Swift Observations of GRB 200711A

F.E. Marshall (NASA/GSFC), J.P. Osborne (U. Leicester) and K.K. Simpson (PSU) for the Swift team

1. Introduction

At 11:04:36 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200711A (trigger=981957) (Marshall et al. GCN Circ. 28078). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 158° from the Sun (11.7 hours East) and 79° from the 64%-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 Stamatikos et al. (GCN Circ. 28084), the BAT ground-calculated position is RA, Dec = 285.975, -0.134 deg which is RA(J2000) = 19h03m53.9s Dec(J2000) = -00°08'04.1" with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 8%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T-5 s and ends at ~T+30 s. The three major peaks occur at ~T0, ~T+5 s, and ~T+20 s, respectively. T90 (15-350 keV) is 29.39 ± 3.07 s (estimated error including systematics).

The time-averaged spectrum from T-4.72 to T+29.73 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.42 ± 0.10. The fluence in the 15-150 keV band is 5.3 ± 0.3 x 10-6 erg cm-2. This fluence is larger than that of 84% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.20 s in the 15-150 keV band is 7.7 ± 1.0 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/981957/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Osborne et al. (GCN Circ. 28081). We have analysed 12 ks of XRT data for GRB 200711A, from 170 s to 166.8 ks after the BAT trigger. The data comprise 29 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=0.44 (+0.19, -0.16), followed by a break at T+3076 s to an α of 1.46 (+0.08, -0.07).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.01 (+0.22, -0.21). The best-fitting absorption column is 1.65 (+0.30, -0.27) x 1022 cm-2, in excess of the Galactic value of 5.7 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 5.6 x 10-11 (1.2 x 10-10) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.65 (+0.30, -0.27) x 1022 cm-2
Galactic foreground: 5.7 x 1021 cm-2
Excess significance: 6.5 σ
Photon index: 2.01 (+0.22, -0.21)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00981957.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200711A 170 s after the BAT trigger (Simpson and Marshall GCN Circ. 28085). No optical afterglow consistent with the BAT 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 0.96 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
19h03m52.85s -00°08'49.5" 1.6" XRT-final UKSSDC
19h03m52.85s -00°08'49.5" 1.6" XRT-refined Osborne et al. GCN Circ. 28081
19h03m53.9s -00°08'04.1" 1.4' BAT-refined Stamatikos et al. GCN Circ. 28084

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 28079 Swift GRB 200711A: Global MASTER-Net
observations report 		MASTER
Optical Zheng and Filippenko 28082 KAIT Optical Upper Limit KAIT upper limits
Optical Pozanenko et al. 28086 AbAO optical upper limit Abastumani Astro. Obs. upper limits
Gamma-ray Fermi 28077 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Poolakkil et al. 28080 Fermi GBM detection Fermi GBM Epeak=287±18 keV
T90=29 seconds
Fluence=1.510±0.042x10-5erg cm-2
(83rd percentile for long GRBs)
Gamma-ray Crnogorcevic et al. 28083 Fermi-LAT marginal detection Fermi LAT Emax=5.7 GeV
Gamma-ray Asaoka et al. 28087 CALET Gamma-Ray Burst Monitor detection CALET
Gamma-ray Xiao et al. 28094 Insight-HXMT/HE detection Insight-HXMT T90=9.15 seconds

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 170 319 147 >20.2
white 170 999 310 >20.6
v 404 4651 255 >18.7
b 503 868 58 >19.1
u 477 5088 80 >19.0
w1 453 5061 255 >19.3
m2 428 448 19 >18.0
w2 379 4446 255 >19.5

Table 3. UVOT observations reported by Simpson and Marshall (GCN Circ. 28085). 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.

July 15, 2020