 이 강착원반은 2015년 6월 26일 갑자기 X-선을 전방향으로 분출 하였다. 분출된 X-선이 전방향으로 퍼져나가 블랙홀과 멀리 떨어진 먼지구름들과 충돌하였다. 영상에서 붉은 환은 블랙홀과 지구 사이의 먼지에 부딧쳐 반사된 X선의 모습이다. 분출된 X선은 전방향의 구형으로 퍼져나갔으나 원형으로 관측된 것은 블랙홀과 지구사이에 놓인 먼지 구름이 일련의 띄를 이루기 때문이다.     

Another example of a low-mass X-ray binary is the X-ray source, X9 in the globular cluster named 47 Tuc. A globular cluster is a dense star cluster that can have many millions of stars. Since the stars are closer to each other than in part of the galaxy where we live, the stars can easily hook up with other stars to form binary systems through dynamical formation.

So, if you were to randomly choose a globular cluster to look at with an X-ray telescope, you'd have a good chance of finding an X-ray binary. The X-ray binary X9 is still classified as a candidate black hole, since its mass has not yet been measured, but the orbital period is very small, only 25 minutes, and the companion star is most likely a white dwarf.

The companion star to the black hole Cygnus X-1 is easily seen as the bright star in the very center of this visible light image of the constellation, Cygnus. Since this is a visible light image, we can't see the accretion disk of the black hole. The red light is coming from glowing hydrogen gas in a nearby star forming region.

Cygnus X-1's companion star is named HDE 226868. But for obvious reasons, we normally call it Cygnus X-1's companion star. The companion is a type O supergiant that has a larger mass than the black hole. The companion star's mass is 19 times larger than the Sun while the black hole's mass is 15 times the Sun's mass.

The two objects orbit their common center of mass, which is closer to the companion once every 5.6 days. Both high-mass and low-mass X-ray binaries are spotted scattered throughout galaxies. They're relatively easy to spot because the black holes have their dinner sitting right there next to them in the binary system.

What happens when we switch up to other size scales? What are the alternative diets for supermassive black holes?

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[인터뷰] [커세라 페이지]

What is a black hole outburst?

Interview with Dr. Aarran Shaw, Astronomer at the University of Alberta

But outburst is a sudden increase in luminosity of a source. A source will go from bubbling around in what we call this low quiescent state, where you either don't see it because it's just not giving off enough x-rays, or optical lights, or it's existing at a lowish luminosity that we can't see. Then, it will increase its luminosity or its brightness by a factor of 1000, and that's what we call an outburst. Then it will start to decay back to this level that is known to be normal for this source. There are a few black holes that jump out or you are being extremely cool, I guess is the word. So two years ago or 2.5 years ago now, there was a very bright outburst of an X-ray binary called V404 Cyg, and this was the first time this source had been seen in outburst for 26 years. So outburst in 1989,

Play video starting at 1 minute 14 seconds and follow transcript1:14

is one of the most famous black holes in the galaxy. The great thing about outbursting in the modern era of astronomy is that, we have a huge array of facilities our disposal. So back in 1989, we looked at it with optical telescopes, and maybe an x-ray telescope. There weren't many around, but now we have all sorts of facilities that we can coordinate via the power of the Internet essentially to look at this thing at the same time. So we have optical telescopes looking at it at the same time as radio telescopes, and all the x-ray observatories that are currently in orbit around the Earth. From the simultaneous astronomy, we can actually find out how the different wavelengths of light interact with each other, what the interplay is there. So that was fascinating. Somewhere that I was involved with, we found that we could measure the base of the jet. So there's a jet that is coming out of the top of the black hole, and coming out of the poles of the black hole, and we can use this simultaneous x-ray and optical view to look at what the interplay is between the x-ray light and the optical light. We can see that the x-ray light, and the optical light are delayed by around 0.1 seconds. This gives us an idea of the size of the base of this jet. I thought that was absolutely fascinating, so that's really cool stuff that happened very recently.

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