Swift Observations of GRB 221201A

E. Troja (NASA/GSFC/UMCP), J.A. Kennea (PSU) and S.R. Oates (U.Birmingham) for the Swift team

1. Introduction

At 12:25:05 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 221201A (trigger=1142847) (Troja et al. GCN Circ. 32998). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 48° from the Sun (1.3 hours East) and 81° from the 60%-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

Analysis of the BAT data was reported by Stamatikos and Barthelmy (GCN Circ. 33013).

The mask-weighted light curve (Figure 1) shows a single peaked pulse.

The time-averaged spectrum from T-20.88 to T+22.92 s is best fit by a simple

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/1142847/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Kennea et al. (GCN Circ. 33002). We have analysed 14 ks of XRT data for GRB 221201A, from 159 s to 110.4 ks after the BAT trigger. The data comprise 375 s in Windowed Timing (WT) mode (the first 10 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 Osborne et al. (GCN Circ. 33001).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=5.38 (+0.21, -0.31). At T+219 s the decay steepens to an α of 6.43 (+0.18, -0.21) before breaking again at T+364 s to a final decay with index α=0.929 (+0.018, -0.020).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 3.19 (+0.07, -0.06). The best-fitting absorption column is 3.42 ± 0.17 x 10²¹ cm^-2, in excess of the Galactic value of 7.5 x 10²⁰ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.99 (+0.12, -0.11) and a best-fitting absorption column of 2.8 ± 0.5 x 10²¹ cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.7 x 10^-11 (5.4 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.8 ± 0.5 x 10²¹ cm^-2
Galactic foreground: 7.5 x 10²⁰ cm^-2
Excess significance: 7.4 σ
Photon index: 1.99 (+0.12, -0.11)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01142847.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 221201A 179 s after the BAT trigger (Oates and Troja GCN Circ. 33003). No optical afterglow consistent with the XRT position (Osborne et al. GCN Circ. 33001) 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 E_B-V of 0.066 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

17^h47^m41.86^s -68°14'56.2" 2.0" XRT-final UKSSDC

17^h47^m41.91^s -68°14'55.8" 2.1" XRT-enhanced Osborne et al. GCN Circ. 33001

17^h47^m36.6^s -68°13'48.2" 1.0' BAT-refined Stamatikos and Barthelmy GCN Circ. 33013

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lipunov et al. 33000 Swift GRB 221201A: Global MASTER-Net
observations report MASTER

Gamma-ray Fermi 32997 Fermi GBM Final Real-time Localization Fermi GBM

Gamma-ray Malacaria and Meegan 33011 Fermi GBM observation Fermi GBM E_peak=144±7 keV
T₉₀=29 seconds
Fluence=1.59±0.03x10^-5erg cm^-2
(brighter than 84% of long GRBs)

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

Filter T_start(s) T_stop(s) Exp(s) Mag

white_FC 179 329 147 >20.1

white 179 1970 225 >20.2

v 1479 2020 78 >17.8

b 1404 2107 85 >18.8

u 337 2094 277 >19.2

w1 1529 2069 78 >18.4

m2 1504 2045 78 >18.5

w2 1454 1996 78 >18.6

RA (J2000)	Dec (J2000)	Error	Note	Reference
17^h47^m41.86^s	-68°14'56.2"	2.0"	XRT-final	UKSSDC
17^h47^m41.91^s	-68°14'55.8"	2.1"	XRT-enhanced	Osborne et al. GCN Circ. 33001
17^h47^m36.6^s	-68°13'48.2"	1.0'	BAT-refined	Stamatikos and Barthelmy GCN Circ. 33013

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lipunov et al.	33000	Swift GRB 221201A: Global MASTER-Net observations report	MASTER
Gamma-ray	Fermi	32997	Fermi GBM Final Real-time Localization	Fermi GBM
Gamma-ray	Malacaria and Meegan	33011	Fermi GBM observation	Fermi GBM	E_peak=144±7 keV T₉₀=29 seconds Fluence=1.59±0.03x10^-5erg cm^-2 (brighter than 84% of long GRBs)

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white_FC	179	329	147	>20.1
white	179	1970	225	>20.2
v	1479	2020	78	>17.8
b	1404	2107	85	>18.8
u	337	2094	277	>19.2
w1	1529	2069	78	>18.4
m2	1504	2045	78	>18.5
w2	1454	1996	78	>18.6

Table 3. UVOT observations reported by Oates and Troja (GCN Circ. 33003). 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.

December 4, 2022