Astronomer tells Wikinews about discovery of closest black hole known so far

Friday, May 22, 2020

Artistic impression showing the orbits of the triplet of the HR 6819 system. The red trajectory is of the black hole. The two stars are located in the Telescopium constellation and visible to the naked eye from the southern hemisphere. (Image: ESO/L. Calçada)

A study published in journal Astronomy & Astrophysics last month reported astronomers from the European Southern Observatory (ESO) and elsewhere discovered a black hole in the Telescopium constellation. The study stated the black hole is about 1010 ± 195 light years (310 ± 60 parsec) away from the Solar System, meaning it is the nearest known black hole from the Earth. The nearest previously known black hole — V616 Mon — the study noted was usually estimated at more than 3000 light years away.

The black hole described in the study is located in the HR 6819 stellar system of Telescopium constellation, making it the first system visible to the naked eye to contain a black hole. HR 6819 contains two stars, and they are visible from the Southern Hemisphere. The astronomers started observing the system in 1999. Initially, they thought it was just a binary system, consisting of two stars. However, upon examination, the researchers concluded there was a third unseen object in the system. One of the two stars in the HR 6819 system is close to the black hole and orbits the black hole in just 40.333 ± 0.004 days.

A supermassive black hole at the centre with matter around it forming an accretion disk. (Image: Event Horizon Telescope)

This newly discovered black hole does not have an accretion disk. A black hole forms an accretion disk when a significant amount of matter orbits the black hole, as depicted in the image. Accretion disks often emit electromagnetic radiation. Since this black hole does not have an accretion disk, researchers had to rely on the gravitational effect of the black hole on the nearby star in order to discover it.

Researcher Thomas Rivinius discussed the findings of this study with Wikinews. (Image: acagastya)

Researchers used the binary mass function to conclude the black hole had a mass of at least 4.2 M (Solar masses; 1 Solar mass = mass of the Sun). Its companion star, which orbits the black hole in about 40 days, is classified as a B3 III star. The outer star is classified as a Be star. Be stars rotate very quickly around their axes. Since the outer star rotates so rapidly, the star is not exactly spherical, but instead oblate, bulged at its equator, forming a gas disk around the equator.

The research suggested HR 6819 was very similar to another system LB-1. The HR 6819 system is estimated to be between 15–75 million years old (myr). The inner star has estimated mass of at least 6.3 ± 0.7 M. Using the mass and the speed at which the inner star rotates, the researchers concluded the black hole had an estimated mass of 5.0 ± 0.4 M. Researcher and co-author of the study Thomas Rivinius told Wikinews the inner star and the black hole are closer than the Sun and the Earth (1au; 150 million km; 93 million miles).

The researchers dedicated the paper to Stanislav Štefl, one of the fellow researchers who died in a car accident in 2014 in Santiago, Chile.

Wikinews caught up with Thomas Rivinius to discuss about this discovery.

Interview with Thomas Rivinius

 

Thank you agreeing for this discussion by the way.

 
The telescope is a part of Very Large Telescope Interferometer (VLTI). VLTI combines the light from multiple telescopes for a detailed view. (Image: European Southern Observatory)

 ((WN )) I would like to ask you about the discoveries that you and the team made. And about the future investigations.

 ((Thomas Rivinius )) Ah, well yes, do you have any specific question? I mean, the future investigation is to make sure that it really is a black hole and we will probably do that by means of Interferometry. What we ultimately want to get is: you probably have seen the animation, it is a sequence of images that looks like this animation. The two stars moving around: not around each other, but with a third object.

 ((WN )) What prompted your interest in astronomy?

 ((Thomas Rivinius )) Oh, astronomy in general. That is very long. You see, when I was maybe ten years old, I was already very interested in astronomy and started observing from backyard with a small telescope. It's actually that got me into physics and mathematics, and then I studied physics. There was a time I was not sure if I would end up in astronomy or particle physics or something else. But in the end, I always decided to continue the astronomy path.

