The “GRB case 070809” has been reopened: the “culprit” is a kilonova

An international team of astronomers identified a kilonova – or a very powerful cosmic explosion – associated with a gamma-ray burst recorded in 2007. The discovery was possible thanks to a careful study of archive data of the Keck and Hubble Space Telescope optical data and with the X-ray telescope, on board the satellite Neil Gehrels Swift Observatory.

Like a group of detectives who reopen an apparently closed case, an international team of astronomers from the Purple Mountain Observatory, the Italian National Institute of Astrophysics/Brera and the University of Guizhou, has identified a kilonova (also known as macronova)- or a very powerful explosion produced by the fusion of very dense celestial objects, such as neutron stars or black holes – associated with a short duration gamma-ray burst identified 12 years ago, called GRB 070809. The results of this work, first-authored by Zhi-Ping Jin of the Purple Mountain Observatory, have been published on 07 October 2019 in the journal Nature Astronomy. The discovery was possible thanks to a new, accurate study that re-analyzed archive data obtained with the Keck optical and Hubble Space Telescope telescopes and with the X-ray Telescope, on board the Neil Gehrels Swift Observatory satellite.

Keck and HST observations of GRB 070809. a, Keck observation in the R band 0.47 d after the burst. The optical afterglow position is marked with black lines, and the X-ray error box is circled in blue. G1 and G2 are the possible host galaxies at z= 0.2187 and z= 0.473, respectively. b, Keck observation in the R band 1.47 d after the burst. c, HST observation in the F606W band 731 d after the burst. Credit: Nature Astronomy, Fan et al. (2019)

“This work is only the latest in a series produced in collaboration with our Chinese colleagues.”, said Stefano Covino, one co-author of this investigation.

“The basic idea is simple. We have a vast archive of data obtained from observations of short gamma-ray bursts revealed by the Swift satellite and followed with telescopes operating at all frequencies. For a long time it was suspected that this class of GRB originated from the fusion of two neutron stars or a neutron star and a black hole, thus giving rise to a signal of gravitational waves and, on the electromagnetic front, to an emission of a kilonova, superimposed on that of the GRB itself. But up until 17 August 2017, no one had ever unambiguously observed a kilonova, nor had it revealed the gravitational signal originating from the coalescence of two neutron stars. Even if it is a single event, the kilonova associated with the 2017 August 17th event was exceptionally well-sampled. So now we have the opportunity to compare past events with that, looking for similarities”.

And indeed, comparing the X and optical emission of the GRB 070809, it was immediately clear that the latter was substantially brighter than expected.

“This led us to think that we were observing two distinct phenomena,” commented by Paolo D’Avanzo, the other co-author from INAF/Brera, “one of thermal origin, dominant in visible light and due to kilonova, together with the emission possibly from the prolonged-activity of GRB central engine, of non-thermal origin, observable in the X rays”.


The possibility that the emissions from kilonovae hide in short-term GRB light curves had already been hypothesized for some time, and confirmed by several other identifications. However the GRB forward shock afterglow usually dominates the emission and consequently identifying the tenuous brightness of the kilonova requires a delicate and complex analysis that explains how these discoveries often require the re-analysis of data obtained with different telescopes, even at a distance of a long time from the studied event.

“Intriguingly, we find the evidence for a tight short GRB-kilonova connection. Now it is clear that the kilonovae are relatively common phenomena resulting from the coalescence of compact binary systems with at least one of the components formed by a neutron star. The detection prospect is more promising for the LIGO/Virgo neutron star merger events because usually the GRB jets are off-beam and the kilonova signals will not be contaminated by the forward shock afterglow emission,” said Yi-Zhong Fan, another co-author from Purple Mountain Observatory.

These findings then prepare the scenario for the possible identification of a new counterpart of a gravitational wave event as soon as the activities of the gravitational wave interferometers LIGO and Virgo are resumed.