Swift Observations of GRB 230325A

M.J. Moss (GWU), K.L. Page (U. Leicester) and N.P. Kuin (UCL/MSSL) for the Swift team

1. Introduction

At 03:15:35 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 230325A (trigger=1161390) (Moss et al. GCN Circ. 33516). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 75° from the Sun (4.4 hours West) and 116° from the 14%-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.

Pieterse et al. (GCN Circ. 33521) determined a redshift of 1.664 from X-shooter. 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 Lien et al. (GCN Circ. 33523), the BAT ground-calculated position is RA, Dec = 296.854, -46.067 deg which is RA(J2000) = 19h47m24.9s Dec(J2000) = -46°03'59.8" with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment). The partial coding was 48%.

The mask-weighted light curve (Figure 1) shows a main pulse with a FRED shape, from approximately T+0 to T+8 seconds. This is preceded by some precursor emission starting around T-10 s, and some low-level emission out to about T+40 s. The spacecraft slewed away from the burst location around T+620 seconds. T90 (15-350 keV) is 38.05 ± 11.56 s (estimated error including systematics).

The time-averaged spectrum from T-8.55 to T+41.70 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.82 ± 0.18. The fluence in the 15-150 keV band is 1.3 ± 0.2 x 10-6 erg cm-2. The 1-s peak photon flux measured from T-0.07 s in the 15-150 keV band is 3.3 ± 0.4 ph cm-2 s-1. This fluence is larger than that of 47% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). 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/1161390/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Page et al. (GCN Circ. 33520). We have analysed 14 ks of XRT data for GRB 230325A, from 83 s to 119.0 ks after the BAT trigger. The data comprise 139 s in Windowed Timing (WT) mode (the first 10 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 Goad et al. (GCN Circ. 33517).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=1.1 ± 0.5. At T+122 s the decay steepens to an α of 3.23 (+0.28, -0.15) before breaking again at T+548 s to a final decay with index α=0.53 ± 0.08.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.06 ± 0.09. The best-fitting absorption column is 2.3 ± 0.4 x 1022 cm-2, at a redshift of 1.664, in addition to the Galactic value of 4.9 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.09 (+0.21, -0.20) and a best-fitting absorption column of 1.7 (+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 3.4 x 10-11 (4.8 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 4.9 x 1020 cm-2
Intrinsic column: 1.7 (+0.7, -0.6) x 1022 cm-2 at z=1.664
Photon index: 2.09 (+0.21, -0.20)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01161390.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 230325A 102 s after the BAT trigger (Kuin and Moss GCN Circ. 33522). No optical afterglow consistent with the XRT position (Goad et al. GCN Circ. 33518) 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 0.046 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
19h47m23.94s -46°04'33.9" 2.1" XRT-final UKSSDC
19h47m23.96s -46°04'34.1" 2.3" XRT-enhanced Goad et al. GCN Circ. 33518
19h47m24.9s -46°03'59.8" 1.5' BAT-refined Lien et al. GCN Circ. 33523

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Pieterse et al. 33521 X-shooter optical afterglow and redshift X-shooter redshift

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 102 252 147 >20.1
uFC 314 540 222 >19.2
white 102 4222 344 >20.8
v 4434 4634 197 >18.9
b 3818 4018 197 >20.0
u 314 5106 277 >19.3
w1 4845 5045 197 >19.9
m2 4640 4839 197 >19.8
w2 4230 4429 197 >20.2

Table 3. UVOT observations reported by Kuin and Moss (GCN Circ. 33522). 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 27, 2023