Swift Observations of GRB 210209A

S. Laha (GSFC/UMBC/CRESST), M. Perri (SSDC and INAF-OAR) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 21:39:44 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210209A (trigger=1031636) (Laha et al. GCN Circ. 29442). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 58° from the Sun (0.3 hours East) and 72° from the 4%-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 Laha et al. (GCN Circ. 29479), the BAT ground-calculated position is RA, Dec = 328.097, 43.545 deg which is RA(J2000) = 21h52m23.3s Dec(J2000) = +43°32'40.4" with an uncertainty of 1.6 arcmin, (radius, sys+stat, 90% containment). The partial coding was 96%.

The mask-weighted light curve (Figure 1) shows a double-peaked structure running from roughly T-35 s to T+140 seconds. The two peaks were at T+3 s and T+65 s. T90 (15-350 keV) is 139.4 ± 21.5 s (estimated error including systematics).

The time-averaged spectrum from T-40.34 to T+134.72 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.52 ± 0.12. The fluence in the 15-150 keV band is 2.4 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 65% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+2.68 s in the 15-150 keV band is 0.6 ± 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/1031636/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Perri et al. (GCN Circ. 29449). We have analysed 14 ks of XRT data for GRB 210209A, from 122 s to 184.2 ks after the BAT trigger. The data comprise 139 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 29443).

The late-time light curve (Figure 2) (from T0+4.3 ks) can be modelled with a power-law decay with a decay index of α=1.04 ± 0.05.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.81 (+0.07, -0.04). The best-fitting absorption column is consistent with the Galactic value of 3.7 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.93 (+0.07, -0.05) and a best-fitting absorption column consistent with the Galactic value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.0 x 10-11 (5.7 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 3.67 (+0.17, -0.00) x 1021 cm-2
Galactic foreground: 3.7 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 1.93 (+0.07, -0.05)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01031636.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210209A 127 s after the BAT trigger (Breeveld and Laha GCN Circ. 29453). No optical afterglow consistent with the XRT position (Evans et al., GCN Circ. 29443) 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 0.286 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
21h52m29.36s +43°33'25.2" 1.4" XRT-final UKSSDC
21h52m29.34s +43°33'25.1" 1.5" XRT-enhanced Evans et al. GCN Circ. 29443
21h52m23.3s +43°32'40.4" 1.6' BAT-refined Laha et al. GCN Circ. 29479

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Hosokawa et al. 29455 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Lipunov et al. 29458 Swift GRB 210209A: Global MASTER-Net
observations report 		MASTER
Optical Hu et al. 29460 BOOTES-4/MET optical upper limit BOOTES-4 upper limits

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 127 276 147 >20.3
uFC 285 535 246 >19.4
white 127 952 275 >20.4
v 615 4730 236 >19.1
b 540 733 39 >18.8
u 285 5225 345 >19.8
w1 664 5140 236 >19.7
m2 639 4934 236 >19.4
w2 590 4525 236 >20.5

Table 3. UVOT observations reported by Breeveld and Laha (GCN Circ. 29453). 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 12, 2021