Swift Observations of GRB 220611A

S.B. Cenko (GSFC), J.P. Osborne (U. Leicester) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 18:01:51 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220611A (trigger=1110090) (Cenko et al. GCN Circ. 32191). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 62° from the Sun (0.9 hours West) and 122° from the 89%-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.

Schneider et al. (GCN Circ. 32595) determined a redshift of 2.3608 from VLT. 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 Markwardt et al. (GCN Circ. 32197), the BAT ground-calculated position is RA, Dec = 66.505, -37.265 deg which is RA(J2000) = 04h26m01.2s Dec(J2000) = -37°15'54.2" with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 70%.

The mask-weighted light curve (Figure 1) shows a weak pulse that starts at ~T0 and ends at ~T+60 s. There might be a possible precursor emission (~ 5 sigma in the 15-350 keV image) from ~T-90 s to ~T-85 s. T90 (15-350 keV) is 57.00 ± 12.37 s (estimated error including systematics).

The time-averaged spectrum from T+0.52 to T+61.52 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.87 ± 0.29. The fluence in the 15-150 keV band is 6.7 ± 1.3 x 10-7 erg cm-2. This fluence is larger than that of 29% 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/1110090/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Osborne et al. (GCN Circ. 32194). We have analysed 67 ks of XRT data for GRB 220611A, from 133 s to 968.9 ks after the BAT trigger. The data comprise 270 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. 32192).

The late-time light curve (Figure 2) (from T0+5.3 ks) can be modelled with an initial power-law decay with an index of α=0.48 (+0.06, -0.07), followed by a break at T+238 ks to an α of 2.4 (+0.5, -0.4).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.31 ± 0.06. The best-fitting absorption column is 1.04 (+0.13, -0.12) x 1021 cm-2, in excess of the Galactic value of 2.7 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.90 (+0.13, -0.07) and a best-fitting absorption column consistent with the Galactic value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.3 x 10-11 (3.5 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.7 (+1.6, -0.0) x 1020 cm-2
Galactic foreground: 2.7 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.90 (+0.13, -0.07)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01110090.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 220611A 153 s after the BAT trigger (Siegel and Cenko GCN Circ. 32201). No optical afterglow consistent with the XRT position 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 0.033 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
04h26m03.51s -37°15'35.6" 2.0" XRT-final UKSSDC
04h26m03.51s -37°15'36.7" 2.4" XRT-enhanced Goad et al. GCN Circ. 32192
04h26m01.2s -37°15'54.2" 2.1' BAT-refined Markwardt et al. GCN Circ. 32197

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 32195 Swift GRB 220611A: Global MASTER-Net
observations report 		MASTER
Optical O'Connor et al. 32203 Gemini South Infrared Detection Gemini detection
Optical Rastinejad et al. 32208 Gemini Ks counterpart detection Gemini detection
Optical O’Connor et al. 32217 Gemini Optical Afterglow Confirmation Gemini detection
Optical O'Connor et al. 32228 Continued Gemini Infrared Observations Gemini detection
Optical Schneider et al. 32595 Host galaxy redshift from VLT/X-shooter VLT redshift
X-ray Levan et al. 32262 X-ray afterglow Chandra sub-arcsecond
localization		 Chandra

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 153 302 147 >19.5
uFC 311 561 246 >19.1
white 153 926 245 >19.5
v 641 835 39 >17.0
b 566 759 39 >18.2
u 311 734 265 >19.1
w1 690 710 19 >17.7
m2 665 685 19 >17.9
w2 616 636 19 >18.7

Table 3. UVOT observations reported by Siegel and Cenko (GCN Circ. 32201). 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 28, 2022