 
The two bright starts are Alpha Centauri on the left and Beta Centuari on the right which is a binary system. (Image: Skatebiker)

 ((WN )) Could you briefly explain this discovery and its findings?

 
Depiction of a Be star. Be stars rotate so quickly that they bulge at the equator, forming an egg-shaped structure and gas disk at the centre. (Image: Fred the Oyster)

 ((Thomas Rivinius )) Well, it is a long story indeed. Because originally we thought it was a binary star — just two objects — not three. And that would have been interesting because these two stars are similar in many senses. Like they have a similar mass. They are similarly old. They are consequently of similar temperature, et cetera. But in one aspect they are very different. Namely, one is rotating very very fast. It is a so-called Be star. Which means it is rotating so fast that it is almost flying apart. There's actually material flowing off the equator. And the other is rotating rather slowly.

So the original investigation was to figure out why two stars, that are so similar — that must have formed together — why they would be they so different. And now this is not something we could answer in the end because the distant system is of a different structure. But instead we found that it is of a different structure and there are three objects, but only two of them visible. And that means, if you put the data we get into Kepler's laws and Newton's laws about gravity and the rotation, you find that the third object must be very massive. And if you don't see it but it is very massive, that means it cannot be a star.

 ((WN )) And that is how you concluded it was a black hole?

 ((Thomas Rivinius )) Yes. How we concluded it's a black hole is we derive the mass from these Newton's and Kepler's laws. And then we see if, given the quality of data we have, if it would be possible to detect the star with such a mass. And we went through several options down to the faintest possible stars we could imagine, with such a mass, and there is no such object. Our data is good enough that we would have seen such a thing. And therefore, if it is not luminous, it must be a black hole.

A wide-field view shows the region of the sky, in the constellation of Telescopium, where HR 6819 can be found. (Image: ESO, IAU and Sky & Telescope)

 ((WN )) What drew the team's attention to this stellar system, HR 6819, in the Telescopium constellation?

 ((Thomas Rivinius )) Well in the first place, because it was in a catalogue of these extraordinarily rapidly rotating stars: the Be stars. They are really so rapidly rotating that they are oblate. They are not round, not spherical anymore, but they are oblate. And they have a gas disk that is flowing off the equator because they rotate almost at critical [velocity]. And that is basically my research, in what is forming this.

 ((WN )) Why did the team start observing HR 6819 specifically?

 ((Thomas Rivinius )) As I said, because it's a Be star. There are about 100–200 Be stars which are of that brightness. And maybe 150 visible from the southern observatory of La Silla. And so over the many years, we have taken, not all, but a fair fraction of them, and we started off with relatively few spectra, with only ten spectra. And with this, we slowly sought our way through which are interesting and which are not and at some point, this [system HR 6819] did strike us interesting because we thought it was binary.

 ((WN )) And when did the research team start observing?

 ((Thomas Rivinius )) The first observations, these few spectra were taken 1999. And the second set of spectra was taken 2004.

 
A supernova explosion often takes place when a massive star dies. It is one of the stages of a star's life cycle (Image: ESO/M. Kornmesser)

 ((WN )) What was the most fascinating aspect of this finding?

 ((Thomas Rivinius )) Well, for me, it's the realisation that black holes are very common. I mean, I knew the numbers: that you get about one black hole per thousand stars. But that actually means there must be some black holes rather nearby. And originally, you would expect most of them single, so that you would never find them, because they are very small, very black. Nothing to see there. But when they are in a multiple system, then you can see them through their gravitational influence on the other stars. And, so that basically was, I think, the best finding, that it is a multiple system — a stable multiple system. Many stars are in multiple systems. But when a black hole is formed via supernova, you would actually think that many of these systems get destroyed: disrupt[ed] by the explosion. This did not happen.

 
Visualisation of doppler effect for the sound wave of a car's horn. (Image: Charly Whisky)
 
Optical spectroscopy of a binary stellar system. (Image: European Southern Observatory)

 ((WN )) What technologies were used for this discovery?

