Swift Observations of GRB 200109A

A. Melandri (INAF-OAB), V. D'Elia (ASDC) and K.K. Simpson (PSU) for the Swift team

1. Introduction

At 01:46:16 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200109A (trigger=948361) (Melandri et al. GCN Circ. 26678). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 77° from the Sun (1.2 hours East) and 97° from the 96%-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. 26692), the BAT ground-calculated position is RA, Dec = 307.116, 52.988 deg which is RA(J2000) = 20h28m27.7s Dec(J2000) = +52°59'17.7" with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a weak structure with several overlapping pulses that start at ~T-30 s to ~T+20 s, followed by a weaker tail emission that lasts till ~T+100 s. T90 (15-350 keV) is 112.0 ± 32.0 s (estimated error including systematics).

The time-averaged spectrum from T-31.45 to T+112.55 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.31 ± 0.14. The fluence in the 15-150 keV band is 2.1 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 60% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+8.05 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/948361/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Elia et al. (GCN Circ. 26682). We have analysed 6.6 ks of XRT data for GRB 200109A, from 91 s to 28.4 ks after the BAT trigger. The data comprise 48 s in Windowed Timing (WT) mode (the first 7 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. 26681).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=1.9 ± 0.3. At T+216 s the decay steepens to an α of 8.0 (+0.0, -3.2) before breaking again at T+269 s to a final decay with index α=1.31 (+0.21, -0.16).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.67 (+0.32, -0.30). The best-fitting absorption column is 7.2 (+2.9, -2.4) x 1021 cm-2, in excess of the Galactic value of 4.0 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 5.4 x 10-11 (7.7 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 7.2 (+2.9, -2.4) x 1021 cm-2
Galactic foreground: 4.0 x 1021 cm-2
Excess significance: 2.2 σ
Photon index: 1.67 (+0.32, -0.30)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00948361.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200109A 110 s after the BAT trigger (Simpson and Melandri GCN Circ. 26690). No optical afterglow 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 0.29 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
20h28m29.88s +52°59'36.9" 1.4" XRT-final UKSSDC
20h28m29.91s +52°59'37.1" 1.7" XRT-enhanced Goad et al. GCN Circ. 26681
20h28m27.7s +52°59'17.7" 1.2' BAT-refined Krimm et al. GCN Circ. 26692

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 26679 Swift GRB200109.07: Global MASTER-Net
observations report 		MASTER
Gamma-ray Fermi 26677 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Hamburg and Meegan 26689 Fermi GBM observation Fermi GBM Epeak=449±109 keV
T90=41 seconds
Fluence=6.9±0.6x10-6erg cm-2
(66th percentile for long GRBs)
Gamma-ray Xiao et al. 26710 Insight-HXMT/HE detection Insight-HXMT T90=36.405 seconds

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 110 259 147 >20.5
uFC 268 518 246 >19.0
white 110 1023 295 >20.9
u 268 518 246 >19.0

Table 3. UVOT observations reported by Simpson and Melandri (GCN Circ. 26690). 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.

January 12, 2020