Swift Observations of GRB 220714B

S. Laha (GSFC/UMBC), M. Perri (SSDC and INAF-OAR) and F.E. Marshall (NASA/GSFC) for the Swift team

1. Introduction

At 13:58:04 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220714B (trigger=1116221) (Laha et al. GCN Circ. 32393). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 77° from the Sun (4.4 hours West) and 91° from the 99%-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.

Kuin and Laha (GCN Circ. 32413) reported the detection with UVOT of an optical afterglow. 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 Laha et al. (GCN Circ. 32403), the BAT ground-calculated position is RA, Dec = 47.086, -19.321 deg which is RA(J2000) = 03h08m20.7s Dec(J2000) = -19°19'13.8" with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 88%.

The mask-weighted light curve (Figure 1) shows a complex structure. The main period of emission starts around T-5 s and continues to T+35 seconds with three main overlapping pulses. There is some precursor activity starting as early as T-45 seconds and low-level emission extending to T+60 seconds. The spacecraft slewed away from the burst location at around T+275 seconds. T90 (15-350 keV) is 50.10 ± 7.96 s (estimated error including systematics).

The time-averaged spectrum from -48.27 to +45.75 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.92 ± 0.07. The fluence in the 15-150 keV band is 4.4 ± 0.1 x 10-6 erg cm-2. The 1-s peak photon flux measured from T+12.89 s in the 15-150 keV band is 3.8 ± 0.2 ph cm-2 s-1. This fluence is larger than that of 80% 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/1116221/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Perri et al. (GCN Circ. 32398). We have analysed 25 ks of XRT data for GRB 220714B, from 71 s to 354.7 ks after the BAT trigger. The data comprise 135 s in Windowed Timing (WT) mode (the first 4 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 Evans et al. (GCN Circ. 32395).

The late-time light curve (Figure 2) (from T0+3.5 ks) can be modelled with a power-law decay with a decay index of α=1.260 ± 0.026.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 3.95 (+0.19, -0.18). The best-fitting absorption column is 2.7 ± 0.3 x 1021 cm-2, in excess of the Galactic value of 3.2 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.03 ± 0.10 and a best-fitting absorption column of 1.47 (+0.28, -0.27) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.4 x 10-11 (4.4 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.47 (+0.28, -0.27) x 1021 cm-2
Galactic foreground: 3.2 x 1020 cm-2
Excess significance: 7.0 σ
Photon index: 2.03 ± 0.10

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01116221.

4. UVOT Observations and Analysis

Analysis of the UVOT data was reported by Marshall and Laha (GCN Circ. 32396). UVOT took a finding chart exposure of 150 seconds with the White filter starting 3552 seconds after the BAT trigger. 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 0.032 mag. in the direction of the GRB (Schlegel et al. 1998).

The Swift/UVOT began settled observations (Kuin and Laha GCN Circ. 32413) of the field of GRB 220714B 3628 s after the BAT trigger and detected a bright afterglow (Marshall & Laha, GCN Circ. 32396). Marshal and Laha provided the UVOT position of GRB 220714B. 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 0.032 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
03h08m20.18s -19°19'45.3" 0.61" UVOT GCN Circ. 32396
03h08m20.13s -19°19'46.6" 1.9" XRT-final UKSSDC
03h08m20.13s -19°19'46.6" 2.1" XRT-enhanced Evans et al. GCN Circ. 32395
03h08m20.7s -19°19'13.8" 1.3' BAT-refined Laha et al. GCN Circ. 32403

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Strausbaugh and Cucchiara 32400 LCO Optical Observations LCO detection
Gamma-ray Fermi 32392 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Dalessi et al. 32402 Fermi GBM observation Fermi GBM Epeak=139±24 keV
T90=28 seconds
Fluence=8.0±0.4x10-6erg cm-2
(brighter than 69% of long GRBs)

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

Table 3. Found no UVOT observations.

July 19, 2022