Swift Observations of GRB 200729A

P.A. Evans (U. Leicester), A. Tohuvavohu (U. Toronto) and S.R. Oates (U.Birmingham) for the Swift team

1. Introduction

At 19:38:05 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200729A (trigger=984929) (Evans et al. GCN Circ. 28165). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 53° from the Sun (3.7 hours East) and 85° from the 75%-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 Krimm et al. (GCN Circ. 28179), the BAT ground-calculated position is RA, Dec = 184.360, 45.571 deg which is RA(J2000) = 12h17m26.5s Dec(J2000) = +45°34'13.9" with an uncertainty of 2.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 46%.

The mask-weighted light curve (Figure 1) shows a weak pulse that starts at ~T0, peaks at ~T+6 s, and ends at ~T+130 s. T90 (15-350 keV) is 122.0 ± 34.2 s (estimated error including systematics).

The time-averaged spectrum from T+0.54 to T+136.54 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 0.96 ± 0.27. The fluence in the 15-150 keV band is 1.6 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 53% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+5.54 s in the 15-150 keV band is 0.7 ± 0.3 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/984929/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Tohuvavohu et al. (GCN Circ. 28176). We have analysed 26 ks of XRT data for GRB 200729A, from 144 s to 609.2 ks after the BAT trigger. The data comprise 213 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 28168).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.44 ± 0.04.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.78 ± 0.13. The best-fitting absorption column is 7.9 (+1.2, -1.1) x 1021 cm-2, in excess of the Galactic value of 1.1 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.38 (+0.22, -0.21) and a best-fitting absorption column of 9.6 (+2.9, -2.5) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 6.8 x 10-11 (8.9 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 9.6 (+2.9, -2.5) x 1021 cm-2
Galactic foreground: 1.1 x 1020 cm-2
Excess significance: 6.4 σ
Photon index: 1.38 (+0.22, -0.21)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00984929.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200729A 145 s after the BAT trigger (Oates and Evans GCN Circ. 28177). No optical afterglow is detected above the flux of NGC 4242, consistent with the XRT position (Osborne et al. GCN Circ. 28168), 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 0.01 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
12h17m30.28s +45°35'41.2" 1.4" XRT-final UKSSDC
12h17m30.30s +45°35'41.1" 1.7" XRT-enhanced Osborne et al. GCN Circ. 28168
12h17m26.5s +45°34'13.9" 2.3' BAT-refined Krimm et al. GCN Circ. 28179

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 28166 Swift GRB 200729A: Global MASTER-Net
observations report 		MASTER
Optical Hu et al. 28167 BOOTES-1 optical upper limit BOOTES-1 upper limits
Optical Malesani et al. 28169 NOT optical upper limits NOT upper limits
Optical D'Avanzo et al. 28170 TNG NIR upper limits TNG upper limits
Optical Kilpatrick et al. 28172 Pan-STARRS pre-explosion source
detection		 Pan-STARRS detection
Optical Anand et al. 28175 HST imaging HST
Optical Malesani et al. 28178 further analysis of the NOT data NOT
Optical Zhu et al. 28180 Nanshan/NEXT optical upper limit Xinjiang Astro. Obs. upper limits
Radio Hajela et al. 28181 VLA Observations VLA
Radio Volvach et al. 28182 RT-22 observations at 36.8 GHz CrAO RT-22
Gamma-ray Malacaria et al. 28174 Fermi-GBM Sub-Threshold candidate for
GRB 200729A		 Fermi GBM Epeak=159±19 keV
Gamma-ray Ridnaia et al. 28192 Konus-Wind detection Konus-Wind Epeak=269 (-65,+105) keV
Fluence=5.23(-1.00,+1.32)x10-6erg cm-2
Neutrino Collaboration 28173 IceCube neutrino search IceCube
Other Laskar et al. 28171 Spitzer archival source detection detection
Other Dichiara et al. 28270 Lowell Discovery Telescope observations

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 145 295 147 >20.0
uFC 303 553 246 >18.9
white 145 1874 431 >20.0
v 633 1749 136 >17.6
b 559 1849 136 >18.7
u 303 1824 362 >18.9
w1 682 1799 97 >18.2
m2 658 1428 39 >17.8
w2 1359 1725 58 >17.8

Table 3. UVOT observations reported by Oates and Evans (GCN Circ. 28177). 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 20, 2020