Mysterious X-rays Emerge in GW170817 3.5 Years Later

Aprajita Hajela

We are excited to welcome Aprajita Hajela, who is finishing her Ph.D. at Northwestern University, as our guest blogger. She is advised by Prof. Raffaella Margutti, and the first author of a study that is the focus of our latest Chandra press release Aprajita uses X-ray and radio observations to constrain the physical properties of a broad set of transients, especially the ones which are known to launch outflows moving at speeds closer to the speed of light. She has worked extensively on the electromagnetic study of GW170817 and has also ventured more recently into the world of long gamma-ray bursts and tidal disruption events. She was also awarded the Future Investigators in NASA Earth and Space Science and Technology Award to fund her research for 3 years during her Ph.D.

Almost 3.5 years after GW170817 went off, I was woken at 4 AM by the notification of the new observations acquired with the Chandra X-ray Observatory and I saw something unexpected, and exciting! As Vsevolod Nedora, a major contributor to the study and a final year Ph.D. student currently at Albert Einstein Institute in Germany, puts it, “this is a complex and very interesting story of 12 photons!” But before diving into this, let’s begin at the beginning and touch base with the prior events linked with GW170817 that made it very special.

The association of short gamma-ray bursts with the colliding compact objects, with at least one of them being a neutron star (NS), existed only in theory, until GW170817 confirmed it with direct observational evidence. GW170817 was the first NS-NS merger ever observed with both gravitational waves (GWs) and across the electromagnetic (EM) spectrum, making it a multi-messenger event. While some more black hole (BH)-NS and NS-NS mergers have been detected with GWs after that, GW170817 still remains the only event of its kind and is a treasure chest of several first observations in our field.

The first detection of light from GW170817 was with gamma-rays two seconds after the merger, followed by strong emission in UV, optical and IR (UVOIR) wavelengths. Emission at X-rays and radio wavelengths was not seen for 1-2 weeks after the merger. These observations were consistent with a picture where:
1) an ultrarelativistic, collimated structured jet (where matter is confined to a cone-like shape and kinetic energy of the material decreases as you go outwards from the axis of the cone) initially pointing away from our line of sight was responsible for the observed gamma-rays,
2) a jet afterglow produced when this jet interacts with the surrounding medium was responsible for the observed non-thermal emission across the EM spectrum,
and,
3) a slower moving less-collimated material (i.e. more spherical than the jet) was inferred to be responsible for the thermal emission produced by a kilonova;
Mergers like these are one of the predominant sites for the production of elements heavier than iron (like silver and gold) in the Universe, and a kilonova is powered by the radioactive decay of these heavy elements usually dominating UVOIR at early times. Both a structured jet and a kilonova were confirmed observationally for the first time to be associated with a NS-NS merger.

Our team at Northwestern University and University of California, Berkeley has monitored the evolution of GW170817 the entire time, and we noted that for almost 900 days (or 2.5 years) post-merger, the X-ray and radio emission was powered by the jet afterglow. Austin McDowell, a 5th year Ph.D. student at NYU, helped with the modeling of the afterglow to constrain the explosion parameters and predict the expectations from this model in the future.

To continue to monitor its evolution, our team led additional observational campaigns for GW170817 with Chandra and the NSF’s Karl Jansky Very Large Array (VLA) after 900 days of the merger. We, as a team, believe that the data should be accessible to everyone and therefore we made all of our observations immediately public without any proprietary period for a better science output. Hence, while you will find different methods and conclusions in different published works, this is an account of what we found.

The observations we acquired with Chandra and the VLA during December 2020 (3.5 years post-merger) were extremely intriguing. While VLA observations showed no radio emission at the site, Chandra’s unexpectedly showed a strong and bright detection. The level of brightness observed in these X-ray observations were approximately four times higher than what was expected from the jet afterglow and has remained at a constant level for around 700 days now.

With the help of an amazing team consisting of scientists from both the US and Europe, we exhausted a list of possible physically motivated scenarios that could explain this likely emergence of a new source of X-rays at such late-times, until only two seemed viable. We discuss both the scenarios in our recent paper. The first one that seemed to explain this excess in X-rays was a kilonova afterglow. This is produced by the interaction of the material moving with 0.6-0.8 times the speed of light.

“It functions similar to a jet afterglow, except the material is slower and has less energy compared to the jet and therefore brightens later in time and is dimmer than the jet afterglow”, says Adithan Kathirgamaraju, a postdoc at Caltech, who performed simulations that predicted the behavior of a kilonova afterglow from GW170817 early on in his published work, and again in this work.

The important implications of observing a kilonova afterglow are multifold. Observing a kilonova afterglow 3.5 years after the merger will confirm the presence of another relativistic component in addition to (yet slower than) the jet in GW170817. Vsevolod performed numerical relativity simulations targeted to GW170817 and found that this could only be possible if a BH wasn’t formed immediately (within 1 millisecond) after the merger, instead the merged product has to remain a NS before collapsing to a BH. The newly formed NS would not be in equilibrium and therefore will bounce to create extremely high-speed winds driving a fraction of the material surrounding the remnant to relativistic speeds, which wouldn’t be possible otherwise. Ore Gottlieb, a postdoc at Northwestern, proposed a scenario of a cocoon shock breakout to explain the origin of gamma-rays in the case of GW170817, which will be confirmed if such high-speed material was launched during the merger. Monitoring the kilonova afterglow evolution will also help us probe the properties of ultra-dense matter under extreme conditions that cannot be replicated in the labs and is one of the biggest mysteries in the community.

Brian Metzger, a professor at Columbia University, proposed an alternate scenario that could explain this strange X-ray behavior where the X-ray emission is powered by the material falling into the black hole formed during the merger. We should note here that if either of these scenarios is found to be true, that would make it to the list of firsts associated with GW170817, as neither a kilonova afterglow nor such a long-term accretion onto a BH formed in the aftermath of a NS-NS merger has ever been observed before.

Even more observations are planned in the future to solve the mystery of GW170817 and to distinguish between the above two scenarios. In the case of a kilonova afterglow the X-rays and radio are expected to eventually brighten with time, but in the case of an accreting BH, the X-rays will decline steeply without any accompanying radio component. Finally, we are really grateful to all the telescope teams worldwide, and especially Chandra and the VLA, for helping us throughout this journey in the timely preparation of these observations.