Swift Observations of GRB 201021C

A.Y. Lien (GSFC/UMBC), D.N. Burrows (PSU) and S.R. Oates (U.Birmingham) for the Swift team

1. Introduction

At 20:27:18 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 201021C (trigger=1001130) (Lien et al. GCN Circ. 28730). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 112° from the Sun (11.1 hours East) and 72° from the 32%-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.

Lien et al. (GCN Circ. 28730) reported the discovery with UVOT of an optical afterglow. Vielfaure et al. (GCN Circ. 28739) determined a redshift of 1.070 from VLT. 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 Stamatikos et al. (GCN Circ. 28755), the BAT ground-calculated position is RA, Dec = 12.503, -55.862 deg which is RA(J2000) = 00h50m00.6s Dec(J2000) = -55°51'44.0" with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 80%.

The mask-weighted light curve (Figure 1) shows a single peak running from ~T-5 s to T+20 s, with a broad peak around T+3 s. T90 (15-350 keV) is 24.38 ± 6.76 s (estimated error including systematics).

The time-averaged spectrum from T-4.19 to T+27.95 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.63 ± 0.15. The fluence in the 15-150 keV band is 1.0 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 38% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-3.62 s in the 15-150 keV band is 1.0 ± 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/1001130/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Burrows et al. (GCN Circ. 28754). We have analysed 4.3 ks of XRT data for GRB 201021C, from 105 s to 58.8 ks after the BAT trigger. The data comprise 62 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.22 (+0.07, -0.06).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.62 (+0.19, -0.16). The best-fitting absorption column is 5 (+21, -5) x 1020 cm-2, at a redshift of 1.07, in addition to the Galactic value of 2.0 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 4.0 x 10-11 (4.2 x 10-11) erg cm-2 count-1.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 201021C 105 s after the BAT trigger (Oates and Lien GCN Circ. 28745). A source consistent with the XRT position (Beardmore et al. GCN Circ. 28736) 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 0.02 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
00h50m06.93s -55°51'56.8" 0.42" UVOT-refined Oates and Lien GCN Circ. 28745
00h50m06.95s -55°51'55.4" 1.4" XRT-final UKSSDC
00h50m06.93s -55°51'55.8" 1.7" XRT-enhanced Beardmore et al. GCN Circ. 28736
00h50m00.6s -55°51'44.0" 1.3' BAT-refined Stamatikos et al. GCN Circ. 28755

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 28733 MASTER optical synchronously
observation 		MASTER
Optical Vielfaure et al. 28739 VLT/X-shooter redshift VLT redshift
Optical Klotz et al. 31752 TAROT Reunion observatory optical light
curve		 TAROT light curve
Gamma-ray Lesage and Meegan 28748 Fermi GBM detection Fermi GBM Epeak=131.1±28.2 keV
T90=35.3 seconds
Fluence=1.7±0.2x10-6erg cm-2
(brighter than 26% of long GRBs)

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 105 255 147 16.66 ± 0.03
white 598 617 19 16.22 ± 0.05
v 647 667 19 16.49 ± 0.16
b 573 593 20 16.80 ± 0.10
uFC 317 567 246 15.76 ± 0.03
u 722 742 19 16.06 ± 0.10
uvm2 672 692 19 16.43 ± 0.24
uvw2 623 643 19 16.68 ± 0.22

Table 3. UVOT observations reported by Oates and Lien (GCN Circ. 28745). 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 14, 2022