GRB 240905B: AstroSat CZTI detection and localization

Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a short-duration GRB 240905B which was also detected by Fermi/GBM (Fermi GBM team, GCN Circ. 37386). Inspection of INTEGRAL/SPI-ACS data also showed the detection of the burst. The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2024-09-05 04:27:16.55 UTC. The measured peak count rate associated with the burst is 3849 (+853, -293) counts/s above the background in the combined data of all quadrants, with a total of 1531 (+95, -118) counts. The local mean background count rate was 326 (+22, -23) counts/s. Using cumulative rates, we measure a T90 of 0.67 (+0.11, -0.06) s. The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2024-09-05 04:27:16.29 UTC. The measured peak count rate associated with the burst is 3645 (+109, -119) counts/s above the background in the combined data of all quadrants, with a total of 4282 (+150, -157) counts. The local mean background count rate was 1276 (+9, -10) counts/s. Due to the intrinsic 1 s binning of veto data, we cannot reliably estimate a T90 from it. We localised the burst using the AstroSat/CZTI localization framework (Saraogi et al., 2024, MNRAS, 530, 2; doi:10.1093/mnras/stae435) and obtained a multi-modal distribution. The global minimum spans a broad swath around Dec=+60, with an RA range of 220-320 degrees. A slightly shallower but better defined minimum is seen at RA, Dec = 260, +15 which is consistent with Fermi/GBM localization (Fermi GBM team, GCN Circ. 37386). The probability contours can be seen at this link: https://bighome.iitb.ac.in/index.php/s/JgJ7s3426kxyRWC 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