Swift Observations of GRB 210421A

M.J. Moss (GWU), P.A. Evans (U. Leicester) and S.R. Oates (U.Birmingham) for the Swift team

1. Introduction

At 00:27:30 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210421A (trigger=1044426) (Moss et al. GCN Circ. 29849). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 76° from the Sun (5.1 hours East) and 32° from the 58%-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 Ukwatta et al. (GCN Circ. 29893), the BAT ground-calculated position is RA, Dec = 104.882, 4.928 deg which is RA(J2000) = 06h59m31.7s Dec(J2000) = +04°55'41.2" with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment). The partial coding was 44%.

The BAT light curve (Figure 1) showed a complex structure with a duration of about ~60 s. T90 (15-350 keV) is 79.52 ± 26.78 s (estimated error including systematics).

The time-averaged spectrum from T-21.74 to T+73.22 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.10 ± 0.72, and Epeak of 35.2 ± 8.5 keV (χ2 61.93 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 2.0 ± 0.2 x 10-6 erg cm-2 and the 1-s peak flux measured from T+2.47 s in the 15-150 keV band is 2.0 ± 0.3 ph cm-2 s-1. This fluence is larger than that of 58% 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 2.10 ± 0.13 (χ2 69.44 for 57 d.o.f.). All the quoted errors are at the 90% confidence level.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 29875). We have analysed 10 ks of XRT data for GRB 210421A, from 91 s to 80.5 ks after the BAT trigger. The data comprise 130 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. 29865).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=2.7 (+0.6, -1.1). At T+111 s the decay steepens to an α of 5.11 (+0.14, -0.15) before breaking again at T+350 s to a final decay with index α=0.73 (+0.09, -0.08).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.74 ± 0.08. The best-fitting absorption column is 5.3 ± 0.3 x 1021 cm-2, in excess of the Galactic value of 4.6 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.17 (+0.19, -0.14) 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 3.7 x 10-11 (6.5 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.6 (+0.8, -0.0) x 1021 cm-2
Galactic foreground: 4.6 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 2.17 (+0.19, -0.14)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01044426.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210421A 96 s after the BAT trigger (Oates and Moss GCN Circ. 29885). No optical afterglow consistent with the XRT position (Evans et al., GCN Circ. 29865). 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 0.318 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
06h59m29.86s +04°55'20.0" 2.1" XRT-final UKSSDC
06h59m29.86s +04°55'20.4" 2.4" XRT-enhanced Evans et al. GCN Circ. 29865
06h59m31.7s +04°55'41.2" 1.2' BAT-refined Ukwatta et al. GCN Circ. 29893

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 29850 Swift GRB 210421A: Global MASTER-Net
observations report 		MASTER
Optical Watson et al. 29857 DDOTI Upper Limit Deca-Degree Optical Imager upper limits
Optical Ito et al. 29882 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Gamma-ray Xie et al. 29889 GECAM detection GECAM

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 96 246 147 >20.4
white 96 1912 191 >20.6
v 1608 1800 39 >17.9
b 1534 1899 58 >19.2
uFC 308 484 173 >19.5
u 308 1874 211 >19.6
uvw1 1658 1850 39 >18.1
uvm2 1633 1825 39 >17.9
uvw2 1584 1776 39 >18.3

Table 3. UVOT observations reported by Oates and Moss (GCN Circ. 29885). 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.

April 24, 2021