Swift Observations of GRB 200917A

E. Sonbas (Adiyaman Univ.), V. D'Elia (ASDC) and K.K. Simpson (PSU) for the Swift team

1. Introduction

At 03:52:28 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200917A (trigger=996184) (Sonbas et al. GCN Circ. 28435). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 61° from the Sun (4.1 hours East) and 64° from the 0%-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 Markwardt et al. (GCN Circ. 28447), the BAT ground-calculated position is RA, Dec = 235.742, -4.695 deg which is RA(J2000) = 15h42m58.0s Dec(J2000) = -04°41'41.4" with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 96%.

The mask-weighted light curve (Figure 1) shows two peaks, a weak peak at ~T-10 s and a stronger peak at ~T+5 s. T90 (15-350 keV) is 19.4 ± 2.9 s (estimated error including systematics).

The time-averaged spectrum from T-15.76 to T+6.18 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.24 ± 0.31. The fluence in the 15-150 keV band is 3.0 ± 0.6 x 10-7 erg cm-2. This fluence is larger than that of 12% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+5.04 s in the 15-150 keV band is 0.6 ± 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/996184/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Elia et al. (GCN Circ. 28444). We have analysed 20 ks of XRT data for GRB 200917A, from 112 s to 500.0 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 28436).

The late-time light curve (Figure 2) (from T0+5.1 ks) can be modelled with a power-law decay with a decay index of α=1.02 (+0.26, -0.23).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.25 (+0.25, -0.24). The best-fitting absorption column is 1.02 (+0.22, -0.19) x 1022 cm-2, in excess of the Galactic value of 1.5 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 4.3 x 10-11 (9.9 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.02 (+0.22, -0.19) x 1022 cm-2
Galactic foreground: 1.5 x 1021 cm-2
Excess significance: 7.5 σ
Photon index: 2.25 (+0.25, -0.24)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00996184.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200917A 104 s after the BAT trigger (Simpson and Sonbas GCN Circ. 28475). No optical afterglow consistent with the XRT position (Goad et al. GCN Circ. 28436) 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 0.19 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
15h42m53.89s -04°42'03.2" 1.5" XRT-final UKSSDC
15h42m53.84s -04°42'02.1" 1.9" XRT-enhanced Goad et al. GCN Circ. 28436
15h42m58.0s -04°41'41.4" 2.0' BAT-refined Markwardt et al. GCN Circ. 28447

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 28442 Swift GRB 200917A: Global MASTER-Net
observations report 		MASTER
Optical Kumar et al. 28450 GROWTH-India Telescope optical upper
limit.		 GROWTH-India upper limits
Gamma-ray Wood 28445 Fermi-GBM Sub-Threshold Detection Fermi GBM Epeak=230 keV

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 104 254 147 >19.9
uFC 262 512 246 >20.1
white 104 1708 412 >20.5
v 592 1758 136 >18.9
b 518 1684 117 >19.5
u 262 1832 382 >20.2
w1 641 1807 136 >18.7
m2 1762 1782 19 >18.2

Table 3. UVOT observations reported by Simpson and Sonbas (GCN Circ. 28475). 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 23, 2020