//////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 31915 SUBJECT: GRB 220418B: Fermi GBM Final Real-time Localization DATE: 22/04/18 17:26:38 GMT FROM: Fermi GBM Team at MSFC/Fermi-GBM The Fermi GBM team reports the detection of a likely SHORT GRB At 17:16:21 UT on 18 Apr 2022, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 220418B (trigger 671994986.650868 / 220418720). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 216.5, Dec = -26.6 (J2000 degrees, equivalent to J2000 14h 26m, -26d 36'), with a statistical uncertainty of 29.8 degrees. The angle from the Fermi LAT boresight is 90.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2022/bn220418720/quicklook/glg_skymap_all_bn220418720.png The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2022/bn220418720/quicklook/glg_healpix_all_bn220418720.fit The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2022/bn220418720/quicklook/glg_lc_medres34_bn220418720.gif //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 31919 SUBJECT: GRB 220418B: Swift/BAT-GUANO candidate arcminute localization of a short burst DATE: 22/04/19 05:52:20 GMT FROM: Aaron Tohuvavohu at U Toronto Aaron Tohuvavohu (U Toronto), James DeLaunay (UAlabama), Gayathri Raman (PSU), Jamie A. Kennea (PSU), report: Swift/BAT did not localize GRB 220418B onboard (T0: 2022-04-18T17:16:21 UTC, Fermi/GBM trig #671994986). The Fermi notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; Tohuvavohu et al. 2020, ApJ, 900, 1). Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground. The BAT likelihood search, NITRATES (DeLaunay + Tohuvavohu, arXiv:2111.01769), detects the burst in a 2.048 s analysis time bin with a sqrt(TS) of 10.4. A candidate arcminute localization is found with DeltaLLHPeak of 11.8. See Section 9.1 and Figure 20 in the NITRATES paper for brief descriptions and interpretations of sqrt(TS), DeltaLLHPeak, and DeltaLLHOut. The BAT position is RA, Dec = 224.329, -17.514 deg which is RA(J2000) = 14h 57m 18.96s Dec(J2000) = -17d 30′ 15.4″ with an estimated uncertainty of 5 arcmin. This position is consistent with the Fermi/GBM localization (GCN 31915). XRT and UVOT follow-up has been requested, but will be delayed due to the position's proximity to the Moon. We encourage prompt followup from other facilities. GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches. A live reporting of Swift/BAT event data recovered by GUANO can be found at: https://www.swift.psu.edu/guano/ //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 31921 SUBJECT: GRB 220418B: Swift/BAT-GUANO refined location of a short burst DATE: 22/04/19 13:26:14 GMT FROM: Aaron Tohuvavohu at U Toronto James DeLaunay (UAlabama), Aaron Tohuvavohu (U Toronto), Gayathri Raman (PSU), Jamie A. Kennea (PSU), report: Further analysis has improved the location of the short GRB reported in GCN 31919, and its confidence. This new position is fully consistent with the originally reported position, with a smaller uncertainty region. The BAT position is RA, Dec = 224.344 -17.539 deg which is RA(J2000) = 14h 57m 22.56s Dec(J2000) = -17d 32′ 20.4″ with an estimated uncertainty of 3 arcmin. We encourage followup from other instruments. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 31923 SUBJECT: GRB 220418B: Swift ToO observations DATE: 22/04/20 00:54:19 GMT FROM: Phil Evans at U of Leicester P. A. Evans (U. Leicester) reports on behalf of the Swift team: Swift has initiated a ToO observation of the Swift/BAT-GUANO GRB 220418B. Automated analysis of the XRT data will be presented online at https://www.swift.ac.uk/ToO_GRBs/00021486 Any uncatalogued X-ray sources detected in this analysis will be reported on this website and via GCN COUNTERPART notices. These are not necessarily related to the Swift/BAT-GUANO event. Any X-ray source considered to be a probable afterglow candidate will be reported via a GCN Circular after manual consideration. Details of the XRT automated analysis methods are detailed in Evans et al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8). This circular is an official product of the Swift-XRT team. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 31928 SUBJECT: GRB 220418B Fermi GBM observation DATE: 22/04/21 01:25:04 GMT FROM: Cori Fletcher at USRA C. Fletcher (USRA), S. Dalessi (UAH) and C. Meegan (UAH) report on behalf of the Fermi GBM Team: "At 17:16:21.65 UT on 18 April 2022, the Fermi Gamma-Ray Burst Monitor (GBM) triggered and located GRB 220418B (trigger 671994986 / 220418720). which was also detected by the Swift-BAT/GUANO (GCN 31919, 31921) The Fermi GBM Final Real-time Localization (GCN 31915) is consistent with the Swift position. The angle from the Fermi LAT boresight at the GBM trigger time is 80 degrees. The GBM light curve shows a single peak with a duration (T90) of about 1.7 s (50-300 keV). The time-averaged spectrum from T0-1.024 s to T0+1.024 s is best fit a simple power law function with index -1.6 +/- 0.1. The event fluence (10-1000 keV) in this time interval is (3.8 +/- 0.6)E-07 erg/cm^2. The 64-ms peak photon flux measured starting from T0-0.57 s in the 10-1000 keV band is 121 +/- 76 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog: https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html For Fermi GBM data and info, please visit the official Fermi GBM Support Page: https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/" //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 31930 SUBJECT: GRB 220418B: Swift-XRT observations DATE: 22/04/21 09:32:41 GMT FROM: Phil Evans at U of Leicester B. Sbarufatti (PSU), D.N. Burrows (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), J. D. Gropp (PSU) and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of the Swift/BAT-GUANO-detected burst GRB 220418B, collecting 4.8 ks of Photon Counting (PC) mode data between T0+113.8 ks and T0+148.8 ks. These observations were delayed due to the Moon observing constraint. Due to a high XRT temperature and consequent high background, the data contain many low significance detections (ranked as "poor") which are likely to be spurious, but no objects are detected within the BAT/GUANO error region. We therefore conclude that no XRT afterglow to this object has been found. The 3-sigma upper limit is 2.3e-3 ct/sec, which, assuming a typical GRB spectral, equates to an observed 0.3-10 keV flux of ~9e-14 erg/cm^2/s. The results of the XRT-team automatic analysis of the XRT observations, including a position-specific upper limit calculator, are available at https://www.swift.ac.uk/ToO_GRBs/00021486. This circular is an official product of the Swift-XRT team.