Chandra Makes First Detection of X-rays from a Gravitational Wave Source: Interview with Chandra Scientist Wen-fai Fong

Wen-fai Fong
Chandra Scientist Wen-fai Fong

Originally from Rochester, NY, Wen-fai Fong received double Bachelor's degrees in Physics and Biology at the Massachusetts Institute of Technology, and earned her Ph.D. in Astronomy & Astrophysics from Harvard University. She was subsequently awarded an Einstein Postdoctoral Fellowship, which she took to the University of Arizona's Steward Observatory. She is currently a Hubble Postdoctoral Fellow at Northwestern University and will begin her appointment as Assistant Professor there in Fall 2018. Wen-fai is excited about unraveling the mysteries enshrouding cosmic explosions, including gamma-ray bursts and gravitational wave sources.

What are gravitational waves? What are neutron stars?

Gravitational waves are best described as ripples in space-time. To envision these merging compact objects, I always try to think of two round objects on a very flexible trampoline, rolling and rolling around each other. For the most flexible of trampolines, they will create some sort of pattern outward, while also spiraling toward each other and eventually colliding. It’s an oversimplified version, but that is how I best imagine what is going on.

In reality, the specific properties of the system — the masses, spins, orbital orientation, and distance — determine the very special pattern of the gravitational waves that are radiated from a system. Scientists then match that pattern against a gigantic bank of patterns by the gravitational wave experts who are able to determine very specific properties of the system. So it is a very neat and elegant problem that is made possible by many years of hard work.

Neutron stars are the leftover cores from supernova explosions, an amazingly powerful event that occurs when some massive stars end their lives. They are extremely dense. Imagine cramming the mass of the Sun, or even two Suns, into an area the size of Manhattan. Astronomers often say that a teaspoon of neutron star stuff weighs about a billion tons. It's no wonder they are called "compact" objects.

How/when did you hear that LIGO had identified a neutron star/neutron star merger and what were your first thoughts?

The announcement happened in the morning, and I was at an extreme gravity conference in Bozeman, Montana. Of all places! I woke up to the news that one, gravitational waves from a neutron star merger had been detected, and two, gamma-ray satellites had detected a gamma-ray burst. My first thought was, "I should go back to sleep. I'm dreaming. Is this real?" I had to pinch myself. As the day wore on, and I went to the conference, it became more and more real. I had confidential conversations with LIGO affiliates and people in the bubble. Since at least half of the conference was not in the bubble, they must have been wondering why we were all staring at our computers and not paying attention.

The very next day, I remember moderating a 3-hour session on how to detect electromagnetic counterparts from gravitational waves. Little did half the audience know that we were in the process of detecting the very first one!

Why is it such a big deal that we’ve discovered an electromagnetic counterpart to a gravitational wave signal?

We always talk about this needle in a haystack problem, in finding the right counterpart among the sea of other things that go bump in the night. At the most basic level, the successful identification and detection of an electromagnetic counterpart can tell you where exactly in the Universe this neutron star merger occurred. It's one thing to know all of the detailed parameters of the system itself — such as masses, spins, the orientation of the binary — all of that falls out beautifully from gravitational waves.

But in my eyes, it's very powerful to be able to say, "This is where the neutron star merger occurred, in this particular galaxy, in our own cosmic neighborhood." So the electromagnetic counterpart really pinpoints the location of the event and its place in the Universe.

Now, with such a precise position, we can go for broke and throw all of the resources we have at it. We can ask all of the instruments with relatively small fields-of-view to take a look and they now have a good shot at detection or placing constraints on emission. Chandra played a key role in this effort, as will come to later.

It is also remarkable how precise our understanding of the expected electromagnetic counterparts was. For two decades, theorists have predicted that we would see visible and infrared light from material ejected from the merger. We saw that. They predicted that heavy elements produced in the merger would create certain spectral features. We detected those. From our observations of short gamma-ray bursts, cosmological versions of these neutron star mergers, we see X-ray and radio emission from their afterglows and predicted we should see the same from neutron star mergers (which we also thought created short gamma-ray bursts). We detected X-ray and radio emission as predicted from this neutron star merger.

It's simply a breakthrough to be able to combine light and gravitational waves for the first time. It is the re-birth of multi-messenger astronomy.

It is almost too good to be true! I do not know if there will be another event like it. There better be!

When you found out that Chandra had initially made a non-detection, did you think that Chandra wouldn’t detect the source at all? That is, were you surprised that it did?

Honestly my gut reaction was to be disappointed that we did not detect anything. We were the first team to take an image of the position with Chandra. We observed the position of the optical counterpart for seven hours with the most sensitive X-ray facility, Chandra, and got no detection!

However, after some closer thought, we realized that this perfectly matches with what we expect. The X-ray emission from neutron star mergers originates from a relativistic jet of material that interacts with the surrounding medium. What does that mean? It means that a fire hose of material close traveling close to the speed of light was pummeling into interstellar space. It created a lot of radiation from the X-ray to radio wavelengths, really across the electromagnetic spectrum.

Now, gravitational wave emission is fairly isotropic — meaning it moves outward in all directions. On the other hand, a neutron star points in a very narrow direction, much like a fire hose. This is why we sometimes call these “relativistic fire hoses” since they are moving at a large fraction of the speed of light, i.e, millions of miles per hour. More often than not, it will not be pointed at us. Over time, the material in the jet slows down and the beam starts to spread, the way water from a fire hose will over some distance. That means that eventually, wherever the jet is pointed, we will see some X-ray emission. The fact that we did not see anything at first means exactly what we predicted: the fire hose was pointed away from us. This gives us a very good handle on the orientation and geometry of the system, and we will be able to match that with the solutions they get from gravitational waves.