 ((Thomas Rivinius )) Spectroscopy. In other words, spectroscopy is a technology by which you spread the light, you resolve the light into the individual wavelengths. And then you can see the signature of the chemical elements as they form in the star. And these signatures — you usually assume they have a fixed wavelength. If you observe them at a slightly different wavelength, this is due to the doppler effect. And so with this doppler effect, we could see that one of the stars was moving, not at a constant speed, but it was moving back and forth with a relatively high velocity. Amplitude of 60km/s [about 37 miles per second]. And a long period of 40 days.

 
The radial velocity method to detect exoplanet. (Image: European Southern Observatory)

As I said, with the combination of Kepler's and Newton's law that is called the [binary] mass function, you can use these numbers to get a mass of what it must orbit about. It's a principle, the same technology we use for finding planets, which is called radial velocity technique.

 ((WN )) How did you get involved in this study?

 ((Thomas Rivinius )) From the very beginning. As I said, I was interested in Be star[s]. Actually most of the team [...] We were originally arranging this observations of many Be stars and it just turned out this one was particular very interesting.

The ESO 1.52m telescope. (Image: ESO)
The ESO 2.2m telescope (Image: ESO)

 ((WN )) Apart from [the] European Southern Observatory, which other observatories were involved in this discovery?

 ((Thomas Rivinius )) Uh, none. So only European Southern Observatory plus the observatory at La Silla. Two different telescopes.

 ((WN )) Okay. Which were the two telescopes?

 ((Thomas Rivinius )) The 1.52 and 2.2m. The point is we always use the same instrument but it was moved at some point from ESO 1.52 to the 2.2m of the Max Planck Society.

 
Aerial view of La Silla Observatory. (Image: European Southern Observatory)

 ((WN )) Were all the observations made from the La Silla Observatory?

 ((Thomas Rivinius )) From La Silla not. There are other observations available, but we have not used them yet in this study. There are observations available that were made by amateur astronomers, there's a data based of amateur spectroscopy, there are observations in the archives that were made from the Paranal Observatory. So at some point, we are going to look at all those observations, but for the discovery purpose, we only used the observations that were taken from La Silla.

Portrait of Petr Hadrava. (Image: Akademie věd České republiky)
Portrait of Marianne Heida. (Image: European Southern Observatory)
Portrait of Dietrich Baade. (Image: European Southern Observatory)
Portrait of Robert Klement. (Image: ResearchGate)
Three of the co-authors of this study.

 ((WN )) How many people were involved in this discovery?

 ((Thomas Rivinius )) Right now the paper has I think five co-authors. And there is a sixth person, Stan Štef, who some years ago died in a car accident. And I believe — the paper is dedicated to him. He is not on the co-author list because this particular paper was written completely new. And if he never had a chance to actually even read the draft, then someone should not be co-author.

 ((WN )) What were the roles of the people involved in the discovery?

 ((Thomas Rivinius )) Marianne Heida and Dietrich Baade were the ones that originally triggered the idea. Well, it's a long study because originally, we studied if for a different purpose. In this different purpose, it was me, Dietrich Baade, and Petr Hadrava. And we wanted to study it as a binary. Robert Klement was a student of Petr Hadrava and he then also later and now assisted in the study of binary parameters. And Marianne Heida is an expert on X-ray binaries, which have a lot of black holes. So she was originally putting us on the track by presenting another paper on a black hole.

 ((WN )) What was the timeline of this study?

 ((Thomas Rivinius )) Well, it took 20 years, as I said; we started observing in 1999. This is a typical scientific discovery: you find something you are not looking for.

 
Stanislav Štefl was one of the researchers on this study. Štefl died in a car crash in 2014 in Santiago, Chile. The authors of this research dedicated the paper to Stanislav Štefl. (Image: European Southern Observatory)

 ((WN )) Which activity took the most time and attention?

 ((Thomas Rivinius )) Well, what took the most time was basically to realise that there is something that unusual in it. As I said, we originally thought it was something different. And then analysed it. Found it was not what we had expected and so on and all that drags it out. And then of course there's that Stan Štefl was a major driver in the study. He died in 2014 which basically stalled the study after that until now.

