Swift Observations of GRB 220305A

R. Caputo (GSFC) and V. D'Elia (SSDC and INAF-OAR) for the Swift team

1. Introduction

At 16:13:08 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220305A (trigger=1095215) (Caputo et al. GCN Circ. 31684). Due to a viewing constraint, Swift did not immediately slew. At the time of the trigger, the initial BAT position was 110° from the Sun (8.1 hours East) and 91° from the 10%-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 Stamatikos et al. (GCN Circ. 31698), the BAT ground-calculated position is RA, Dec = 107.610, -37.703 deg which is RA(J2000) = 07^h10^m26.4^s Dec(J2000) = -37°42'09.2" with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment). The partial coding was 25%.

The mask-weighted light curve (Figure 1) shows a single-pulse structure that starts at ~T-15 s, peaks at ~T₀, and ends at ~T+11 s. The burst went out of the BAT FOV at T+225.7 s. T₉₀ (15-350 keV) is 21.12 ± 3.65 s (estimated error including systematics).

The time-averaged spectrum from T-15.04 to T+11.02 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.66 ± 0.15. The fluence in the 15-150 keV band is 2.0 ± 0.2 x 10^-6 erg cm^-2. This fluence is larger than that of 58% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.16 s in the 15-150 keV band is 2.1 ± 0.5 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/1095215/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Elia et al. (GCN Circ. 31695). We have analysed 11 ks of XRT data for GRB 220305A, from 3.0 ks to 78.5 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 31688).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.14 ± 0.08.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.30 (+0.26, -0.24). The best-fitting absorption column is 7.7 (+1.8, -1.6) x 10²¹ cm^-2, in excess of the Galactic value of 2.6 x 10²¹ cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.8 x 10^-11 (8.6 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 7.7 (+1.8, -1.6) x 10²¹ cm^-2
Galactic foreground: 2.6 x 10²¹ cm^-2
Excess significance: 5.3 σ
Photon index: 2.30 (+0.26, -0.24)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01095215.

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.

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

07^h10^m23.65^s -37°40'49.2" 2.0" XRT-final UKSSDC

07^h10^m23.65^s -37°40'49.2" 2.2" XRT-enhanced Beardmore et al. GCN Circ. 31688

07^h10^m26.4^s -37°42'09.2" 1.5' BAT-refined Stamatikos et al. GCN Circ. 31698

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lipunov et al. 31686 Swift GRB 220305A: Global MASTER-Net
observations report MASTER

Optical Lipunov et al. 31691 Fermi GRB 220305A: Global MASTER-Net
observations report MASTER

Gamma-ray Dunwoody et al. 31689 Fermi GBM observation Fermi GBM E_peak=60.9±9.1 keV
T₉₀=172.8 seconds
Fluence=6.8±0.8x10^-7erg cm^-2
(brighter than 8% of long GRBs)

Gamma-ray Dunwoody et al. 31690 Correction to GCN Circ. 31689: Fermi GBM
Report of GRB 220305A should be GRB
220305B Fermi GBM E_peak=60.9±9.1 keV
T₉₀=172.8 seconds
Fluence=6.8±0.8x10^-7erg cm^-2
(brighter than 8% of long GRBs)

Gamma-ray Wood 31696 Fermi GBM Sub-Threshold Detection Fermi GBM E_peak=230 keV

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

RA (J2000)	Dec (J2000)	Error	Note	Reference
07^h10^m23.65^s	-37°40'49.2"	2.0"	XRT-final	UKSSDC
07^h10^m23.65^s	-37°40'49.2"	2.2"	XRT-enhanced	Beardmore et al. GCN Circ. 31688
07^h10^m26.4^s	-37°42'09.2"	1.5'	BAT-refined	Stamatikos et al. GCN Circ. 31698

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lipunov et al.	31686	Swift GRB 220305A: Global MASTER-Net observations report	MASTER
Optical	Lipunov et al.	31691	Fermi GRB 220305A: Global MASTER-Net observations report	MASTER
Gamma-ray	Dunwoody et al.	31689	Fermi GBM observation	Fermi GBM	E_peak=60.9±9.1 keV T₉₀=172.8 seconds Fluence=6.8±0.8x10^-7erg cm^-2 (brighter than 8% of long GRBs)
Gamma-ray	Dunwoody et al.	31690	Correction to GCN Circ. 31689: Fermi GBM Report of GRB 220305A should be GRB 220305B	Fermi GBM	E_peak=60.9±9.1 keV T₉₀=172.8 seconds Fluence=6.8±0.8x10^-7erg cm^-2 (brighter than 8% of long GRBs)
Gamma-ray	Wood	31696	Fermi GBM Sub-Threshold Detection	Fermi GBM	E_peak=230 keV

March 7, 2022