Swift Observations of GRB 200410A

K.K. Simpson (PSU), J.D. Gropp (PSU) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 02:25:10 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200410A (trigger=965638) (Simpson et al. GCN Circ. 27518). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 77° from the Sun (5.3 hours West) and 91° from the 94%-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.

Simpson et al. (GCN Circ. 27518) 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 Laha et al. (GCN Circ. 27578), the BAT ground-calculated position is RA, Dec = 299.644, 51.745 deg which is RA(J2000) = 19h58m34.6s Dec(J2000) = +51°44'40.9" with an uncertainty of 2.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 53%.

The mask-weighted light curve (Figure 1) shows several weak pulses. The two main pulses occur at ~T-20 s and ~T+2 s, respectively, followed by some weak emission that lasts till ~T+50 s. T90 (15-350 keV) is 108.8 ± 40.7 s (estimated error including systematics).

The time-averaged spectrum from T-90.88 to T+50.62 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.41 ± 0.31. 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+1.43 s in the 15-150 keV band is 1.8 ± 0.3 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/965638/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Gropp et al. (GCN Circ. 27523). We have analysed 16 ks of XRT data for GRB 200410A, from 86 s to 139.1 ks after the BAT trigger. The data comprise 36 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 Goad et al. (GCN Circ. 27522).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=4.2 (+1.7, -0.7). At T+160 s the decay flattens to an α of 0.41 (+0.12, -0.14) before breaking again at T+5741 s to a final decay with index α=0.98 (+0.12, -0.08).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.07 ± 0.15. The best-fitting absorption column is 3.0 ± 0.6 x 1021 cm-2, in excess of the Galactic value of 2.2 x 1021 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10-11 (5.4 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 3.0 ± 0.6 x 1021 cm-2
Galactic foreground: 2.2 x 1021 cm-2
Excess significance: 2.4 σ
Photon index: 2.07 ± 0.15

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00965638.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200410A 104 s after the BAT trigger (Kuin and Simpson GCN Circ. 27532). A source consistent with the XRT position (Goad et al. GCN Circ. 27522) 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 0.15 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
19h58m35.32s +51°46'53.0" 0.6" UVOT-initial Simpson et al. GCN Circ. 27518
19h58m35.10s +51°46'53.7" 1.4" XRT-final UKSSDC
19h58m35.02s +51°46'50.8" 1.9" XRT-enhanced Goad et al. GCN Circ. 27522
19h58m34.6s +51°44'40.9" 2.9' BAT-refined Laha et al. GCN Circ. 27578

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 27517 Swift GRB200410.10: Global MASTER-Net
observations report 		MASTER
Optical Jelinek et al. 27520 Ondrejov D50 afterglow confirmation D50 detection
Optical Kumar et al. 27603 Optical upper limit Nainital upper limits
Optical Steele et al. 27621 Liverpool Telescope First Hour
Observations		 Liverpool Telescope light curve
Gamma-ray Cannady et al. 27584 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 104 254 147 19.51 ± 0.14
v 5051 6869 396 21.23 ± 0.90
b 572 1519 98 19.61 ± 0.32
u 317 567 246 20.39 ± 0.37
w1 696 1294 57 >19.8
m2 671 1220 57 >20.0
w2 622 1270 57 >19.9

Table 3. UVOT observations reported by Kuin and Simpson (GCN Circ. 27532). 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.

April 23, 2020