Swift Observations of GRB 200411A

A. Tohuvavohu (U. Toronto), J.P. Osborne (U. Leicester) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 04:29:02 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200411A (trigger=965784) (Tohuvavohu et al. GCN Circ. 27536). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 65° from the Sun (1.8 hours East) and 108° from the 87%-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 Krimm et al. (GCN Circ. 27545), the BAT ground-calculated position is RA, Dec = 47.686, -52.320 deg which is RA(J2000) = 03h10m44.7s Dec(J2000) = -52°19'12.6" with an uncertainty of 1.6 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a couple overlapping pulses that starts at ~T-0.02 s and ends at ~T+0.25 s. The highest peak occurs at ~ T+0.05 s. T90 (15-350 keV) is 0.22 ± 0.04 s (estimated error including systematics).

The time-averaged spectrum from T-0.02 to T+0.25 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 0.95 ± 0.19. The fluence in the 15-150 keV band is 8.6 ± 1.1 x 10-8 erg cm-2. This fluence is larger than that of 66% of the short GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.38 s in the 15-150 keV band is 1.1 ± 0.2 ph cm-2 s-1. All the quoted errors are at the 90% confidence level.

Using the 4-ms binned light curve (Figure 1) , the spectral lag of the short pulse is 1 (+7, -4) ms for the 100-350 keV to 25-50 keV bands, and 8 (+6, -6) ms for the 50-100 keV to 15-25 keV bands. These values are more consistent with those of short GRBs.

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/965784/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Osborne et al. (GCN Circ. 27540). We have analysed 15 ks of XRT data for GRB 200411A, from 39 s to 76.1 ks after the BAT trigger. The data comprise 8 s in Windowed Timing (WT) mode (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. 27538).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.96 ± 0.08.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.77 (+0.23, -0.22). The best-fitting absorption column is 1.5 (+0.8, -0.7) x 1021 cm-2, in excess of the Galactic value of 1.6 x 1020 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.9 x 10-11 (4.7 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.5 (+0.8, -0.7) x 1021 cm-2
Galactic foreground: 1.6 x 1020 cm-2
Excess significance: 3.2 σ
Photon index: 1.77 (+0.23, -0.22)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00965784.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200411A 58 s after the BAT trigger (Kuin and Tohuvavohu GCN Circ. 27539). No optical afterglow consistent with the XRT position (Osborne et al., GCN Circ. 27538) 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 0.02 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
03h10m39.39s -52°19'03.3" 1.4" XRT-final UKSSDC
03h10m39.40s -52°19'03.5" 1.5" XRT-enhanced Osborne et al. GCN Circ. 27538
03h10m44.7s -52°19'12.6" 1.6' BAT-refined Krimm et al. GCN Circ. 27545

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 27542 Swift GRB 200411A: Global MASTER-Net
observations report 		MASTER
Radio Anderson et al. 27573 ATCA 5/9 GHz radio observations ATCA
X-ray Escorial et al. 27589 Chandra observations indicative of
temporal steepening		 Chandra
Gamma-ray Fermi 27535 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Bissaldi et al. 27543 Fermi GBM observation Fermi GBM Epeak=420±120 keV
T90=1.4 seconds
Fluence=4.7±0.6x10-7erg cm-2
(64th percentile for short GRBs)

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 58 208 147 >20.8
uFC 270 520 246 >20.4
white 58 1540 392 >21.2
v 601 1417 97 >18.5
b 525 1343 78 >20.3
u 270 1490 343 >21.0
w1 651 1466 97 >18.8
m2 626 646 19 >18.1
w2 576 1220 58 >19.2

Table 3. UVOT observations reported by Kuin and Tohuvavohu (GCN Circ. 27539). 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 16, 2020