Swift Observations of GRB 230205A

E. Ambrosi (INAF-IASFPA), K.L. Page (U. Leicester) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 10:29:51 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 230205A (trigger=1152764) (Ambrosi et al. GCN Circ. 33271). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 120° from the Sun (7.8 hours West) and 58° from the 100%-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 Sakamoto et al. (GCN Circ. 33280), the BAT ground-calculated position is RA, Dec = 202.057, 46.686 deg which is RA(J2000) = 13h28m13.7s Dec(J2000) = +46°41'08.1" with an uncertainty of 2.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a short pulse that starts at ~T0, peaks at ~T+2 s, and ends at ~T+3 s. In addition, there may be some hint of extended emission that lasts till ~T+80 s. T90 (15-350 keV) is 2.30 ± 0.36 s (estimated error including systematics).

The time-averaged spectrum from T+0.32 to T+2.87 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.44 ± 0.29. The fluence in the 15-150 keV band is 1.6 ± 0.3 x 10-7 erg cm-2. This fluence is larger than that of 4.1% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+1.74 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/1152764/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Page et al. (GCN Circ. 33277).

We have analysed 22 ks of XRT data for GRB 230205A, from 80 s to 275.1 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 Goad et al. (GCN Circ. 33275).

The late-time light curve (Figure 2) (from T0+5.1 ks) can be modelled with a power-law decay with a decay index of α=0.91 (+0.14, -0.13).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.89 (+0.22, -0.21). The best-fitting absorption column is 6.1 (+1.6, -1.4) x 1021 cm-2, in excess of the Galactic value of 2.1 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 4.5 x 10-11 (7.1 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 6.1 (+1.6, -1.4) x 1021 cm-2
Galactic foreground: 2.1 x 1020 cm-2
Excess significance: 7.0 σ
Photon index: 1.89 (+0.22, -0.21)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01152764.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 230205A 100 s after the BAT trigger (Siegel and Ambrosi GCN Circ. 33289). No optical afterglow consistent with the XRT position (Goad et al. GCN Circ. 33275) 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 0.036 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
13h28m17.01s +46°43'31.1" 1.9" XRT-final UKSSDC
13h28m17.03s +46°43'30.7" 1.9" XRT-enhanced Goad et al. GCN Circ. 33275
13h28m13.7s +46°41'08.1" 2.3' BAT-refined Sakamoto et al. GCN Circ. 33280

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Hu et al. 33274 BOOTES-5/JGT early optical upper limit BOOTES upper limits
Optical Turpin et al. 33282 OHP/T193 optical observations Obs.de Haute-Provence
Optical Wu et al. 33283 SVOM/C-GFT optical upper limit C-GFT upper limits
Optical Strausbaugh and Cucchiara 33286 LCOGT Optical Upper Limits Las Cumbres Obs. Global Tele.Network upper limits
Optical Urata 33307 SUBARU HSC pre-imaging for host galaxy
search		 SUBARU
Optical O'Connor et al. 33334 Lowell Discovery Telescope Observations Lowell Discovery Telescope
Radio Schroeder et al. 33309 6 GHz VLA observations VLA
Radio Schroeder and Laskar 33372 VLA radio afterglow confirmation VLA 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 100 250 147 >21.3
uFC 258 508 246 >20.3
white 100 5198 526 >21.5
v 588 1581 117 >19.5
b 514 1680 117 >20.1
u 258 1481 343 >20.2
w1 638 657 19 >19.2

Table 3. UVOT observations reported by Siegel and Ambrosi (GCN Circ. 33289). 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.

September 9, 2023