Swift Observations of GRB 221016A

K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 23:39:24 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 221016A (trigger=1129775) (Page et al. GCN Circ. 32774). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 133° from the Sun (10.8 hours West) and 89° from the 57%-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.

Siegel and Page (GCN Circ. 32785) reported the detection with UVOT of an optical afterglow. 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

Analysis of the BAT data was reported by Laha and Barthelmy (GCN Circ. 32786).

The light curve (Figure 1) shows a single peaked pulse. T90 (15-350 keV) is 21.86 ± 5.70 s

The time-averaged spectrum from T-3.18 to T+29.18 s is best fit by a simple

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/1129775/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Beardmore et al. (GCN Circ. 32784). We have analysed 12 ks of XRT data for GRB 221016A, from 76 s to 78.7 ks after the BAT trigger. The data comprise 83 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 Goad et al. (GCN Circ. 32776).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=2.19 (+0.20, -0.18), followed by a break at T+258 s to an α of 0.77 (+0.04, -0.05).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.49 (+0.23, -0.21). The best-fitting absorption column is 7.9 (+4.2, -3.7) x 1020 cm-2, in excess of the Galactic value of 3.5 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.04 (+0.20, -0.19) and a best-fitting absorption column of 1.5 (+0.6, -0.5) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.3 x 10-11 (4.4 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.5 (+0.6, -0.5) x 1021 cm-2
Galactic foreground: 3.5 x 1020 cm-2
Excess significance: 3.5 σ
Photon index: 2.04 (+0.20, -0.19)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01129775.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 221016A 97 s after the BAT trigger (Siegel and Page GCN Circ. 32785). A source consistent with the enhanced XRT position (Goad et al. GCN Circ. 32776) 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 0.026 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
02h35m46.70s -34°36'37.6" 2.0" XRT-final UKSSDC
02h35m46.70s -34°36'38.6" 2.4" XRT-enhanced Goad et al. GCN Circ. 32776
02h35m45.2s -34°36'54.2" 1.0' BAT-refined Laha and Barthelmy GCN Circ. 32786

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 32772 Swift GRB221016.99: Global MASTER-Net
observations report 		MASTER
Optical Strausbaugh and Cucchiara 32775 LCOGT Optical Afterglow Detection Las Cumbres Obs. Global Tele.Network detection
Gamma-ray Fermi 32773 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Bissaldi et al. 32782 Fermi GBM observation Fermi GBM Epeak=119±12 keV
T90=13 seconds
Fluence=6.3±0.3x10-6erg cm-2
(brighter than 63% of long GRBs)
Gamma-ray Ridnaia et al. 32783 Konus-Wind detection Konus-Wind Epeak=207 (-46,+98) keV
Duration=~12.5 seconds
Fluence=5.46(-0.74,+1.07)x10-6erg cm-2
Other Malesani et al. 32778 z-band upper limit, possible afterglow
brightening		 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
white 97 247 147 20.20 ± 0.23
b 3736 3861 123 >19.8
u 309 349 39 >18.8

Table 3. UVOT observations reported by Siegel and Page (GCN Circ. 32785). The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary detections and 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.

October 18, 2022