Swift Observations of GRB 210807A

A.Y. Lien (GSFC/UMBC), A.P. Beardmore (U. Leicester) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

At 10:03:40 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210807A (trigger=1064221) (Lien et al. GCN Circ. 30600). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 61° from the Sun (4.0 hours West) and 48° from the 1%-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.

Lien et al. (GCN Circ. 30600) reported the discovery 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 Ukwatta et al. (GCN Circ. 30623), the BAT ground-calculated position is RA, Dec = 76.666, 58.257 deg which is RA(J2000) = 05^h06^m39.7^s Dec(J2000) = +58°15'25.6" with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment). The partial coding was 28%.

The BAT light curve (Figure 1) showed a complex structure with a duration of about ~200 s. T₉₀ (15-350 keV) is 156.30 ± 214.60 s (estimated error including systematics).

The time-averaged spectrum from -19.544 to 640.052 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.26 ± 0.08. The fluence in the 15-150 keV band is 9.5 ± 0.4 x 10^-6 erg cm^-2. This fluence is larger than that of 92.2% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+70.52 s in the 15-150 keV band is 1.8 ± 0.4 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/1064221/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Beardmore et al. (GCN Circ. 30617). We have analysed 26 ks of XRT data for GRB 210807A, from 208 s to 476.1 ks after the BAT trigger. The data comprise 488 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 30603).

The late-time light curve (Figure 2) (from T0+5.5 ks) can be modelled with a power-law decay with a decay index of α=1.60 ± 0.04.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.82 ± 0.03. The best-fitting absorption column is 5.52 (+0.24, -0.23) x 10²¹ cm^-2, in excess of the Galactic value of 3.4 x 10²¹ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.71 (+0.15, -0.12) 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 4.6 x 10^-11 (5.9 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 3.4 (+0.6, -0.0) x 10²¹ cm^-2
Galactic foreground: 3.4 x 10²¹ cm^-2
Excess significance: <1.6 σ
Photon index: 1.71 (+0.15, -0.12)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01064221.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210807A 210 s after the BAT trigger (LaPorte and Lien GCN Circ. 30618). A source 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 E_B-V of 0.409 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

05^h06^m43.89^s +58°14'59.2" 0.43" UVOT-refined LaPorte and Lien GCN Circ. 30618

05^h06^m44.17^s +58°14'59.1" 2.0" XRT-final UKSSDC

05^h06^m44.10^s +58°14'59.3" 2.1" XRT-enhanced Evans et al. GCN Circ. 30603

05^h06^m39.7^s +58°15'25.6" 1.5' BAT-refined Ukwatta et al. GCN Circ. 30623

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Hu et al. 30602 BOOTES-5/JGT optical afterglow detection BOOTES detection

Optical Lipunov et al. 30605 Swift GRB210807.78: Global MASTER-Net
observations report MASTER

Optical Lipunov et al. 30606 Swift GRB210807.79: Global MASTER-Net
observations report MASTER

Optical Lipunov et al. 30607 Swift GRB 210807A: Global MASTER-Net
observations report MASTER

Optical Lipunov et al. 30611 Swift GRB210807.96: Global MASTER-Net
observations report MASTER

Optical Lipunov et al. 30613 Swift GRB210807.74: Global MASTER-Net
observations report MASTER

Optical Kim et al. 30616 correction to the GCN Circ. 30614 Assy
optical observations Assy-Turgen Obs.

Optical Schmidl et al. 30628 Tautenburg observations Tautenburg

Optical Belkin et al. 30637 continued Assy, AbAO and Mondy
observations Abastumani Astro. Obs. detection

Gamma-ray Ripa et al. 30624 Detection by GRBAlpha GRBAlpha

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

b 489 683 38 >17.84

u 639 659 19 >17.04

v 390 583 39

uvw1 440 633 38 >17.12

uvw2 539 721 26 >17.08

white 234 383 147

white 234 708 184 17.19± 0.05

RA (J2000)	Dec (J2000)	Error	Note	Reference
05^h06^m43.89^s	+58°14'59.2"	0.43"	UVOT-refined	LaPorte and Lien GCN Circ. 30618
05^h06^m44.17^s	+58°14'59.1"	2.0"	XRT-final	UKSSDC
05^h06^m44.10^s	+58°14'59.3"	2.1"	XRT-enhanced	Evans et al. GCN Circ. 30603
05^h06^m39.7^s	+58°15'25.6"	1.5'	BAT-refined	Ukwatta et al. GCN Circ. 30623

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Hu et al.	30602	BOOTES-5/JGT optical afterglow detection	BOOTES	detection
Optical	Lipunov et al.	30605	Swift GRB210807.78: Global MASTER-Net observations report	MASTER
Optical	Lipunov et al.	30606	Swift GRB210807.79: Global MASTER-Net observations report	MASTER
Optical	Lipunov et al.	30607	Swift GRB 210807A: Global MASTER-Net observations report	MASTER
Optical	Lipunov et al.	30611	Swift GRB210807.96: Global MASTER-Net observations report	MASTER
Optical	Lipunov et al.	30613	Swift GRB210807.74: Global MASTER-Net observations report	MASTER
Optical	Kim et al.	30616	correction to the GCN Circ. 30614 Assy optical observations	Assy-Turgen Obs.
Optical	Schmidl et al.	30628	Tautenburg observations	Tautenburg
Optical	Belkin et al.	30637	continued Assy, AbAO and Mondy observations	Abastumani Astro. Obs.	detection
Gamma-ray	Ripa et al.	30624	Detection by GRBAlpha	GRBAlpha

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
b	489	683	38	>17.84
u	639	659	19	>17.04
v	390	583	39
uvw1	440	633	38	>17.12
uvw2	539	721	26	>17.08
white	234	383	147
white	234	708	184	17.19± 0.05

Table 3. UVOT observations reported by LaPorte and Lien (GCN Circ. 30618). The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary detections and 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.

August 13, 2021