Swift Observations of GRB 230123A

M.A. Williams (PSU), S. Dichiara (PSU) and A. Belles (PSU) for the Swift team

1. Introduction

At 10:19:44 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 230123A (trigger=1150429) (Williams et al. GCN Circ. 33206). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 75° from the Sun (6.4 hours East) and 67° from the 4%-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 Palmer et al. (GCN Circ. 33214), the BAT ground-calculated position is RA, Dec = 40.905, -65.661 deg which is RA(J2000) = 02h43m37.3s Dec(J2000) = -65°39'39.9" with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment). The partial coding was 88%.

The mask-weighted light curve (Figure 1) shows a single-peaked structure that starts at ~T-2 s, peaks at ~T+1, and ends at ~T+6 s. T90 (15-350 keV) is 5.37 ± 1.03 s (estimated error including systematics).

The time-averaged spectrum from T-1.25 to T+4.75 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.06 ± 0.24. The fluence in the 15-150 keV band is 2.7 ± 0.4 x 10-7 erg cm-2. This fluence is larger than that of 11% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.12 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/1150429/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Dichiara et al. (GCN Circ. 33210). We have analysed 10 ks of XRT data for GRB 230123A, from 73 s to 45.8 ks after the BAT trigger. The data comprise 8 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 Beardmore et al. (GCN Circ. 33209).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.71 (+0.07, -0.06).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.2 (+0.4, -0.3). The best-fitting absorption column is 1.5 (+1.0, -0.8) x 1021 cm-2, in excess of the Galactic value of 3.0 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 3.1 x 10-11 (4.4 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.5 (+1.0, -0.8) x 1021 cm-2
Galactic foreground: 3.0 x 1020 cm-2
Excess significance: 2.4 σ
Photon index: 2.2 (+0.4, -0.3)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01150429.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 230123A 93 s after the BAT trigger (Belles and Williams GCN Circ. 33211). No optical afterglow consistent with the XRT position (Beardmore et al. GCN Circ. 33209) 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 0.022 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
02h43m25.84s -65°41'10.2" 2.2" XRT-final UKSSDC
02h43m26.02s -65°41'11.3" 4.1" XRT-enhanced Beardmore et al. GCN Circ. 33209
02h43m37.3s -65°39'39.9" 1.8' BAT-refined Palmer et al. GCN Circ. 33214

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Gendre et al. 33208 Zadko observatory - Gingin optical
observations		 Zadko
Optical Strausbaugh and Cucchiara 33217 LCOGT Optical Upper Limits Las Cumbres Obs. Global Tele.Network upper limits

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 93 243 147 >20.6
uFC 305 555 246 >20.6
white 93 1704 412 >21.0
v 637 1754 136 >19.4
b 562 1679 117 >20.0
u 305 1825 359 >20.5
w1 686 1804 117 >20.8
m2 1412 1605 39 >18.7
w2 612 1209 78 >20.1

Table 3. UVOT observations reported by Belles and Williams (GCN Circ. 33211). 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.

January 25, 2023