Swift Observations of GRB 230204A

A. D'Ai (INAF-IASFPA), J.P. Osborne (U. Leicester) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

At 02:56:57 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 230204A (trigger=1152509) (D'Ai et al. GCN Circ. 33260). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 70° from the Sun (5.3 hours West) and 127° 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 Parsotan et al. (GCN Circ. 33279), the BAT ground-calculated position is RA, Dec = 238.166, -50.848 deg which is RA(J2000) = 15h52m39.8s Dec(J2000) = -50°50'52.8" with an uncertainty of 2.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 48%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T-10 and ends at ~T+200 s. The most prominent peak occurs at ~T+148 s. T90 (15-350 keV) is 182.05 ± 16.73 s (estimated error including systematics).

The time-averaged spectrum from T-6.81 to T+195.62 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.72 ± 0.13. The fluence in the 15-150 keV band is 3.7 ± 0.3 x 10-6 erg cm-2. This fluence is larger than that of 76% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+148.12 s in the 15-150 keV band is 0.9 ± 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/1152509/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Osborne et al. (GCN Circ. 33264). We have analysed 7.0 ks of XRT data for GRB 230204A, from 136 s to 52.2 ks after the BAT trigger. The data comprise 250 s in Windowed Timing (WT) mode (the first 8 s were 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. 33261).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=2.37 (+0.17, -0.14).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.64 ± 0.10. The best-fitting absorption column is 2.61 (+0.21, -0.20) x 1022 cm-2, in excess of the Galactic value of 1.0 x 1022 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.5 ± 0.4 and a best-fitting absorption column of 2.1 (+0.7, -0.6) x 1022 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 7.6 x 10-11 (1.2 x 10-10) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.1 (+0.7, -0.6) x 1022 cm-2
Galactic foreground: 1.0 x 1022 cm-2
Excess significance: 2.8 σ
Photon index: 1.5 ± 0.4

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01152509.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 230204A 156 s after the BAT trigger (LaPorte and D'Ai GCN Circ. 33266). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 33261) 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 2.516 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
15h52m47.91s -50°55'00.2" 2.0" XRT-final UKSSDC
15h52m47.91s -50°54'59.5" 2.2" XRT-enhanced Evans et al. GCN Circ. 33261
15h52m39.8s -50°50'52.8" 2.4' BAT-refined Parsotan et al. GCN Circ. 33279

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Strausbaugh and Cucchiara 33263 Swift J1552.8-5055: LCOGT Optical
Counterpart Candidate		 Las Cumbres Obs. Global Tele.Network detection

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 156 306 147 >17.5
uFC 314 564 246 >19.1
white 156 1372 373 >17.8
v 645 1596 117 >15.9
b 570 1521 97 >17.9
u 314 5553 382 >19.1
w1 695 5489 274 >19.1
m2 670 1446 97 >18.8
w2 621 1571 117 >18.5

Table 3. UVOT observations reported by LaPorte and D'Ai (GCN Circ. 33266). 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.

February 6, 2023