Swift Observations of GRB 221120A

R.A.J. Eyles-Ferris (U. Leicester), S. Dichiara (PSU) and P. Kuin (MSSL/UCL) for the Swift team

1. Introduction

At 21:29:28 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 221120A (trigger=1141030) (Eyles-Ferris et al. GCN Circ. 32955). Swift slewed immediately to the burst. 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. 32970), the BAT ground-calculated position is RA, Dec = 41.363, 43.241 deg which is RA(J2000) = 02h45m27.1s Dec(J2000) = +43°14'26.5" with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment). The partial coding was 97%.

The BAT light curve (Figure 1) shows several peaks within a duration of ~1 s T90 (15-350 keV) is 0.79 ± 0.16 s (estimated error including systematics).

The time-averaged spectrum from T-0.60 to T+0.27 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 0.85 ± 0.27. The fluence in the 15-150 keV band is 1.1 ± 0.2 x 10-7 erg cm-2. This fluence is larger than that of 76% of the short GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.67 s in the 15-150 keV band is 1.2 ± 0.2 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/1141030/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Dichiara et al. (GCN Circ. 32963). We have analysed 16 ks of XRT data for GRB 221120A, from 103 s to 182.9 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 32962).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.6 ± 0.3.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.9 (+1.1, -0.9). The best-fitting absorption column is 3.3 (+2.5, -1.7) x 1022 cm-2, in excess of the Galactic value of 1.2 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 8.0 x 10-11 (1.8 x 10-10) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 3.3 (+2.5, -1.7) x 1022 cm-2
Galactic foreground: 1.2 x 1021 cm-2
Excess significance: 3.0 σ
Photon index: 1.9 (+1.1, -0.9)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01141030.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 221120A 208 s after the BAT trigger (Kuin and Eyles-Ferris GCN Circ. 32958). An optical detection in r' has been reported by O'Connor et al. (GCN Circ. 32957). 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 0.093 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
02h45m21.56s +43°14'35.4" 2.4" XRT-final UKSSDC
02h45m21.56s +43°14'35.4" 2.4" XRT-enhanced Goad et al. GCN Circ. 32962
02h45m27.1s +43°14'26.5" 1.8' BAT-refined Sakamoto et al. GCN Circ. 32970

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Swain et al. 32956 GIT upper limits on optical afterglow GROWTH-India Tel. upper limits
Optical O'Connor et al. 32957 LDT Optical Afterglow Candidate Lowell Discovery Telescope detection
Optical Conti et al. 32961 Montarrenti Observatory Upper Limit Montarrenti upper limits
Optical Gompertz et al. 32966 Liverpool Telescope Upper Limits Liverpool Telescope upper limits
Optical M. Odeh 32968 AKO Telescope Upper Limits Al-Khatim Obs. upper limits
Optical O'Connor et al. 32972 GTC Confirmation of an Underlying Galaxy GTC
Optical Querrard et al. 32976 VIRT Optical Upper Limit Virgin Island Robotic Telescope upper limits
Optical Strausbaugh et al. 32978 TURBO Optical Upper Limit upper limits
Gamma-ray Veres and Meegan 32964 Fermi GBM observation Fermi GBM Epeak=379±65 keV
T90=0.64 seconds
Fluence=7.8±0.9x10-7erg cm-2
(brighter than 75% of short GRBs)
Gamma-ray Ripa et al. 32982 Detection by VZLUSAT-2 VZLUSAT
Other 32959 is a short burst
Other Karambelkar et al. 32967 WIRC near-IR imaging of the afterglow
candidate		 detection

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
uFC 208 458 246 >19.9
white 761 911 295 >21.0
u 208 458 246 >19.9

Table 3. UVOT observations reported by Kuin and Eyles-Ferris (GCN Circ. 32958). 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.

November 26, 2022