It was now that we picked it out because there was another paper appearing on a similar object, which, looking at that paper, I realised that HR 6819 must be of the same type.

 ((WN )) What was the most difficult part of this investigation?

 ((Thomas Rivinius )) As it basically is for any investigation: you very early on have an idea what it might be. And it is always, the most difficult part is to always to make it sure in such a way that also the colleagues would believe it. You have to explore any other options, any possibilities and exclude them one-by-one.

 ((WN )) What was your first reaction when confirming the unseen member of the triplet was a black hole?

 ((Thomas Rivinius )) Well it depends. I mean, the first suggestion that it was a black hole surfaced in 2010. But then we were very cautious. Because we were not quite sure. At that point we hadn't really excluded everything else. And so, this was a very cautious time. And then Stan died, but when I looked at it back in 2019, I managed to convince myself relatively quickly now, whatever I had learned in the past six years.

 ((WN )) How old is this system?

 ((Thomas Rivinius )) Judging from the two other stars that still exist in there, these two other stars must [be] between 15 and 70 million years old (myr). Probably closer to the high upper end. Probably closer to 70 million years. That is probably the age of the system.

 ((WN )) How rare is the occurrence of the stellar systems with black holes?

 ((Thomas Rivinius )) Well, when we are talking about black holes of the stellar mass, not supermassive black holes like are in the hearts of the galaxies, only 50 of them are known. And most of them are in binaries. Because, as I say, a single black hole is almost impossible to find. It is, however, one of the first we are certain of, or at least the one we are most certain that it is not in binary — one companion — but a triple, with two companions.

 ((WN )) How different are the stellar systems with black holes as compared to the ones without it?

 ((Thomas Rivinius )) Well, black hole is the end phase of the stellar evolution. So every triple system that has a star with higher than 12 solar masses, at some point must have a supernova explosion that results in either a neutron star or a black hole. The question is not — these progenitor systems, they are very abundant, they exist everywhere. The question is how frequently they survive these explosions. That is something completely unknown. But I suspect there are quite many of them.

 
Artistic depiction of a black hole devouring on a nearby star. (Image: ESA, NASA, and Felix Mirabe)

 ((WN )) This black hole does not have an accretion disk. What does this tell us about the system?

 ((Thomas Rivinius )) Not very much. It will eventually acquire an accretion disk. The two other stars are not very evolved. They are still small and compact. As soon as they start evolving — in particular, the inner one — it will grow, and once it has grown enough, the black hole will start to accrete from it. But there is a couple of million years to go still, at least.

Artist's animation of HR 6819. The red-coloured trajectory shows the motion of the black hole. (Image: ESO/L. Calçada)

 ((WN )) Tell us about the orbits of the two stars in HR 6819.

 ((Thomas Rivinius )) Well the inner orbit is relatively fast. It's 40 days. That's not an unusual orbital period, for two stars. But, the amplitude is very high: the speed with which they orbit each other is high: it is 60 km/s. And together with the 40 days, that makes a minimum mass of the other object. The outer orbit we don't know. It is very far away. It's probably, judging from the similarity to other systems [...], it is probably several decades. It could be even long[er].

 ((RS )) How close do these stars get?

 ((Thomas Rivinius )) Since the orbit is circular, they will always stay at the same distance — at least the inner two, the black hole and the other star. For now, they will stay at the same distance. They will not get closer or farther from each other. They are about as far as, closer than the Sun and the Earth (1au, 150 million km (93 million miles). The precise value is not known because it depends on the inclination angle of the system: whether we see the system edge-on, or whether we see it face-on.

 ((WN )) Is the black hole assigned an official name?

 ((Thomas Rivinius )) For naming, naming is the job of International Astronomical Union. So. Traditionally you would call it as Component A, Component B, Component Aa: something like that. But even that, I am not entirely sure how that would go. Because whether you do this by mass, or sometimes you call the "A" which the most massive, sometimes you call "A" which is the most brightest, so.

