GRB221009A: the brightest GRB ever observed

Telescopio Nazionale Galileo participates in the observing campaign of the BOAT GRB

Gamma-ray bursts (GRBs) are explosions that release collimated outflows of matter with speeds close to the speed of light. They are the most luminous events known in the Universe and astronomers observe them as extremely bright flashes at gamma-ray frequencies, so intense that they overwhelm any other high-energy source. GRBs are divided into long and short classes on the basis of their duration. In particular, short GRBs last from a few tens of milliseconds up to about 2 seconds, and are usually associated with the merger of two compact objects, with at least one neutron star.

They are also emitters of gravitational waves, as spectacularly demonstrated in August 2017 with the joint detection of the binary neutron star merger GW 170817 and of the short GRB 170817A. The majority of long GRBs last longer than 2 seconds and are associated with the collapse of stars with masses greater than 5 to10 times that of the Sun that explode as Supernovae.

The brightness decay over time of the NIR afterglow of GRB 221009A as observed by TNG with NICS. Credits: P. D’Avanzo

The long-duration GRB 221009A is the brightest GRB ever recorded in more than 50 years of sky-monitoring and out of almost 10.000 GRBs, that is the Brightest Of All Time (BOAT). Rate estimates suggest bursts like this should occur only once every 1000 years thanks to a unique combination of high intrinsic luminosity and proximity, providing a once-in-a-lifetime opportunity to study its properties in great detail. Detected for the first time on October 9th, 2022 by X-ray and gamma-ray satellites, this burst was at least 10-20 times brighter than any of the thousands observed by NASA Fermi or Swift satellites and was sufficiently bright to cause detectable disturbances to the upper ionosphere. It is also the first GRB with photons attaining the record energy of 18 TeV. Observations also showed the redshift to be z= 0.151, corresponding to a distance of 2.4 billion light years, a very local event for standard GRBs, as the average distance of GRBs is usually larger than this.

TNG participated in the follow-up observing campaign of this GRB at IR wavelengths with NICS (Near Infrared Camera Spectrometer) from Oct 16 to Nov 21 2022, providing a monitoring of its luminosity evolution. In particular, the observations of Oct 22 were obtained quasi-simultaneously with a JWST spectroscopic observations, also at IR wavelengths and almost 13 days after the burst. The combined JWST + TNG dataset allows a detailed investigation of the nature of this unique event, providing tight limits on the presence of an associated supernova.

The JWST NIRSPEC + MIRI spectrum of GRB 221009A as observed at 13 days post burst. The upper panel shows the observed spectrum (with the black points near-simultaneous photometry taken with NICS instaled at the TNG), while the lower panel is corrected for a foreground extinction of AV= 4.9. (more details at figure 2 of the paper: https://arxiv.org/abs/2302.07761v2

Astronomers are now characterizing the properties of the host galaxy in which it resides, of the cosmic explosion that originated it and of its environment, with unprecedented details thanks to the uniqueness of this event.

More information at the paper: https://arxiv.org/abs/2302.07761v2