Swift Observations of GRB 200907B

A.P. Beardmore (U. Leicester), M. Capalbi (INAF-IASFPA) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 18:51:11 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200907B (trigger=995004) (Beardmore et al. GCN Circ. 28384). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 77° from the Sun (5.2 hours West) and 44° from the 74%-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 Krimm et al. (GCN Circ. 28398), the BAT ground-calculated position is RA, Dec = 89.019, 6.905 deg which is RA(J2000) = 05h56m04.5s Dec(J2000) = +06°54'17.7" with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment). The partial coding was 78%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T-0.6 and ends at ~T+0.3 s. The main peak occurs at ~T+0.1 s. T90 (15-350 keV) is 0.83 ± 0.13 s (estimated error including systematics).

The time-averaged spectrum from T-0.64 to T+0.28 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 0.68 ± 0.25. The fluence in the 15-150 keV band is 1.6 ± 0.2 x 10-7 erg cm-2. This fluence is larger than that of 82% of the short GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.68 s in the 15-150 keV band is 1.5 ± 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/995004/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Capalbi et al. (GCN Circ. 28394). We have analysed 10 ks of XRT data for GRB 200907B, from 83 s to 62.3 ks after the BAT trigger. The data comprise 5 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. 28385).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=2.09 ± 0.18.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.0 (+0.5, -0.4). The best-fitting absorption column is 5.8 (+3.0, -1.5) x 1021 cm-2, consistent with the Galactic value of 4.2 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 4.4 x 10-11 (7.1 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 5.8 (+3.0, -1.5) x 1021 cm-2
Galactic foreground: 4.2 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 2.0 (+0.5, -0.4)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00995004.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200907B 84 s after the BAT trigger (Siegel and Beardmore GCN Circ. 28400). No optical afterglow consistent with the XRT position (Evans GCN Circ. 28385) 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 0.61 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
05h56m06.97s +06°54'22.5" 1.7" XRT-final UKSSDC
05h56m06.97s +06°54'22.5" 1.8" XRT-enhanced Evans et al. GCN Circ. 28391
05h56m04.5s +06°54'17.7" 1.8' BAT-refined Krimm et al. GCN Circ. 28398

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Kumar et al. 28388 Correction to GCN Circ. 28387: GRB 200907B
GROWTH-India optical upper limit		 GROWTH-India upper limits
Optical Lipunov et al. 28392 Swift GRB 200907B: Global MASTER-Net
observations report 		MASTER
Optical Hosokawa et al. 28393 MITSuME Okayama optical upper limits MITSuME Okayama upper limits
Gamma-ray Gupta et al. 28395 AstroSat CZTI detection CZTI

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 84 233 147 >20.4
uFC 296 546 246 >19.7
white 84 4892 383 >21.0
v 625 819 39 >17.7
b 551 4687 236 >19.4
u 296 4482 462 >20.4
w1 675 4277 216 >19.2
m2 650 670 19 >18.3
w2 601 5048 186 >19.2

Table 3. UVOT observations reported by Siegel and Beardmore (GCN Circ. 28400). 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.

September 9, 2020