//////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 20302 SUBJECT: GRB 161220A: Swift detection of a burst DATE: 16/12/20 08:56:40 GMT FROM: Kim Page at U.of Leicester V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), T. Sakamoto (AGU), B. Sbarufatti (INAF-OAB/PSU) and T. N. Ukwatta (LANL) report on behalf of the Swift Team: At 08:33:30 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 161220A (trigger=727610). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 350.817, -47.502 which is RA(J2000) = 23h 23m 16s Dec(J2000) = -47d 30' 07" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a FRED-like structure with a duration of about 12 sec. The peak count rate was ~1285 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 08:34:39.2 UT, 69.1 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source located at RA, Dec 350.84774, -47.49773, which is equivalent to: RA(J2000) = 23h 23m 23.46s Dec(J2000) = -47d 29' 51.8" with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 76 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 consistent with the Galactic value of 1.19 x 10^20 cm^-2 (Willingale et al. 2013). The initial flux in the 2.5 s image was 1.94e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 78 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. Results from the list of sources generated on-board are not available at this time. No correction has been made for the expected extinction corresponding to E(B-V) of 0.01. Burst Advocate for this burst is V. D'Elia (delia AT asdc.asi.it). 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: 20307 SUBJECT: GRB 161220A: Swift/UVOT Upper Limits DATE: 16/12/20 17:10:33 GMT FROM: Frank Marshall at GSFC F. E. Marshall (NASA/GSFC) and V. D'Elia (ASDC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 161220A 78 s after the BAT trigger (D'Elia et al., GCN Circ. 20302). No optical afterglow consistent with the XRT position (D'Elia et al. GCN Circ. 20302) 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 78 228 147 >20.5 white 78 1012 295 >21.0 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.01 in the direction of the burst (Schlegel et al. 1998). //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 20312 SUBJECT: GRB 161220A: GROND Upper Limits DATE: 16/12/21 01:58:47 GMT FROM: Philip Wiseman at MPE/Swift P. Wiseman, T. Kruehler, and J. Greiner (all MPE Garching) report on behalf of the GROND team: We observed the field of GRB 161220A (Swift trigger 727610; D'Elia et al., GCN #20302) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2m MPG telescope at the ESO La Silla Observatory (Chile). Observations started at 00:36 UT on 2016-12-21, 16 hrs after the GRB trigger. They were performed at an average seeing of 0".9, and at an average airmass of 1.2. Similar to Marshall & D'Elia (GCN #20307), we detect no credible optical/NIR afterglow consistent with the XRT position. Based on combined images with 14.5 min of total exposures in g'r'i'z' and 12.0 min in JHK at a mid-time of 00:45 UT on 2016-12-21, we derive the following upper limits (all in AB system). g' > 22.7 mag r' > 22.7 mag i' > 21.3 mag z' > 21.8 mag J > 20.8 mag H > 20.4 mag K > 17.2 mag Given magnitudes are calibrated against USNO as well as 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.005 mag in the direction of the burst (Schlafly & Finkbeiner 2011). //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 20315 SUBJECT: GRB 161220A: LCO-FTS observations DATE: 16/12/21 06:57:45 GMT FROM: Cristiano Guidorzi at Ferrara U,Italy C. Guidorzi (U. Ferrara), S. Kobayashi, I.A. Steele (LJMU), A. Gomboc (U. Nova Gorica), C.G. Mundell (U. Bath) on behalf of a large collaboration report: We observed Swift GRB 161220A (D'Elia et al. GCN 20302) on December 20, from 11:02 to 11:13 UT (2.5-2.7 hours since the GRB) with the 2-m LCO Faulkes Telescope South in Siding Springs with the SDSS i filter under poor sky transparency conditions. Within the XRT error circle we do not find any object brighter than i~20 mag (total exposure of 5x120s) as calibrated against nearby USNOB1 stars. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 20316 SUBJECT: GRB 161220A: Swift-XRT refined Analysis DATE: 16/12/21 08:56:04 GMT FROM: Phil Evans at U of Leicester A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), P. D'Avanzo (INAF-OAB), A. D'Ai (INAF-IASFPA), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), S.L. Gibson (U. Leicester) and V. D'Elia report on behalf of the Swift-XRT team: We have analysed 3.4 ks of XRT data for GRB 161220A (D'Elia et al. GCN Circ. 20302), from 60 s to 58.3 ks after the BAT trigger. The data comprise 85 s in Windowed Timing (WT) mode (the first 7 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The refined XRT position is RA, Dec = 350.8479, -47.4978 which is equivalent to: RA (J2000): 23 23 23.49 Dec(J2000): -47 29 52.2 with an uncertainty of 3.6 arcsec (radius, 90% confidence). The light curve can be modelled with a series of power-law decays. The light curve initially rises, with an index alpha=-1.5 (+0.0, -2.0). At T+71.0 s it breaks to an alpha of 4.75 (+0.51, -0.29) before breaking again at T+217 s to a final decay with index alpha=0.82 (+0.13, -0.11). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 3.57 (+0.20, -0.19). The best-fitting absorption column is 3.2 (+/-0.4) x 10^21 cm^-2, in excess of the Galactic value of 1.2 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 2.1 x 10^-11 (9.9 x 10^-11) erg cm^-2 count^-1. A summary of the WT-mode spectrum is thus: Total column: 3.2 (+/-0.4) x 10^21 cm^-2 Galactic foreground: 1.2 x 10^20 cm^-2 Excess significance: 12.4 sigma Photon index: 3.57 (+0.20, -0.19) If the light curve continues to decay with a power-law decay index of 0.82, the count rate at T+24 hours will be 2.7 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.7 x 10^-14 (2.6 x 10^-13) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00727610. This circular is an official product of the Swift-XRT team. //////////////////////////////////////////////////////////////////////// TITLE: GCN CIRCULAR NUMBER: 20325 SUBJECT: GRB 161220A: Swift-BAT refined analysis DATE: 16/12/21 18:35:58 GMT FROM: Tilan Ukwatta at LANL T. Sakamoto (AGU), S. D. Barthelmy (GSFC), J. R. Cummings (CPI), V. D'Elia (ASDC), N. Gehrels (GSFC), 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-61 to T+243 sec from the recent telemetry downlink, we report further analysis of BAT GRB 161220A (trigger #727610) (D'Elia, et al., GCN Circ. 20302). The BAT ground-calculated position is RA, Dec = 350.841, -47.480 deg which is RA(J2000) = 23h 23m 21.7s Dec(J2000) = -47d 28' 47.3" with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%. The mask-weighted light curve shows a single episode that starts around T-4 sec, peaks around T+3 sec and ends at ~T+4 sec. T90 (15-350 keV) is 6 +- 1 sec (estimated error including systematics). The time-averaged spectrum from T-2.24 to T+4.66 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.01 +- 0.25. The fluence in the 15-150 keV band is 1.7 +- 0.3 x 10^-07 erg/cm2. The 1-sec peak photon flux measured from T+2.42 sec in the 15-150 keV band is 0.5 +- 0.1 ph/cm2/sec. 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/727610/BA/