Swift Observations of GRB 210724A

P. D'Avanzo (INAF-OAB), P.A. Evans (U. Leicester) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

At 20:14:09 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210724A (trigger=1061482) (D'Avanzo et al. GCN Circ. 30497). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 105° from the Sun (6.9 hours East) and 86° from the 99%-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.

de Ugarte Postigo et al. (GCN Circ. 30511) reported the position from GTC for the optical afterglow of this GRB. 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. 30513), the BAT ground-calculated position is RA, Dec = 227.425, -6.279 deg which is RA(J2000) = 15h09m41.9s Dec(J2000) = -06°16'42.9" with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 34%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T-40 s and ends at ~T+25 s. The main peak occurs at ~T0. T90 (15-350 keV) is 50.57 ± 9.54 s (estimated error including systematics).

The time-averaged spectrum from T-38.55 to T+25.50 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.30 ± 0.28. The fluence in the 15-150 keV band is 1.8 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 56% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.51 s in the 15-150 keV band is 2.5 ± 0.8 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/1061482/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 30515). We have analysed 24 ks of XRT data for GRB 210724A, from 123 s to 509.7 ks after the BAT trigger. The data comprise 263 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode.

The late-time light curve (Figure 2) (from T0+4.9 ks) can be modelled with a power-law decay with a decay index of α=0.85 ± 0.11.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.48 ± 0.08. The best-fitting absorption column is 2.08 (+0.23, -0.22) x 1021 cm-2, in excess of the Galactic value of 8.2 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 (4.9 x 10-11) erg cm-2 count-1.

A summary of the WT-mode spectrum is thus:
Total column: 2.08 (+0.23, -0.22) x 1021 cm-2
Galactic foreground: 8.2 x 1020 cm-2
Excess significance: 9.5 σ
Photon index: 2.48 ± 0.08

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01061482.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210724A 125 s after the BAT trigger (LaPorte and D'Avanzo GCN Circ. 30529). No optical afterglow consistent with the XRT position 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 0.097 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
15h09m39.51s -06°17'26.5" 3.0" XRT-final UKSSDC
15h09m39.51s -06°17'26.5" 3.0" XRT Evans and D'Avanzo GCN Circ. 30547
15h09m41.9s -06°16'42.9" 1.9' BAT-refined Laha et al. GCN Circ. 30513

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 30496 Swift GRB210724.84: Global MASTER-Net
observations report 		MASTER
Optical de Wet et al. 30498 MeerLICHT upper limits MeerLICHT upper limits
Optical de Ugarte Postigo et al. 30511 Afterglow candidate from OSIRIS/GTC GTC detection
Optical Belkin et al. 30518 Zeiss-1000 of Koshka observatory,
optical upper limit		 Koshka Obs. upper limits
Optical Kann et al. 30551 Early CAHA 2.2m limit CAHA 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 125 275 147 >20.8
uFC 283 513 226 >20.1
white 125 275 147 >20.8
u 283 513 227 >20.1

Table 3. UVOT observations reported by LaPorte and D'Avanzo (GCN Circ. 30529). 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.

July 31, 2021