Swift Observations of GRB 200509A

E. Troja (NASA/GSFC/UMCP), A. D'Ai (INAF-IASFPA) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 06:47:17 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200509A (trigger=970886) (Troja et al. GCN Circ. 27701). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 73° from the Sun (4.7 hours East) and 130° from the 95%-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.

Troja et al. (GCN Circ. 27701) 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

As reported by Palmer et al. (GCN Circ. 27713), the BAT ground-calculated position is RA, Dec = 116.415, -4.634 deg which is RA(J2000) = 07h45m39.6s Dec(J2000) = -04°38'02.5" with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 39%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T0 and ends at ~T+850 s. The three prominent peaks occur at ~T0, ~T+600 s, and ~T+820 s, respectively. There might be additional burst emission beyond the available event data range (i.e., > T+963 s). T90 (15-350 keV) is 828.2 ± 13.4 s (estimated error including systematics).

The time-averaged spectrum from T-0.80 to T+848.99 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.67 ± 0.09. The fluence in the 15-150 keV band is 7.3 ± 0.4 x 10-6 erg cm-2. This fluence is larger than that of 88% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+822.69 s in the 15-150 keV band is 1.5 ± 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/970886/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Ai et al. (GCN Circ. 27704). We have analysed 20 ks of XRT data for GRB 200509A, from 104 s to 131.2 ks after the BAT trigger. The data comprise 7 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 Evans et al. (GCN Circ. 27703).

The late-time light curve (Figure 2) (from T0+5.0 ks) can be modelled with a power-law decay with a decay index of α=0.39 ± 0.06.

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.725 ± 0.020. The best-fitting absorption column is 1.89 (+0.08, -0.07) 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 1.64 (+0.10, -0.07) and a best-fitting absorption column consistent with the Galactic value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.3 x 10-11 (5.0 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.6 (+0.3, -0.0) x 1021 cm-2
Galactic foreground: 1.6 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 1.64 (+0.10, -0.07)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00970886.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200509A 123 s after the BAT trigger (Kuin and Troja GCN Circ. 27709). A source 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 0.13 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
07h45m41.28s -04°37'49.3" 0.48" UVOT-refined Kuin and Troja GCN Circ. 27709
07h45m41.35s -04°37'49.7" 1.4" XRT-final UKSSDC
07h45m41.37s -04°37'49.5" 1.7" XRT-enhanced Evans et al. GCN Circ. 27703
07h45m39.6s -04°38'02.5" 1.7' BAT-refined Palmer et al. GCN Circ. 27713

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Gendre et al. 27702 Zadko observatory - Gingin optical
upper limits		 Zadko upper limits
Optical Strausbaugh and Cucchiara 27705 LCO Optical Afterglow Detection LCO detection
Optical Lipunov et al. 27706 Swift GRB 200509A: Global MASTER-Net
observations report 		MASTER
Optical Kumar et al. 27710 GROWTH-India r band upper limits GROWTH-India 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
white 123 273 147 18.92 ± 0.08
v 611 1738 122 >18.6
b 537 1678 117 >19.8
u 281 531 246 18.46 ± 0.10
w1 660 1628 117 >19.0
m2 636 1604 117 >19.0
w2 587 1728 136 >19.6

Table 3. UVOT observations reported by Kuin and Troja (GCN Circ. 27709). 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.

May 11, 2020