Swift Observations of GRB 200612A

E. Sonbas (Adiyaman Univ.), A. D'Ai (INAF-IASFPA) and S.J. LaPorte (PSU) for the Swift team

1. Introduction

At 04:39:37 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200612A (trigger=977310) (Sonbas et al. GCN Circ. 27915). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 139° from the Sun (9.2 hours West) and 47° from the 60%-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.

Lipunov et al. (GCN Circ. 27917) reported the position from MASTER for the optical afterglow of this GRB. 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

Analysis of the BAT data was reported by Markwardt et al. (GCN Circ. 27924). Sonbas et al., GCN Circ. 27915). The BAT ground-calculated position is RA, Dec = 302.691, -45.354 deg which is RA(J2000) = 20h10m45.8s Dec(J2000) = -45°21'12.7" with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment). The partial coding was 96%.

The mask weighted light curve (Figure 1) shows a complex structure. The overall structure starts at ~T-20 s and lasts till ~T+200 s. T90 (15-350 keV) is 171.48 ± 19.74 s (estimated error including systematics).

The time-averaged spectrum from T-11 to T+193 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.34 ± 0.09. The fluence in the 15-150 keV band is 3.8 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 76% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+4.03 s in the 15-150 keV band is 0.7 ± 0.2 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/977310/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Ai et al. (GCN Circ. 27923). We have analysed 10 ks of XRT data for GRB 200612A, from 93 s to 92.0 ks after the BAT trigger. The data comprise 281 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. 27921).

The late-time light curve (Figure 2) (from T0+3.7 ks) can be modelled with an initial power-law decay with an index of α=1.22 (+0.12, -0.13), followed by a break at T+27.2 ks to an α of 3.8 (+1.4, -0.8).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.07 ± 0.05. The best-fitting absorption column is 9.1 (+2.3, -2.1) x 1020 cm-2, in excess of the Galactic value of 3.7 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.06 (+0.19, -0.17) and a best-fitting absorption column of 5.7 (+4.1, -2.0) x 1020 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.1 x 10-11 (3.6 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 5.7 (+4.1, -2.0) x 1020 cm-2
Galactic foreground: 3.7 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 2.06 (+0.19, -0.17)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00977310.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200612A 96 s after the BAT trigger (LaPorte and Sonbas GCN Circ. 27929). No optical afterglow consistent with the optical position (Lipunov et al. GCN Circ. 27917) 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 0.03 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
20h10m48.49s -45°20'07.4" 1.5" XRT-final UKSSDC
20h10m48.49s -45°20'07.8" 1.5" XRT-enhanced Osborne et al. GCN Circ. 27921
20h10m45.8s -45°21'12.7" 1.5' BAT-refined Markwardt et al. GCN Circ. 27924

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 27916 Swift GRB200612.19: Global MASTER-Net
observations report 		MASTER
Optical Lipunov et al. 27917 Swift GRB200612.19: Global MASTER-Net
OT detection		 MASTER detection
Optical Hu et al. 27919 BOOTES-3/YA optical upper limit BOOTES-3 upper limits
Optical 27920 Late Optical Maximum Deteted by MASTER MASTER
Gamma-ray Svinkin et al. 27922 Konus-Wind observation of ultra-long
GRB 200612A		 Konus-Wind Epeak=253 (-65,+95) keV
Fluence=4.08(-0.34,+1.04)x10-5erg cm-2
Gamma-ray Malacaria et al. 27934 Fermi GBM detection Fermi GBM T90=88 seconds
Fluence=4.187±0.109x10-5erg cm-2
(93rd percentile for long GRBs)
Other 27925 Draft Circular for GRB 200612A

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 96 245 147 >20.6
white 96 245 147 >20.6
u 308 375 67 >19.2

Table 3. UVOT observations reported by LaPorte and Sonbas (GCN Circ. 27929). 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.

June 14, 2020