//////////////////////////////////////////////////////////////////////// TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3654 SUBJECT: GRB050721A: Swift-BAT detection of a weak burst DATE: 05/07/21 06:04:14 GMT FROM: Angelo Antonelli at Obs. Astro. di Roma GRB050721A: Swift-BAT detection of a weak burst L.A. Antonelli (INAF/OAR), K. Page (U. Leicester), D. Morris (PSU), P. Romano (INAF/OAB), N. Gehrels (GSFC), P. Roming (PSU), P. Schady (MSSL), G. Tagliaferri (INAF/OAB) S. Barthelmy (GSFC), H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), D. Palmer (LANL), on behalf of the Swift team: At 04:29:14.28 UT, Swift-BAT triggered and located GRB050721 (trigger=146970). The spacecraft slewed immediately. The BAT on-board calculated location is RA, Dec 253.445d,-28.390d {+16h 53m 47s,-28d 23' 22"} (J2000), with an uncertainty of 3 arcmin (radius, 90% containment, stat+sys). The BAT light curve shows a peak with a total duration of ~10 sec. The peak count rate was ~1000 counts/sec (15-350 keV), at ~5 seconds after the trigger. The XRT started the observation at 04:32:20 UT with XRT in automatic state, 186 sec after the BAT Trigger. An uncatalogued X-ray source was detected at the following position RA, Dec 16h 53m 44.7s, -28d 22' 53''.3 with an uncertainty of about 6 arcsec (90% containment). This is 42 arcsec from the BAT position reported above. Based on the UVOT finding chart there is no new source at the XRT position. //////////////////////////////////////////////////////////////////////// TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3655 SUBJECT: GRB 050721a: Optical Observations DATE: 05/07/21 06:25:56 GMT FROM: Ken ichi Torii at RIKEN K. Torii (Osaka U.) reports: "The error region of the Swift GRB 050721a (Antonelli et al. GCN 3654) was imaged with the 0.3 m telescope in the New Mexico Skies observatory. The observation started at 2005 July 21, 04:35:23 UT (369 s after the trigger) and several 120 s integrations in I band were obtained. We identify a bright (I~15 mag) fading object whose position is consistent with the XRT afterglow candidate reported in GCN 3654. However, this source does not seem to be spatially resolved with a nearby 2MASS star in our frames. Further observations and analyses are in progress." //////////////////////////////////////////////////////////////////////// TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3657 SUBJECT: GRB 050721: optical afterglow confirmed DATE: 05/07/21 06:59:54 GMT FROM: Daniele Malesani at SISSA-ISAS,Trieste,Italy S. Covino, P. D'Avanzo (INAF/OABr), S. Bagnulo, T. Szeifert (ESO), D. Malesani (SISSA), G. Tagliaferri (INAF/OABr), G. Chincarini (Univ. Milano-Bicocca), S. Piranomonte, L.A. Antonelli, and L. Stella (INAF/OAR), on behalf of the MISTICI collaboration, report: We observed the field of GRB 050721A (Antonelli et al., GCN 3654) with the ESO-VLT UT2. Observations were carried out with the FORS1 instrument in the R filter, starting on 2005 Jul 21, 4:54 UT (~25 minutes after the GRB) and are still in progress. Inside the XRT error box, we find a fiding source at the approximate coordinates: alpha = 16:53:44.53 delta = -28:22:51.8 with an error of ~1". This lies 1.1" E, 0.7" N of the USNO star U0600_22125777, and is likely the same source reported by Torii (GCN 3655). This rapid response by an 8m-class telescope was possibile thanks to the special Rapid Response Mode provided by ESO. This message can be cited. //////////////////////////////////////////////////////////////////////// TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3658 SUBJECT: GRB050721A: optical decay DATE: 05/07/21 08:02:10 GMT FROM: Paolo D'Avanzo at INAF-OAB P. D'Avanzo, S. Covino, (INAF/OABr), D. Malesani (SISSA), S. Bagnulo, T. Szeifert (ESO), G. Tagliaferri (INAF/OABr), G. Chincarini (Univ. Milano- Bicocca), S. Piranomonte, L.A. Antonelli, and L. Stella (INAF/OAR), on behalf of the MISTICI collaboration, report: We performed a preliminary analysis on our R-band images (Covino et al., GCN 3657) of the field of GRB050721 (Antonelli et al., GCN 3654). The afterglow indicated by Torii (GCN 3655) and confirmed by Covino et al. (GCN 3657) faded by ~ 0.9 mag in the R band between July 21.204 UT and July 21.226 UT (25 and 56 minutes after the GRB respectively). This implies a decay index of ~ 1. At our last epoch, the afterglow magnitude was R ~ 18.6, with reference to USNO star UO600_22128314 