INTEGRAL sees once in half a millennium Gamma-Ray Burst
An exceptionally luminous and nearby (z=0.151) Gamma-Ray Burst, GRB
221009A, was observed on the 9th of October 2022 by INTEGRAL, as well as
numerous other space and ground telescopes. It was estimated
(GCN #32793)
that such an event only occurs once in half a millenium, and the chance
of it happening in INTEGRAL lifetime was only about 3%. The capacity of
INTEGRAL for nearly continuous all-sky monitoring ensured that it would
not miss this once in a lifetime event. Note that this capacity is also
crucial for the forthcoming LIGO/Virgo/Kagra O4 run.
Luminous GRBs sometimes reveal a prolonged bright hard X-ray and
gamma-ray afterglow, and for a previous GRBs INTEGRAL detected it up to
few hours after the event (see GRB 120711A,
Martin-Carillo et al. 2012). INTEGRAL was well prepared to search for this emission
in new bright events, and owing to a fast follow-up strategy recently
optimized for multi-messenger transient counterpart searches, the
observation of GRB 221009A started only a few hours after the
Target-of-Opportunity request, managing to catch an early bright afterglow.
The intensity of the GRB was so high, that its afterglow was detected in
INTEGRAL/JEM-X for almost a week, far exceeding the previous record
of a few hours. Long-lasting afterglows can also appear without prompt
GRB detection in case the collimated source (jet) is not pointing
straight towards us (as it was also the case in GW170817/GRB 170817A).
At the later stages, as the jet decelerates, the emission beam is
progressively widening, and may start to reach the observer. In the
future, hard X-ray follow-ups of a rapidly expanding population of
Gravitational Waves (GW) sources will possibly reveal these so-called
"orphan afterglows", providing precise localizations for sources located
at distances which will be probed by the GW detectors in the coming
years (z<~0.1).
The image shows the GRB 221009A light curve recorded by several of the
INTEGRAL instruments in days since T0 = 2022-10-09 13:17:00. On the left
- the prompt emission phase as measured by SPI-ACS, showing the
brightest part of the GRB and an extended tail lasting a few hours,
detected even by a non-imaging detector. On the right - three
revolutions of ISGRI and JEM-X observations of the hard X-ray afterglow.
For the last point, only JEM-X yields a significant detection. The
larger inset shows combined JEM-X image for the entire observation (1 to
10 days since the event). The smaller inset shows the last observation
with significant detection (5.3 to 6.6 days since T0).
Credits:
V. Savchenko, C. Ferrigno, E. Bozzo, D. Gotz, S. Mereghetti, A. Martin
Carrillo, L. Hanlon, E. Jourdain, J.-P. Roques, T. Siegert, E. Kuulkers,
C. Sanchez 2022, ATel #15663
D. Gotz, S. Mereghetti, V. Savchenko, C. Ferrigno, E. Bozzo, on behalf
of the IBAS team 2022, GCN #32660
Special thanks to the ESA Ground Segment Team for the rapid scheduling
of the follow-up observations.
