{
  "circularId": 44987,
  "format": "text/plain",
  "bibcode": "2026GCN.44987....1W",
  "subject": "GRB 260616A: NuSTAR detection of prompt emission",
  "body": "G. Waratkar (Caltech) and B. Grefenstette (Caltech) report on behalf of the NuSTAR Search for INteresting Gamma-ray Signals (SINGS) working group:\n\nThe NuSTAR SINGS working group reports the detection of prompt emission from the long-duration GRB 260616A in both the NuSTAR CsI anti-coincidence shields. Details of the search algorithm will be described in a future paper.\n\nThe NuSTAR SINGS algorithm, triggered at 2026-06-16T03:21:25.62 UTC, shows a detection of GRB 260616A, also detected by Fermi-GBM (Fermi GBM Team, GCN 44948), GECAM-B (Guo et al., GCN 44964), Konus-Wind (Ridnaia et al., GCN 44967), and Insight-HXMT (Wang et al., GCN 44980).\n\nThe NuSTAR CsI shield data are recorded at 1 Hz. We detect a burst lasting for about 100-s. The peak count rate is ~1500-cps with a baseline rate of ~1000-cps during this time period. We do not see any evidence in the signal above 100 keV in the CZT detectors.\n\nThe Fermi-LAT localization (Lopez et al., GCN 44960) at RA = 93.53, Dec = -32.07 implies an offset from the NuSTAR boresight of 133-deg (i.e. from the backside) and an offset from the geocenter of 86-deg.\n\nLightcurves and analysis for this GRB can be found here:\nhttps://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2026/260616A  \n\nInformation on NuSTAR SINGS can be found here: \nhttps://nustarsoc.caltech.edu/NuSTAR_Public/grbs/     \n\nNuSTAR is a NASA Small Explorer mission led by Caltech and managed by JPL for NASA's Science Mission Directorate in Washington.\n",
  "eventId": "GRB 260616A",
  "submitter": "Gaurav Waratkar at Caltech <gauravwaratkar@iitb.ac.in>",
  "createdOn": 1781840074280,
  "submittedHow": "web"
}