Swift Observations of GRB 210514A

E. Ambrosi (INAF-IASFPA), P.A. Evans (U. Leicester) and A. Belles (PSU) for the Swift team

1. Introduction

Ambrosi et al. (GCN Circ. 30008) reported the initial Swift results. The Swift Burst Alert Telescope (BAT) has triggered on a new event and Swift has slewed to the location of this trigger. 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 Markwardt et al. (GCN Circ. 30018), the BAT ground-calculated position is RA, Dec = 2.947, -21.898 deg which is RA(J2000) = 00h11m47.2s Dec(J2000) = -21°53'54.0" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 79%.

The mask-weighted light curve (Figure 1) showed a complex structure of five main peaks with sub-peaks. Emission started around T-1 s and continued to T+75 s. T90 (15-350 keV) is 70.21 ± 0.74 s (estimated error including systematics).

The time-averaged spectrum from T-1.08 to T+75.13 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.65 ± 0.04. The fluence in the 15-150 keV band is 7.4 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 88% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.52 s in the 15-150 keV band is 8.7 ± 0.4 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/1048372/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 30012). We have analysed 18 ks of XRT data for GRB 210514A, from 73 s to 247.4 ks after the BAT trigger. The data comprise 1.4 ks in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 30011).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=6.05 ± 0.20. At T+126 s the decay flattens to an α of 1.24 (+0.17, -0.13). The light curve breaks again at T+276 s to a decay with α=-0.14 ± 0.05, before a final break at T+1016 s s after which the decay index is 1.220 (+0.018, -0.016).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.06 ± 0.04. The best-fitting absorption column is 4.79 ± 0.22 x 1021 cm-2, in excess of the Galactic value of 2.0 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.95 (+0.12, -0.11) and a best-fitting absorption column of 4.7 (+0.7, -0.6) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.1 x 10-11 (6.5 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.7 (+0.7, -0.6) x 1021 cm-2
Galactic foreground: 2.0 x 1020 cm-2
Excess significance: 12.2 σ
Photon index: 1.95 (+0.12, -0.11)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01048372.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210514A 80 s after the BAT trigger (Belles and Ambrosi GCN Circ. 30025). No optical afterglow consistent with the XRT position (Osborne et al., GCN Circ. 30011) 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 0.022 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
00h11m50.84s -21°53'40.3" 2.0" XRT-final UKSSDC
00h11m50.84s -21°53'40.3" 2.0" XRT-enhanced Osborne et al. GCN Circ. 30011
00h11m47.2s -21°53'54.0" 1.0' BAT-refined Markwardt et al. GCN Circ. 30018

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 30014 Swift GRB210514.71: Global MASTER-Net
observations report 		MASTER
Gamma-ray Sakamoto et al. 30035 CALET Gamma-Ray Burst Monitor detection CALET
Gamma-ray Ridnaia et al. 30048 Konus-Wind detection Konus-Wind Epeak=118 (-24,+40) keV
Duration=~74.2 seconds
Fluence=1.97(-0.43,+0.56)x10-5erg cm-2
Other Lien and Ambrosi 30009 update on the BAT trigger time and
burst duration		 light curve

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 80 229 147 >20.85
uFC 293 543 245 >20.12
white 573 1366 225 >21.00
v 622 1415 97 >18.87
b 548 1342 77 >19.73
u 696 5458 210 >19.99
w1 672 5299 278 >19.59
m2 646 1440 97 >18.41
w2 598 1391 97 >18.52

Table 3. UVOT observations reported by Belles and Ambrosi (GCN Circ. 30025). 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