Swift Observations of GRB 200806A

E. Ambrosi (INAF-IASFPA), K.L. Page (U. Leicester) and F.E. Marshall (NASA/GSFC) for the Swift team

1. Introduction

At 15:28:49 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200806A (trigger=987016) (Ambrosi et al. GCN Circ. 28211). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 75° from the Sun (5.6 hours West) and 73° from the 91%-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.

Marshall and Ambrosi (GCN Circ. 28222) reported the detection 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 Lien et al. (GCN Circ. 28223), the BAT ground-calculated position is RA, Dec = 52.933, 37.076 deg which is RA(J2000) = 03h31m43.9s Dec(J2000) = +37°04'33.5" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 57%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure, with a sharp rise close to T+0 s. The emission peaked at T+22 s and declined to background by T+50 s. T90 (15-350 keV) is 38.8 ± 1.1 s (estimated error including systematics).

The time-averaged spectrum from T-0.13 to T+61.22 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.72 ± 0.12, and Epeak of 148.1 ± 18.6 keV (χ2 40.06 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 1.6 ± 0.02 x 10-5 erg cm-2 and the 1-s peak flux measured from T+22.24 s in the 15-150 keV band is 14.1 ± 0.4 ph cm-2 s-1. This fluence is larger than that of 96.1% 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 1.27 ± 0.03 (χ2 111.98 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/987016/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Page et al. (GCN Circ. 28221). We have analysed 17 ks of XRT data for GRB 200806A, from 84 s to 270.6 ks after the BAT trigger. The data comprise 349 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. 28214).

The late-time light curve (Figure 2) (from T0+5.8 ks) can be modelled with a power-law decay with a decay index of α=1.14 ± 0.07.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.58 ± 0.07. The best-fitting absorption column is 3.6 ± 0.4 x 1021 cm-2, in excess of the Galactic value of 2.6 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.85 ± 0.14 and a best-fitting absorption column of 4.8 (+0.9, -0.8) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.4 x 10-11 (6.6 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.8 (+0.9, -0.8) x 1021 cm-2
Galactic foreground: 2.6 x 1021 cm-2
Excess significance: 4.5 σ
Photon index: 1.85 ± 0.14

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00987016.

4. UVOT Observations and Analysis

Analysis of the UVOT data was reported by Marshall and Ambrosi (GCN Circ. 28222). 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 0.42 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
03h31m43.73s +37°04'43.5" 1.4" XRT-final UKSSDC
03h31m43.71s +37°04'43.5" 1.7" XRT-enhanced Beardmore et al. GCN Circ. 28214
03h31m43.9s +37°04'33.5" 1.0' BAT-refined Lien et al. GCN Circ. 28223

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 28212 Swift GRB 200806A: Global MASTER-Net
observations report 		MASTER
Optical Murata et al. 28215 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Belkin et al. 28218 AbAO optical upper limit Abastumani Astro. Obs. upper limits
Optical Gokuldass et al. 28224 VIRT optical observations Virgin Island Robotic Telescope
Optical Kann et al. 28227 CAHA optical afterglow confirmation CAHA detection
Optical Fu et al. 28229 Nanshan/NEXT optical upper limit Xinjiang Astro. Obs. upper limits
Gamma-ray Gupta et al. 28216 AstroSat CZTI detection CZTI
Gamma-ray Shimizu et al. 28234 CALET Gamma-Ray Burst Monitor detection CALET

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

Filter Tstart(s) Tstop(s) Exp(s) Mag
white 86 236 147 20.26 ± 0.28
v 629 821 39 >17.9
b 555 747 39 >19.3
u 300 722 265 >19.5
w1 678 870 39 >18.8
m2 653 845 39 >19.4
w2 604 1052 58 >19.0

Table 3. UVOT observations reported by Marshall and Ambrosi (GCN Circ. 28222). 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 10, 2020