Swift Observations of GRB 210504A

A.P. Beardmore (U. Leicester), D.N. Burrows (PSU) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

Beardmore et al. (GCN Circ. 29929) reported the initial Swift results. Swift Burst Alert Telescope (BAT) triggered and located GRB 210504A (trigger=1046782), with the trigger time most likely between 13:40 and 13:58 UT. 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.

Breeveld and Beardmore (GCN Circ. 29933) reported the detection with UVOT of an optical afterglow. Heintz et al. (GCN Circ. 29937) reported the position from NOT for the optical afterglow of this GRB. Xu et al. (GCN Circ. 29944) determined a redshift of 2.077 from VLT. 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 Laha et al. (GCN Circ. 29939), the BAT ground-calculated position is RA, Dec = 222.379, -30.557 deg which is RA(J2000) = 14h49m31.0s Dec(J2000) = -30°33'23.5" with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 53%.

The mask-weighted light curve (Figure 1) shows several overlapping pulses that start at ~T-30 s and end at ~T+150 s. There might be additional burst emission before the GRB came into the BAT FOV at ~T-43 s. T90 (15-350 keV) is 135.06 ± 9.57 s (estimated error including systematics).

The time-averaged spectrum from T-27.55 to T+134.76 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.64 ± 0.13. The fluence in the 15-150 keV band is 2.7 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 69% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+87.68 s in the 15-150 keV band is 0.9 ± 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/1046782/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Burrows et al. (GCN Circ. 29942). We have analysed 10.0 ks of XRT data for GRB 210504A, from 197 s to 116.3 ks after the BAT trigger. The data comprise 146 s in Windowed Timing (WT) mode (the first 11 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode.

The late-time light curve (Figure 2) (from T0+11.6 ks) can be modelled with a power-law decay with a decay index of α=1.9 (+0.7, -0.5).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.00 (+0.11, -0.10). The best-fitting absorption column is 2.3 (+4.4, -2.3) x 1021 cm-2, at a redshift of 2.077, in addition to the Galactic value of 1.3 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.11 (+0.20, -0.10) and a best-fitting absorption column consistent with the Galactic value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.0 x 10-11 (4.0 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.3 x 1021 cm-2
Intrinsic column: 1.3 (+3.8, -0.0) x 1021 cm-2 at z=2.077
Photon index: 2.11 (+0.20, -0.10)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01046782.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210504A 218 s after the BAT trigger (Breeveld and Beardmore GCN Circ. 29933). A fading source 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).

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
14h49m34.03s -30°32'02.2" 0.6" UVOT-refined Breeveld and Beardmore GCN Circ. 29933
14h49m33.89s -30°32'00.5" 2.0" XRT-final UKSSDC
14h49m33.88s -30°32'00.4" 2.0" XRT-refined Burrows et al. GCN Circ. 29942
14h49m31.0s -30°33'23.5" 2.1' BAT-refined Laha et al. GCN Circ. 29939

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 29931 Swift GRB210504.58: Global MASTER-Net
observations report 		MASTER
Optical Hu et al. 29932 BOOTES-4/MET optical upper limit BOOTES-4 upper limits
Optical Sun et al. 29934 AST3-3 YaoAn Optical Upper Limit Antarctic Survey Tel. upper limits
Optical Hosokawa et al. 29936 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Heintz et al. 29937 NOT optical afterglow detection NOT detection
Optical Xu et al. 29944 VLT X-shooter redshift VLT redshift
Optical O'Connor et al. 29946 Lowell Discovery Telescope optical
observations		 detection
Optical Strausbaugh and Cucchiara 30027 LCO Optical Afterglow Detection LCO detection
Other Lien et al. 29930 update on the BAT trigger time and
burst duration

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 218 368 147 19.75 0.19
white 1168 1877 97 20.08 0.31
v 374 739 58 >18.26
b 473 837 58 >19.29
b 1144 1852 97 19.17 0.25
u 448 812 58 >18.89
uvw1 424 788 58 >18.49
uvm2 399 418 20 >17.24
uvw2 523 1040 59 >18.60

Table 3. UVOT observations reported by Breeveld and Beardmore (GCN Circ. 29933). 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 17, 2021