Swift Observations of GRB 200522A

P.A. Evans (U. Leicester), A.P. Beardmore (U. Leicester) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 11:41:34 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200522A (trigger=973628) (Evans et al. GCN Circ. 27778). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 56° from the Sun (3.6 hours West) and 54° 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.

Dichiara et al. (GCN Circ. 28038) determined a redshift of 0.554 from Gemini. 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 Ukwatta et al. (GCN Circ. 27793), the BAT ground-calculated position is RA, Dec = 5.668, -0.264 deg which is RA(J2000) = 00h22m40.3s Dec(J2000) = -00°15'49.9" with an uncertainty of 1.6 arcmin, (radius, sys+stat, 90% containment). The partial coding was 86%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~ T-0.3 s and ends at ~T+0.7 s. The main peak occur at ~T0. T90 (15-350 keV) is 0.62 ± 0.08 s (estimated error including systematics).

The time-averaged spectrum from T-0.02 to T+0.67 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.45 ± 0.17. The fluence in the 15-150 keV band is 1.1 ± 0.1 x 10-7 erg cm-2. This fluence is larger than that of 76% of the short GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.17 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/973628/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Beardmore et al. (GCN Circ. 27785). We have analysed 17 ks of XRT data for GRB 200522A, from 406 s to 246.4 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 27780).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.65 ± 0.09.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.38 (+0.43, -0.29). The best-fitting absorption column is 4 (+11, -1) x 1020 cm-2, consistent with the Galactic value of 2.9 x 1020 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.0 x 10-11 (5.2 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 4 (+11, -1) x 1020 cm-2
Galactic foreground: 2.9 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.38 (+0.43, -0.29)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00973628.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200522A 448 s after the BAT trigger (Kuin and Evans GCN Circ. 27783). No optical afterglow 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 0.03 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
00h22m43.68s -00°16'59.4" 2.2" XRT-final UKSSDC
00h22m43.64s -00°16'57.8" 3.2" XRT-enhanced Beardmore et al. GCN Circ. 27780
00h22m40.3s -00°15'49.9" 1.6' BAT-refined Ukwatta et al. GCN Circ. 27793

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Hu et al. 27784 BOOTES-3/YA optical upper limit BOOTES-3 upper limits
Optical Lipunov et al. 27789 Swift GRB 200522A: Global MASTER-Net
observations report 		MASTER
Optical Strausbaugh and Cucchiara 27792 LCO Optical Afterglow Detection LCO detection
Optical Dichiara et al. 27822 Gemini North optical imaging Gemini
Optical Fong et al. 27826 Hubble Space Telescope Near-IR F160W
Observations		 Hubble Space Telescope light curve
Optical Fong et al. 27827 HST F125W new point source detection HST detection
Optical Kilpatrick et al. 27904 Further HST F125W Observations HST
Optical Dichiara et al. 28038 Gemini North spectroscopy of the
putative host galaxy		 Gemini spectroscopy
Optical O'Connor et al. 28100 Candidate Optical/nIR Counterpart from
Gemini and HST Imaging		 Gemini detection
Optical Fong et al. 28127 HST NIR counterpart confirmation HST detection
Radio Schroeder et al. 27786 6 GHz VLA radio source detection VLA detection
Other Fong et al. 27779 Coincident catalogued galaxy at z~0.4
Strausbaugh and Cucchiara 27794 Correction to GCN Circ. 27792 corrects Circ. 27792

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 448 598 147 >19.5
white 448 598 147 >19.5
v 605 624 19 >17.2

Table 3. UVOT observations reported by Kuin and Evans (GCN Circ. 27783). 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 17, 2020