Swift Observations of GRB 220623A

B. Sbarufatti (PSU), V. D'Elia (SSDC and INAF-OAR) and A. Belles (PSU) for the Swift team

1. Introduction

At 07:04:14 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220623A (trigger=1111672) (Sbarufatti et al. GCN Circ. 32243). Swift did not slew immediately to the burst. At the time of the trigger, the initial BAT position was 59° from the Sun (3.6 hours East) and 89° from the 29%-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 Barthelmy et al. (GCN Circ. 32264), the BAT ground-calculated position is RA, Dec = 145.459, 75.823 deg which is RA(J2000) = 09h41m50.1s Dec(J2000) = +75°49'22.9" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 94%.

The BAT light curve (Figure 1) showed a triple-peaked structure. One possible precursor peak occurred prior to the trigger, another occurred at the trigger time, and the final occurred 10 s post trigger. At ~40 s there may be broad emission as well. T90 (15-350 keV) is 57.11 ± 8.53 s (estimated error including systematics).

The time-averaged spectrum from -3.84 to 73.652 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.33 ± 0.07. The fluence in the 15-150 keV band is 3.5 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 74% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.15 s in the 15-150 keV band is 4.5 ± 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/1111672/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Elia et al. (GCN Circ. 32254). We have analysed 10 ks of XRT data for GRB 220623A, from 4.0 ks to 68.4 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 32248).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.81 (+0.13, -0.12).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.92 (+0.19, -0.18). The best-fitting absorption column is 2.0 (+0.7, -0.6) x 1021 cm-2, in excess of the Galactic value of 2.5 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 3.7 x 10-11 (4.8 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.0 (+0.7, -0.6) x 1021 cm-2
Galactic foreground: 2.5 x 1020 cm-2
Excess significance: 4.7 σ
Photon index: 1.92 (+0.19, -0.18)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01111672.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 220623A 4034 s after the BAT trigger (Belles and Sbarufatti GCN Circ. 32252). No optical afterglow consistent with the XRT position (Kennea et al. GCN Circ. 32244) 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
09h41m34.51s +75°49'15.6" 2.0" XRT-final UKSSDC
09h41m34.50s +75°49'15.6" 2.1" XRT-enhanced Beardmore et al. GCN Circ. 32248
09h41m50.1s +75°49'22.9" 1.0' BAT-refined Barthelmy et al. GCN Circ. 32264

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical 32245 iTelescope optical upper limit iTelescope upper limits
Optical Hu et al. 32251 BOOTES-5/JGT optical afterglow detection BOOTES detection
Optical Malesani et al. 32253 NOT optical observations NOT
Optical Pankov et al. 32255 AbAO optical upper limit Abastumani Astro. Obs. upper limits
Optical Schneider et al. 32271 T193-OHP optical upper limit upper limits
Optical Swain et al. 32273 GIT optical upper limits. GROWTH-India Tel. upper limits
Gamma-ray Gopalakrishnan et al. 32246 AstroSat CZTI detection CZTI
Gamma-ray Ursi et al. 32247 AGILE detection AGILE
Gamma-ray Frederiks et al. 32258 Konus-Wind detection Konus-Wind Epeak=869 (-224,+363) keV

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 4034 4184 147 >19.9
white 4034 5415 344 >20.1
v 4190 9642 326 >18.9
b 5010 5210 197 >19.4
u 4805 5005 197 >19.0
w1 4600 4800 197 >19.3
m2 4395 4595 197 >19.9
w2 5421 5621 197 >19.6

Table 3. UVOT observations reported by Belles and Sbarufatti (GCN Circ. 32252). 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.

June 28, 2022