Swift Observations of GRB 200529A

V. D'Elia (SSDC), A. D'Ai (INAF-IASFPA) and K.K. Simpson (PSU) for the Swift team

1. Introduction

At 00:56:35 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200529A (trigger=974942) (D'Elia et al. GCN Circ. 27845). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 167° from the Sun (11.5 hours East) and 95° from the 38%-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.

Malesani et al. (GCN Circ. 27846) reported the position from NOT for the optical afterglow of this GRB. 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

Analysis of the BAT data was reported by Krimm et al. (GCN Circ. 27868). D'Elia, et al., GCN Circ. 27845). The BAT ground-calculated position is RA, Dec = 238.756, -11.076 deg which is RA(J2000) = 15h55m01.3s Dec(J2000) = -11°04'34.5" with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment). The partial coding was 50%.

The mask weighted light curve (Figure 1) shows a complex structure. The overall structure starts at ~T-5 s and lasts till ~T+100 s. T90 (15-350 keV) is 79.57 ± 7.26 s (estimated error including systematics).

The time-averaged spectrum from T-1.24 to T+92.92 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.89 ± 0.10. The fluence in the 15-150 keV band is 3.2 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 72% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+1.58 s in the 15-150 keV band is 1.8 ± 0.3 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/974942/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Ai et al. (GCN Circ. 27863).

We have analysed 3.7 ks of XRT data for GRB 200529A, from 197 s to 44.3 ks after the BAT trigger. The data comprise 51 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 27848).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=4.0 ± 0.6, followed by a break at T+580 s to an α of 0.44 (+0.19, -0.17).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.29 (+0.29, -0.20). The best-fitting absorption column is consistent with the Galactic value of 2.4 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 2.8 x 10-11 (4.5 x 10-11) erg cm-2 count-1.

A summary of the WT-mode spectrum is thus:
Total column: 2.4 ± 0.8 x 1021 cm-2
Galactic foreground: 2.4 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 2.29 (+0.29, -0.20)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00974942.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200529A 195 s after the BAT trigger (Simpson and D'Elia GCN Circ. 27884). No optical afterglow consistent with the optical position (Malesani et al. GCN Circ. 27846) 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 0.29 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
15h55m03.34s -11°04'25.1" 1.8" XRT-final UKSSDC
15h55m03.34s -11°04'25.1" 1.8" XRT-enhanced Goad et al. GCN Circ. 27848
15h55m01.3s -11°04'34.5" 1.2' BAT-refined Krimm et al. GCN Circ. 27868

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 27843 Swift GRB200529.04: Global MASTER-Net
observations report 		MASTER
Optical Malesani et al. 27846 NOT optical afterglow NOT
Optical Zheng and Filippenko 27847 KAIT Optical Afterglow Confirmation KAIT detection
Optical Hu et al. 27853 BOOTES-2/TELMA early optical upper limit BOOTES-2 upper limits
Optical Strausbaugh and Cucchiara 27858 LCO Optical Afterglow Detection LCO detection
Optical Ogawa et al. 27878 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Gamma-ray Fermi 27844 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Roberts and Meegan 27859 Fermi GBM Detection Fermi GBM Epeak=73.99 (+11.10/-8.18) keV
T90=85 seconds
Fluence=4.08±0.27x10-6erg cm-2
(52nd percentile for long GRBs)

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 216 360 142 >19.8
white 216 360 142 >19.8
v 195 3771 209 >19.1
b 4392 4495 102 >19.5
u 4187 4386 197 >19.7
w1 3982 4182 197 >19.0
m2 3777 3977 197 >20.2

Table 3. UVOT observations reported by Simpson and D'Elia (GCN Circ. 27884). 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.

June 3, 2020