Black holes, those "cosmic ghosts" on which literature and cinema have based many of their stories, were no longer so mysterious when Phil Charles, Professor of Astronomy at the University of Southampton (United Kingdom), found in 1992, along with the IAC researcher Jorge Casares, the first empirical evidence of their existence in the Milky Way. From that moment, everything changed in the field of High Energy Astrophysics. Even today, he continues to investigate these binary systems, although he is also interested in all cosmic objects that are X-ray sources. Linked to the Instituto de Astrofísica de Canarias (IAC) since its beginnings more than three decades ago, Phil Charles has returned to this center as visiting researcher of the Severo Ochoa Program, willing to work on new projects with old and new colleagues.
By Elena Mora (IAC)
“If the most massive neutron stars we know are around two or just over two solar masses and the lowest mass black hole we found so far is around five, where are the low mass black holes and the high mass neutron stars?”
“By knowing the equation of state of a neutron star, we can work out how big it can become before it must then collapse to become a black hole”
“When matter is approaching the event horizon at significant fractions of the speed of light, we are seeing matter behaving in the most extreme physical conditions we know of, conditions we cannot possibly replicate on Earth”
Question: This is not your first time here at the IAC. When did you start collaborating with the institute and why?
Answer: There’s a long history to this. I first came to the IAC more than 30 years ago, shortly after the new IAC building was opened and in the early days of setting up the Roque de los Muchachos Observatory in La Palma. Then, in the late eighties, I had a 5-year spell as head of astronomy operations at the Isaac Newton Group of Telescopes (ING). It was during that time that I had a Spanish graduate student from the University of La Laguna (ULL), who was Jorge Casares, and he and I started work on optical observations of X-ray sources, and have kept doing that for most of the last thirty years. During that time most of our work has been associated with the black hole X-ray binaries.
Through my scientific links with people here at the IAC, I continued to work with both the ING Group and now with the Gran Telescopio Canarias (GTC). I’m delighted to see its success, but I’m also very excited to hear that La Palma might become host to the TMT, which I think will be a fantastic recognition of what a superb site La Palma is.
Q: What are you doing now at the institute? Which groups or researchers are you working with?
A: During this time I’ve been working with Jorge Casares’ X-ray binaries group, which is being looked after by Teodoro Muñoz Darias whilst Jorge is temporarily away in Oxford. But I have other friends and colleagues here as well, like the researcher Artemio Herrero. We are talking about a project on a high mass X-ray binary which uses his intrinsic expertise in very massive stars. During this last month it’s also been amazing because two of our black hole transients have been observed doing something interesting, and I’m hoping that we, Teodoro Muñoz and some PhD students, will get publications out of that. It’s been extremely exciting and very productive so far.
Q: You discovered, along with Jorge Casares and Tim Naylor, the first black hole in our galaxy in 1992. I’ve heard that it was a breakthrough in the field of high energy astrophysics. What was the impact of this discovery on your career?
A: I’ve always been interested in X-ray binaries although I did my PhD on X-ray emission from supernova remnants. But of course, those remnants include neutron stars and black holes and many close X-ray binaries. My first postdoc position was in the University of California in Berkeley (USA) in the late seventies and I started working there in looking at these objects, with both X-ray satellites and from the ground at the same time. Now, everybody does it, but not back in that time.
There are several groups around the world that were always been keen on observing what we think might be a neutron star or black hole. We can usually show whether it is a neutron star because it has characteristics that only neutron stars have. One of the most obvious is pulsations and there are other more subtle properties such as X-ray bursts. But it is much more difficult to show that you are dealing with a black hole because the X-ray properties are often mimiced by neutron stars, so the only way we can really do it is to measure the orbital period, and, in particular, the orbital motion of the black holes companion, and use basically Kepler’s laws of binary motion (to say, the mass of this object has to be at least greater than a certain amount).
The “Holy Grail” in the eighties was to find an object whose mass is at least greater than around three times that of our Sun, which theorists are confident is close to the maximum mass that a neutron star could have. There were several objects found around those minimum values and people said: “Well, maybe it’s a black hole, maybe it isn’t”. Then, when Jorge Casares and I started working on a particular new transient, called V404 Cyg, we discovered that the minimum mass of its compact object was over six solar masses and, assuming the inclination and the companion mass (which have to be greater than zero obviously) the net number is at least nine or ten. At that point, there was a definite change in the mindset, from “Well, we have no real evidence for there being black holes in our galaxy” to “Alright, we give up. There are black holes”.
