Swift Observations of GRB 220701A

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

1. Introduction

At 04:52:58 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220701A (trigger=1114034) (Evans et al. GCN Circ. 32306). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 95° from the Sun (9.5 hours East) and 84° from the 4%-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 Sakamoto et al. (GCN Circ. 32328), the BAT ground-calculated position is RA, Dec = 242.474, 55.436 deg which is RA(J2000) = 16h09m53.7s Dec(J2000) = +55°26'08.9" with an uncertainty of 2.5 arcmin, (radius, sys+stat, 90% containment). The partial coding was 56%.

The BAT mask-weighted light curve (Figure 1) shows a fairly weak peak extending from Roughly T-2 to T+8 seconds. T90 (15-350 keV) is 5.77 ± 1.13 s (estimated error including systematics).

The time-averaged spectrum from T-1.40 to T+4.83 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.37 ± 0.34. The fluence in the 15-150 keV band is 1.8 ± 0.4 x 10-7 erg cm-2. The 1-s peak photon flux measured from T+3.65 s in the 15-150 keV band is 0.7 ± 0.2 ph cm-2 s-1. This fluence is larger than that of 5.8% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). 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/1114034/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Gropp et al. (GCN Circ. 32314).

We have analysed 6.2 ks of XRT data for GRB 220701A, from 178 s to 52.7 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 32308).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.03 (+0.15, -0.12).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.3 (+0.7, -0.6). The best-fitting absorption column is 1.2 (+0.7, -0.5) x 1022 cm-2, in excess of the Galactic value of 1.0 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 4.4 x 10-11 (1.1 x 10-10) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.2 (+0.7, -0.5) x 1022 cm-2
Galactic foreground: 1.0 x 1020 cm-2
Excess significance: 4.0 σ
Photon index: 2.3 (+0.7, -0.6)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01114034.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 220701A 175 s after the BAT trigger (Oates and Evans GCN Circ. 32310). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 32308) 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 0.008 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
16h10m01.32s +55°24'29.1" 2.3" XRT-final UKSSDC
16h10m01.20s +55°24'29.5" 2.6" XRT-enhanced Evans et al. GCN Circ. 32308
16h09m53.7s +55°26'08.9" 2.5' BAT-refined Sakamoto et al. GCN Circ. 32328

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical O'Connor et al. 32309 Lowell Discovery Telescope Optical
Observations		 Lowell Discovery Telescope
Optical Hu et al. 32311 BOOTES-5/JGT early optical upper limit BOOTES upper limits
Optical Strausbaugh and Cucchiara 32313 LCO Optical Upper Limits LCO upper limits
Optical Zheng et al. 32315 Keck/LRIS Optical Upper Limit Keck upper limits
Optical Belkin et al. 32317 AbAO optical upper limit Abastumani Astro. Obs. upper limits
Optical Watson et al. 32318 COATLI Optical Upper Limit COATLI upper limits
Optical Takamatsu et al. 32320 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Gokuldass et al. 32325 VIRT Optical Upper Limit Virgin Island Robotic Telescope 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 175 325 147 >21.0
uFC 333 583 246 >20.2
white 175 1884 424 >21.6
v 665 1758 136 >19.2
b 589 1858 136 >20.2
u 333 1833 363 >20.5
w1 714 1808 117 >19.4
w2 640 1906 55 >18.5

Table 3. UVOT observations reported by Oates and Evans (GCN Circ. 32310). 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 7, 2022