 ((WN )) Does the team refer to the name by an unofficial name?

 ((Thomas Rivinius )) No. Not really.

 ((WN )) What are some of the properties of this black hole?

 ((Thomas Rivinius )) Ah, black holes don't really have properties. It's mass. That's it. It has at least the mass of 4.3 Solar [Mass], which means it has a diameter of maybe about 30 km (18.64 miles). That's about it. There's not many more properties a black hole can have.

 ((WN )) When I asked about the properties of the black hole, I was hoping you would remark on the spin, Swartzchild radii and the charge.

 ((Thomas Rivinius )) The radius is about 15 km (9.3 miles), as I said. Nothing is known about spin or charge, but it is likely that charge is zero or close to that.

 ((WN )) What do we know about the history of this black hole?

 ((Thomas Rivinius )) Only that it comes from a supernova, and that the triple system still survives. That is basically telling us something about the explosion must have been symmetric in a sense. Because if it had been asymmetric, the system would have been disrupted.

 ((WN )) What are the affects of the black hole on the star closer to it?

 ((Thomas Rivinius )) None. It's pulling — the gravity — but another star would just pull the same. So it has no radiation. It has not accrete. It is only forcing it into an orbit. Nothing else.

 ((WN )) What were the challenges the team faced to discover this black hole?

 ((Thomas Rivinius )) Well, as I said before, it is basically to make sure that you have excluded all other options. It's really: science is really about excluding the alternatives and, so that finally only one possibility remains.

 
A computer simulation of a black hole with an accretion disk.
Image: acagastya.

 ((WN )) How are the black holes with the accretion disk different form those which don't have one?

 ((Thomas Rivinius )) Ah, if they have something to accrete, then something falls into the black hole. And in that process, they produce all sorts of emission. They produce X-ray emission, radio emission. And ours does too, but it really is just black.

 ((WN )) As you mentioned, x-ray emission helps detecting the black holes with accretion disk. What are some of the other ways we can detect black holes without accretion disks?

 ((Thomas Rivinius )) Ah, I don't think there's many other ways. You wouldn't get them unless they have another object which you can detect. And then you have to infer it from the gravitational effects, like its orbit.

 ((WN )) Do these stars orbiting the black hole have any known planets?

 ((Thomas Rivinius )) Ah, no, and considering their type, it is rather unlikely. These kinds [...] as far as I know don't really have planets. They are too violent, the deformation process is too massive. Planets are normally around older or less massive stars.

 
Srtistic impression of the LB-1 system (Image: YU Jingchuan, Beijing Planetarium)

 ((WN )) The report mentioned the spectroscopic time series of HR 6819 was similar to LB-1. Tell us more about the system LB-1.

 ((Thomas Rivinius )) It's basically the same. This was a press release form the Chinese National Academy of Sciences in November. And back then they claimed it is only a binary, one is a normal star, the other is a black hole. It has an accretion disk. And from this they infer it could have had very high mass. But from a technical point of view, this type of analysis is incorrect. It was, at least to me, looking at the system it looks fairly clear that the effect they ascribe to the black hole which they thought was an accretion disk must be an outer object — a third star nearby — of this Be type, which makes the emission line. So it is basically looking at the paper and realising that they are not right in their result but I know what it is, and that realising I have something similar just in my drawer.

 ((WN )) In what ways are LB-1 and HR 6819 similar and dissimilar?

 ((Thomas Rivinius )) They are similar in almost every aspect. The inner B star, as far as we can tell at the moment. The B star that is close to the black hole [of LB-1] is rotating even slower than in HR 6819. It could be that it has been a bit modified: that it already has either transferred mass to the black hole or the progenitor of the black hole has transferred mass[...] For [HR] 6819, there's no such indication. But otherwise, we believe they are essentially the same.

 ((WN )) In what ways would the Solar System be different if the Sun was in the inner star's orbit, orbiting this black hole?

 ((Thomas Rivinius )) It would not exist. As I said, planets simply would not form around such a massive binary. Planet formation around binary in principle is possible. But the inner star would have been a star with at least 12, maybe 20 solar masses. And these stars form very quickly, burn very quickly, die very quickly. Planets would simply not have had the time to form along such an object.

