Swift Observations of GRB 220118A

N.J. Klingler (GSFC/UMBC/CRESSTII), M.R. Goad (U. Leicester) and A. Belles (PSU) for the Swift team

1. Introduction

At 18:20:38 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220118A (trigger=1093742) (Klingler et al. GCN Circ. 31490). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 114° from the Sun (7.2 hours West) and 56° from the 99%-illuminated Moon. Table 1 contains the best reported positions from Swift.

Klingler et al. (GCN Circ. 31490) 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

As reported by Sakamoto et al. (GCN Circ. 31499), the BAT ground-calculated position is RA, Dec = 192.277, 22.911 deg which is RA(J2000) = 12h49m06.4s Dec(J2000) = +22°54'37.8" with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 22%.

The mask-weighted light curve (Figure 1) shows a single-peaked structure that starts at ~T-4 s, peaks at ~T+1 s, and ends at ~T+10 s. T90 (15-350 keV) is 10.61 ± 1.78 s (estimated error including systematics).

The time-averaged spectrum from T-3.90 to T+9.54 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.67 ± 0.12. The fluence in the 15-150 keV band is 1.8 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 56% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.68 s in the 15-150 keV band is 3.3 ± 0.5 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/1093742/BA/.

3. XRT Observations and Analysis

We have analysed 1.8 ks of XRT data for GRB 220118A, from 79 s to 11.9 ks after the BAT trigger. The data comprise 32 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. 31495).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=2.95 (+0.31, -0.22), followed by a break at T+174 s to an α of 1.10 (+0.08, -0.07).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.82 (+0.27, -0.15). The best-fitting absorption column is consistent with the Galactic value of 2.3 x 1020 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10-11 (3.7 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.3 ± 5.6 x 1020 cm-2
Galactic foreground: 2.3 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.82 (+0.27, -0.15)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01093742.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 220118A 99 s after the BAT trigger (Belles and Klingler GCN Circ. 31498). A fading source consistent with the XRT position (Goad et al., GCN Circ. 31495) 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 0.025 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
12h49m04.52s +22°54'56.2" 0.44" UVOT-refined Belles and Klingler GCN Circ. 31498
12h49m04.44s +22°54'55.0" 2.0" XRT-final UKSSDC
12h49m04.44s +22°54'55.0" 2.2" XRT-enhanced Goad et al. GCN Circ. 31495
12h49m06.4s +22°54'37.8" 1.4' BAT-refined Sakamoto et al. GCN Circ. 31499

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 31491 Swift GRB 220118A: Global MASTER-Net
observations report 		MASTER
Optical Hosokawa et al. 31493 MITSuME Akeno optical observation MITSuME Akeno detection
Optical Lipunov et al. 31494 Swift GRB220118.50: Global MASTER-Net
observations report 		MASTER
Optical Kumar et al. 31496 GIT optical observations GROWTH-India Tel. detection
Optical Komesh et al. 31502 NUTTelA-TAO / BSTI Early Optical Limits
(Preliminary)		 NUTTelA upper limits
Optical Pereyra et al. 31505 COATLI Optical Observations COATLI
Optical Kann et al. 31507 2.2m CAHA/CAFOS detections CAHA detection
Optical Koyanagi et al. 31519 TARGET optical upper limit upper limits
Gamma-ray Fermi 31489 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Fletcher et al. 31506 Fermi GBM detection Fermi GBM Epeak=148.30±17.50 keV
T90=19 seconds

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
wh 119 268 145 17.73±0.07
wh 11382 11797 404 >20.26
u 276 454 175 17.33±0.12
b 10470 11377 885 >20.56

Table 3. UVOT observations reported by Belles and Klingler (GCN Circ. 31498). 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.

January 24, 2022