Swift Observations of GRB 210210A

S. Laha (GSFC/UMBC/CRESST), V. D'Elia (SSDC and INAF-OAR) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 02:00:27 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210210A (trigger=1031728) (Laha et al. GCN Circ. 29444). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 67° from the Sun (4.1 hours West) and 55° from the 3%-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.

Laha et al. (GCN Circ. 29444) reported the discovery with UVOT of an optical afterglow. de Ugarte Postigo et al. (GCN Circ. 29450) determined a redshift of 0.715 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 Lien et al. (GCN Circ. 29467), the BAT ground-calculated position is RA, Dec = 262.764, 14.657 deg which is RA(J2000) = 17h31m03.4s Dec(J2000) = +14°39'26.8" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 43%.

The mask-weighted light curve (Figure 1) shows a double-peaked structure running approximately from T-1 s to T+7 s. The first pulse peaks close to T+0 s and the second pulse peaks at T+4.5 s. T90 (15-350 keV) is 6.60 ± 0.59 s (estimated error including systematics).

The time-averaged spectrum from T-0.75 to T+6.93 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.53 ± 0.58, and Epeak of 19.9 ± 13.9 keV (χ2 44.33 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 9.8 ± 0.8 x 10-7 erg cm-2 and the 1-s peak flux measured from T+4.21 s in the 15-150 keV band is 7.0 ± 0.5 ph cm-2 s-1. This fluence is larger than that of 38% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). A fit to a simple power law gives a photon index of 2.46 ± 0.12 (χ2 53.94 for 57 d.o.f.). 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/1031728/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Elia et al. (GCN Circ. 29452). We have analysed 26 ks of XRT data for GRB 210210A, from 85 s to 385.4 ks after the BAT trigger. The data comprise 58 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 29448).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.31 ± 0.06, followed by a break at T+4508 s to an α of 1.70 (+0.14, -0.13).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.89 ± 0.11. The best-fitting absorption column is 7.8 (+7.0, -6.6) x 1020 cm-2, at a redshift of 0.715, in addition to the Galactic value of 8.8 x 1020 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.7 x 10-11 (4.6 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 8.8 x 1020 cm-2
Intrinsic column: 7.8 (+7.0, -6.6) x 1020 cm-2 at z=0.715
Photon index: 1.89 ± 0.11

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01031728.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210210A 85 s after the BAT trigger (Breeveld and Laha GCN Circ. 29457). A fading source consistent with the XRT position (Goad et al. GCN Circ. 29448) is detected in the initial UVOT exposures, as also detected by Lipunov et al. (GCN Circ. 29447) and de Ugarte Postigo et al. (GCN Circ. 29450). 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 0.097 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
17h31m05.01s +14°39'46.4" 0.42" UVOT-refined Breeveld and Laha GCN Circ. 29457
17h31m04.88s +14°39'45.1" 1.4" XRT-final UKSSDC
17h31m04.90s +14°39'45.2" 1.7" XRT-enhanced Goad et al. GCN Circ. 29448
17h31m03.4s +14°39'26.8" 1.0' BAT-refined Lien et al. GCN Circ. 29467

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 29445 Swift GRB 210210A: Global MASTER-Net
observations report 		MASTER
Optical Lipunov et al. 29447 MASTER afterglow observation MASTER
Optical de Ugarte Postigo et al. 29450 Redshift from OSIRIS/GTC GTC redshift
Optical Strausbaugh and Cucchiara 29459 LCO Optical Afterglow Detection LCO detection
Optical Jelinek et al. 29469 FRAM-ORM afterglow detection FRAM detection
Optical Kann et al. 29476 CAHA detection and likely jet break CAHA detection
Optical Dimple et al. 29488 1.3m DFOT optical observations Devasthal Opt.Tel. detection
Optical Kann et al. 29502 CAHA 2nd epoch and jet break
confirmation		 CAHA detection
Gamma-ray Frederiks et al. 29517 Konus-Wind detection and joint
Konus-Wind + Swift-BAT spectral
analysis		 Konus-Wind Epeak=16.6 (-10.7,+7.2) keV
Other Ruffini et al. 29481 as a BdHN II

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 85 235 147 16.04 ± 0.03
white 878 1028 147 16.57 ± 0.06
v 629 648 20 16.95 ± 0.19
b 553 573 20 16.99 ± 0.11
u 298 547 246 15.91 ± 0.03
w1 679 699 19 16.05 ± 0.15
m2 827 847 19 16.13 ± 0.20
w2 779 798 20 16.35 ± 0.18

Table 3. UVOT observations reported by Breeveld and Laha (GCN Circ. 29457). 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 18, 2021