Swift Observations of GRB 200205B

P.A. Evans (U. Leicester), J.P. Osborne (U. Leicester) and K.K. Simpson (PSU) for the Swift team

1. Introduction

At 19:21:43 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200205B (trigger=954519 and 954520) (Evans et al. GCN Circ. 26993). Swift slewed to the burst after a short delay. At the time of the trigger, the initial BAT position was 103° from the Sun (9.9 hours East) and 81° from the 85%-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.

Evans et al. (GCN Circ. 26993) reported the discovery with UVOT of an optical afterglow. Vielfaure et al. (GCN Circ. 26998) determined a redshift of 1.465 from VLT. 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 Ukwatta et al. (GCN Circ. 27012). We have combined the data from two triggers (#954519 and #954520) for the analysis. All the times in this circular are relative to the trigger time from #954519.

The mask-weighted light curve (Figure 1) shows several pulses that start from ~T+20 s and end at ~T+550 s. The first prominent pulse occurs at ~T+60 s, followed by another significant structure with several overlapping pulses from ~T+380 s to ~T+520 s. There is a data gap from T+63 s to T+160 s, likely due to problems of saving data because of the double triggers. T90 (15-350 keV) is 458.0 ± 5.4 s (estimated error including systematics).

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/954520/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Osborne et al. (GCN Circ. 27001). We have analysed 6.5 ks of XRT data for GRB 200205B, from 332 s to 46.4 ks after the BAT trigger. The data comprise 360 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 late-time light curve (Figure 2) (from T0+5.3 ks) can be modelled with a power-law decay with a decay index of α=0.76 (+0.14, -0.13).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.129 (+0.027, -0.026). The best-fitting absorption column is 4.9 (+1.2, -1.1) x 1021 cm-2, at a redshift of 1.465, in addition to the Galactic value of 1.0 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.93 (+0.19, -0.13) and a best-fitting absorption column of 1 (+35, -1) x 1020 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10-11 (4.2 x 10-11) erg cm-2 count-1.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200205B 352 s after the BAT trigger (Simpson and Evans GCN Circ. 27006). A source consistent with the XRT position 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 0.11 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.

RA (J2000) Dec (J2000) Error Note Reference
07h11m09.15s -56°29'15.3" 0.42" UVOT-refined Simpson and Evans GCN Circ. 27006
07h11m09.03s -56°29'14.7" 1.6" XRT-enhanced Beardmore et al. GCN Circ. 26999
07h11m02.3s -56°29'18.1" 1.8' BAT-refined Ukwatta et al. GCN Circ. 27012

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Strausbaugh and Cucchiara 26995 LCO Optical Detection LCO detection
Optical Vielfaure et al. 26998 VLT/X-shooter redshift VLT redshift
Optical Malesani et al. 27004 ESO/NTT near-infrared imaging detection
Optical Mazaeva et al. 27027 Chilescope optical observations marginal detection
Gamma-ray Tsvetkova et al. 27034 Konus-Wind observation Konus-Wind Fluence=1.12(-0.05,+0.26)x10-5erg cm-2

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 6386 6586 196 18.92± 0.10
white 383 1042 478 16.81± 0.03
v 540 733 38 16.88 ±
b 638 833 38 17.74 ±
u 614 808 38 17.25 ±
w1 589 783 38 16.88± 0.20
m2 564 758 38 17.12 ± 0.28
w2 689 1056 46 >17.77

Table 3. UVOT observations reported by Simpson and Evans (GCN Circ. 27006). 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.

February 12, 2020