Radio Reports from GW170817 1,200 Days Since the Kilonova

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Title: Continued radio observations of GW170817 3.5 years post-merger
Authors: Arvind Balasubramanian et al.
First Author’s Institution: Texas Tech University
Status: Published in ApJL

More than 3 years since GW170817, astronomers have reported the latest updates from the post-merger kilonova, as seen through X-ray and radio telescopes.

Illustration of a radio overlaid on top of an image of colliding neutron stars.

What can we learn from the radio emission produced years after two neutron stars collide? [University of Warwick/Mark Garlick/ESO; modified by Sumeet Kulkarni]

On August 17, 2017, astronomers worldwide sprang to their feet following an alert from LIGO and Virgo. These gravitational-wave observatories had already made a series of groundbreaking detections involving pairs of black holes plunging into each other, but this new trigger was what their astronomer colleagues had been waiting for: a merger involving two neutron stars. Unlike binary black hole mergers, this event, labelled GW170817, was expected to light up and provide signatures of an explosion — a kilonova — in the electromagnetic (EM) spectrum. Indeed, just 1.3 seconds after the LIGO/Virgo trigger, the Fermi and Swift space-based observatories recorded a short-duration gamma ray burst (sGRB). What followed was a frantic hunt using optical telescopes, and the EM counterpart was finally located in the galaxy NGC 4993.

The wealth of science that came out of this multi-messenger observation was immense — including explaining how sGRBs occur, exploring the nucleosynthesis of heavy elements such as gold and platinum, and verifying that gravitational waves travel close to the speed of light.

Anatomy of a Kilonova

As the first-ever kilonova observed in association with gravitational waves, GW170817 also helped us understand the astrophysical processes that emit radiation in different parts of the EM spectrum after a neutron star merger. Within 24 hours of the merger, early light in the optical and UV bands showed signs of emission from the radioactive decay of heavy elements from the tidal tails of the disrupting neutron stars. This optical and UV light dimmed out within a couple of days, and was followed by a brightening of the signal in X-rays a week later. Combined with radio emission that emerged two weeks after the merger, this afterglow indicated that matter was ejected from the merger as a structured jet, where the velocity of the ejected material varied away from the jet axis.

Soon after the initial observations, astronomers were able to confirm or constrain various structured-jet kilonova models and predict how the EM radiation — particularly in radio and X-rays — would evolve over time. This is shown as the solid black line in Figure 1, below.

Plot showing the flux density of different wavelengths for the kilonova over time. The final few data points deviate from the model.

Figure 1: X-ray and radio observations of the flux density of the kilonova emission over time. The predicted evolution of the radiation is shown by the solid black line, and all observations confirmed this — until now. The kilonova emission has recently seen a re-brightening in X-ray emission (purple points), while most sensitive radio observations reported in this paper (yellow points) do not see a corresponding increase. [Adapted from Balasubramanian et al. 2021]

What New X-ray Scans Show

Recent X-ray observations (purple data points in Figure 1) show evidence of signals in excess of the afterglow predicted by the structured-jet model. While the X-ray emission used in the model is due to ejected particles moving at relativistic speeds (speeds close to the speed of light), this re-brightening could be a signature of ejecta moving at non-relativistic speeds interacting with the surrounding interstellar medium. Alternatively, it could be the initial sGRB that was seen a few seconds after the merger, scattering off interstellar dust! The uncertainties in the X-ray measurements are too large to definitively say which of these is correct, but continued combined observations in both the X-ray and radio spectra can help us figure it out.

Results from ‘Deep’ Radio Observations Using VLA

Today’s authors follow-up the new kilonova observations in the radio spectrum, by reporting the latest set of ‘deep’ observations made using the Very Large Array (VLA) of radio telescopes. They increased the sensitivity of of the search by increasing the observing time up to 32 hours. Previous observations of GW170817 were ‘shallow’, taken only over a duration of a few hours at a time.

The new radio observations (yellow points in Figure 1) show no radiation in excess to what is expected from the structured-jet afterglow model (black line). The radio emission is still following the model and not showing the flattening observed in X-rays, and astronomers are trying to find out why. If the X-rays are just a back-scatter from previous emission, re-brightening is not expected to be seen in radio. But if the new X-ray emission is from a transition to slower, non-relativistic particles, the radio emission should follow suit; whether it happens now or is delayed by a period of time remains to be seen.

More than 3.5 years since the neutron stars first chirped in gravitational waves, the ensuing fireworks continue to excite us!

Original astrobite edited by Roan Haggar.

About the author, Sumeet Kulkarni:

I’m a third-year PhD candidate at the University of Mississippi. My research revolves around various aspects of gravitational wave astrophysics as well as noise characterization of the LIGO detectors. It involves a lot of coding, and I like to keep tapping my fingers on a keyboard even in my spare time, creating tunes instead of bugs. I run a science cafe featuring monthly public talks for the local community here in Oxford, MS, and I also love writing popular science articles. My other interests include reading, cooking, cats, and coffee.