Swift Observations of GRB 200205A

P.A. Evans (U. Leicester) and J.P. Osborne (U. Leicester) for the Swift team

1. Introduction

At 06:35:51 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200205A (trigger=954304) (Evans et al. GCN Circ. 26982). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 128° from the Sun (11.5 hours East) and 73° from the 80%-illuminated Moon. Table 1 contains the best reported positions from Swift.

Lipunov et al. (GCN Circ. 26984) 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

As reported by Stamatikos et al. (GCN Circ. 27010), the BAT ground-calculated position is RA, Dec = 130.168, -35.275 deg which is RA(J2000) = 08h40m40.4s Dec(J2000) = -35°16'30.9" with an uncertainty of 3.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 94%.

The mask weighted light curve (Figure 1) shows some weak emission which started before the source came into BAT field of view (which is ~ T-100 s). Kennea et al. 2020 (Atel #13452) discuss the properties of this newly discovered X-ray transient. The lower limit of the duration of the burst as detected by BAT is ~ 200 s. The source went out of BAT field of view at ~T+260 s, with a possibility of continuing emission which may be detected by other telecopes.

The time-averaged spectrum from -81.88 to 132.86 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 0.84 ± 0.91, and Epeak of 31.5 ± 8.4 keV (χ2 67.17 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is (1.3 ± 0.1) x 10-6 erg cm-2 and the 1-s peak flux measured from T-81.90 s in the 15-150 keV band is 0.3 ± 0.2 ph cm-2 s-1. This fluence is larger than that of 47% 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 2.26 ± 0.17 (χ2 77.17 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/954304/BA/.

3. XRT Observations and Analysis

We have analysed 14 ks of XRT data for GRB 200205A, from 127 s to 219.2 ks after the BAT trigger. The data comprise 863 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 26996).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=2.29 ± 0.18. At T+666 s the decay flattens to an α of -1.50 (+0.00, -0.20). The light curve breaks again at T+5675 s to a decay with α=8.00 (+0.00, -0.13), again at T+6208 s s to α=3.12 ± 0.08, and again at T+38.0 ks s to α=-0.8 (+0.4, -0.6), before a final break at T+159 ks s after which the decay index is 3.0 (+1.8, -0.5).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.06 (+0.06, -0.04). The best-fitting absorption column is consistent with the Galactic value of 4.3 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.12 (+0.21, -0.20) and a best-fitting absorption column of 6.8 (+1.5, -1.3) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.1 x 10-11 (7.7 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 6.8 (+1.5, -1.3) x 1021 cm-2
Galactic foreground: 4.3 x 1021 cm-2
Excess significance: 3.2 σ
Photon index: 2.12 (+0.21, -0.20)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00954304.

4. UVOT Observations and Analysis

UVOT results are not available.

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
08h40m40.94s -35°16'24.6" 0.8" UVOT ?
08h40m40.84s -35°16'24.8" 1.9" XRT-final UKSSDC
08h40m40.81s -35°16'25.2" 1.9" XRT-enhanced Osborne et al. GCN Circ. 26996
08h40m40.4s -35°16'30.9" 3.0' BAT-refined Stamatikos et al. GCN Circ. 27010

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 26983 Swift GRB 200205A: Global MASTER-Net
observations report 		MASTER
Optical Lipunov et al. 26984 MASTER Ot detection MASTER detection
Optical Lipunov et al. 26985 MAXI GRB200205.29: Global MASTER-Net
observations report 		MASTER
Optical Melandri et al. 26986 REM optical observations REM detection
Optical Malesani et al. 26989 Swift trigger 954304: VLT optical
observations		 VLT
Optical Buckley et al. 26991 Swift GRB200205.81: Global MASTER-Net
OT detection		 MASTER detection
Optical Lipunov et al. 26992 Swift GRB200205.81: Global MASTER-Net
observations report 		MASTER
Optical Mazaeva et al. 27003 = Swift J0840.7-3516: optical
photometry in Chilescope observatory		 detection
X-ray Niwano et al. 26987 MAXI/GSC detection MAXI detection

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

February 8, 2020