Swift Observations of GRB 220708A

V. D'Elia (SSDC and INAF-OAR), A. Tohuvavohu (U. Toronto) and A. Belles (PSU) for the Swift team

1. Introduction

At 04:39:29 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220708A (trigger=1115156) (D'Elia et al. GCN Circ. 32343). Swift did not slew immediately to the burst. At the time of the trigger, the initial BAT position was 50° from the Sun (0.2 hours West) and 104° from the 62%-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 Barthelmy et al. (GCN Circ. 32361), the BAT ground-calculated position is RA, Dec = 103.038, 72.199 deg which is RA(J2000) = 06h52m09.1s Dec(J2000) = +72°11'57.4" with an uncertainty of 2.2 arcmin, (radius, sys+stat, 90% containment). The partial coding was 75%.

The BAT mask-weighted light curve (Figure 1) shows a peak starting from T-1 s to T+3 s. T90 (15-350 keV) is 4.4 ± 1.0 s (estimated error including systematics).

The time-averaged spectrum from T-1.55 to T+3.44 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.31 ± 0.32. The fluence in the 15-150 keV band is 1.3 ± 0.3 x 10-7 erg cm-2. This fluence is larger than that of 2.7% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+1.54 s in the 15-150 keV band is 0.8 ± 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/1115156/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Tohuvavohu et al. (GCN Circ. 32352).

We have analysed 6.3 ks of XRT data for GRB 220708A, from 2.5 ks to 49.4 ks after the BAT trigger. The data comprise 257 s 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. 32347).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.1 (+nan, -nan).

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01115156.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 220708A 2473 s after the BAT trigger (Belles and D'Elia GCN Circ. 32354). No optical afterglow consistent with the XRT position (Osborne et al. GCN Circ. 32347) 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 0.169 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
06h52m30.17s +72°08'28.0" 2.5" XRT-final UKSSDC
06h52m30.17s +72°08'28.0" 2.6" XRT-enhanced Osborne et al. GCN Circ. 32347
06h52m09.1s +72°11'57.4" 2.2' BAT-refined Barthelmy et al. GCN Circ. 32361

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Hu et al. 32349 BOOTES-5/JGT early optical upper limit BOOTES 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 2473 2622 147 >19.8
white 2473 8325 402 >20.2
v 2934 3134 197 >18.9
b 2680 3954 216 >20.1
u 2655 3749 216 >19.9
w1 2630 3544 216 >19.3
m2 3139 3339 197 >19.3
w2 2730 2930 197 >19.5

Table 3. UVOT observations reported by Belles and D'Elia (GCN Circ. 32354). 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 11, 2022