Swift Observations of GRB 210730A

E. Sonbas (Adiyaman Univ.), J.A. Kennea (PSU) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 04:57:29 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210730A (trigger=1062150) (Sonbas et al. GCN Circ. 30548). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 53° from the Sun (1.4 hours East) and 97° from the 63%-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

As reported by Sakamoto et al. (GCN Circ. 30554), the BAT ground-calculated position is RA, Dec = 149.570, 69.703 deg which is RA(J2000) = 09h58m16.8s Dec(J2000) = +69°42'12.4" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 55%.

The mask-weighted light curve (Figure 1) shows a single-peaked structure that starts at ~T-0.5, peaks at ~T+0.5, and ends at ~T+5. T90 (15-350 keV) is 3.86 ± 0.66 s (estimated error including systematics).

The time-averaged spectrum from T-0.32 to T+5.01 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.41 ± 0.11. The fluence in the 15-150 keV band is 1.3 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 47% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.17 s in the 15-150 keV band is 7.1 ± 0.5 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/1062150/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Kennea et al. (GCN Circ. 30553). We have analysed 51 ks of XRT data for GRB 210730A, from 87 s to 549.4 ks after the BAT trigger. The data comprise 71 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 30550).

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=5.6 ± 1.5, followed by a break at T+110 s to an α of 1.14 ± 0.04.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.41 (+0.32, -0.29). The best-fitting absorption column is 6.7 (+1.9, -1.6) x 1021 cm-2, in excess of the Galactic value of 6.3 x 1020 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10-11 (8.3 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 6.7 (+1.9, -1.6) x 1021 cm-2
Galactic foreground: 6.3 x 1020 cm-2
Excess significance: 6.2 σ
Photon index: 2.41 (+0.32, -0.29)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01062150.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 210730A 90 s after the BAT trigger (Kuin and Sonbas GCN Circ. 30552). No optical afterglow consistent with the XRT position (Beardmore et al. GCN Circ. 30550) 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 0.097 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
09h58m22.14s +69°41'21.9" 2.2" XRT-final UKSSDC
09h58m21.63s +69°41'21.7" 4.5" XRT-enhanced Beardmore et al. GCN Circ. 30550
09h58m16.8s +69°42'12.4" 1.0' BAT-refined Sakamoto et al. GCN Circ. 30554

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical D'Avanzo et al. 30560 TNG optical observations TNG
Optical Lipunov et al. 30562 Swift GRB 210730A: Global MASTER-Net
observations report 		MASTER
Optical Hu et al. 30563 BOOTES-5/JGT and 2.2m CAHA telescopes
optical limit		 BOOTES upper limits
Optical De Luise et al. 30564 Campo Imperatore optical observations
Optical Schmidl et al. 30629 Tautenburg observations Tautenburg
Gamma-ray Prasad et al. 30555 AstroSat CZTI detection CZTI
Gamma-ray Wood 30557 Fermi GBM observation Fermi GBM Epeak=170±11 keV
T90=5 seconds
Fluence=2.9±0.1x10-6erg cm-2
(40th percentile for 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 90 204 112 >17.0
white 90 4145 308 >16.7
v 4355 4555 197 >15.5
b 3740 3940 197 >16.4
u 3535 5146 370 >16.5
w1 4765 4965 197 >18.0
m2 4560 4760 197 >19.7
w2 4151 4351 197 >18.6

Table 3. UVOT observations reported by Kuin and Sonbas (GCN Circ. 30552). 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.

August 12, 2021