Swift Observations of GRB 220907A

K.L. Page (U. Leicester) and S.R. Oates (U. Birmingham) for the Swift team

1. Introduction

At 14:05:25 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220907A (trigger=1123129) (Page et al. GCN Circ. 32526). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 104° from the Sun (6.9 hours East) and 38° 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.

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 90%. The mask-weighted light curve (Figure 1) shows a single peak of ten seconds approximate duration starting at T-1 s. A spacecraft maneuver took the source out of the BAT field of view at approximately 400 seconds. T₉₀ (15-350 keV) is 8.49 ± 1.56 s (estimated error including systematics). The time-averaged spectrum from T-0.60 to T+10.29 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 0.99 ± 0.24. The fluence in the 15-150 keV band is 4.1 ± 0.6 x 10^-7 erg cm^-2. The 1-s peak photon flux measured from T+0.70 s in the 15-150 keV band is 1.0 ± 0.2 ph cm^-2 s^-1. This fluence is larger than that of 18% 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/1123129/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Page (GCN Circ. 32530).

We have analysed 5.6 ks of XRT data for GRB 220907A, from 100 s to 21.5 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The source is fading with α >0.4.

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01123129.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 220907A 95 s after the BAT trigger (Oates and Page GCN Circ. 32531). No optical afterglow consistent with the enhanced XRT position (Page and Evans GCN Circ. 32530) 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 E_B-V of 1.637 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

17^h55^m29.56^s -20°17'22.2" 4.0" XRT-final UKSSDC

17^h55^m29.56^s -20°17'22.2" 4.0" XRT-refined Page GCN Circ. 32530

17^h55^m28.6^s -20°18'10.8" 1.9' BAT-refined Stamatikos et al. GCN Circ. 32537

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lipunov et al. 32524 Swift GRB220907.59: Global MASTER-Net
observations report MASTER

Optical Zhu et al. 32532 Nanshan/NEXT optical upper limits Xinjiang Astro. Obs. upper limits

Gamma-ray Fermi 32525 Fermi GBM Final Real-time Localization Fermi GBM

Gamma-ray Dalessi et al. 32533 Fermi GBM observation Fermi GBM E_peak=320±170 keV
T₉₀=9 seconds
Fluence=1.13±0.12x10^-6erg cm^-2
(brighter than 17% of long GRBs)

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

Filter T_start(s) T_stop(s) Exp(s) Mag

white_FC 95 245 147 >20.2

white 95 5001 298 >20.6

v 3823 4023 197 >18.6

b 4644 4844 197 >19.7

u 4438 4638 197 >19.4

w1 4234 4433 197 >19.2

m2 4028 4228 197 >19.2

w2 3618 3818 197 >19.2

RA (J2000)	Dec (J2000)	Error	Note	Reference
17^h55^m29.56^s	-20°17'22.2"	4.0"	XRT-final	UKSSDC
17^h55^m29.56^s	-20°17'22.2"	4.0"	XRT-refined	Page GCN Circ. 32530
17^h55^m28.6^s	-20°18'10.8"	1.9'	BAT-refined	Stamatikos et al. GCN Circ. 32537

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lipunov et al.	32524	Swift GRB220907.59: Global MASTER-Net observations report	MASTER
Optical	Zhu et al.	32532	Nanshan/NEXT optical upper limits	Xinjiang Astro. Obs.	upper limits
Gamma-ray	Fermi	32525	Fermi GBM Final Real-time Localization	Fermi GBM
Gamma-ray	Dalessi et al.	32533	Fermi GBM observation	Fermi GBM	E_peak=320±170 keV T₉₀=9 seconds Fluence=1.13±0.12x10^-6erg cm^-2 (brighter than 17% of long GRBs)

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white_FC	95	245	147	>20.2
white	95	5001	298	>20.6
v	3823	4023	197	>18.6
b	4644	4844	197	>19.7
u	4438	4638	197	>19.4
w1	4234	4433	197	>19.2
m2	4028	4228	197	>19.2
w2	3618	3818	197	>19.2

Table 3. UVOT observations reported by Oates and Page (GCN Circ. 32531). 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 9, 2022