Swift Observations of GRB 221226B

S.B. Cenko (GSFC), M. Capalbi (INAF-IASFPA) and N. Klingler (UMBC/NASA-GSFC/CRESST II) for the Swift team

1. Introduction

At 22:41:20 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 221226B (trigger=1145959) (Cenko et al. GCN Circ. 33108). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 87° from the Sun (7.2 hours East) and 52° from the 17%-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.

Xu et al. (GCN Circ. 33110) reported the position from VLT for the optical afterglow of this GRB. Klingler (GCN Circ. 33114) reported the detection with UVOT of an optical afterglow. Xu et al. (GCN Circ. 33110) determined a redshift of 2.694 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 Laha et al. (GCN Circ. 33115), the BAT ground-calculated position is RA, Dec = 22.903, -41.515 deg which is RA(J2000) = 01h31m36.7s Dec(J2000) = -41°30'52.5" with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment). The partial coding was 97%.

The mask-weighted light curve (Figure 1) shows a pulse of approximately 4 seconds total duration and some substructure. T90 (15-350 keV) is 3.44 ± 0.57 s (estimated error including systematics).

The time-averaged spectrum from T+0.22 to T+4.28 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.16 ± 0.13. The fluence in the 15-150 keV band is 5.4 ± 0.5 x 10-7 erg cm-2. The 1-s peak photon flux measured from T+0.24 s in the 15-150 keV band is 2.6 ± 0.3 ph cm-2 s-1. This fluence is larger than that of 23% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). All the quoted errors are at the 90% confidence level. The results of the batgrbproduct analysis are available at http://gcn.gsfc.nasa.gov/notices_s/1145959/BA/

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/1145959/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Capalbi et al. (GCN Circ. 33111). We have analysed 9.4 ks of XRT data for GRB 221226B, from 56 s to 68.1 ks after the BAT trigger. The data comprise 14 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. 33109).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.57 (+0.13, -0.12).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.6 (+0.4, -0.3). The best-fitting absorption column is 1.4 (+2.5, -1.4) x 1022 cm-2, at a redshift of 2.694, in addition to the Galactic value of 1.7 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 4.3 x 10-11 (4.7 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.7 x 1020 cm-2
Intrinsic column: 1.4 (+2.5, -1.4) x 1022 cm-2 at z=2.694
Photon index: 1.6 (+0.4, -0.3)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01145959.

4. UVOT Observations and Analysis

Analysis of the UVOT data was reported by Klingler (GCN Circ. 33114). The Swift-UVOT began settled observations of the field of GRB 221226B 76 s after the BAT trigger. A source consistent with the XRT position (Goad et al., GCN Circ. 33109) and consistent with the optical counterpart reported by VLT/X-shooter (Xu et al., GCN Circ. 33110) 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 0.017 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
01h31m38.07s -41°31'36.0" 2.0" XRT-final UKSSDC
01h31m38.33s -41°31'36.1" 2.6" XRT-enhanced Goad et al. GCN Circ. 33109
01h31m36.7s -41°30'52.5" 1.2' BAT-refined Laha et al. GCN Circ. 33115

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Xu et al. 33110 VLT/X-shooter redshift and host galaxy VLT redshift
Optical Strausbaugh and Cucchiara 33119 LCOGT Optical Upper Limits Las Cumbres Obs. Global Tele.Network upper limits
Gamma-ray Fermi 33107 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Lesage and Meegan 33112 Fermi GBM Observation Fermi GBM Epeak=104±8 keV
T90=5 seconds
Fluence=7.8±0.5x10-7erg cm-2
(brighter than 10% of long GRBs)

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 76 226 147 20.54 ± 0.22
v 4408 4608 197 >19.3
b 3793 3993 197 >20.0
u 288 5132 278 >20.1
w1 4818 5018 197 >20.0
m2 4613 4813 197 >19.5
w2 4204 4404 197 >20.0

Table 3. UVOT observations reported by Klingler (GCN Circ. 33114). 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.

December 29, 2022