Swift Observations of GRB 210211A

E. Troja (NASA/GSFC/UMCP), A. D'Ai (INAF-IASFPA) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 08:43:18 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210211A (trigger=1032024) (Troja et al. GCN Circ. 29472). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 56° from the Sun (3.7 hours West) and 50° from the 0%-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.

D'Avanzo et al. (GCN Circ. 29474) reported the position from REM for the optical afterglow of this GRB. Kuin and Troja (GCN Circ. 29485) reported the detection with UVOT of an optical afterglow. 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 Markwardt et al. (GCN Circ. 29483), the BAT ground-calculated position is RA, Dec = 269.408, -46.270 deg which is RA(J2000) = 17h57m38.0s Dec(J2000) = -46°16'11.0" with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a single broad peak starting at T-1 s, and decaying to background by T+8 s. T90 (15-350 keV) is 6.1 ± 0.6 s (estimated error including systematics).

The time-averaged spectrum from T-0.70 to T+5.90 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.09 ± 0.16. The fluence in the 15-150 keV band is 4.2 ± 0.4 x 10-7 erg cm-2. This fluence is larger than that of 18% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+4.28 s in the 15-150 keV band is 1.0 ± 0.2 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/1032024/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Ai et al. (GCN Circ. 29484). We have analysed 25 ks of XRT data for GRB 210211A, from 81 s to 298.6 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. 29478).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.25 (+0.09, -0.08).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.5 (+0.7, -0.6). The best-fitting absorption column is 6.5 (+6.3, -3.5) x 1021 cm-2, in excess of the Galactic value of 1.2 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 3.4 x 10-11 (8.2 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 6.5 (+6.3, -3.5) x 1021 cm-2
Galactic foreground: 1.2 x 1021 cm-2
Excess significance: 2.5 σ
Photon index: 2.5 (+0.7, -0.6)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01032024.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210211A 78 s after the BAT trigger (Kuin and Troja GCN Circ. 29485). A source consistent with the XRT position (Osborne et al. GCN Circ. 29478) and the REM detection (d'Avanzo et al., GCN Circ. 29474)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 0.127 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
17h57m45.68s -46°16'01.6" 0.47" UVOT-refined Kuin and Troja GCN Circ. 29485
17h57m45.55s -46°16'02.1" 1.9" XRT-final UKSSDC
17h57m45.54s -46°16'02.3" 2.4" XRT-enhanced Osborne et al. GCN Circ. 29478
17h57m38.0s -46°16'11.0" 1.4' BAT-refined Markwardt et al. GCN Circ. 29483

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 29471 Swift GRB210211.36: Global MASTER-Net
observations report 		MASTER
Optical D'Avanzo et al. 29474 REM optical/NIR afterglow detection REM detection
Optical Lipunov et al. 29477 MASTER optical observation of REM OT MASTER
Optical Harvey et al. 29515 ePESSTO+ NTT optical observations PESSTO
Gamma-ray Fermi 29470 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Wood et al. 29482 Fermi GBM observation Fermi GBM Epeak=95±35 keV
T90=29 seconds
Fluence=3.1±0.2x10-6erg cm-2
(43rd percentile for long GRBs)

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
white 78 227 147 17.89 ± 0.06
white 596 611 15 >18.6
white 5391 5587 197 >20.3
v 4239 4438 197 >19.8
b 545 565 19 >18.4
u 290 489 196 >18.6
m2 4444 4491 47 >18.3
w2 4034 4234 197 >19.4

Table 3. UVOT observations reported by Kuin and Troja (GCN Circ. 29485). The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary detections and 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.

February 17, 2021