Credit: NASA/Swift/Cruz deWilde
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INTEGRAL/IBIS-PICsIT Study of the GRB 221009A Spectral and Time Evolution
The gamma-ray burst (GRB) 221009A was likely the brightest GRB ever detected and
possibly happens about once every 10,000 years (E. Burns et al 2023, ApJL, 946, L31).
It has provided the opportunity to explore GRB prompt and afterglow emission behaviour
on short time scales with high statistics. Studies of this event have shed light on
the emission processes at work in both the initial phases of GRBs emission and during
the afterglow with a detection up to very high-energy gamma-rays (Y. Aimuratov et al. 2022,
GRB Coordinates Network, Circular Service, No. 32802, 32802).
INTEGRAL/IBIS's soft gamma-ray detector, PICsIT, has a geometric area of about 3200 cm
2
and covers the energy band 200–2600 keV and provides 7.8-ms time resolution when using its
spectral-timing data. This data type is capable of detecting short, impulsive events even outside
the instrument's imaging field of view, even if it does not have imaging capabilities. Therefore,
PICsIT functions also as a gamma-ray all-sky monitor for GRBs.
GRB 221009A was outside the PICsIT imaging field of view (about 65.8 degrees off-axis), and
we studied the temporal and spectral evolution during the prompt phase and the early afterglow
period. Note that due to the brightness, the PICsIT data were not fully telemetered to ground
during the highest flux periods of GRB 221009A, so one is unable to study the event for these
portions of the GRB.
A "flux-tracking" behaviour with the source spectrum becoming "softer" when the source gets brighter
for the early parts of the prompt emission. However, the relationship between the spectral index
and the flux changes during the burst.
In the light curve, the red star tracks the light curve evolution, and its size varies with the
GRB flux. The photon indexes from the spectral fits are shown in the panels below the light curves.
The corresponding spectral shapes are shown in the inset panels. The photon indexes are plotted in
different colors with index greater than 2 shown in red, less than 1.7 shown in green, and values
between 1.7 and 2 shown in yellow.
The PICsIT light curve shows that the afterglow emission begins to dominate at about T0 + 630 s and
then decays with a slope of 1.6 ± 0.2, consistent with the slopes reported at soft X-rays (see
Rodi & Ubertini, 2023, A&A, in press).
Credits:
- "GRB 221009A: The Boat",
Burns, E., Svinkin, D., Fenimore, E., et al.
2023, ApJL, 946, L31
DOI:10.3847/2041-8213/acc39c
- "GRB 221009A X-ray light-curve and the indication of TeV light-curve",
Aimuratov, Y., Becerra, L., Bianco, C.~L., et al.
2022, GRB Coordinates Network, Circular Service, No. 32802, 32802
- "Soft Gamma-Ray Spectral and Time evolution of the GRB 221009A: prompt and afterglow emission with INTEGRAL/IBIS-PICsIT",
Rodi, J & Ubertini, P.,
A&A forthcoming article Accepted: 28 March 2023, eprint arXiv:2303.16943,
DOI: 10.1051/0004-6361/202346373
- Artist illustration movie:
NASA / Swift / Cruz deWilde
https://svs.gsfc.nasa.gov/14227
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