I think I was already at Oxford at that point (I’ve been there since 1980) and this result probably did help me to get my professorial title. It’s also been a fantastic support for Jorge’s very successful career here in Spain and it’s given tremendous visibility to the power of the La Palma telescopes, which had only been operating for about six years at that time. It was clear it had quickly become one of the world’s major observatories, at comparable level to the Paranal, Hawaiian and other Chilean observatories. Jorge and I were delighted in being involved in this. I think it helped in the establishment of the La Palma facilities and helped the whole institute here on Tenerife.
Q: What is so fascinating about this type of objects? Will we ever know what happens inside them, which processes take place?
A: This is a classic question. It does come down to the physics as to why it is so interesting. By just measuring the masses of this objects, when we have found enough of them, we will have a distribution of their masses. After almost 50 years of effort, we now know something like 20-30 neutron stars with accurately measured masses. But so far, there are only a couple of neutron stars around two or just over two solar masses and they are very interesting because it is definitely difficult to get them to be that heavy. However, the lowest mass black hole we have found so far is around five solar masses. So, where are the objects in between i.e. the low mass black hole or the high mass neutron star?
It turns out that answering that question is actually tackling some of the most important questions in fundamental physics, which is quite simply “what is the equation of state of nuclear matter”? This equation would allow us to describe the behavior of nuclear matter in different conditions of density and temperature, so its knowledge is vital to describe both the formation of the Universe after the Big Bang and the evolution of stellar bodies.
People are always surprised when I tell them we actually don’t know what the equation of state is. Yet, a neutron star is almost pure nuclear matter. By knowing its equation of state, we can work out how big it can become before it must then collapse to become a black hole. These are the kind of questions that the particle physicists are tackling in a different way using giant particle accelerators, in particular the Large Hadron Collider at CERN. I think it is fantastic that astronomy can tackle these questions from studies on the largest scales.
With these objects, in finding them, we are watching processes that produce powerful X-ray outbursts, and they happen when the accretion disk around them becomes unstable and starts dumping matter inwards, which means that matter then passes through the Event Horizon of the Black Hole. Sadly, your question about what is beyond that, I’m afraid I don’t know the answer and neither does Stephen Hawking although we would love to know. I think we all would love to know. I think the answer will probably come from the theorist’s great attempts to combine general relativity and quantum mechanics.
I’m just a simple observer at these things. When matter is approaching the Event Horizon it is actually travelling incredibly fast, at significant fractions of the speed of light, so the material is getting very hot, and this is what produces these copious sudden flashes of X-rays that we observe. This way you are seeing matter behaving in the most extreme physical condition we know of, conditions we cannot possibly replicate on Earth and that is what makes it such an exciting field to work in.
Q: What are your next challenges?
A: I’ve partly alluded to it in my previous answer. We’ve got only about 20 or so accurate mass measurements so far and Jorge Casares has made a great advance in this field by working out a possible way of finding these objects when they are quiescent, so they are dormant. Until now we only find them when they have one of the big transient X-ray outbursts, but they can be dormant for decades. We only get one or two of these each year and they are very hard to follow up. Adding to this number is very difficult so Jorge found a possible way of finding them throughout our galaxy whilst they are dormant. I think that has great potential for future research.
There are also new X-ray satellites being brought into operation. We’re involved in projects with the Indian astronomers who have their new Astrosat satellite, which was launched last year and is now getting into normal operations. There are also all-sky monitors operating on the International Space Station which tell us about new transients as they go off. We have got a lot of work to do still in this field and we are looking at ways to study them in even greater number. I’m actually formally retired from my academic position but I have received a research fellowship, so that I’m able to concentrate on research again. I think there are really interesting prospects for the future.
Q: Would you recommend these visiting fellowships? If so, why?
A: Absolutely. My Severo Ochoa fellowship is only for three months, but the group here will tell you that I have always been coming through here. So you’ll be seeing me hopefully for a long time to come. Now that I have this new research fellowship starting in January (for the next two years) I will continue to return here frequently.
The IAC is very strong in this field, it has an excellent X-ray binary group, plus people working in high mass systems such as Artemio, not to mention the Director, Rafael Rebolo, who has an interest in these objects and their binary evolution. There is a broad mix of research and it is that mix which can be so productive, and that ends up being positive for everybody. That’s why certainly in Spain this is the place to come.
