Swift Observations of GRB 210731A

E. Troja (NASA/GSFC/UMCP), A. Tohuvavohu (U. Toronto) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 22:21:08 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210731A (trigger=1062336) (Troja et al. GCN Circ. 30568). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 166° from the Sun (11.3 hours East) and 106° 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.

Kuin and Troja (GCN Circ. 30572) reported the detection with UVOT of an optical afterglow. Kann et al. (GCN Circ. 30583) determined a redshift of 1.2525 from VLT. 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. 30580), the BAT ground-calculated position is RA, Dec = 300.311, -28.056 deg which is RA(J2000) = 20^h01^m14.5^s Dec(J2000) = -28°03'20.0" with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 39%.

The mask-weighted light curve (Figure 1) shows a single-pulse structure that starts at ~T-1 s, peaks at ~T+11 s, and ends at ~T+30 s. T₉₀ (15-350 keV) is 22.51 ± 2.79 s (estimated error including systematics).

The time-averaged spectrum from T-0.78 to T+25.50 s is best fit by a power law with an exponential cutoff. This fit gives a photon index -0.25 ± 0.59, and E_peak of 106.5 ± 26.8 keV (χ² 50.28 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 2.2 ± 0.2 x 10^-6 erg cm^-2 and the 1-s peak flux measured from T+11.28 s in the 15-150 keV band is 1.6 ± 0.3 ph cm^-2 s^-1. This fluence is larger than that of 62% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). A fit to a simple power law gives a photon index of 1.01 ± 0.11 (χ² 67.96 for 57 d.o.f.). 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/1062336/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Tohuvavohu et al. (GCN Circ. 30576). We have analysed 11 ks of XRT data for GRB 210731A, from 207 s to 157.6 ks after the BAT trigger. The data comprise 62 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=3.5 (+0.8, -0.5), followed by a break at T+368 s to an α of 0.96 (+0.14, -0.13).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 3.35 (+0.27, -0.25). The best-fitting absorption column is 1.9 (+1.6, -1.4) x 10²¹ cm^-2, at a redshift of 1.2525, in addition to the Galactic value of 1.1 x 10²¹ cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.06 (+0.21, -0.18) and a best-fitting absorption column of 8 (+23, -8) x 10²⁰ cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.3 x 10^-11 (4.5 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.1 x 10²¹ cm^-2
Intrinsic column: 8 (+23, -8) x 10²⁰ cm^-2 at z=1.2525
Photon index: 2.06 (+0.21, -0.18)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01062336.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210731A 210 s after the BAT trigger (Kuin and Troja GCN Circ. 30572). An optical afterglow consistent with the XRT position (D'Ai et al. GCN Circ. 30569) and discovered by Meerlicht (de Wet ert al, GCN Circ. 30570) is also 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 0.092 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

20^h01^m13.14^s -28°03'39.4" 2.2" XRT-final UKSSDC

20^h01^m13.20^s -28°03'39.8" 2.6" XRT-enhanced D'Ai et al. GCN Circ. 30569

20^h01^m14.5^s -28°03'20.0" 1.7' BAT-refined Stamatikos et al. GCN Circ. 30580

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Lipunov et al. 30567 Swift GRB210731.93: Global MASTER-Net
observations report MASTER

Optical de Wet et al. 30570 MeerLICHT optical afterglow detection MeerLICHT detection

Optical Lipunov et al. 30571 Swift GRB210731.62: Global MASTER-Net
observations report MASTER

Optical Guelbenzu et al. 30574 GROND observations GROND detection

Optical Lipunov et al. 30578 MASTER optical afterglow detection MASTER detection

Optical Zheng and Filippenko 30582 KAIT Optical Detection KAIT detection

Optical Kann et al. 30583 VLT/X-shooter redshift VLT redshift

Optical Guelbenzu et al. 30584 continued GROND observations GROND detection

Gamma-ray Lesage and Meegan 30573 Fermi GBM Observation Fermi GBM E_peak=175±11 keV
T₉₀=26 seconds
Fluence=4.9±0.2x10^-6erg cm^-2
(56^th percentile for 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 210 273 62 20.2 ± 0.38

b 17547 18204 587 19.02 ± 0.08

u 29863 30504 624 18.66 ± 0.09

v 11787 12145 348 18.60 ± 0.17

uvw1 28956 29857 886 19.33 ± 0.14

uvw2 34767 35667 886 20.41 ± 0.27

uvm2 23308 23846 530 19.40 ± 0.21

RA (J2000)	Dec (J2000)	Error	Note	Reference
20^h01^m13.14^s	-28°03'39.4"	2.2"	XRT-final	UKSSDC
20^h01^m13.20^s	-28°03'39.8"	2.6"	XRT-enhanced	D'Ai et al. GCN Circ. 30569
20^h01^m14.5^s	-28°03'20.0"	1.7'	BAT-refined	Stamatikos et al. GCN Circ. 30580

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Lipunov et al.	30567	Swift GRB210731.93: Global MASTER-Net observations report	MASTER
Optical	de Wet et al.	30570	MeerLICHT optical afterglow detection	MeerLICHT	detection
Optical	Lipunov et al.	30571	Swift GRB210731.62: Global MASTER-Net observations report	MASTER
Optical	Guelbenzu et al.	30574	GROND observations	GROND	detection
Optical	Lipunov et al.	30578	MASTER optical afterglow detection	MASTER	detection
Optical	Zheng and Filippenko	30582	KAIT Optical Detection	KAIT	detection
Optical	Kann et al.	30583	VLT/X-shooter redshift	VLT	redshift
Optical	Guelbenzu et al.	30584	continued GROND observations	GROND	detection
Gamma-ray	Lesage and Meegan	30573	Fermi GBM Observation	Fermi GBM	E_peak=175±11 keV T₉₀=26 seconds Fluence=4.9±0.2x10^-6erg cm^-2 (56^th percentile for long GRBs)

Filter	T_start(s)	T_stop(s)	Exp(s)	Mag
white	210	273	62	20.2 ± 0.38
b	17547	18204	587	19.02 ± 0.08
u	29863	30504	624	18.66 ± 0.09
v	11787	12145	348	18.60 ± 0.17
uvw1	28956	29857	886	19.33 ± 0.14
uvw2	34767	35667	886	20.41 ± 0.27
uvm2	23308	23846	530	19.40 ± 0.21

Table 3. UVOT observations reported by Kuin and Troja (GCN Circ. 30572). 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 3, 2021