Swift Observations of GRB 220930A

S.B. Cenko (GSFC), V. D'Elia (SSDC and INAF-OAR) and A. Belles (PSU) for the Swift team

1. Introduction

At 11:11:52 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220930A (trigger=1125809) (Cenko et al. GCN Circ. 32603). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 121° from the Sun (8.1 hours West) and 170° from the 23%-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. 32612), the BAT ground-calculated position is RA, Dec = 64.725, 13.353 deg which is RA(J2000) = 04h18m54.0s Dec(J2000) = +13°21'12.0" with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 11%.

The BAT light curve (Figure 1) showed a complex structure with a duration of about 200 s. T90 (15-350 keV) is 180.47 ± 104.20 s (estimated error including systematics).

The time-averaged spectrum from T-0.42 to T+235.67 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.59 ± 0.13. The fluence in the 15-150 keV band is 5.5 ± 0.4 x 10-6 erg cm-2. This fluence is larger than that of 84% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.31 s in the 15-150 keV band is 6.2 ± 1.0 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/1125809/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Elia et al. (GCN Circ. 32611). We have analysed 15 ks of XRT data for GRB 220930A, from 159 s to 212.0 ks after the BAT trigger. The data comprise 390 s in Windowed Timing (WT) mode (the first 9 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. 32605).

The late-time light curve (Figure 2) (from T0+5.1 ks) can be modelled with a power-law decay with a decay index of α=1.23 (+0.08, -0.07).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.59 ± 0.08. The best-fitting absorption column is 4.9 (+0.6, -0.5) x 1021 cm-2, in excess of the Galactic value of 2.8 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.82 (+0.16, -0.15) and a best-fitting absorption column of 6.4 (+1.2, -1.1) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.7 x 10-11 (7.3 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 6.4 (+1.2, -1.1) x 1021 cm-2
Galactic foreground: 2.8 x 1021 cm-2
Excess significance: 5.4 σ
Photon index: 1.82 (+0.16, -0.15)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01125809.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 220930A 180 s after the BAT trigger (Belles and Cenko GCN Circ. 32609). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 32608) 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 0.639 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
04h18m52.11s +13°20'56.8" 2.0" XRT-final UKSSDC
04h18m52.12s +13°20'56.5" 2.2" XRT-enhanced Evans et al. GCN Circ. 32608
04h18m54.0s +13°21'12.0" 1.7' BAT-refined Barthelmy et al. GCN Circ. 32612

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Watson et al. 32604 COATLI Upper Limits COATLI upper limits
Optical Hu et al. 32606 BOOTES-5/JGT early optical upper limit BOOTES upper limits
Optical Lipunov et al. 32607 Swift GRB 220930A: Global MASTER-Net
observations report 		MASTER
Optical Strausbaugh and Cucchiara 32610 LCOGT Optical Upper Limits Las Cumbres Obs. Global Tele.Network upper limits
Optical Pankov et al. 32617 AbAO Optical Upper Limit Abastumani Astro. Obs. upper limits
Gamma-ray Gopalakrishnan et al. 32613 AstroSat CZTI detection CZTI
Gamma-ray Ridnaia et al. 32614 Konus-Wind detection Konus-Wind Epeak=198 (-58,+181) keV
Duration=~76.0 seconds
Fluence=1.78(-0.64,+0.98)x10-5erg cm-2

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

Filter Tstart(s) Tstop(s) Exp(s) Mag
whFC 180 330 147 >20.71
uFC 338 588 245 >19.95
wh 618 1368 225 >20.93
v 669 1418 97 >18.73
b 594 1343 77 >19.59
u 744 1318 58 >18.99
w1 719 1467 77 >18.83
m2 693 1442 77 >18.59
w2 644 1393 97 >19.10

Table 3. UVOT observations reported by Belles and Cenko (GCN Circ. 32609). 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.

October 3, 2022