INTEGRAL Picture Of the Month
May 2023

INTEGRAL POM
Credit: NASA/Swift/Cruz deWilde

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INTEGRAL POM
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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 cm2 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).

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