Swift Observations of GRB 220306B

R. Caputo (GSFC), A. D'Ai (INAF-IASFPA) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 18:15:37 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220306B (trigger=1095288) (Caputo et al. GCN Circ. 31697). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 78° from the Sun (1.2 hours East) and 62° from the 17%-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.

Klose et al. (GCN Circ. 31711) reported the position from Tautenburg for the optical afterglow of this GRB. 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 Barthelmy et al. (GCN Circ. 31707), the BAT ground-calculated position is RA, Dec = 5.380, 71.384 deg which is RA(J2000) = 00h21m31.3s Dec(J2000) = +71°23'04.1" with an uncertainty of 1.6 arcmin, (radius, sys+stat, 90% containment). The partial coding was 78%.

The mask-weighted light curve (Figure 1) shows a single-peaked structure that starts at ~T-2 s, peaks at ~T0, and ends at ~T+12 s. T90 (15-350 keV) is 12.34 ± 3.24 s (estimated error including systematics).

The time-averaged spectrum from T-1.82 to T+11.66 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.30 ± 0.20. The fluence in the 15-150 keV band is 6.0 ± 0.7 x 10-7 erg cm-2. This fluence is larger than that of 24% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.05 s in the 15-150 keV band is 2.3 ± 0.3 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/1095288/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Ai et al. (GCN Circ. 31703).

We have analysed 6.6 ks of XRT data for GRB 220306B, from 111 s to 24.3 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 31700).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.88 (+0.17, -0.12).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.80 (+0.32, -0.29). The best-fitting absorption column is 7.1 (+2.9, -2.3) x 1021 cm-2, in excess of the Galactic value of 4.1 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 4.9 x 10-11 (7.6 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 7.1 (+2.9, -2.3) x 1021 cm-2
Galactic foreground: 4.1 x 1021 cm-2
Excess significance: 2.2 σ
Photon index: 1.80 (+0.32, -0.29)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01095288.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 220306B 89 s after the BAT trigger (Siegel and Caputo GCN Circ. 31759). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 31700) 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 EB-V of 0.578 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
00h21m33.10s +71°21'49.0" 2.1" XRT-final UKSSDC
00h21m33.10s +71°21'49.0" 2.3" XRT-enhanced Evans et al. GCN Circ. 31700
00h21m31.3s +71°23'04.1" 1.6' BAT-refined Barthelmy et al. GCN Circ. 31707

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 31699 Swift GRB 220306B: Global MASTER-Net
observations report 		MASTER
Optical Hu et al. 31704 BOOTES-1 optical upper limit BOOTES-1 upper limits
Optical Belkin et al. 31705 AbAO optical observations, possible
afterglow candidate		 Abastumani Astro. Obs. possible detection
Optical Klose et al. 31711 Tautenburg observations Tautenburg
Optical Belkin et al. 31714 Mondy optical observations,
confirmation afterglow candidate		 Mondy detection
Optical Fu et al. 31737 Nanshan/NEXT optical upper limit Xinjiang Astro. Obs. upper limits
Gamma-ray Wood 31702 Fermi GBM observation Fermi GBM Epeak=160±122 keV
T90=10.2 seconds
Fluence=1.7±0.2x10-6erg cm-2
(brighter than 26% of long GRBs)

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
whiteFC 89 239 147 >19.9
uFC 302 552 246 >20.1
white 89 1716 402 >20.4
v 631 1601 117 >18.7
b 557 1701 117 >19.9
u 302 1676 343 >20.2
w1 680 1650 117 >19.3
m2 656 1626 117 >19.4
w2 607 5204 213 >19.4

Table 3. UVOT observations reported by Siegel and Caputo (GCN Circ. 31759). 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.

March 16, 2022