Swift Observations of GRB 230228A

S. Dichiara (PSU), A. Tohuvavohu (U. Toronto) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 05:50:50 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 230228A (trigger=1156572) (Dichiara et al. GCN Circ. 33378). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 63° from the Sun (2.5 hours East) and 50° from the 59%-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

Analysis of the BAT data was reported by Krimm and Barthelmy (GCN Circ. 33393).

The mask-weighted light curve (Figure 1) shows a Fast Rise Exponential Decay single pulse.

The time-averaged spectrum from T-0.05 to T+2.53 s is best fit by a simple

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/1156572/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Tohuvavohu et al. (GCN Circ. 33384). We have analysed 10 ks of XRT data for GRB 230228A, from 102 s to 97.7 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 Beardmore et al. (GCN Circ. 33379).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.94 ± 0.07.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.82 (+0.27, -0.25). The best-fitting absorption column is 2.1 (+1.1, -0.9) x 1021 cm-2, in excess of the Galactic value of 9.7 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 3.9 x 10-11 (5.0 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.1 (+1.1, -0.9) x 1021 cm-2
Galactic foreground: 9.7 x 1020 cm-2
Excess significance: 1.9 σ
Photon index: 1.82 (+0.27, -0.25)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01156572.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 230228A 122 s after the BAT trigger (Breeveld and Dichiara GCN Circ. 33392). No optical afterglow consistent with the enhanced XRT position (Beardmore et al., GCN Circ. 33382) 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 0.085 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
01h13m30.74s +44°28'58.5" 2.1" XRT-final UKSSDC
01h13m30.75s +44°28'58.5" 2.1" XRT-enhanced Beardmore et al. GCN Circ. 33382
01h13m35.3s +44°29'41.9" 1.5' BAT-refined Krimm and Barthelmy GCN Circ. 33393

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lim et al. 33380 Miryang Arirang Astronomical
Observatory (MAAO) 0.7-m telescope
optical upper limit		 Miryang Arirang Astro.Obs. upper limits
Optical Lu et al. 33381 Nanshan/NEXT optical upper limit Xinjiang Astro. Obs. upper limits
Optical Belkin et al. 33383 Mondy optical upper limit Mondy upper limits
Optical Hu et al. 33387 OSN optical upper limit Obs.de Sierra Nevada upper limits
Optical Noonan et al. 33426 VIRT Optical Upper Limit Virgin Island Robotic Telescope upper limits
Optical Komesh et al. 33436 NUTTelA-TAO / BSTI Optical Limits NUTTelA upper limits
Optical Hu et al. 33557 detection by GTC of the potential host
galaxy		 GTC
Radio Schroeder et al. 33394 6 GHz VLA radio upper limit VLA upper limits
Gamma-ray Fletcher et al. 33389 Fermi GBM Observation Fermi GBM Epeak=110±20 keV
T90=2 seconds
Fluence=5.9±0.6x10-7erg cm-2
(brighter than 7% of long GRBs)

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 122 272 147 >20.2
uFC 335 585 246 >19.5
white 122 1706 240 >20.6
v 665 1752 97 >18.8
b 591 1851 88 >19.8
u 335 1830 341 >19.7
w1 714 1806 117 >20.0
m2 689 1780 97 >19.7
w2 640 1731 78 >20.1

Table 3. UVOT observations reported by Breeveld and Dichiara (GCN Circ. 33392). 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.

April 3, 2023