Swift Observations of GRB 201013A

K.L. Page (U. Leicester), P.A. Evans (U. Leicester) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 03:46:30 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 201013A (trigger=999948) (Page et al. GCN Circ. 28607). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 97° from the Sun (6.1 hours West) and 53° from the 19%-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. 28614), the BAT ground-calculated position is RA, Dec = 107.392, 57.031 deg which is RA(J2000) = 07h09m34.1s Dec(J2000) = +57°01'52.4" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 64%.

The mask-weighted light curve (Figure 1) shows a double-peaked structure that starts at ~T-1 s and ends at ~T+10 s. The two main peaks occur at ~T+0.5 s and ~T+6.5 s, respectively. T90 (15-350 keV) is 7.36 ± 0.18 s (estimated error including systematics).

The time-averaged spectrum from T-0.85 to T+9.64 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.22 ± 0.04. The fluence in the 15-150 keV band is 6.7 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 87% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.05 s in the 15-150 keV band is 24.6 ± 0.7 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/999948/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 28611). We have analysed 15 ks of XRT data for GRB 201013A, from 89 s to 127.7 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 28608).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=0.54 (+0.25, -0.34), followed by a break at T+609 s to an α of 0.99 (+0.15, -0.05).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.02 (+0.28, -0.27). The best-fitting absorption column is 4.3 ± 0.7 x 1022 cm-2, in excess of the Galactic value of 5.4 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 7.8 x 10-11 (2.1 x 10-10) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.3 ± 0.7 x 1022 cm-2
Galactic foreground: 5.4 x 1020 cm-2
Excess significance: 10.4 σ
Photon index: 2.02 (+0.28, -0.27)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00999948.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 201013A 74 s after the BAT trigger (Breeveld and Page GCN Circ. 28613). There is a bright optical source (USNO B1.0 1470-0207304) 0.5 arcsec from the enhanced XRT position (Beardmore et al. GCN Circ. 28608). 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 0.05 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
07h09m34.48s +57°02'03.4" 1.4" XRT-final UKSSDC
07h09m34.62s +57°02'03.2" 1.9" XRT-enhanced Beardmore et al. GCN Circ. 28608
07h09m34.1s +57°01'52.4" 1.0' BAT-refined Laha et al. GCN Circ. 28614

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 28612 Swift GRB 201013A: Global MASTER-Net
observations report 		MASTER
Optical Jelinek et al. 28615 FRAM-ORM possible afterglow candidate FRAM possible detection
Optical Belkin et al. 28621 TSHAO and AbAO optical observations,
afterglow confirmation		 Abastumani Astro. Obs. detection
Optical Gupta et al. 28686 3.6m DOT Optical Observations Devasthal Opt.Tel.
Gamma-ray Tamura et al. 28618 CALET Gamma-Ray Burst Monitor detection CALET
Gamma-ray Gupta et al. 28620 AstroSat CZTI detection CZTI
Gamma-ray Zheng et al. 28625 Insight-HXMT/HE detection Insight-HXMT T90=6.77 seconds
Gamma-ray Tsvetkova et al. 28652 Konus-Wind observation Konus-Wind Epeak=212 (-31,+32) keV
Fluence=2.77(-0.33,+0.37)x10-5erg 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
whiteFC 74 223 147 >16.8
uFC 286 535 246 >16.7
white 74 1712 412 >16.8
v 616 1763 136 >16.2
b 541 1686 117 >16.6
u 286 1662 343 >16.7
w1 665 1804 127 >16.6
m2 640 1611 58 >16.7
w2 591 1738 136 >17.1

Table 3. UVOT observations reported by Breeveld and Page (GCN Circ. 28613). 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.

October 20, 2020