Swift Observations of GRB 211025A

J.D. Gropp (PSU), J.P. Osborne (U. Leicester) and S.R. Oates (U.Birmingham) for the Swift team

1. Introduction

At 05:10:20 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 211025A (trigger=1081113) (Gropp et al. GCN Circ. 30986). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 63° from the Sun (3.1 hours West) and 70° from the 82%-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.

Oates and Gropp (GCN Circ. 30991) reported the detection with UVOT of an optical afterglow. 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. 30997), the BAT ground-calculated position is RA, Dec = 162.407, 31.506 deg which is RA(J2000) = 10h49m37.8s Dec(J2000) = +31°30'20.8" with an uncertainty of 2.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a single broad pulse beginning at T+0, peaking around T+50 s and declining to background by T+150 s. T90 (15-350 keV) is 103.5 ± 27.6 s (estimated error including systematics).

The time-averaged spectrum from T+12.32 to T+128.5 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.96 ± 0.28. The fluence in the 15-150 keV band is 7.7 ± 1.4 x 10-7 erg cm-2. This fluence is larger than that of 32% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+51.66 s in the 15-150 keV band is 0.5 ± 0.1 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/1081113/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Osborne et al. (GCN Circ. 30996). We have analysed 12 ks of XRT data for GRB 211025A, from 113 s to 137.9 ks after the BAT trigger. The data comprise 203 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 30987).

The late-time light curve (Figure 2) (from T0+4.7 ks) can be modelled with a power-law decay with a decay index of α=1.16 (+0.25, -0.23).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.91 ± 0.06. The best-fitting absorption column is 7.1 ± 1.4 x 1020 cm-2, in excess of the Galactic value of 2.4 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.33 (+0.22, -0.12) and a best-fitting absorption column consistent with the Galactic value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 2.8 x 10-11 (3.1 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.4 (+3.5, -0.0) x 1020 cm-2
Galactic foreground: 2.4 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 2.33 (+0.22, -0.12)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01081113.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 211025A 131 s after the BAT trigger (Oates and Gropp GCN Circ. 30991). A source consistent with the XRT position (Evans et al. GCN Circ. 30987) 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 0.027 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
10h49m31.93s +31°30'44.3" 0.9" UVOT-refined Oates and Gropp GCN Circ. 30991
10h49m31.91s +31°30'44.2" 2.0" XRT-final UKSSDC
10h49m31.93s +31°30'44.4" 2.1" XRT-enhanced Evans et al. GCN Circ. 30987
10h49m37.8s +31°30'20.8" 2.3' BAT-refined Markwardt et al. GCN Circ. 30997

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 31001 Swift GRB 211025A: Global MASTER-Net
observations report 		MASTER
Optical Strausbaugh and Cucchiara 31009 Optical Afterglow Detection LCO detection

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 131 281 147 > 20.2
white 131 943 220 19.90 ± 0.25
white 5965 6164 197 > 20.4
v 4940 6509 329 > 19.0
b 5760 5960 197 > 19.8
uvw1 669 688 19 > 17.5
uvw1 5350 5550 197 > 19.5
uvm2 5145 5345 197 > 19.4
uvw2 4735 6370 393 > 20.0

Table 3. UVOT observations reported by Oates and Gropp (GCN Circ. 30991). The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary detections and 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.

October 27, 2021