Swift Observations of GRB 210517A

V. D'Elia (SSDC and INAF-OAR), K.L. Page (U. Leicester) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

At 05:28:01 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210517A (trigger=1048783) (D'Elia et al. GCN Circ. 30032). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 78° from the Sun (3.7 hours West) and 128° from the 25%-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.

Tanvir et al. (GCN Circ. 30037) reported the position from VLT for the optical afterglow of this GRB. Tanvir et al. (GCN Circ. 30037) determined a redshift of 2.486 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 Sakamoto et al. (GCN Circ. 30043), the BAT ground-calculated position is RA, Dec = 358.190, -39.104 deg which is RA(J2000) = 23h52m45.5s Dec(J2000) = -39°06'15.0" with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 78%.

The mask-weighted light curve (Figure 1) shows a single peak of three-second duration near the trigger time. T90 (15-350 keV) is 3.06 ± 1.4 s (estimated error including systematics).

The time-averaged spectrum from T-0.17 to T+3.13 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.85 ± 0.25. The fluence in the 15-150 keV band is 1.5 ± 0.3 x 10-7 erg cm-2. This fluence is larger than that of 3.9% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.18 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/1048783/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Page et al. (GCN Circ. 30042). We have analysed 15 ks of XRT data for GRB 210517A, from 85 s to 230.0 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. 30034).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.93 ± 0.05.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.71 (+0.18, -0.14). The best-fitting absorption column is 1.4 (+7.0, -1.4) x 1021 cm-2, at a redshift of 2.486, in addition to the Galactic value of 1.3 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 3.7 x 10-11 (3.9 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.3 x 1020 cm-2
Intrinsic column: 1.4 (+7.0, -1.4) x 1021 cm-2 at z=2.486
Photon index: 1.71 (+0.18, -0.14)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01048783.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210517A 69 s after the BAT trigger (LaPorte and D'Elia GCN Circ. 30040). No optical afterglow consistent with the optical position (Tanvir et al. GCN Circ. 30037) 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 0.011 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
23h52m53.81s -39°06'06.7" 2.2" XRT-final UKSSDC
23h52m53.87s -39°06'07.9" 2.2" XRT-enhanced Beardmore et al. GCN Circ. 30034
23h52m45.5s -39°06'15.0" 2.0' BAT-refined Sakamoto et al. GCN Circ. 30043

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 30033 Swift GRB 210517A: Global MASTER-Net
observations report 		MASTER
Optical Tanvir et al. 30037 VLT/X-shooter optical afterglow and
redshift		 VLT redshift
Optical Paek et al. 30057 KCT optical upper limit upper limits
Gamma-ray Ursi et al. 30036 AGILE detection AGILE

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 69 219 147 >19.4
uFC 282 531 246 >19.7
white 69 1356 373 >19.9
v 611 5047 274 >19.0
b 537 1332 78 >19.5
u 282 1307 304 >19.7
w1 661 1109 39 >18.4
m2 5052 5070 18 >18.7
w2 4642 4841 197 >21.1

Table 3. UVOT observations reported by LaPorte and D'Elia (GCN Circ. 30040). 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 21, 2021