Swift Observations of GRB 211023B

S. Dichiara (NASA/GSFC/UMCP), A.P. Beardmore (U. Leicester) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 21:05:52 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 211023B (trigger=1080859) (Dichiara et al. GCN Circ. 30960). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 62° from the Sun (2.5 hours West) and 88° from the 91%-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.

Dichiara et al. (GCN Circ. 30960) reported the discovery with UVOT of an optical afterglow. Rossi et al. (GCN Circ. 31107) determined a redshift of 0.862 from LBT. 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 Laha et al. (GCN Circ. 30979), the BAT ground-calculated position is RA, Dec = 170.314, 39.133 deg which is RA(J2000) = 11h21m15.3s Dec(J2000) = +39°07'58.7" with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 48%.

The mask-weighted light curve (Figure 1) s shows a single peak from approximately T+0 to T+1.5 seconds. T90 (15-350 keV) is 1.30 ± 0.26 s (estimated error including systematics).

The time-averaged spectrum from T-0.01 to T+1.45 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.97 ± 0.24. The fluence in the 15-150 keV band is 1.7 ± 0.3 x 10-7 erg cm-2. This fluence is larger than that of 82% of the short GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.24 s in the 15-150 keV band is 2.2 ± 0.3 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/1080859/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Beardmore et al. (GCN Circ. 30967). We have analysed 27 ks of XRT data for GRB 211023B, from 79 s to 396.3 ks after the BAT trigger. The data comprise 8 s in Windowed Timing (WT) mode (taken while Swift was slewing), with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 30966).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.95 (+0.10, -0.08).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.12 (+0.23, -0.21). The best-fitting absorption column is 1.1 (+0.6, -0.5) x 1021 cm-2, in excess of 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 3.1 x 10-11 (4.1 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.1 (+0.6, -0.5) x 1021 cm-2
Galactic foreground: 2.3 x 1020 cm-2
Excess significance: 2.9 σ
Photon index: 2.12 (+0.23, -0.21)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01080859.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 211023B 98 s after the BAT trigger (Kuin and Dichiara GCN Circ. 30975). A source consistent with the XRT position (Beardmore et al. GCN Circ. 30966) 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 0.024 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
11h21m14.33s +39°08'08.8" 0.53" UVOT-refined Kuin and Dichiara GCN Circ. 30975
11h21m14.09s +39°08'07.2" 2.7" XRT-final UKSSDC
11h21m14.09s +39°08'07.2" 2.7" XRT-enhanced Beardmore et al. GCN Circ. 30966
11h21m15.3s +39°07'58.7" 2.0' BAT-refined Laha et al. GCN Circ. 30979

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 30964 Swift GRB 211023B: Global MASTER-Net
observations report 		MASTER
Optical Pozanenko et al. 30972 TSHAO optical observation Zeiss-1000
Optical Watson et al. 30973 RATIR Optical and NIR Upper Limits RATIR upper limits
Optical Hu et al. 30976 BOOTES-4/MET and 1.5m OSN optical upper
limit		 BOOTES-4 upper limits
Optical Kann et al. 30992 Possible Short GRB 211023B: CAHA 2.2m
Detection		 CAHA possible detection
Optical O'Connor et al. 31000 LDT Optical Afterglow Detection Lowell Discovery Telescope detection
Optical Rastinejad et al. 31002 Gemini-North Optical Observations Gemini
Optical Pozanenko et al. 31003 Mondy optical afterglow confirmation Mondy detection
Optical Grossan et al. 31016 NUTTelA-TAO / BSTI Early Optical Limits
(Preliminary)		 NUTTelA upper limits
Optical Hosokawa et al. 31031 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Sakamoto et al. 31038 Subaru optical observation Subaru
Optical Rossi et al. 31107 LBT likely spectroscopic redshift of
the host galaxy		 LBT redshift

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 98 247 147 19.44 ± 0.11
v 640 1753 136 >18.8
b 566 1679 117 >19.9
u 310 734 265 19.50 ± 0.20
w1 690 1803 117 >19.3
w2 615 1039 39 >19.9

Table 3. UVOT observations reported by Kuin and Dichiara (GCN Circ. 30975). 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 24, 2021