Swift Observations of GRB 200819A

S. Dichiara (NASA/GSFC/UMCP), V. D'Elia (ASDC) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 15:43:52 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200819A (trigger=992099) (Dichiara et al. GCN Circ. 28263). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 94° from the Sun (8.5 hours West) and 96° 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. 28276), the BAT ground-calculated position is RA, Dec = 21.527, 63.061 deg which is RA(J2000) = 01h26m06.4s Dec(J2000) = +63°03'39.3" with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 96%.

The mask-weighted light curve (Figure 1) shows a double-peaked structure that starts at ~T0 and ends at ~T+7 s, followed by some tail emission that lasts until ~T+27 s. T90 (15-350 keV) is 23.62 ± 6.74 s (estimated error including systematics).

The time-averaged spectrum from T-0.72 to T+27.12 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.08 ± 0.31. The fluence in the 15-150 keV band is 3.2 ± 0.6 x 10-7 erg cm-2. This fluence is larger than that of 13% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.31 s in the 15-150 keV band is 0.6 ± 0.1 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/992099/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Elia et al. (GCN Circ. 28272). We have analysed 9.7 ks of XRT data for GRB 200819A, from 94 s to 69.3 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. 28265).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.97 (+0.07, -0.06).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.6 (+0.6, -0.5). The best-fitting absorption column is 2.3 (+1.1, -0.9) x 1022 cm-2, in excess of the Galactic value of 9.8 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 7.4 x 10-11 (1.3 x 10-10) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.3 (+1.1, -0.9) x 1022 cm-2
Galactic foreground: 9.8 x 1021 cm-2
Excess significance: 2.4 σ
Photon index: 1.6 (+0.6, -0.5)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00992099.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200819A 89 s after the BAT trigger (Siegel and Dichiara GCN Circ. 28281). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 28265) 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 1.59 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
01h25m41.76s +63°03'39.7" 1.7" XRT-final UKSSDC
01h25m41.81s +63°03'39.3" 2.2" XRT-enhanced Evans et al. GCN Circ. 28265
01h26m06.4s +63°03'39.3" 2.1' BAT-refined Markwardt et al. GCN Circ. 28276

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 28264 Swift GRB 200819A: Global MASTER-Net
observations report 		MASTER
Optical Belkin et al. 28266 Kitab, TSHAO optical upper limit Zeiss-1000 upper limits
Optical Jelinek et al. 28267 Swift J0125.7+6303: Ondrejov Perek-2m
optical limit		 upper limits
Optical Murata et al. 28269 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Horiuchi et al. 28271 MITSuME Ishigaki optical upper limits MITSuME upper limits
Optical Dutta et al. 28273 Optical Observations from HCT Himalayan Chandra Telescope

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 89 239 147 >20.4
uFC 301 551 246 >19.0
white 89 601 167 >20.3
b 557 577 19 >17.7
u 301 551 246 >19.0

Table 3. UVOT observations reported by Siegel and Dichiara (GCN Circ. 28281). 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.

August 22, 2020