Swift Observations of GRB 211225B

K.L. Page (U. Leicester) for the Swift team

1. Introduction

At 13:12:19 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 211225B (trigger=1090853) (Page et al. GCN Circ. 31305). Swift did not slew because of Sun constraint. At the time of the trigger, the initial BAT position was 43° from the Sun (1.0 hours West) and 87° from the 66%-illuminated Moon. Table 1 contains the best reported positions from Swift.

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. 31312), the BAT ground-calculated position is RA, Dec = 258.545, 17.277 deg which is RA(J2000) = 17^h14^m10.9^s Dec(J2000) = +17°16'37.2" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T-25 s and likely lasts beyond ~T+152 s when the burst went out of the BAT field of view. The main peak occurs at ~T+50 s. T₉₀ (15-350 keV) estimated from the available data when the burst was inside the BAT field of view is 121.54 ± 2.94 s (estimated error including systematics).

The time-averaged spectrum from T-24.78 to T+139.82 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.31 ± 0.14, and E_peak of 139.9 ± 51.5 keV (χ² 52.29 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 1.5 ± 0.03 x 10^-5 erg cm^-2 and the 1-s peak flux measured from T+50.19 s in the 15-150 keV band is 6.1 ± 0.3 ph cm^-2 s^-1. This fluence is larger than that of 95.6% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). A fit to a simple power law gives a photon index of 1.59 ± 0.03 (χ² 64.87 for 57 d.o.f.). 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/1090853/BA/.

3. XRT Observations and Analysis

XRT refined results are not available.

4. UVOT Observations and Analysis

UVOT results are not available.

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.

RA (J2000) Dec (J2000) Error Note Reference

17^h14^m10.9^s +17°16'37.2" 1.0' BAT-refined Laha et al. GCN Circ. 31312

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lipunov et al. 31308 Swift GRB 211225B: Global MASTER-Net
observations report MASTER

Gamma-ray Cannady et al. 31332 CALET Gamma-Ray Burst Monitor detection CALET

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

RA (J2000)	Dec (J2000)	Error	Note	Reference
17^h14^m10.9^s	+17°16'37.2"	1.0'	BAT-refined	Laha et al. GCN Circ. 31312

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lipunov et al.	31308	Swift GRB 211225B: Global MASTER-Net observations report	MASTER
Gamma-ray	Cannady et al.	31332	CALET Gamma-Ray Burst Monitor detection	CALET

December 30, 2021