//////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 22568 SUBJECT: GRB 180331A: Swift detection of a burst DATE: 18/03/31 04:43:56 GMT FROM: Amy Lien at GSFC C. B. Markwardt (GSFC), A. Deich (PSU), A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester) and A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 04:14:55 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180331A (trigger=820347). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 66.017, +13.398 which is RA(J2000) = 04h 24m 04s Dec(J2000) = +13d 23' 52" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a multi-peaked structure with a duration of about 50 sec. The peak count rate was ~1000 counts/sec (15-350 keV), at ~7 sec after the trigger. The XRT began observing the field at 04:16:48.0 UT, 112.3 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 66.0242, 13.3979 which is equivalent to: RA(J2000) = 04h 24m 05.81s Dec(J2000) = +13d 23' 52.5" with an uncertainty of 2.2 arcseconds (radius, 90% containment). This location is 25 arcseconds from the BAT onboard position, within the BAT error circle. This position may be improved as more data are received; the latest position is available at http://www.swift.ac.uk/sper. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (2.85 x 10^21 cm^-2, Willingale et al. 2013), with an excess column of 3 (+3.23/-2.75) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 115 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.60. Burst Advocate for this burst is C. B. Markwardt (craig.b.markwardt AT nasa.gov). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/too.html.) //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 22569 SUBJECT: GRB 180331A: Enhanced Swift-XRT position DATE: 18/03/31 06:21:42 GMT FROM: Phil Evans at U of Leicester P.A. Evans, M.R. Goad, J.P. Osborne and A.P. Beardmore (U. Leicester) report on behalf of the Swift-XRT team. Using 1180 s of XRT Photon Counting mode data and 1 UVOT images for GRB 180331A, we find an astrometrically corrected X-ray position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 66.02468, +13.39893 which is equivalent to: RA (J2000): 04h 24m 5.92s Dec (J2000): +13d 23' 56.1" with an uncertainty of 1.8 arcsec (radius, 90% confidence). This position may be improved as more data are received. The latest position can be viewed at http://www.swift.ac.uk/xrt_positions. Position enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans et al. (2009, MNRAS, 397, 1177). This circular was automatically generated, and is an official product of the Swift-XRT team. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 22570 SUBJECT: GRB 180331A: Swift/UVOT Upper Limits DATE: 18/03/31 11:26:21 GMT FROM: Paul Kuin at MSSL N. P. M. Kuin (UCL-MSSL) and C. B. Markwardt (GSFC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 180331A 116 s after the BAT trigger (Markwardt et al., GCN Circ. 22568). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 22569) is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white_FC 116 266 147 >20.2 u_FC 274 524 246 >19.9 white 116 1373 371 >20.4 v 604 1250 78 >18.6 b 530 1350 78 >19.4 u 274 524 246 >19.9 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.60 in the direction of the burst (Schlegel et al. 1998). //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 22572 SUBJECT: GRB 180331A: Swift-XRT refined Analysis DATE: 18/03/31 13:46:11 GMT FROM: Phil Evans at U of Leicester P.A. Evans (U. Leicester) and C. B. Markwardt (GSFC) report on behalf of the Swift-XRT team: We have analysed 4.2 ks of XRT data for GRB 180331A (Markwardt et al. GCN Circ. 22568), from 115 s to 23.0 ks after the BAT trigger. The data comprise 70 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 22569). The light curve can be modelled with an initial power-law decay with an index of alpha=0.64 (+0.21, -0.19), followed by a break at T+368 s to an alpha of 3.47 (+0.30, -0.25); it then breaks again at T+1.2 ks to a decay of alpha=1.2 (+0.5, -2.4). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.35 (+0.21, -0.20). The best-fitting absorption column is 4.9 (+1.7, -1.5) x 10^21 cm^-2, in excess of the Galactic value of 2.9 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 6.0 x 10^-11 (7.3 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 4.9 (+1.7, -1.5) x 10^21 cm^-2 Galactic foreground: 2.9 x 10^21 cm^-2 Excess significance: 2.3 sigma Photon index: 1.35 (+0.21, -0.20) If the light curve continues to decay with a power-law decay index of 1.2, the count rate at T+24 hours will be 3.3e-4 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.3 x 2.0^-14 (2.4 x 10^-14) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00820347. This circular is an official product of the Swift-XRT team. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 22574 SUBJECT: GRB 180331A: Swift-BAT refined analysis DATE: 18/03/31 16:01:42 GMT FROM: Amy Lien at GSFC T. Sakamoto (AGU), S. D. Barthelmy (GSFC), J. R. Cummings (CPI), H. A. Krimm (NSF/USRA), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), D. M. Palmer (LANL), M. Stamatikos (OSU), T. N. Ukwatta (LANL) (i.e. the Swift-BAT team): Using the data set from T-239 to T+963 sec from the recent telemetry downlink, we report further analysis of BAT GRB 180331A (trigger #820347) (Markwardt et al., GCN Circ. 22568). The BAT ground-calculated position is RA, Dec = 66.005, 13.386 deg which is RA(J2000) = 04h 24m 01.1s Dec(J2000) = +13d 23' 09.2" with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 81%. The mask-weighted light curve shows a weak pulse that starts at ~T0, peaks at ~T+9 s, and ends at ~T+50 s. T90 (15-350 keV) is 48.0 +- 22.6 sec (estimated error including systematics). The time-averaged spectrum from T+0.58 to T+48.58 sec is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.11 +- 1.04, and Epeak of 66.1 +- 28.3 keV (chi squared 57.70 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 6.1 +- 1.0 x 10^-7 erg/cm2 and the 1-sec peak flux measured from T+8.08 sec in the 15-150 keV band is 0.5 +- 0.1 ph/cm2/sec. A fit to a simple power law gives a photon index of 1.46 +- 0.19 (chi squared 64.59 for 57 d.o.f.). All the quoted errors are at the 90% confidence level. The results of the batgrbproduct analysis are available at http://gcn.gsfc.nasa.gov/notices_s/820347/BA/