Swift Observations of GRB 221216A

S. Dichiara (PSU), E. Ambrosi (INAF-IASFPA) and P. Kuin (MSSL/UCL) for the Swift team

1. Introduction

At 11:21:20 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 221216A (trigger=1144698) (Dichiara et al. GCN Circ. 33067). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 55° from the Sun (4.2 hours East) and 139° from the 49%-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

As reported by Krimm et al. (GCN Circ. 33254), the BAT ground-calculated position is RA, Dec = 326.023, -34.394 deg which is RA(J2000) = 21^h44^m05.6^s Dec(J2000) = -34°23'37.4" with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows two broad peaks, one centered around T+0, and the other at T+165 seconds. There is a third, more narrow and weaker peak at T+50 seconds and some possible subsidiary peaks. Note that not all of the event data is available, so it is possible that the burst extended before or after this period. T₉₀ (15-350 keV) is 174.48 ± 5.26 s (estimated error including systematics).

The time-averaged spectrum from -9.68 to T+175.22 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.02 ± 0.36, and E_peak of 91.0 ± 44.0 keV (χ² 37.14 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 4.6 ± 0.3 x 10^-6 erg cm^-2 and the 1-s peak flux measured from T+0.23 s in the 15-150 keV band is 1.8 ± 0.2 ph cm^-2 s^-1. This fluence is larger than that of 81% 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 1.56 ± 0.08 (χ² 44.55 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/1144698/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Ambrosi et al. (GCN Circ. 33076). We have analysed 23 ks of XRT data for GRB 221216A, from 61 s to 382.3 ks after the BAT trigger. The data comprise 263 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 Evans et al. (GCN Circ. 33071).

The late-time light curve (Figure 2) (from T0+4.6 ks) can be modelled with an initial power-law decay with an index of α=0.30 (+0.16, -0.17), followed by a break at T+12.8 ks to an α of 2.42 (+0.21, -0.19).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.58 ± 0.03. The best-fitting absorption column is 1.49 (+0.13, -0.12) x 10²¹ cm^-2, in excess of the Galactic value of 2.8 x 10²⁰ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.72 (+0.11, -0.10) and a best-fitting absorption column of 4.3 (+2.8, -1.5) x 10²⁰ cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.7 x 10^-11 (4.0 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.3 (+2.8, -1.5) x 10²⁰ cm^-2
Galactic foreground: 2.8 x 10²⁰ cm^-2
Excess significance: <1.6 σ
Photon index: 1.72 (+0.11, -0.10)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01144698.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 221116A 113 s after the BAT trigger (Kuin and Dichiara GCN Circ. 33078). No optical afterglow consistent with the enhanced XRT position 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 0.042 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

21^h44^m07.21^s -34°25'16.1" 2.0" XRT-final UKSSDC

21^h44^m07.25^s -34°25'16.5" 2.3" XRT-enhanced Evans et al. GCN Circ. 33071

21^h44^m05.6^s -34°23'37.4" 1.3' BAT-refined Krimm et al. GCN Circ. 33254

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lu et al. 33068 Nanshan/NEXT optical upper limits Xinjiang Astro. Obs. upper limits

Optical Gendre et al. 33069 Zadko observatory - Gingin optical
observations Zadko

Optical Lipunov et al. 33070 Swift GRB 221216A: Global MASTER-Net
observations report MASTER

Optical M. Odeh 33073 AKO Upper Limit Al-Khatim Obs. upper limits

Gamma-ray Fermi 33066 Fermi GBM Final Real-time Localization Fermi GBM

Gamma-ray Dunwoody et al. 33077 Fermi GBM observation Fermi GBM E_peak=94.6±8.4 keV
T₉₀=184 seconds
Fluence=6.31±0.40x10^-6erg cm^-2
(brighter than 63% of long GRBs)

Gamma-ray Ripa et al. 33088 Detection by VZLUSAT-2 VZLUSAT

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 113 263 147 >20.3

white 4797 4997 197 >20.4

u 325 358 32 >18.3

b 4592 6041 197 >19.7

uvw1 5618 5818 197 >19.4

uvm2 5413 5613 197 >19.4

uvw2 5003 5203 197 >19.6

RA (J2000)	Dec (J2000)	Error	Note	Reference
21^h44^m07.21^s	-34°25'16.1"	2.0"	XRT-final	UKSSDC
21^h44^m07.25^s	-34°25'16.5"	2.3"	XRT-enhanced	Evans et al. GCN Circ. 33071
21^h44^m05.6^s	-34°23'37.4"	1.3'	BAT-refined	Krimm et al. GCN Circ. 33254

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lu et al.	33068	Nanshan/NEXT optical upper limits	Xinjiang Astro. Obs.	upper limits
Optical	Gendre et al.	33069	Zadko observatory - Gingin optical observations	Zadko
Optical	Lipunov et al.	33070	Swift GRB 221216A: Global MASTER-Net observations report	MASTER
Optical	M. Odeh	33073	AKO Upper Limit	Al-Khatim Obs.	upper limits
Gamma-ray	Fermi	33066	Fermi GBM Final Real-time Localization	Fermi GBM
Gamma-ray	Dunwoody et al.	33077	Fermi GBM observation	Fermi GBM	E_peak=94.6±8.4 keV T₉₀=184 seconds Fluence=6.31±0.40x10^-6erg cm^-2 (brighter than 63% of long GRBs)
Gamma-ray	Ripa et al.	33088	Detection by VZLUSAT-2	VZLUSAT

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white	113	263	147	>20.3
white	4797	4997	197	>20.4
u	325	358	32	>18.3
b	4592	6041	197	>19.7
uvw1	5618	5818	197	>19.4
uvm2	5413	5613	197	>19.4
uvw2	5003	5203	197	>19.6

Table 3. UVOT observations reported by Kuin and Dichiara (GCN Circ. 33078). 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.

February 3, 2023