Swift Observations of GRB 230322B

S.B. Cenko (GSFC), J.D. Gropp (PSU) and S.R. Oates (U. of Birmingham) for the Swift team

1. Introduction

At 21:10:37 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 230322B (trigger=1161127) (Cenko et al. GCN Circ. 33505). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 50° from the Sun (1.0 hours East) and 53° from the 2%-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.

Cenko et al. (GCN Circ. 33505) reported the discovery with UVOT of an optical afterglow. 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 Laha et al. (GCN Circ. 33519), the BAT ground-calculated position is RA, Dec = 16.550, -47.709 deg which is RA(J2000) = 01h06m12.1s Dec(J2000) = -47°42'31.2" with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 92%.

The mask-weighted light curve (Figure 1) shows a single-peaked FRED-like structure that starts at ~T-3 s, peaks at ~T0, and ends at ~T+11 s. T90 (15-350 keV) is 11.61 ± 2.17 s (estimated error including systematics).

The time-averaged spectrum from T-2.41 to T+11.12 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.07 ± 0.26. The fluence in the 15-150 keV band is 3.7 ± 0.6 x 10-7 erg cm-2. This fluence is larger than that of 16% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.02 s in the 15-150 keV band is 1.1 ± 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/1161127/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Gropp et al. (GCN Circ. 33508). We have analysed 22 ks of XRT data for GRB 230322B, from 97 s to 264.3 ks after the BAT trigger. The data comprise 10 s in Windowed Timing (WT) mode (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. 33507).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.72 ± 0.03.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.00 (+0.19, -0.18). The best-fitting absorption column is 7.6 (+4.8, -4.2) x 1020 cm-2, in excess of the Galactic value of 2.5 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 (3.8 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 7.6 (+4.8, -4.2) x 1020 cm-2
Galactic foreground: 2.5 x 1020 cm-2
Excess significance: 2.0 σ
Photon index: 2.00 (+0.19, -0.18)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01161127.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 230322B 119 s after the BAT trigger (Oates and Cenko GCN Circ. 33512). A source consistent with the XRT position (Evans et al. GCN Circ. 33507) 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 0.017 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
01h06m12.84s -47°42'02.8" 0.43" UVOT-refined Oates and Cenko GCN Circ. 33512
01h06m12.83s -47°42'02.8" 2.0" XRT-final UKSSDC
01h06m12.80s -47°42'02.8" 2.3" XRT-enhanced Evans et al. GCN Circ. 33507
01h06m12.1s -47°42'31.2" 1.7' BAT-refined Laha et al. GCN Circ. 33519

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 33506 Swift GRB 230322B: Global MASTER-Net
observations report 		MASTER

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
white 119 269 147 17.60 ± 0.04
v 661 1061 58 >18.3
b 758 778 19 18.55 ± 0.36
u 332 581 246 17.63 ± 0.08
w1 710 1095 19 16.90 ± 0.28
m2 1064 1084 19 17.17 ± 0.35
w2 635 655 19 17.36 ± 0.32

Table 3. UVOT observations reported by Oates and Cenko (GCN Circ. 33512). The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary detections and 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.

March 26, 2023