Swift Observations of GRB 210515C

A. D'Ai (INAF-IASFPA), J.P. Osborne (U. Leicester) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

At 20:02:01 UT, the Swift Burst Alert Telescope (BAT) triggered and located a GRB 210515C (trigger=1048618) (D'Ai et al. GCN Circ. 30020). 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 Palmer et al. (GCN Circ. 30021), the BAT ground-calculated position is RA, Dec = 1.106, -14.972 deg which is RA(J2000) = 00h04m25.4s Dec(J2000) = -14°58'18.2" with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a weak pulse that starts at ~T-3 s, peaks at ~T+1 s, and ends at ~T+11 s, with some tail emission lasting until ~T+60 s. T90 (15-350 keV) is 55.7 ± 26.7 s (estimated error including systematics).

The time-averaged spectrum from T-2.53 to T+56.90 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.26 ± 0.28. The fluence in the 15-150 keV band is 5.9 ± 1.1 x 10-7 erg cm-2. This fluence is larger than that of 24% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.84 s in the 15-150 keV band is 0.6 ± 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/1048618/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Osborne et al. (GCN Circ. 30023). We have analysed 21 ks of XRT data for GRB 210515C, from 76 s to 178.2 ks after the BAT trigger. The data comprise 98 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=2.63 (+0.13, -0.12), followed by a break at T+772 s to an α of 0.81 (+0.10, -0.09).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 3.06 (+0.24, -0.21). The best-fitting absorption column is 1.7 ± 0.4 x 1021 cm-2, in excess of the Galactic value of 2.6 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.12 (+0.26, -0.24) and a best-fitting absorption column of 6.7 (+5.7, -4.1) x 1020 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.1 x 10-11 (3.7 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 6.7 (+5.7, -4.1) x 1020 cm-2
Galactic foreground: 2.6 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 2.12 (+0.26, -0.24)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01048618.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations (LaPorte and D'Ai GCN Circ. 30039) of the field of GRB 210515C 40 s after the BAT trigger (A. D'Ai et al., GCN Circ. 30020). 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
00h04m22.60s -14°57'12.0" 2.3" XRT-final UKSSDC
00h04m22.60s -14°57'12.0" 2.3" XRT-refined Osborne et al. GCN Circ. 30023
00h04m25.4s -14°58'18.2" 2.0' BAT-refined Palmer et al. GCN Circ. 30021

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Strausbaugh and Cucchiara 30028 LCO Optical Upper Limit LCO 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
b 486 851 58 >18.60
uvm2 758 5041 216 >18.99
u 231 5285 311 >19.33
v 561 4836 235 >18.60
uvw1 610 5246 235 >18.89
uvw2 4432 4631 196 >18.90
white 72 942 271 >20.12

Table 3. UVOT observations reported by LaPorte and D'Ai (GCN Circ. 30039). 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 18, 2021