Swift Observations of GRB 220715B

E. Troja (NASA/GSFC/UMCP), V. D'Elia (SSDC and INAF-OAR) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 22:24:42 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220715B (trigger=1116441) (Troja et al. GCN Circ. 32405). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 143° from the Sun (9.3 hours East) and 66° from the 93%-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

The partial coding was 95%. The mask-weighted light curve (Figure 1) shows an initial pulse with a FRED shape, Beginning at just before T+0 and extending to about T+20. There is a second weaker, softer pulse centered just after T+40 s. There are a few gaps in the light curve data, but none appear to correspond to burst activity. T90 (15-350 keV) is 40.40 ± 1.24 s (estimated error including systematics). The time-averaged spectrum from T-0.15 to T+42.17 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.36 ± 0.11. The fluence in the 15-150 keV band is 1.2 ± 0.1 x 10-6 erg cm-2. The 1-s peak photon flux measured from T-0.08 s in the 15-150 keV band is 1.3 ± 0.2 ph cm-2 s-1. This fluence is larger than that of 45% 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/1116441/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Elia et al. (GCN Circ. 32411). We have analysed 15 ks of XRT data for GRB 220715B, from 81 s to 143.7 ks after the BAT trigger. The data comprise 11 s in Windowed Timing (WT) mode (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. 32406).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.81 ± 0.05.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.6 (+0.5, -0.4). The best-fitting absorption column is 10.0 (+4.0, -3.2) x 1021 cm-2, in excess of the Galactic value of 3.6 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 3.7 x 10-11 (1.2 x 10-10) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 10.0 (+4.0, -3.2) x 1021 cm-2
Galactic foreground: 3.6 x 1021 cm-2
Excess significance: 3.3 σ
Photon index: 2.6 (+0.5, -0.4)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01116441.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 220715B 103 s after the BAT trigger (Kuin and Troja GCN Circ. 32410). No optical afterglow consistent with the XRT position (Goad et al. GCN Circ. 32409) 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 0.525 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
16h59m30.09s -33°35'51.7" 2.2" XRT-final UKSSDC
16h59m30.08s -33°35'51.9" 2.5" XRT-enhanced Goad et al. GCN Circ. 32409
16h59m26.5s -33°35'10.3" 1.1' BAT-refined Lien et al. GCN Circ. 32414

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 32407 Swift GRB 220715B: Global MASTER-Net
observations report 		MASTER
Optical Hu et al. 32408 BOOTES-network optical upper limits BOOTES upper limits
Optical Noonan et al. 32422 VIRT Optical Upper Limit Virgin Island Robotic Telescope upper limits
Gamma-ray Fermi 32404 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Roberts et al. 32412 Fermi GBM detection Fermi GBM Epeak=118±28 keV
T90=40 seconds
Fluence=2.2±0.3x10-6erg cm-2
(brighter than 31% of long GRBs)

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 103 253 147 >20.1
uFC 316 565 246 >20.0
white 103 1369 373 >20.4
v 645 1419 97 >18.9
b 571 5005 274 >20.8
u 316 1488 319 >20.1
w1 695 1468 78 >18.8
m2 670 1443 97 >19.8
w2 621 5144 227 >19.9

Table 3. UVOT observations reported by Kuin and Troja (GCN Circ. 32410). 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 24, 2022