Swift Observations of GRB 220706A

A. D'Ai (INAF-IASFPA), J.A. Kennea (PSU) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

At 16:09:14 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220706A (trigger=1114937) (D'Ai et al. GCN Circ. 32329). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 110° from the Sun (7.0 hours West) and 161° from the 46%-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 Stamatikos et al. (GCN Circ. 32348), the BAT ground-calculated position is RA, Dec = 0.577, -16.411 deg which is RA(J2000) = 00h02m18.5s Dec(J2000) = -16°24'38.8" with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment). The partial coding was 73%.

The BAT mask-weighted light curve (Figure 1) shows a bright peak starting from T-3 s to T+30 s. The weak emission is evident in the 10-s binned light curve starting from T-80 s to T+70 s. T90 (15-350 keV) is 86.9 ± 18.0 s (estimated error including systematics).

The time-averaged spectrum from T-71.84 to T+36.15 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.75 ± 0.17. The fluence in the 15-150 keV band is 1.4 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 49% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+3.72 s in the 15-150 keV band is 0.9 ± 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/1114937/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Kennea et al. (GCN Circ. 32340). We have analysed 11 ks of XRT data for GRB 220706A, from 87 s to 201.4 ks after the BAT trigger. The data comprise 590 s in Windowed Timing (WT) mode (the first 8 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. 32330).

The late-time light curve (Figure 2) (from T0+4.4 ks) can be modelled with a power-law decay with a decay index of α=1.21 (+0.23, -0.21).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.35 (+0.15, -0.14). The best-fitting absorption column is 6.0 ± 0.8 x 1021 cm-2, in excess of the Galactic value of 2.4 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 3.5 x 10-11 (7.6 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 6.0 ± 0.8 x 1021 cm-2
Galactic foreground: 2.4 x 1020 cm-2
Excess significance: 12.6 σ
Photon index: 2.35 (+0.15, -0.14)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01114937.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 220706A 107 s after the BAT trigger (LaPorte and D'Ai GCN Circ. 32351). No optical afterglow consistent with the XRT position (Goad et al. GCN Circ. 32333) 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 0.025 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
00h02m25.06s -16°24'39.7" 1.9" XRT-final UKSSDC
00h02m25.03s -16°24'40.4" 2.2" XRT-enhanced Goad et al. GCN Circ. 32333
00h02m18.5s -16°24'38.8" 1.8' BAT-refined Stamatikos et al. GCN Circ. 32348

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Gendre et al. 32331 Zadko observatory - Gingin optical
observations		 Zadko
Optical Lipunov et al. 32332 Swift GRB 220706A: Global MASTER-Net
observations report 		MASTER
Optical Lipunov et al. 32334 MAXI GRB220706.73: Global MASTER-Net
observations report 		MASTER
Optical Jiang et al. 32335 Nanshan/NEXT optical upper limits Xinjiang Astro. Obs. upper limits
Optical de Ugarte Postigo et al. 32338 Optical observations from NOT NOT
Optical Guelbenzu et al. 32339 GROND observations GROND upper limits
Optical Strausbaugh and Cucchiara 32342 LCO Optical Upper Limits LCO upper limits
X-ray Serino et al. 32337 MAXI/GSC detection 84 minutes after the
Swift Trigger		 MAXI detection

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 107 257 147 >20.5
uFC 320 570 246 >20.0
white 107 1025 334 >20.5
b 751 771 19 >18.5
u 320 570 246 >20.0
w1 701 720 19 >17.8
m2 849 869 19 >18.6

Table 3. UVOT observations reported by LaPorte and D'Ai (GCN Circ. 32351). 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.

July 9, 2022