Swift Observations of GRB 220117B

A. Melandri (INAF-OAB), J.P. Osborne (U. Leicester) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 20:05:28 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220117B (trigger=1093611) (Melandri et al. GCN Circ. 31468). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 94° from the Sun (7.2 hours West) and 89° from the 100%-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 Parsotan et al. (GCN Circ. 31486), the BAT ground-calculated position is RA, Dec = 191.989, -28.762 deg which is RA(J2000) = 12h47m57.3s Dec(J2000) = -28°45'42.8" with an uncertainty of 2.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 96%.

The mask-weighted light curve (Figure 1) shows a weak structure that starts at ~T-3 s and ends at ~T+23 s. T90 (15-350 keV) is 24.37 ± 4.63 s (estimated error including systematics).

The time-averaged spectrum from T-2.73 to T+23.29 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.94 ± 0.32. The fluence in the 15-150 keV band is 2.9 ± 0.6 x 10-7 erg cm-2. This fluence is larger than that of 12% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.13 s in the 15-150 keV band is 0.5 ± 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/1093611/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Osborne et al. (GCN Circ. 31478).

We have analysed 6.7 ks of XRT data for GRB 220117B, from 88 s to 28.6 ks after the BAT trigger. The data comprise 203 s in Windowed Timing (WT) mode (the first 6 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 Evans et al. (GCN Circ. 31473).

The late-time light curve (Figure 2) (from T0+3.5 ks) can be modelled with a power-law decay with a decay index of α=0.79 ± 0.09.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.65 (+0.11, -0.10). The best-fitting absorption column is 4.5 ± 0.4 x 1021 cm-2, in excess of the Galactic value of 7.6 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.04 (+0.19, -0.18) and a best-fitting absorption column of 3.0 (+0.8, -0.7) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10-11 (5.4 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 3.0 (+0.8, -0.7) x 1021 cm-2
Galactic foreground: 7.6 x 1020 cm-2
Excess significance: 5.2 σ
Photon index: 2.04 (+0.19, -0.18)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01093611.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 220117B 104 s after the BAT trigger (Breeveld and Melandri GCN Circ. 31483). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 31473) 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 0.075 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
12h47m48.72s -28°44'43.6" 2.0" XRT-final UKSSDC
12h47m48.70s -28°44'43.4" 2.3" XRT-enhanced Evans et al. GCN Circ. 31473
12h47m57.3s -28°45'42.8" 2.7' BAT-refined Parsotan et al. GCN Circ. 31486

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 31469 Swift GRB 220117B: Global MASTER-Net
observations report 		MASTER
Optical Ito et al. 31474 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Fu et al. 31475 Nanshan/NEXT optical upper limit Xinjiang Astro. Obs. upper limits
Optical Koyanagi et al. 31518 TARGET optical upper limit upper limits
Other Rossi et al. 31492 z-band limit 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
whiteFC 104 254 147 >19.7
white 104 3930 344 >20.5
v 4141 4341 197 >19.2
b 3525 5048 281 >20.7
u 262 4957 239 >19.8
w1 4552 4752 197 >20.0
m2 4346 4546 197 >20.3
w2 3936 4136 197 >20.4

Table 3. UVOT observations reported by Breeveld and Melandri (GCN Circ. 31483). 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.

January 24, 2022