Swift Observations of GRB 201001A

K.K. Simpson (PSU), D.N. Burrows (PSU) and A. Belles (PSU) for the Swift team

1. Introduction

At 09:58:51 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 201001A (trigger=998344) (Simpson et al. GCN Circ. 28539). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 78° from the Sun (5.2 hours West) and 105° from the 100%-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. 28545), the BAT ground-calculated position is RA, Dec = 110.050, -2.213 deg which is RA(J2000) = 07h20m12.0s Dec(J2000) = -02°12'48.3" with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 38%.

The mask-weighted light curve (Figure 1) shows a broad single peak of duration roughly 20 seconds, from ~T-5 to ~T+15 s. T90 (15-350 keV) is 15.2 ± 2.7 s (estimated error including systematics).

The time-averaged spectrum from T-4.02 to T+12.98 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 0.96 ± 0.23. The fluence in the 15-150 keV band is 7.6 ± 1.1 x 10-7 erg cm-2. This fluence is larger than that of 31% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+10.79 s in the 15-150 keV band is 1.0 ± 0.3 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/998344/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Burrows et al. (GCN Circ. 28544). We have analysed 11 ks of XRT data for GRB 201001A, from 129 s to 93.0 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 28540). The source is fading with α >0.3.

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00998344.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 201001A 120 s after the BAT trigger (Belles and Simpson GCN Circ. 28555). No optical afterglow 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 0.12 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
07h20m19.44s -02°12'39.4" 3.2" XRT-final UKSSDC
07h20m19.41s -02°12'39.7" 3.1" XRT-enhanced Evans et al. GCN Circ. 28540
07h20m12.0s -02°12'48.3" 1.9' BAT-refined Ukwatta et al. GCN Circ. 28545

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Hosokawa et al. 28548 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Fu et al. 28549 Xinglong-2.16m optical upper limit Xinglong upper limits
Optical Belkin et al. 28550 TSHAO optical upper limit Zeiss-1000 upper limits
Gamma-ray Fermi 28538 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Veres et al. 28542 Fermi GBM observation Fermi GBM Epeak=71±21 keV
T90=15.6 seconds
Fluence=9.6±5.6x10-7erg cm-2
(13rd percentile for 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 120 270 147 >21.1
uFC 279 529 246 >19.8
white 120 1525 392 >21.1
v 611 1575 117 >18.8
b 535 1501 97 >18.9
u 279 1476 324 >19.8
w1 660 1452 97 >19.1
m2 635 1427 97 >19.7
w2 586 1550 117 >19.6

Table 3. UVOT observations reported by Belles and Simpson (GCN Circ. 28555). 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 4, 2020