Swift Observations of GRB 220319A

K.L. Page (U. Leicester) and J.D. Gropp (PSU) for the Swift team

1. Introduction

At 17:40:33 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220319A (trigger=1098132) (Page et al. GCN Circ. 31769). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 112° from the Sun (9.4 hours West) and 67° from the 98%-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. 31781), the BAT ground-calculated position is RA, Dec = 218.139, 61.291 deg which is RA(J2000) = 14^h32^m33.4^s Dec(J2000) = +61°17'26.6" with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 43%.

The BAT light curve (Figure 1) showed a single-peaked structure with a duration of about 10 s. T₉₀ (15-350 keV) is 6.44 ± 1.54 s (estimated error including systematics).

The time-averaged spectrum from T-0.36 to T+6.56 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.15 ± 0.28. The fluence in the 15-150 keV band is 2.3 ± 0.4 x 10^-7 erg cm^-2. This fluence is larger than that of 8.0% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.24 s in the 15-150 keV band is 1.5 ± 0.2 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/1098132/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Gropp et al. (GCN Circ. 31772). We have analysed 16 ks of XRT data for GRB 220319A, from 108 s to 127.7 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 31771).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.00 (+0.08, -0.07).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.26 (+0.29, -0.27). The best-fitting absorption column is 2.8 (+1.0, -0.9) x 10²¹ cm^-2, in excess of the Galactic value of 1.5 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 3.2 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 (+1.0, -0.9) x 10²¹ cm^-2
Galactic foreground: 1.5 x 10²⁰ cm^-2
Excess significance: 4.8 σ
Photon index: 2.26 (+0.29, -0.27)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01098132.

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

14^h32^m53.86^s +61°17'43.3" 2.1" XRT-final UKSSDC

14^h32^m53.81^s +61°17'41.9" 2.2" XRT-enhanced Goad et al. GCN Circ. 31771

14^h32^m33.4^s +61°17'26.6" 1.9' BAT-refined Krimm et al. GCN Circ. 31781

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lipunov et al. 31770 MASTER Global Robotic Net optical
observations MASTER

Optical Belkin et al. 31773 Mondy optical uper limit Mondy upper limits

Optical Strausbaugh and Cucchiara 31774 LCO Optical Upper Limit LCO upper limits

Optical de Ugarte Postigo and Jimenez 31775 Optical observations from NOT NOT

Optical Hosokawa et al. 31776 MITSuME Akeno optical upper limits MITSuME Akeno upper limits

Other Bizouard et al. 31785 GRANDMA observations

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

RA (J2000)	Dec (J2000)	Error	Note	Reference
14^h32^m53.86^s	+61°17'43.3"	2.1"	XRT-final	UKSSDC
14^h32^m53.81^s	+61°17'41.9"	2.2"	XRT-enhanced	Goad et al. GCN Circ. 31771
14^h32^m33.4^s	+61°17'26.6"	1.9'	BAT-refined	Krimm et al. GCN Circ. 31781

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lipunov et al.	31770	MASTER Global Robotic Net optical observations	MASTER
Optical	Belkin et al.	31773	Mondy optical uper limit	Mondy	upper limits
Optical	Strausbaugh and Cucchiara	31774	LCO Optical Upper Limit	LCO	upper limits
Optical	de Ugarte Postigo and Jimenez	31775	Optical observations from NOT	NOT
Optical	Hosokawa et al.	31776	MITSuME Akeno optical upper limits	MITSuME Akeno	upper limits
Other	Bizouard et al.	31785	GRANDMA observations

March 25, 2022