Swift Observations of GRB 221110A

M.A. Williams (PSU), J.D. Gropp (PSU) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

At 02:28:17 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 221110A (trigger=1136936) (Williams et al. GCN Circ. 32925). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 133° from the Sun (10.9 hours East) and 61° from the 97%-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.

Lipunov et al. (GCN Circ. 32924) reported the position from MASTER for the optical afterglow of this GRB. Williams et al. (GCN Circ. 32925) reported the detection with UVOT of an optical afterglow. Izzo et al. (GCN Circ. 32928) determined a redshift of 4.06 from VLT. 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. 32931), the BAT ground-calculated position is RA, Dec = 29.106, -27.298 deg which is RA(J2000) = 01h56m25.5s Dec(J2000) = -27°17'51.4" with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment). The partial coding was 75%.

The mask-weighted light curve (Figure 1) shows a single-pulse structure that starts at ~T0, peaks at ~T+1 s, and ends at ~T+10 s. T90 (15-350 keV) is 8.98 ± 2.11 s (estimated error including systematics).

The time-averaged spectrum from T-0.41 to T+9.74 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.41 ± 0.16. The fluence in the 15-150 keV band is 5.1 ± 0.5 x 10-7 erg cm-2. This fluence is larger than that of 21% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.52 s in the 15-150 keV band is 1.1 ± 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/1136936/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Gropp et al. (GCN Circ. 32930). We have analysed 29 ks of XRT data for GRB 221110A, from 64 s to 434.7 ks after the BAT trigger. The data comprise 55 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 Evans et al. (GCN Circ. 32929).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.950 (+0.023, -0.022).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.88 ± 0.15. The best-fitting absorption column is 8 (+14, -8) x 1021 cm-2, at a redshift of 4.06, in addition to the Galactic value of 1.5 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 3.4 x 10-11 (3.6 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.5 x 1020 cm-2
Intrinsic column: 8 (+14, -8) x 1021 cm-2 at z=4.06
Photon index: 1.88 ± 0.15

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01136936.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 221110A 83 s after the BAT trigger (LaPorte and Williams GCN Circ. 32932). 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 0.013 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
01h56m23.90s -27°17'36.9" 0.5" UVOT-initial Williams et al. GCN Circ. 32925
01h56m23.81s -27°17'44.5" 2.1" XRT-final UKSSDC
01h56m23.69s -27°17'42.7" 2.2" XRT-enhanced Evans et al. GCN Circ. 32929
01h56m25.5s -27°17'51.4" 1.5' BAT-refined Parsotan et al. GCN Circ. 32931

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 32923 Swift GRB221110.10: Global MASTER-Net
observations report 		MASTER
Optical Lipunov et al. 32924 Swift GRB221110.10: Global MASTER-Net
OT detection		 MASTER detection
Optical Strausbaugh and Cucchiara 32926 LCOGT Optical Afterglow Detection Las Cumbres Obs. Global Tele.Network detection
Optical Watson et al. 32927 COATLI Afterglow detection COATLI detection
Optical Izzo et al. 32928 VLT/X-shooter redshift VLT redshift
Optical 32933 GMG upper limit Gao-Mei-Gu upper limits
Optical Watson et al. 32935 COATLI Second-Epoch Upper Limits COATLI upper limits
Optical Coughlin et al. 32939 GRANDMA observations GRANDMA

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
b 3773 3973 196 >19.91
m2 4593 4793 196 >19.35
u 326 439 110 >19.33
v 4357 4557 197 >19.5
w1 4798 4998 196 >19.48
w2 4184 4383 196 >19.55
whiteFC 114 264 147 18.68± 0.08
white 3978 4178 196 >20.66

Table 3. UVOT observations reported by LaPorte and Williams (GCN Circ. 32932). 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.

November 16, 2022