Chandra Makes First Detection of X-rays from a Gravitational Wave Source: Interview with Chandra Scientist Raffaella Margutti

Chandra Scientist Raffaella Margutti

Raffaella Margutti obtained a PhD degree in Physics and Astronomy from the University of Milano Bicocca, Italy, in 2010, working on the broad-band (radio to gamma-ray) emission from relativistic jets in gamma-ray bursts within the Swift team. She then worked as a postdoctoral fellow at the Institute for Theory and Computation (ITC) at Harvard University, and then moved in 2015 for one year to New York University as James Arthur Fellow. Raffaella began a faculty position at Northwestern University (Physics and Astronomy) in 2016. She has been working in the field of Astronomical transients for more than a decade, with a wide range of expertise including, Stellar Explosions, Gamma-Ray bursts, Tidal Disruption Events, Stellar Outburst, and now, counterparts to GW.

What are gravitational waves?

Gravitational Waves are ripples in space-time that become particularly strong when very violent event in our Universe happen, like the merge of two very peculiar stars that we call neutron stars (NS). NS are what get left behind after a big star like 10 times the Sun ends its life with a big explosion.

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

I still remember the excitement of that Thursday (August 17) morning, opening up the email inbox and the plowing through all the emails that arrived in the last few hours or so and the finding the “alert”. “Alert” in this case meant that there has been a likely NS-NS merger identified by LIGO and I remember thinking: “WOW this is it!” This is something we have been waiting for years and here it was.

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

Identifying the electromagnetic (EM) counterpart to a gravitational wave (GW) event means to be able for the first time to be able to pinpoint the exact location in the sky where that dramatic event happened. So not just being able to say: “it happened in this or that galaxy,” but even within a galaxy. We can now say “HERE”, in this particular galaxy, two neutron stars merged.

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?

As soon as we realized we had an optical counterpart to the GW detection we started thinking how to get Chandra on target as soon as possible. We were able to observe the source 2.4 days after the merger and we had a non-detection. We saw literally zero X-ray photons coming from the source. Was this unexpected? I would say no. In our mind we knew that that could be the most likely outcome of our observations could very well be a non-detection. And I say that as we went into the game thinking that the most likely source of X-rays from the event would be originating from a jet. A jet is something collimated, that might as well point away from our line of sight. The most likely scenario is actually the one where the jet is not pointing directly at us, but needs some time to decelerate so that the radiation can reach us. So the very early non-detection was definitely expected.

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

The X-ray detection and observations of the GW source allow us to understand the propertied of the merger: (Was it able to launch a relativistic jet? What are the properties of this jet? How wide is the jet? How energetic?) Not only we can learn about the properties of the merger, but we can also learn about the properties of the environment. So we can really understand what was left behind around the two NS just before they merged. X-ray observations are an extremely powerful tool to learn both about the merger itself and its nearby environment

How does this add to or change what we know?

For the very first time we have an X-ray detection at a location of a GW event. This is the dawn of a new era of exploration of our Universe.

What questions remain? What will additional observations tell us?

Most of the questions actually remain open, and maybe the most important of all is how does this event compare to the rest of the population? Is this event an outlier or is this representative of all NS-NS mergers? And of course a very big open question is: how does a BH-NS merger look like across the spectrum?

How will this affect future work?

This discovery is going to have a major impact into the lives of a lot of astronomers. There is going to be for sure an explosion of workshops and conferences about NS-NS as well as other opportunities related to NS-NS mergers. We will witness an increase of interest by a number of scientists in the filed on NS-NS mergers.

What gets you motivated to work on this?

I would say that the real engine behind any research is the unknown. So there was for sure a big excitement about something we did not know before. However it is also true that it is a highly competitive field, so there was always the thought in our mind to move swiftly as somebody else otherwise might find the same results before we do.

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