Chandra Makes First Detection of X-rays from a Gravitational Wave Source: Interview with Chandra Scientist Eleonora Nora Troja

Chandra Scientist Eleonora Nora Troja

Eleonora Nora Troja was born and raised in Palermo, Sicily. After completing her PhD at the University of Palermo, she moved to NASA Goddard Space Flight Center as a NASA Postdoctoral Program Fellow. Her main research interest is the study of gamma-ray bursts, and in particular the connection between short duration gamma-ray bursts, neutron star mergers and gravitational waves. Beside her studies, she also leads the Swift Guest Investigator Program and serves as co-chair for the Athena mission working groups.

What are gravitational waves?

Gravitational waves are ripples in the fabric of space and time. According to Einstein’s theory of general relativity, every moving object actually emits a tiny amount of gravitational waves, but the signal is usually too small for us to detect. In order for us to detect gravitational waves, we need to wait for catastrophic events like the collision of two neutron stars. This brings to the second question, what are neutron stars? Neutron stars are some of the most exotic and dense objects in our Universe. These stars weight as much as our Sun but they are much smaller in size. A neutron star can extend up to 15-17 miles across, this is more or less the size of Washington D.C.

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

The LIGO and Virgo collaborations sent an alert to the community of astronomers informing us of their discovery. In this message, they told us that they were seeing this very clear signal in the LIGO data and, surprisingly, this signal was coincident in time with a gamma-ray burst. My first thought after I read this message was that I had to cancel all my plans for the weekend because it was clear from the very beginning that this was a big deal. However, what happened next far exceeded any of my expectations, for sure.

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

We did not just discover an electromagnetic counterpart, we discovered all of them! We saw a gamma-ray burst, we saw a bright “kilonova” and then, finally, the afterglow showed up. Astronomers working in this field, including myself, had been looking for this perfect combination of events for over a decade. Now not only did we see all three of these phenomena together in the same event, but we can also associate them with the merger of two neutron stars thanks to the gravitational wave data. This was so important and exciting because it is solving a mystery that lasted for over three decades and is opening a new window into our Universe. We now have a new way to look at our cosmos – not only through photons, but using gravitational waves and photons.

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?

No, after the first non-detection I did not lose my hope, because our knowledge and understanding of gamma-ray bursts was telling us that there were still some chance to see X-ray emission at a later time. I definitely thought it was worth observing. However, as always, one thing is theory and expectations based on models, and a completely different thing is reality. To actually observe something that, up to that moment, existed only in our minds is extremely exciting. I remember when I opened the first Chandra image and saw that the source was emitting X-rays, I could not believe my eyes.

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

This Chandra detection is very important because it is the first evidence that sources of gravitational waves are also sources of X-ray emission. We were hoping for it and we were investigating possible ways to produce gravitational waves and X-ray emission at the same time, but we did not know it for sure. We did not know until Chandra saw it. This detection is teaching us a great deal of information about the collision and its remnant. For example, it is telling us that after the two neutron stars collided, a powerful jet of energy and matter was launched into the outer space. It is this jet that we are observing in the X-rays. The X-ray observations are also telling us that this jet was not directly pointing toward us, but it was tilted in a different direction. We got very, very lucky that we managed to observe the faint emission from this jet. In the future, if there will be another collision and the jet is pointed toward us, the X-ray emission could be thousands of times brighter than what we detected this time.

How does this add to or change what we know?

The way we discover gamma-ray bursts so far is always because we saw this bright gamma-ray emission coming from these powerful jets, and we detect it when the jet is pointing straight toward us. For nearly two decades we thought that it was possible to see these phenomena even if the jet was pointing away from us. We have been trying to prove this theory for over two decades by looking for evidence in the X-rays, optical and radio, but we actually never found a good candidate. This is the very first time that we can confidently say that even if the jet of the gamma-ray burst is pointing away from us, we are still able to detect some emission, at least at X-ray energies. This is important for several reasons. First, it gives us an important confirmation that gamma-ray bursts are actually beamed into narrow, pencil-thin jets. It also allows us to study the geometry and the structure of these extreme jets. What is even more exciting, in my opinion, is that this gives us a new way to find these cosmic explosions because so far we were relying only on the gamma-ray detection, and we were losing all the explosions that were not directed toward us. Instead now we can think of finding gamma-ray bursts and sources of gravitational waves just by looking in the X-rays. An X-ray mission that is sensitive enough and has a wide field of view can actually catch hundreds of these events even if the jet is not pointing toward us.

What questions remain? What will additional observations tell us?

Most of them! We have a ton of questions that still need to be answered because we just saw one event. This is the first event of hopefully a very long series, and we want to look for more of them. The X-ray emission could look completely different in future events – maybe there is more than a way to produce X-rays. Personally, I am very curious to observe a different type of collision. This time we saw the collision of two neutron stars, very likely. Next time, I would like to observe the collision of a neutron star with a black hole. I’d like to see if the type of electromagnetic radiation that these collisions emit is different from what we observed this time, and if there is any way for us to distinguish between these catastrophic collisions just by looking at the light that they emit.

How will this affect your future work?

In the short term, I know that I am going to be very busy. It was so exciting to be part of this discovery that I want to talk about it all the time, with everyone. I think that the near future is very important to disseminate this result and let other people be aware of what amazing findings we had. Despite the fact that we worked so much, I have so many ideas now for papers I want to write because this discovery opened a new window on these cosmic explosions and the way we study them. I have tons of ideas in my mind and I feel I should publish them all in the next one or two months!

In the longer term, it is much harder to predict. Usually this type of results should help scientists to find a more stable position. I do not know if this is going to happen in my case as my life is very much settled in Washington D.C. and I am not planning to move in the near future, but who knows! Maybe this will bring me an offer that I cannot refuse.

What gets you motivated to work on this?

We worked 24/7 for four or five weeks. It was very hard, and it is not something that we normally do. We were all so very excited and we worked a lot, but we could have worked even more! The motivation was that this discovery was so beautiful by itself and we had these great results that we wanted to release them as soon as possible. We wanted to be ready to announce our results at the same time as the LIGO and Virgo collaborations because we think that in this case the electromagnetic discovery is an important component, and is as important as the gravitational wave discovery. We think that X-ray observations are adding a lot to the understanding of this phenomenon. And, of course, there was also a lot of competition because in our field once we make the observations we release part of the results to help other astronomers to plan their observations. It is very important to share the results with the community, but this also implies that all the community knows what is going on. This is a motivation to publish the results as quickly as possible.

For more images, animations, and information about this exciting discovery, visit: http://chandra.si.edu/photo/2017/2nstars/