Swift Observations of GRB 200416A

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

1. Introduction

At 07:05:17 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200416A (trigger=966554) (Evans et al. GCN Circ. 27591). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 52° from the Sun (3.3 hours West) and 24° from the 37%-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. 27591) reported the discovery 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

Analysis of the BAT data was reported by Lien et al. (GCN Circ. 27601). A. Evans et al., GCN Circ. 27591). The BAT ground-calculated position is RA, Dec = 335.691, -7.525 deg which is RA(J2000) = 22h22m45.8s Dec(J2000) = -07°31'29.5" with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 33%.

The mask weighted light curve (Figure 1) shows multi-peaked structure. The overall structure starts at ~T-2 s and lasts till ~T+6 s. T90 (15-350 keV) is 5.04 ± 0.73 s (estimated error including systematics).

The time-averaged spectrum from T-1.31 to T+4.69 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.76 ± 0.15. The fluence in the 15-150 keV band is 8.7 ± 0.8 x 10-7 erg cm-2. This fluence is larger than that of 35% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.05 s in the 15-150 keV band is 3.8 ± 0.5 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/966554/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Evans (GCN Circ. 27594). We have analysed 677 s of XRT data for GRB 200416A, from 84 s to 773 s after the BAT trigger. The data comprise 43 s in Windowed Timing (WT) mode (the first 9 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=2.9 ± 0.5, followed by a break at T+167 s to an α of 0.25 (+0.19, -0.25).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.83 (+0.20, -0.16). The best-fitting absorption column is 8.5 (+5.4, -2.0) x 1020 cm-2, consistent with the Galactic value of 6.5 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 3.7 x 10-11 (4.3 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 8.5 (+5.4, -2.0) x 1020 cm-2
Galactic foreground: 6.5 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.83 (+0.20, -0.16)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00966554.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200416A 102 s after the BAT trigger (Simpson and Evans GCN Circ. 27602). A source consistent with the XRT position (Evans GCN Circ. 27594) 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 0.06 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
22h22m47.84s -07°31'06.0" 0.43" UVOT-refined Simpson and Evans GCN Circ. 27602
22h22m47.64s -07°31'04.4" 3.5" XRT-final UKSSDC
22h22m47.66s -07°31'04.3" 3.5" XRT-refined Evans GCN Circ. 27594
22h22m45.8s -07°31'29.5" 1.4' BAT-refined Lien et al. GCN Circ. 27601

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Gamma-ray Bissaldi 27592 Fermi GBM observation Fermi GBM Epeak=74±6 keV
T90=9 seconds
Fluence=1.43±0.08x10-6erg cm-2
(22nd percentile for long 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
white 102 252 147 16.57 ± 0.03
v 644 664 19 17.05 ± 0.32
b 570 590 20 17.48 ± 0.20
u 314 564 246 16.44 ± 0.04
w1 694 714 20 16.59 ± 0.23
m2 668 688 19 >16.3
w2 620 640 19 >17.5

Table 3. UVOT observations reported by Simpson and Evans (GCN Circ. 27602). 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 19, 2020