Swift Observations of GRB 201221D

K.L. Page (U. Leicester), P.A. Evans (U. Leicester) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 23:06:34 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 201221D (trigger=1014037) (Page et al. GCN Circ. 29112). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 112° from the Sun (6.6 hours West) and 141° from the 50%-illuminated Moon. Table 1 contains the best reported positions from Swift, and the latest XRT position can be viewed at http://www.swift.ac.uk/xrt_positions.

de Ugarte Postigo et al. (GCN Circ. 29132) determined a redshift of 1.046 from GTC. Table 2 is a summary of GCN Circulars about this GRB from observatories other than Swift.

Standard analysis products for this burst are available at https://gcn.gsfc.nasa.gov/swift_gnd_ana.html.

2. BAT Observations and Analysis

As reported by Krimm et al. (GCN Circ. 29139), the BAT ground-calculated position is RA, Dec = 171.055, 42.152 deg which is RA(J2000) = 11h24m13.2s Dec(J2000) = +42°09'08.4" with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 23%.

The mask-weighted light curve (Figure 1) shows a single-peaked structure that starts at ~T-0.06 s, peaks at ~T+0.02 s, and ends at ~T+0.2 s. T90 (15-350 keV) is 0.16 ± 0.04 s (estimated error including systematics).

The time-averaged spectrum from T-0.06 to T+0.17 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.56 ± 0.13. The fluence in the 15-150 keV band is 3.9 ± 0.4 x 10-7 erg cm-2. This fluence is larger than that of 92% of the short GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.44 s in the 15-150 keV band is 5.6 ± 0.5 ph cm-2 s-1. All the quoted errors are at the 90% confidence level.

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/1014037/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 29124). We have analysed 11 ks of XRT data for GRB 201221D, from 97 s to 430.0 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 29113). There is evidence that the source may be fading, however we cannot constrain the decay at the current time

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01014037.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 201221D 92 s after the BAT trigger (Breeveld and Page GCN Circ. 29129). No optical afterglow consistent with the NOT or XRT position (Malesani et al. GCN Circ. 29117; Evans et al. GCN Circ. 29119) is detected in the initial UVOT exposures. Table 3 gives preliminary magnitudes using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc., 1358, 373). No correction has been made for the expected extinction in the Milky Way corresponding to a reddening of EB-V of 0.02 mag. in the direction of the GRB (Schlegel et al. 1998).

BAT light curve

Figure 1. The BAT mask-weighted light curve in the four individual and total energy bands. The units are counts s-1 illuminated-detector-1. The vertical green dash-dotted lines show the T50 interval, the vertical black dashed lines show the T90 interval, and vertical blue (orange) solid lines show the start (stop) of slews.

XRT light curve

Figure 2. The XRT light curve. Any data from a crosshatched region are not included in the fit.

RA (J2000) Dec (J2000) Error Note Reference
11h24m14.15s +42°08'36.6" 4.3" XRT-final UKSSDC
11h24m14.19s +42°08'35.5" 3.9" XRT-enhanced Evans et al. GCN Circ. 29119
11h24m13.2s +42°09'08.4" 1.3' BAT-refined Krimm et al. GCN Circ. 29139

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Malesani and Knudstrup 29117 candidate optical counterpart from the
NOT		 NOT detection
Optical Xu et al. 29118 Nanshan/NEXT early optical upper limit Xinjiang Astro. Obs. upper limits
Optical Lipunov et al. 29122 Swift GRB 201221D: Global MASTER-Net
observations report 		MASTER
Optical Jelinek et al. 29125 FRAM-ORM optical limit FRAM upper limits
Optical de Ugarte Postigo et al. 29132 Short GRB201221D: High Redshift from
OSIRIS/GTC		 GTC redshift
Optical Andreoni and Coughlin 29141 LCOGT upper limits Las Cumbres Obs. Global Tele.Network upper limits
Optical Rastinejad et al. 29142 MMT MMIRS detection of marginally
extended source		 MMT detection
Optical Dimple et al. 29148 3.6m DOT optical observations Devasthal Opt.Tel.
Optical Rossi 29311 near-infrared observation with LBT LBT
Radio Huang et al. 29144 SMA submm observation SMA
Gamma-ray Frederiks et al. 29130 Konus-Wind detection of GRB 201221D
(short)		 Konus-Wind Epeak=148 (-37,+86) keV
Gamma-ray Hamburg et al. 29140 Fermi GBM detection Fermi GBM Epeak=108±5 keV
T90=0.14 seconds
Fluence=1.076±0.046x10-6erg cm-2
(83rd percentile for short GRBs)
Other Dichiara et al. 29128 Lowell Discovery Telescope afterglow
observations		 detection
Other Kilpatrick et al. 29133 Archival PS1 Imaging of the Putative
Host Galaxy

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

Filter Tstart(s) Tstop(s) Exp(s) Mag
whiteFC 92 242 147 >20.3
uFC 305 554 246 >19.3
white 4047 4247 197 >21.0
v 634 4657 216 >19.2
u 305 5110 282 >19.4
w1 684 5068 216 >19.7
m2 4663 4863 197 >19.5
w2 4253 4453 197 >19.9

Table 3. UVOT observations reported by Breeveld and Page (GCN Circ. 29129). The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.

January 18, 2021