Why is the detection of this source with Chandra important? What did it reveal about this cosmic collision?

It is actually the evolution of this source that makes it so interesting. The fact that we did not detect something, and then suddenly did 10 days later, means that the source was brightening. It was doing something entirely different than what it was doing at visible wavelengths, which came from the production of heavy elements. This is because the X-rays were coming from something else. The changes in the X-ray data revealed that we are really seeing a neutron star merger with a jet pointed away from us. Combined with all of the things happening at other wavelengths, it really revealed just how colorful and dynamic these merging systems are.

As for Chandra in particular, Chandra had the necessary resolution to detect the neutron star merger. Why was this important? Well, every galaxy has X-ray emission, whether it’s from an actively feeding supermassive black hole at its center, or from stellar systems called X-ray binaries, or whatever else. In other words, if you have a galaxy that’s in our own cosmic neighborhood, you better believe you will see some X-rays coming from something there. Imagine if you now have a neutron star merger that occurred in that galaxy. You need a high-resolution camera, an HDTV at X-ray wavelengths, to be able to pick out the neutron star merger. Chandra did the job perfectly.

How does this add to or change what we know?

It gives me so much confidence in our previous theories and observations and really solidifies that we are doing things right! First, we have known about these extremely energetic explosions called gamma-ray bursts, for a very long time. A subset of these gamma-ray bursts that are very short in duration (called “short-duration gamma-ray bursts”) have thus far been mysterious in origin. I have been working for a long time on tracking the origin of short gamma-ray bursts. Through observations of them and their host galaxies, I have tried to convince myself and the scientific community that the collision of two neutron stars, or a neutron star and a black hole, is the source of short gamma-ray bursts. The fact that there was a neutron star merger and then something that looked like a gamma-ray burst detected at the same time is just remarkable. It is that “smoking gun” that we have been waiting for, linking short gamma-ray bursts to neutron star mergers.

Now, whenever we detect a bright short gamma-ray burst, it usually means that the relativistic firehose of material is pointed straight at us. So we would see very bright X-ray emission and it would fade in brightness right from the beginning over the course of hours and even days.

However, in this case, we think the jet from the neutron star merger is pointed away from us, so the observed X-ray behavior will be very different. Chandra observations of the neutron star merger were essential in telling us what exactly is happening. A very detailed study of our Chandra observations led by my collaborator Professor Raffaella Margutti demonstrates that this event looks very much like a cosmological short gamma-ray burst, except that it’s pointed away from us. It is amazing!

What questions remain? What will additional observations tell us?

Currently the neutron star merger is being blocked by the Sun and it will not be observable by our telescopes until December. Every day I wake up and think, "I wonder what the neutron star merger is doing! I wonder what it looks like." As soon as it comes out from behind the Sun, we will be able to take additional images at all wavelengths. In the X-rays, additional images will give us very precise constraints on our orientation with respect to the firehose, the kinetic energy (energy from motion) that is in the jet, and the density of the environment that the neutron stars merged in. (For instance, we want to know whether they merged in a region of dense gas, or they merged in a very diffuse, low-density medium.) Through the gathering of additional observations, we will gain a much better understanding of the explosion properties and geometry of the system.

How will this affect your future work?

I will be starting my faculty job about a year from now. It is extremely exciting to be at my current career stage, fairly early in my career, and be able to experience a discovery like this. I have worked for a long time on short gamma-ray bursts, and in some ways, have felt like I have spent years planning the best ways to look for electromagnetic counterparts to gravitational waves. With this event, this is no longer a field of planning. This is a field of discovery. Every detected event has the potential to change the ballgame and bring new surprises. The prospect of bringing younger scientists, students and post-doctoral researchers, into this revolutionary field of discovery is so exciting. I am thrilled to share the joy of this discovery, as well as some of the hardships it took to get here, with the next generation.

What gets you motivated to work on this?

Since the morning of August 17th when I first found out about this event, it has been such a whirlwind. I am motivated by two things: the team of people that I have been fortunate enough to work with, and the feeling that we are really making history.

For the team: For the Chandra effort, I cannot underscore enough that this was the effort of many people. My colleague, Raffaella Margutti, went above and beyond to make the Chandra observations happen and communicate with the Chandra staff. The Chandra leadership team were on-call and extremely responsive even as our requests did not stop for the weekend or the solar eclipse. Many Chandra staff likely woke up in the middle of the night, had to stay at work late, missed family dinners, etc., so that we could get these observations. It was really the combined effort of many people that made these observations happen.

I have been extremely fortunate to be working with a team of people who are extraordinary in their scientific abilities and positivity. This positive energy has really fueled me. Some days I would wake up to an email from a team member who had stayed up until 3am trying to solve a problem, and it would motivate me to work with them to solve it. There was exceptional teamwork in acquiring and understanding every single data point. It was especially exciting to be working with several younger scientists, who often had novel and interesting viewpoints that I had not thought of before.

For making history: There is an undeniable feeling in studying this landmark event that we, as observers, are making history. This neutron star merger will be written about in Astronomy 101 textbooks, it will be disseminated to high school and college classrooms across the country 5, 10, 50 years down the line. I am so excited to be part of that movement. To read about it in a book someday and know that I played even a very small part in the observations and interpretation of this event is all of the motivation I need!

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