Swift Observations of GRB 220730A

S. Dichiara (PSU), A. D'Ai (INAF-IASFPA) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 15:48:55 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220730A (trigger=1118357) (Dichiara et al. GCN Circ. 32431). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 109° from the Sun (6.4 hours East) and 100° from the 3%-illuminated Moon. Table 1 contains the best reported positions from Swift.

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

The partial coding was 60%. The mask-weighted light curve (Figure 1) shows a single FRED-shaped pulse from approximately T-0.05 to T+0.25 seconds. T90 (15-350 keV) is 0.19 ± 0.20 s (estimated error including systematics). The time-averaged spectrum from T-0.04 to T+0.28 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 0.61 ± 0.25. The fluence in the 15-150 keV band is 1.6 ± 0.2 x 10-7 erg cm-2. The 1-s peak photon flux measured from T-0.35 s in the 15-150 keV band is 1.6 ± 0.3 ph cm-2 s-1. This fluence is larger than that of 82% of the short 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/1118357/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Dichiara et al. (GCN Circ. 32436). Of the sources reported by D'Ai et al. (GCN Circ. 32434), "Source 5" is fading with >3-sigma significance and is believed to be the afterglow.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 220730A 112 s after the BAT trigger (Kuin and Dichiara GCN Circ. 32435). No optical afterglow consistent with any of the positions in GCN Circ. 32434 (D'Ai et al.) has been 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 0.161 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.

RA (J2000) Dec (J2000) Error Note Reference
15h00m03.43s -69°29'45.4" 6.6" XRT Dichiara et al. GCN Circ. 32436
14h59m57.1s -69°29'44.5" 1.7' BAT-refined Markwardt et al. GCN Circ. 32438

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 32432 Swift GRB 220730A: Global MASTER-Net
observations report 		MASTER
Gamma-ray Poolakkil and Meegan 32437 Fermi GBM detection Fermi GBM Epeak=488±84 keV
T90=0.16 seconds
Fluence=1.06±0.09x10-6erg cm-2
(brighter than 83% of short GRBs)
Gamma-ray Frederiks et al. 32439 Konus-Wind detection of GRB 220730A
(short)		 Konus-Wind Epeak=369 (-59,+92) keV

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 112 261 147 >20.5
uFC 324 574 246 >19.8
white 112 623 167 >20.6
v 655 675 19 >17.8
b 579 599 19 >18.7
u 324 574 246 >19.8
w2 630 650 19 >18.4

Table 3. UVOT observations reported by Kuin and Dichiara (GCN Circ. 32435). 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.

August 3, 2022