GRB 231015A

P. K. Navaneeth (IUCAA), G. Waratkar (IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:

Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a long GRB 231015A which was also detected by Konus-Wind (reported in IPN_RAW GCN notices).

The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2023-10-15 23:49:53.50 UTC. The measured peak count rate associated with the burst is 477 (+43, -43) counts/s above the background in the combined data of two quadrants (out of four), with a total of 5641 (+323, -329) counts. The local mean background count rate was 239 (+3, -3) counts/s. Using cumulative rates, we measure a T90 of 24 (+1, -1) s. 

The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2023-10-15 23:49:53.42 UTC. The measured peak count rate associated with the burst is 864 (+76, -87) counts/s above the background in the combined data of all quadrants, with a total of 10039 (+610, -910) counts. The local mean background count rate was 1514 (+7, -6) counts/s. We measure a T90 of 25 (+4, -2) s from the cumulative Veto light curve.

CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.

CZTI GRB detections are reported regularly on the payload site at:
http://astrosat.iucaa.in/czti/?q=grb

M. Kerr, C.C. Cheung, J. E. Grove, R. Woolf (NRL), A. Goldstein (USRA), C.A. Wilson-Hodge (MSFC), and M.S. Briggs (UAH) report:

The Glowbug gamma-ray telescope [1,2], operating on the International Space Station, reports the detection of GRB 231015A, which was also detected by AstroSat and Konus-Wind (GCN 34832
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 ). AstroSat CZTI detected multiple peaks of emission with the strongest peak at 2023-10-15 23:49:53.50 UTC.

Using an adaptive window with a resolution of 32-ms, the onset for the multi-peaked emission reported by AstroSat is determined to be 2023-10-15 23:49:21.128 with a duration of 36.9 s and a total significance of about 95.4 sigma. The light curve comprises two primary peaks separated by about 15 s.

Using a standard power-law function with an exponential high-energy cutoff [3] to model the emission over this duration results in a photon index dN/dE~E^x of x=0.1 and a cutoff energy ("Epeak") of 292 keV.  The modeled 10-10000 keV fluence is 3.4e-05 erg/cm^2.

Additional analysis of Glowbug data revealed an earlier component to the two main emission peaks. Using an adaptive window with a resolution of 32-ms, the burst onset of this earlier emission is determined to be 2023-10-15 23:47:03.912 with a duration of 10.3 s and a total significance of about 13.4 sigma.  The light curve comprises a single broad peak.

Using a standard power-law function with an exponential high-energy cutoff [3] to model the emission in the earlier component over this duration results in a photon index dN/dE~E^x of x=2.0 and a cutoff energy ("Epeak") of 193 keV.  The modeled 10-10000 keV fluence is 8.9e-07 erg/cm^2.

The analysis results presented here are preliminary and use a response function that lacks a detailed characterization of the surrounding passive structure of the ISS.

Glowbug is a NASA-funded technology demonstrator for sensitive, low-cost gamma-ray transient telescopes developed, built, and operated by the U.S. Naval Research Laboratory (NRL) with support from the University of Alabama in Huntsville, USRA, and NASA MSFC.  It was launched on 2023 March 15 aboard the Department of Defense Space Test Program’s STP-H9 to the ISS.  The detector comprises 12 large-area (15 cm x 15 cm) CsI:Tl panels covering the surface of a half cube, and two hexagonal (5-cm diameter, 10-cm length) CLLB scintillators, giving it a large field of view (instantaneous FoV ~2/3 sky) over a wide energy band of 50 keV to >2 MeV.

[1] Grove, J.E. et al. 2020, Proc. Yamada Conf. LXXI, arXiv:2009.11959
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[2] Woolf, R.S. et al. 2022, Proc. SPIE, 12181, id. 121811O
[3] Goldstein, A. et al. 2020, ApJ 895, 40, arXiv :1909.03006

B. Grefenstette reports on behalf of the NuSTAR Search for INteresting Gamma-ray Signals (SINGS) working group:

The NuSTAR SINGS working group reports the discovery of prompt emission from a potential GRB 231015A in the NuSTAR CsI anti-coincidence shields. This GRB was identified through a blind search using the CsI shield rates. Details of the search algorithm will be described in a future paper. 

The CsI data are recorded at 1 Hz and show a broad burst with multiple peaks. The initial NuSTAR trigger had a trigger time of 2023-10-15T23:49:35, which is one the rising edge of the slow, initial peak and is consistent with the onset time reported by GlowBug (Cheung et al, GCN Circ 3485
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 ). A stronger second peak is also seen, consistent with the strongest peak identified in the AstroSat data (2023-10-15T23:49:53, Waratkar et al, GCN Circ 3482
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 ).

The stronger second burst resulted in peak 1-sec count rates of ~3,000 counts per second in both the FPMA and FPMB shield units. Typical background rates are ~1,000 counts per second. The peak of the second burst is unresolved (e.g., shorter than the 1-s bins) with a decay back to quiescence of ~5-s. The first peak had a rise time of ~10-s and a longer overall duration.

The burst was detected in both the CdZnTe detectors. We see some emission associated with multiple peaks during the burst above 40 keV, though the number of counts is relatively modest (~10s of counts above 40 keV) so that more detailed analysis is limited by counting statistics.

The automated light curve report for this GRB, discovery report, off-line analysis of the shield data, and the CdZnTe lightcurves can be found here:

https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2023/231015A/

Information on NuSTAR SINGS can be found here: 

https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/

NuSTAR is a NASA Small Explorer mission led by Caltech and managed by JPL for NASA’s Science Mission Directorate in Washington.