Swift Observations of GRB 230217A

M.J. Moss (GWU), M. Capalbi (INAF-IASFPA) and F. T_start (s) for the Swift team

1. Introduction

At 21:53:10 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 230217A (trigger=1154967) (Moss et al. GCN Circ. 33339). Swift did not slew to the burst due to an observing constraint. At the time of the trigger, the initial BAT position was 49° from the Sun (3.3 hours West) and 16° from the 8%-illuminated Moon. Table 1 contains the best reported positions from Swift.

D'Avanzo et al. (GCN Circ. 33374) reported the position from VLT/FORS2 for the optical afterglow of this GRB. 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 Barthelmy et al. (GCN Circ. 33363), the BAT ground-calculated position is RA, Dec = 280.759, -28.840 deg which is RA(J2000) = 18h43m02.2s Dec(J2000) = -28°50'23.0" with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 21%.

The mask-weighted light curve (Figure 1) shows a double-peaked event running from T -0.2 s to about T+1.2 s. T90 (15-350 keV) is 1.30 ± 0.45 s. (estimated error including systematics). The T90 vs. hardness diagram shows the burst well within the short-hard burst region.

The time-averaged spectrum from T-0.26 to T+1.75 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 0.99 ± 0.12. The fluence in the 15-150 keV band is 2.3 ± 0.2 x 10-6 erg cm-2. The 1-s peak photon flux measured from T-0.24 s in the 15-150 keV band is 20.2 ± 1.5 ph cm-2 s-1. This fluence is larger than that of 100% of the short 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 http://gcn.gsfc.nasa.gov/notices_s/1154967/BA/

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/1154967/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Capalbi et al. (GCN Circ. 33365). Of the sources reported by Capalbi et al. (GCN Circ. 33348), "Source 3" is fading with >3-sigma significance, and is therefore likely the GRB afterglow.

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.1 (+0.6, -0.4).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.6 (+0.8, -0.4). The best-fitting absorption column is consistent with the Galactic value of 1.6 x 1021 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.4 x 10-11 (5.1 x 10-11) erg cm-2 count-1.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 230217A 45.8 ks after the Swift/BAT trigger (T_start et al. GCN Circ. 33370). No optical afterglow consistent with the XRT position (Capalbi et al., GCN Circ. 33365) 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 0.15 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
18h43m04.80s -28°50'14.6" 4.7" XRT-final UKSSDC
18h43m04.80s -28°50'14.6" 4.7" XRT Capalbi et al. GCN Circ. 33365
18h43m02.2s -28°50'23.0" 1.3' BAT-refined Barthelmy et al. GCN Circ. 33363

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical O'Connor and Troja 33356 Gemini-South Optical Observation Gemini
Optical Troja and O’Connor 33360 Gemini South optical upper limits Gemini upper limits
Optical Gillanders et al. 33368 Continued Gemini Observations Gemini
Optical D'Avanzo et al. 33374 VLT/FORS2 optical observations and
candidate counterpart		 VLT/FORS2 detection
Radio Schroeder et al. 33358 6 GHz VLA observations VLA
Radio Anderson et al. 33433 ATCA rapid-response radio detection ATCA detection
Gamma-ray Fermi 33338 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Casentini et al. 33343 AGILE/MCAL detection AGILE
Gamma-ray Svinkin et al. 33349 Konus-Wind detection Konus-Wind Epeak=1367 (-240,+309) keV
Duration=~0.9 seconds
Fluence=2.74(-0.35,+0.39)x10-5erg cm-2
Gamma-ray Veres 33353 Fermi GBM observation Fermi GBM Epeak=1497±63 keV
T90=0.90 seconds
Fluence=1.81±0.02x10-5erg cm-2
(brighter than 100% of short GRBs)
Gamma-ray Navaneeth et al. 33361 AstroSat CZTI detection CZTI

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

Filter Tstart(s) Tstop(s) Exp(s) Mag
wh 45886 84347 2643 >21.37
u 329962 342875 4884 >20.78

Table 3. UVOT observations reported by T_start et al. (GCN Circ. 33370). 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 10, 2023