Swift Observations of GRB 200425A

M.G. Bernardini (INAF-OAB), V. D'Elia (ASDC) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 00:16:21 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200425A (trigger=967774) (Bernardini et al. GCN Circ. 27638). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 58° from the Sun (3.5 hours East) and 43° from the 3%-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 Markwardt et al. (GCN Circ. 27650), the BAT ground-calculated position is RA, Dec = 85.217, -13.399 deg which is RA(J2000) = 05h40m52.2s Dec(J2000) = -13°23'58.1" with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a weak pulse that starts at ~T-10 s, peaks at ~T+3 s, and ends at ~T+10 s. In addition, there is some weak emission that starts at ~T-50 s and lasts till ~T+60 s. T90 (15-350 keV) is 81.0 ± 21.7 s (estimated error including systematics).

The time-averaged spectrum from T-47.59 to T+58.41 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.30 ± 0.21. The fluence in the 15-150 keV band is 1.2 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 45% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+2.41 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/967774/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Elia et al. (GCN Circ. 27641). We have analysed 12 ks of XRT data for GRB 200425A, from 78 s to 97.2 ks after the BAT trigger. The data comprise 145 s in Windowed Timing (WT) mode (the first 6 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 Beardmore et al. (GCN Circ. 27639).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=2.47 (+0.33, -0.17). At T+105 s the decay flattens to an α of -1.5 (+0.0, -0.4). The light curve breaks again at T+125 s to a decay with α=2.71 (+0.45, -0.25), before a final break at T+268 s s after which the decay index is 1.18 ± 0.05.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.11 (+0.17, -0.16). The best-fitting absorption column is 5.8 (+1.0, -0.9) x 1021 cm-2, in excess of the Galactic value of 1.6 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.05 (+0.26, -0.24) and a best-fitting absorption column of 7.2 (+2.0, -1.7) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.3 x 10-11 (7.8 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 7.2 (+2.0, -1.7) x 1021 cm-2
Galactic foreground: 1.6 x 1021 cm-2
Excess significance: 5.4 σ
Photon index: 2.05 (+0.26, -0.24)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00967774.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200425A 94 s after the BAT trigger (Kuin and Bernardini GCN Circ. 27643). No optical afterglow consistent with the XRT positionis 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 0.16 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
05h40m52.68s -13°26'06.4" 1.5" XRT-final UKSSDC
05h40m52.68s -13°26'06.4" 1.6" XRT-enhanced Beardmore et al. GCN Circ. 27639
05h40m52.2s -13°23'58.1" 2.0' BAT-refined Markwardt et al. GCN Circ. 27650

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 27645 Swift GRB 200425A: Global MASTER-Net
observations report 		MASTER
Optical Strausbaugh and Cucchiara 27646 LCO Optical Upper Limits LCO upper limits

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 94 244 147 >20.7
uFC 253 503 246 >19.9
white 94 728 186 >20.5
v 585 778 39 >18.6
b 510 704 39 >19.1
u 253 679 265 >19.9
w1 635 827 39 >18.4
m2 610 802 39 >18.9
w2 560 753 39 >18.7

Table 3. UVOT observations reported by Kuin and Bernardini (GCN Circ. 27643). 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.

April 27, 2020