 ((WN )) No, considering our Solar System was there.

 ((Thomas Rivinius )) Right now? Well, if you just put a Solar System there, that exists, it would probably destroy Mercury, it wouldn't exist, it would be not in a stable orbit. Probably also Venus would not be in a stable orbit. In the region of Earth, the orbit could be stable. So maybe Earth could just so barely exist. The seasons would be different. There would be a strong 40-day modulation, which makes every 20 days — or every 40 days you get a lot of radiation. Every 40 days, you get only much less radiation. So it would be very hostile to life.

 
Barycentric radial velocities of the inner B3 III star (He iλ4026) (Image: acagastya)

 ((WN )) What were the different radiation emissions observed in this system?

 ((Thomas Rivinius )) There is only visible light so far. We know of no other [radiation spectrum].

 ((WN )) What are some of the other observed systems that have similar radiation emission?

 ((Thomas Rivinius )) As I said, this is from the point-of-view of the stars, these are just normal stars. And the black hole does not contribute anything to the emission of the radiation.

 ((WN )) What does the future of these stars look like?

 ((Thomas Rivinius )) The outer star will just evolve very normally just like any other star of its mass. The inner star will become a super giant as well. But when it ends its supergiant phase, it will be big enough to get accounted into the black hole and then the black hole will start accreting. It is hard to say what happens then. It may get big enough for the black hole to actually fall into the star, and then shortly after the star falls into the black hole, or it might end with the accretion.

 ((WN )) So you are saying there is a chance the black hole may create an accretion disk feeding on this inner star?

 ((Thomas Rivinius )) It will certainly. But it will take another few million years.

 ((WN )) Were you expecting a black hole to be so close to the Solar System?

 ((Thomas Rivinius )) Ah yes, as I said, there must be many black holes. I mean, there are certainly black holes which are closer. It's just not very likely to find them. They are not infrequent objects. They are interestingly very frequent objects. There are just very hard to find.

 ((WN )) What future investigations is your team planning for?

 ((Thomas Rivinius )) With interferometry, we want to image that, and maybe we want to look in the catalogues if we can find a similar system. We are looking for candidates where we might reasonably expect that it might be similar and then we can have a look in detail.

 ((WN )) What lies ahead in this system to be discovered?

 ((Thomas Rivinius )) Well, basically what we still have to do with this system is to make the numbers certain. Right now, what we have is limits. We have a lower limit for the mass. But what we can do is we get precise value for the mass. We only have a rough distance, we can get a precise distance. We can measure the chemical composition of the stars that still exists, if they in any way have been altered or affected by the original primary, when the most massive star went supernova. So it should tell us a lot, in principle, about heterogeneous systems about how such massive stars evolve.

 ((WN )) Are there any plans to observe the nearby binary stars for black holes?

 ((Thomas Rivinius )) As I said, we are looking into the catalogues. If we can find similar systems that could give us hints, that might be promising candidates. But even if they are frequent, they are not that frequent that we could observe every star and just look for black holes. That is not an efficient use of telescope time.

 ((WN )) Are there any other observations you have been working on?

 ((Thomas Rivinius )) Ah, well, there's a lot of things I'm working on. But they are unrelated to this particular project.

 ((WN )) What is your role at the ESO?

 ((Thomas Rivinius )) At the ESO, I am a science operations astronomer. Which means that I run the telscopes; I support other astronomers when they come here to conduct the observations, and when they cannot come, or do not want to come which also happens, I do the observations for them. And I do this about 60% of my working time, I work for the observatory in this capacity, and one third of my working time I do my own research.

 ((WN )) Well, those were all the questions I had for you. Is there something you would like to add?

 ((Thomas Rivinius )) No, I think that was fine, thank you.

 ((WN )) Thank you for agreeing for this interview. It had been a great pleasure discussing this with you.

 ((Thomas Rivinius )) Thank you. Ciao.

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