Swift Observations of GRB 210308A

V. D'Elia (SSDC) and J.D. Gropp (PSU) for the Swift team

1. Introduction

At 06:37:56 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210308A (trigger=1036227) (D'Elia et al. GCN Circ. 29619). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 84° from the Sun (5.2 hours East) and 144° from the 27%-illuminated Moon. Table 1 contains the best reported positions from Swift.

D'Elia et al. (GCN Circ. 29619) reported the discovery 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 Palmer et al. (GCN Circ. 29633), the BAT ground-calculated position is RA, Dec = 67.083, 37.429 deg which is RA(J2000) = 04h28m19.9s Dec(J2000) = +37°25'42.9" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 32%.

The mask-weighted light curve (Figure 1) shows a double-peaked structure from T-0.5 s to T+9 s. The first pulse peaks at T+0.2 s, and the second pulse peaks at T+3 s. T90 (15-350 keV) is 5.3 ± 0.6 s (estimated error including systematics).

The time-averaged spectrum from T-0.04 to T+7.36 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.76 ± 0.28, and Epeak of 113.6 ± 31.0 keV (χ2 52.89 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 3.2 ± 0.1 x 10-6 erg cm-2 and the 1-s peak flux measured from T+2.12 s in the 15-150 keV band is 13.6 ± 0.7 ph cm-2 s-1. This fluence is larger than that of 72% 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 1.38 ± 0.06 (χ2 69.63 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/1036227/BA/.

3. XRT Observations and Analysis

We have analysed 13 ks of XRT data for GRB 210308A, from 91 s to 84.9 ks after the BAT trigger. The data comprise 45 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.79 (+0.09, -0.15), followed by a break at T+729 s to an α of 1.29 (+0.23, -0.08).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.79 (+0.19, -0.18). The best-fitting absorption column is 8.5 (+1.8, -1.6) x 1021 cm-2, in excess of the Galactic value of 4.5 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 5.2 x 10-11 (8.2 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 8.5 (+1.8, -1.6) x 1021 cm-2
Galactic foreground: 4.5 x 1021 cm-2
Excess significance: 4.1 σ
Photon index: 1.79 (+0.19, -0.18)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01036227.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210308A 95 s after the BAT trigger (Gropp and D'Elia GCN Circ. 29630). A source consistent with the XRT position 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 1.296 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
04h28m17.73s +37°26'02.1" 0.46" UVOT-refined Gropp and D'Elia GCN Circ. 29630
04h28m17.83s +37°26'02.1" 1.4" XRT-final UKSSDC
04h28m18.34s +37°26'07.11" 4.3" XRT-initial D'Elia et al. GCN Circ. 29619
04h28m19.9s +37°25'42.9" 1.0' BAT-refined Palmer et al. GCN Circ. 29633

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Ogata et al. 29621 MITSuME Okayama optical upper limits MITSuME Okayama upper limits
Optical Zhu et al. 29623 Nanshan/NEXT optical upper limits Xinjiang Astro. Obs. upper limits
Optical Lipunov et al. 29627 Swift GRB 210308A: Global MASTER-Net
observations report 		MASTER
Optical Belkin et al. 29634 Mondy and AbAO optical observations Abastumani Astro. Obs. detection
Gamma-ray Lesage et al. 29626 Fermi GBM detection Fermi GBM Epeak=142±5 keV
T90=5.4 seconds
Fluence=6.3±0.1x10-6erg cm-2
(63rd percentile for long GRBs)
Gamma-ray Svinkin et al. 29643 Konus-Wind detection Konus-Wind Epeak=150 (-19,+25) keV
Duration=~8.9 seconds
Fluence=5.00(-0.48,+0.53)x10-6erg cm-2

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
b 765 1189 38 >18.62
uvm2 864 1115 38 >17.52
u 338 1164 284 >19.50
v 667 1090 58 >17.84
uvw1 1120 1140 19 >17.25
uvw2 815 1231 50 >17.91
white 126 275 292
white 618 1213 205 >20.20

Table 3. UVOT observations reported by Gropp and D'Elia (GCN Circ. 29630). 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.

March 11, 2021