Swift Observations of GRB 221028A

A.Y. Lien (U. Tampa) and V. D'Elia (SSDC and INAF-OAR) for the Swift team

1. Introduction

At 13:16:27 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 221028A (trigger=1131910) (Lien et al. GCN Circ. 32874). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 107° from the Sun (6.8 hours East) and 87° from the 12%-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 Palmer et al. (GCN Circ. 32893), the BAT ground-calculated position is RA, Dec = 314.198, 41.082 deg which is RA(J2000) = 20h56m47.5s Dec(J2000) = +41°04'56.0" with an uncertainty of 1.6 arcmin, (radius, sys+stat, 90% containment). The partial coding was 53%.

The mask-weighted light curve (Figure 1) shows a short pulse that starts and peaks at ~T0 and ends at ~T+2, followed by several overlapping pulses that lasts till ~T+84 s. The structure resembles those of short GRBs with extended emission. T90 (15-350 keV) is 57.1 ± 15.0 s (estimated error including systematics).

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/1131910/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Elia et al. (GCN Circ. 32899).

We have analysed 5.6 ks of XRT data for GRB 221028A, from 92 s to 28.1 ks after the BAT trigger. The data comprise 23 s in Windowed Timing (WT) mode (the first 2 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=2.04 (+0.13, -0.12).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.5 (+0.5, -0.4). The best-fitting absorption column is 2.4 (+1.0, -0.8) x 1022 cm-2, in excess of the Galactic value of 7.8 x 1021 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 8.2 x 10-11 (1.3 x 10-10) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.4 (+1.0, -0.8) x 1022 cm-2
Galactic foreground: 7.8 x 1021 cm-2
Excess significance: 3.3 σ
Photon index: 1.5 (+0.5, -0.4)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01131910.

4. UVOT Observations and Analysis

UVOT results are not available.

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
20h56m48.64s +41°05'23.3" 3.5" XRT-final UKSSDC
20h56m48.64s +41°05'23.3" 3.5" XRT-refined D'Elia et al. GCN Circ. 32899
20h56m47.5s +41°04'56.0" 1.6' BAT-refined Palmer et al. GCN Circ. 32893

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Kumar et al. 32876 GIT optical upper limit GROWTH-India Tel. upper limits
Optical Hu et al. 32878 BOOTES-4/MET optical upper limit BOOTES-4 upper limits
Optical Lipunov et al. 32879 Swift GRB 221028A: Global MASTER-Net
observations report 		MASTER
Optical Strausbaugh and Cucchiara 32881 LCOGT Optical Upper Limits Las Cumbres Obs. Global Tele.Network upper limits
Optical Niwano et al. 32883 MITSuME Okayama optical upper limits MITSuME Okayama upper limits
Optical Kumar et al. 32886 HCT optical upper limit Himalayan Chandra Telescope upper limits
Optical Watson et al. 32888 COATLI Upper Limits COATLI upper limits
Optical Belkin et al. 32889 CrAO/ZTSh optical upper limit CrAO upper limits
Optical 32894 AKO Optical Upper Limit upper limits
Optical Komesh et al. 32896 NUTTelA-TAO / BSTI Early Optical Limits NUTTelA upper limits
Optical Komesh et al. 32897 NUTTelA-TAO / BSTI Early Optical Limits NUTTelA upper limits
Optical Ror et al. 32898 1.3m DFOT Optical upper limit DFOT upper limits
Optical Kochiashvili et al. 32906 GRANDMA observations GRANDMA
Gamma-ray Ursi et al. 32885 AGILE detection AGILE
Gamma-ray Ripa et al. 32904 Detection by VZLUSAT-2 VZLUSAT

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

November 4, 2022