Swift Observations of GRB 210527A

S. Laha (GSFC/UMBC/CRESST), V. D'Elia (SSDC and INAF-OAR) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 07:07:48 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210527A (trigger=1051453) (Laha et al. GCN Circ. 30081). Swift did not slew immediately due to an observing constraint. 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 Stamatikos et al. (GCN Circ. 30092), the BAT ground-calculated position is RA, Dec = 172.346, 31.627 deg which is RA(J2000) = 11h29m23.0s Dec(J2000) = +31°37'37.0" with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment). The partial coding was 90%.

The BAT light curve (Figure 1) showed a complex structure with a duration of about ~70 s. T90 (15-350 keV) is 63.21 ± 11.15 s (estimated error including systematics).

The time-averaged spectrum from T-17.07 to T+62.45 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.33 ± 0.26. The fluence in the 15-150 keV band is 6.5 ± 1.0 x 10-7 erg cm-2. This fluence is larger than that of 27% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+17.86 s in the 15-150 keV band is 0.4 ± 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/1051453/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Elia et al. (GCN Circ. 30085). We have analysed 16 ks of XRT data for GRB 210527A, from 3.2 ks to 100.9 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. 30083).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.77 ± 0.11.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.9 (+0.5, -0.4). The best-fitting absorption column is 6.2 (+2.2, -1.8) x 1021 cm-2, in excess of the Galactic value of 1.8 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 2.8 x 10-11 (1.0 x 10-10) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 6.2 (+2.2, -1.8) x 1021 cm-2
Galactic foreground: 1.8 x 1020 cm-2
Excess significance: 5.4 σ
Photon index: 2.9 (+0.5, -0.4)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01051453.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210527A 3164 s after the BAT trigger (Siegel and Laha GCN Circ. 30089). No optical afterglow consistent with the XRT position (Evans et al., GCN Circ. 30083) 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 0.023 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
11h29m17.00s +31°37'34.2" 2.3" XRT-final UKSSDC
11h29m17.00s +31°37'34.3" 2.3" XRT-enhanced Evans et al. GCN Circ. 30083
11h29m23.0s +31°37'37.0" 1.8' BAT-refined Stamatikos et al. GCN Circ. 30092

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Butler et al. 30084 RATIR Optical and NIR Observations RATIR upper limits
Optical Lipunov et al. 30086 Swift GRB 210527A: Global MASTER-Net
observations report 		MASTER
Optical Imai et al. 30087 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Kumar et al. 30088 GIT optical upper limit GROWTH-India upper limits

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 3164 3313 147 >20.2
white 3164 3932 344 >21.2
v 4143 4343 197 >19.4
b 3528 3727 197 >20.4
u 3322 4895 331 >20.4
w1 4553 4753 197 >20.2
m2 4348 4548 197 >20.0
w2 3938 4138 197 >20.5

Table 3. UVOT observations reported by Siegel and Laha (GCN Circ. 30089). 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.

May 29, 2021