Swift Observations of GRB 210725A

M.G. Bernardini (INAF-OAB), M. Perri (SSDC and INAF-OAR) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 03:47:02 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210725A (trigger=1061511) (Bernardini et al. GCN Circ. 30501). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 91° from the Sun (6.1 hours East) and 103° 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.

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 Lien et al. (GCN Circ. 30514), the BAT ground-calculated position is RA, Dec = 215.381, -1.177 deg which is RA(J2000) = 14h21m31.4s Dec(J2000) = -01°10'37.2" with an uncertainty of 2.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 92%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T0 and ends at ~T+60 s. The main peak occurs at ~T+1 s. T90 (15-350 keV) is 53.54 ± 11.25 s (estimated error including systematics).

The time-averaged spectrum from T-0.82 to T+59.86 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.42 ± 0.32. The fluence in the 15-150 keV band is 8.1 ± 1.2 x 10-7 erg cm-2. This fluence is larger than that of 33% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+1.01 s in the 15-150 keV band is 1.0 ± 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/1061511/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Perri et al. (GCN Circ. 30517). We have analysed 8.6 ks of XRT data for GRB 210725A, from 114 s to 85.6 ks after the BAT trigger. The data comprise 74 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.99 ± 0.08.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.48 (+0.21, -0.19). The best-fitting absorption column is 8.0 (+6.8, -3.9) x 1020 cm-2, consistent with the Galactic value of 4.1 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 4.5 x 10-11 (4.9 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 8.0 (+6.8, -3.9) x 1020 cm-2
Galactic foreground: 4.1 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.48 (+0.21, -0.19)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01061511.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210725A 114 s after the BAT trigger (Siegel and Bernardini GCN Circ. 30527). No optical afterglow consistent with the XRT position (Perri et al. GCN Circ. 30517) 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 0.047 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
14h21m29.79s -01°11'33.6" 1.9" XRT-final UKSSDC
14h21m29.79s -01°11'33.6" 1.9" XRT-refined Perri et al. GCN Circ. 30517
14h21m31.4s -01°10'37.2" 2.3' BAT-refined Lien et al. GCN Circ. 30514

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 30500 Swift GRB210725.16: Global MASTER-Net
observations report 		MASTER
Optical Zheng and Filippenko 30503 KAIT Optical Upper Limit KAIT upper limits
Optical Niwano et al. 30509 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Pankov et al. 30530 AbAO optical upper limit Abastumani Astro. Obs. upper limits
Gamma-ray Torii et al. 30507 CALET Gamma-Ray Burst Monitor detection CALET
Gamma-ray Poolakkil and Meegan 30519 Fermi GBM detection Fermi GBM Epeak=460±104 keV
T90=47 seconds
Fluence=3.14±0.35x10-6erg cm-2
(43rd percentile for long GRBs)

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 (fc) 114 264 147 >20.7
u (fc) 273 523 246 >20.2
white 114 1545 392 >21.0
v 603 1595 117 >19.1
b 529 1519 97 >20.4
u 273 1495 343 >20.4
w1 653 1470 97 >19.2

Table 3. UVOT observations reported by Siegel and Bernardini (GCN Circ. 30527). 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 28, 2021