Swift Observations of GRB 210212A

E. Troja (NASA/GSFC/UMCP), A. Tohuvavohu (U. Toronto) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 04:25:18 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210212A (trigger=1032183) (Troja et al. GCN Circ. 29487). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 108° from the Sun (7.1 hours East) and 102° 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.

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 Palmer et al. (GCN Circ. 29491), the BAT ground-calculated position is RA, Dec = 72.249, 7.215 deg which is RA(J2000) = 04h48m59.7s Dec(J2000) = +07°12'52.6" with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) showed a broad peak with some underlying structure. It began around T-10 s, peaked around T+5 s and returned to background by T+90 s. T90 (15-350 keV) is 80.0 ± 35.8 s (estimated error including systematics).

The time-averaged spectrum from T-15.51 to T+80.49 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.85 ± 0.23. The fluence in the 15-150 keV band is 8.0 ± 1.2 x 10-7 erg cm-2. This fluence is larger than that of 33% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+7.99 s in the 15-150 keV band is 0.5 ± 0.1 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/1032183/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Tohuvavohu et al. (GCN Circ. 29495). We have analysed 24 ks of XRT data for GRB 210212A, from 95 s to 577.9 ks after the BAT trigger. The data comprise 337 s in Windowed Timing (WT) mode (the first 6 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 29489).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=2.52 (+0.15, -0.14), followed by a break at T+331 s to an α of 7.2 (+0.8, -0.6).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.55 (+0.13, -0.12). The best-fitting absorption column is 1.6 (+0.6, -0.5) x 1021 cm-2, consistent with the Galactic value of 1.1 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 4.4 x 10-11 (5.0 x 10-11) erg cm-2 count-1.

A summary of the WT-mode spectrum is thus:
Total column: 1.6 (+0.6, -0.5) x 1021 cm-2
Galactic foreground: 1.1 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 1.55 (+0.13, -0.12)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01032183.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210212A 112 s after the BAT trigger (Kuin and Troja GCN Circ. 29492). No optical afterglow consistent with the XRT position (Beardmore et al. GCN Circ. 29489) 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 0.094 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
04h49m03.99s +07°16'10.8" 3.6" XRT-final UKSSDC
04h49m03.99s +07°16'10.8" 3.7" XRT-enhanced Beardmore et al. GCN Circ. 29489
04h48m59.7s +07°12'52.6" 1.9' BAT-refined Palmer et al. GCN Circ. 29491

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Hu et al. 29494 BOOTES-4/MET optical upper limit BOOTES-4 upper limits
Optical Murata et al. 29496 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Lipunov et al. 29497 Swift GRB 210212A: Global MASTER-Net
observations report 		MASTER
Optical Heintz et al. 29498 Possible host galaxy counterpart with
the NOT		 NOT
Optical Dichiara and Troja 29507 Gemini South optical observations Gemini
Optical Gupta et al. 29510 1.3m DFOT Optical upper limit Devasthal Opt.Tel. upper limits
Optical Belkin et al. 29522 Mondy optical observations Mondy
Optical D'Avanzo et al. 29527 REM early-time optical/NIR upper limits REM upper limits
Other Dichiara and Troja 29540 Lowell Discovery Telescope observations

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 112 262 147 >20.6
uFC 271 520 246 >20.3
white 112 930 242 >20.8
v 600 4588 236 >19.0
b 526 720 39 >19.3
u 271 868 285 >20.4
w1 650 4851 90 >19.1
m2 626 4793 235 >19.7
w2 93 4383 245 >19.7

Table 3. UVOT observations reported by Kuin and Troja (GCN Circ. 29492). 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 20, 2021