assumed to be R = 17.4 This message can be cited. //////////////////////////////////////////////////////////////////////// TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3659 SUBJECT: GRB 050721A: Early Swift XRT analysis results DATE: 05/07/21 10:25:27 GMT FROM: Pat Romano at OAB-Swift P. Romano (INAF-OAB), L.A. Antonelli (INAF-OAR), G. Chincarini, A. Moretti, G. Tagliaferri (INAF-OAB), K. Page (U. Leicester), F. Marshall (GSFC), P. Boyd (GSFC-UMBC) on behalf of the Swift-XRT team: We have analyzed the Swift XRT data from the first orbit observation of GRB 050721A (Antonelli et al., GCN 3654). The new refined coordinates are: RA(J2000) = 16h 53m 44.9s Dec(J2000) = -28 22 51.7 This position is 41 arcseconds from the BAT position given in GCN 3654. We estimate an uncertainty of 6 arcseconds radius (90% containment). A preliminary spectral fit (simple absorbed power-law) to the combined PC and WT data yields a photon index of 1.7+/-0.1 in the [0.5-10] keV band. The derived NH is 3.16E21 cm^-2, which is higher than the Galactic value (1.57E21 cm^-2; Dickey & Lockman 1990). The average (194-323 seconds from trigger for WT and 324-463 seconds from trigger for PC) estimated unabsorbed 0.5-10 keV flux is then 3.5E-10 ergs/s/cm^2. The light curves in Windowed Timing (WT) and Photon Counting (PC) mode start 194 and 324 seconds from the BAT trigger (T0), and they show a fading afterglow with a preliminary decay slope -1.99+/-0.07 (first orbit only). The unabsorbed 0.5-10.0 keV flux at 1 hour after the burst is estimated to be 5E-12 ergs/s/cm^2. //////////////////////////////////////////////////////////////////////// TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3661 SUBJECT: GRB 050721A: Swift-BAT refined analysis DATE: 05/07/21 16:41:48 GMT FROM: Hans Krimm at NASA-GSFC E. Fenimore (LANL), L. Barbier (GSFC), S. Barthelmy (GSFC), J. Cummings (GSFC/NRC), N. Gehrels (GSFC), D. Hullinger (UMD), H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), T. McMahon (Langston U), T. Mitani (ISAS), D. Palmer (LANL), A. Parsons (GSFC), T. Sakamoto (GSFC/NRC), G. Sato (ISAS), M. Suzuki (Saitama), J. Tueller (GSFC), N. White (GSFC), on behalf of the Swift/BAT team: At 04:29:14.28 UT Swift-BAT detected GRB 050721 (trigger=146970) (GCN Circ 3654, A. Antonelli et al.). This burst has been called GRB 050721A in some circulars, although it is so far the only bona fide burst of the day. The refined BAT ground position is (RA,Dec) = 253.452, -28.386 {16h 53m 48.5s, -28d 23' 10"} (J2000) +- 3 arcmin (radius, 90% containment, stat+sys). The partial coding was 13%. The masked-weighted light curve shows a FRED-like structure with a single large peak starting to rise at T-5 seconds, peaking at T+3.7 seconds and decaying back to background levels by T+50 seconds. The peak is visible in the 15-100 keV energy band, but not at higher energies. The calculated T90 (15-350 keV) is 39 +- 2 sec (estimated error including systematics). The power-law photon index of the time-averaged spectrum is 1.81 +- 0.08. The fluence in the 15-350 keV band is (5.05 +- 0.25) x 10^-6 erg/cm2. The 1-sec peak photon flux measured from T+3.7 sec in the 15-350 band is (3.4 +- 0.8) ph/cm2/sec. All the quoted errors are at the 90% confidence level. //////////////////////////////////////////////////////////////////////// TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3662 SUBJECT: GRB 050721: early afterglow light curve DATE: 05/07/22 23:00:24 GMT FROM: Daniele Malesani at SISSA-ISAS,Trieste,Italy V. Testa, L.A. Antonelli (INAF/OAR), D. Malesani (SISSA), S. Covino, P. D'Avanzo, G. Tagliaferri (INAF/OABr), G. Chincarini (Univ. Milano-Bicocca), G.L. Israel, S. Piranomonte, L. Stella (INAF/OAR), report on behalf of the MISTICI collaboration: We analyzed the full dataset of the early light curve of the afterglow of GRB 050721 (Antonelli et al., GCN 3654; Torii, GCN 3655; Covino et al., GCN 3657), obtained with the ESO VLT in rapid response mode. The observation, performed in the R band, started 25 min after the GRB and lasted for 107 minutes (till the object set). It was composed by several short (30 s), repeated exposures. The light curve is well described by a single power law, with decay index 1.20 +- 0.01. The decay is smooth, without any significant rebrightening or deep. We acknowledge the excellent support provided by the ESO staff in performing the observations. This message can be cited.