Swift Observations of GRB 210205A

S. Dichiara (NASA/GSFC/UMCP), P.A. Evans (U. Leicester) and A. Belles (PSU) for the Swift team

1. Introduction

At 11:11:17 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210205A (trigger=1030629) (Dichiara et al. GCN Circ. 29397). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 76° from the Sun (1.9 hours East) and 117° from the 42%-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.

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 Barthelmy et al. (GCN Circ. 29409), the BAT ground-calculated position is RA, Dec = 347.264, 56.311 deg which is RA(J2000) = 23h09m03.3s Dec(J2000) = +56°18'41.2" with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 18%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~ T-7 s and ends at ~T+20 s. The main peak in the structure occurs at ~T+6 s. T90 (15-350 keV) is 22.70 ± 4.18 s (estimated error including systematics).

The time-averaged spectrum from T-7.35 to T+20.07 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.27 ± 0.31. The fluence in the 15-150 keV band is 8.7 ± 1.6 x 10-7 erg cm-2. This fluence is larger than that of 35% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+5.98 s in the 15-150 keV band is 1.6 ± 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/1030629/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 29407). We have analysed 9.6 ks of XRT data for GRB 210205A, from 143 s to 39.7 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 29399).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.10 (+0.09, -0.10).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.2 (+0.5, -0.4). The best-fitting absorption column is 8.9 (+4.0, -3.1) x 1021 cm-2, in excess of the Galactic value of 5.1 x 1021 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.2 x 10-11 (9.0 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 8.9 (+4.0, -3.1) x 1021 cm-2
Galactic foreground: 5.1 x 1021 cm-2
Excess significance: 2.0 σ
Photon index: 2.2 (+0.5, -0.4)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01030629.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210205A 139 s after the BAT trigger (Belles and Dichiara GCN Circ. 29403). No optical afterglow consistent with the XRT position (Osborne et al. GCN Circ. 29399) 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 0.716 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
23h08m52.64s +56°17'47.5" 1.8" XRT-final UKSSDC
23h08m52.58s +56°17'47.4" 2.0" XRT-enhanced Osborne et al. GCN Circ. 29399
23h09m03.3s +56°18'41.2" 2.1' BAT-refined Barthelmy et al. GCN Circ. 29409

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Xin et al. 29398 ： GWAC-F60A optical upper limit Xinglong Obs. upper limits
Optical Hu et al. 29400 BOOTES-4/MET optical upper limit BOOTES-4 upper limits
Optical Fu et al. 29401 Nanshan/NEXT optical upper limit Xinjiang Astro. Obs. upper limits
Optical Lipunov et al. 29402 Swift GRB 210205A: Global MASTER-Net
observations report 		MASTER
Optical Horiuchi et al. 29406 MITSuME Ishigaki optical observation MITSuME
Optical Pandey et al. 29526 observations with the 3.6m DOT, a
potential dark burst?		 DOT light curve

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 139 289 147 >20.5
white 139 4057 344 >20.6
v 4268 4468 197 >19.4
b 3652 5159 265 >20.1
u 297 5084 255 >19.7
w1 4678 4878 197 >19.9
m2 4473 4673 197 >20.8
w2 4063 4263 197 >19.3

Table 3. UVOT observations reported by Belles and Dichiara (GCN Circ. 29403). 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.

February 19, 2021