Swift Observations of GRB 200519A

B. Sbarufatti (PSU), D.N. Burrows (PSU) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 11:20:23 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200519A (trigger=973140) (Sbarufatti et al. GCN Circ. 27756). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 160° from the Sun (10.7 hours West) and 124° from the 10%-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.

Sbarufatti et al. (GCN Circ. 27756) reported the discovery with UVOT of an optical afterglow. 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. 27771), the BAT ground-calculated position is RA, Dec = 255.319, -30.393 deg which is RA(J2000) = 17h01m16.6s Dec(J2000) = -30°23'35.6" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 99%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T0 and ends at ~T+100 s. The three main peaks occur at ~T+1 s, ~T+24 s, and ~T+75 s, respectively. T90 (15-350 keV) is 71.88 ± 1.28 s (estimated error including systematics).

The time-averaged spectrum from T-0.45 to T+103.00 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.42 ± 0.03. The fluence in the 15-150 keV band is 1.2 ± 0.02 x 10-5 erg cm-2. This fluence is larger than that of 93.7% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+23.53 s in the 15-150 keV band is 20.6 ± 0.5 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/973140/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Burrows et al. (GCN Circ. 27763). We have analysed 12 ks of XRT data for GRB 200519A, from 55 s to 154.8 ks after the BAT trigger. The data comprise 953 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 27761).

The late-time light curve (Figure 2) (from T0+4.8 ks) can be modelled with an initial rise, with a power-law index of α=-0.29 (+0.59, -0.22), followed by a break at T+6208 s to an α of 1.31 (+0.05, -0.04).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.902 ± 0.029. The best-fitting absorption column is 4.35 (+0.17, -0.16) x 1021 cm-2, in excess of the Galactic value of 3.3 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.76 ± 0.08 and a best-fitting absorption column of 4.7 ± 0.5 x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.6 x 10-11 (6.6 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.7 ± 0.5 x 1021 cm-2
Galactic foreground: 3.3 x 1021 cm-2
Excess significance: 4.7 σ
Photon index: 1.76 ± 0.08

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00973140.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200519A 58 s after the BAT trigger (Siegel and Sbarufatti GCN Circ. 27769). A source 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 0.47 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
17h01m20.01s -30°22'41.1" 0.42" UVOT-refined Siegel and Sbarufatti GCN Circ. 27769
17h01m19.94s -30°22'43.3" 1.4" XRT-final UKSSDC
17h01m19.96s -30°22'41.7" 1.8" XRT-enhanced Osborne et al. GCN Circ. 27761
17h01m16.6s -30°23'35.6" 1.0' BAT-refined Stamatikos et al. GCN Circ. 27771

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Kong 27757 iTelescope optical afterglow detection iTelescope detection
Optical Strausbaugh and Cucchiara 27759 LCO Optical Afterglow Detection LCO detection
Optical Hu et al. 27760 BOOTES-3/YA optical counterpart
observations		 BOOTES-3 detection
Optical Kumar et al. 27764 1.3m DFOT Optical Afterglow Detection Nainital detection
Optical Lipunov et al. 27765 Swift GRB 200519A: Global MASTER-Net
observations report 		MASTER
Optical Kumar et al. 27767 GROWTH-India optical follow-up GROWTH-India
Optical Kumar et al. 27772 R-band follow-up from HCT Himalayan Chandra Telescope detection
Gamma-ray Gupta et al. 27766 AstroSat CZTI detection CZTI
Gamma-ray Xiao et al. 27770 Insight-HXMT/HE detection Insight-HXMT T90=8.97 seconds
Gamma-ray Marrocchesi et al. 27773 CALET Gamma-Ray Burst Monitor detection CALET

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

Filter Tstart(s) Tstop(s) Exp(s) Mag
white 58 208 147 15.43 ± 0.02
v 599 619 20 16.30 ± 0.16
b 525 545 19 17.13 ± 0.13
u 270 519 246 16.27 ± 0.04
w1 648 840 39 17.46 ± 0.25
w2 747 766 19 >17.5

Table 3. UVOT observations reported by Siegel and Sbarufatti (GCN Circ. 27769). The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary detections and 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.

May 22, 2020