Swift Observations of GRB 180709A

F.E. Marshall (NASA/GSFC), S.J. LaPorte (PSU) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 10:52:00 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180709A (trigger=846868) (Marshall et al. GCN Circ. 22930). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 61° from the Sun (4.7 hours West) and 47° from the 19%-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 Stamatikos et al. (GCN Circ. 22938), the BAT ground-calculated position is RA, Dec = 38.086, 60.333 deg which is RA(J2000) = 02h32m20.7s Dec(J2000) = +60°19'59.4" with an uncertainty of 1.6 arcmin, (radius, sys+stat, 90% containment). The partial coding was 81%.

The mask-weighted light curve (Figure 1) shows a single-peaked structure that starts at ~T0 and peaks at ~T+10 s. The main structure ends at ~T+40 s, with some tail emission lasting till ~T+240 s. T90 (15-350 keV) is 215.1 ± 28.9 s (estimated error including systematics).

The time-averaged spectrum from T+0.54 to T+243.46 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.37 ± 0.23. The fluence in the 15-150 keV band is 1.4 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 49% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+8.73 s in the 15-150 keV band is 0.6 ± 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/846868/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by LaPorte et al. (GCN Circ. 22936). We have analysed 18 ks of XRT data for GRB 180709A, from 95 s to 139.3 ks after the BAT trigger. The data comprise 277 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 Beardmore et al. (GCN Circ. 22933).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.95 (+0.09, -0.08).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.41 ± 0.20. The best-fitting absorption column is 1.9 ± 0.4 x 1022 cm-2, in excess of the Galactic value of 8.9 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.7 (+0.5, -0.4) and a best-fitting absorption column of 3.5 (+1.2, -1.0) x 1022 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 8.4 x 10-11 (1.6 x 10-10) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 3.5 (+1.2, -1.0) x 1022 cm-2
Galactic foreground: 8.9 x 1021 cm-2
Excess significance: 4.2 σ
Photon index: 1.7 (+0.5, -0.4)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00846868.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180709A 115 s after the BAT trigger (Breeveld et al. GCN Circ. 22937). No optical afterglow consistent with the XRT position (Beardmore et al., GCN Circ. 22933) 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 2.72 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
02h32m28.28s +60°20'56.8" 1.5" XRT-final UKSSDC
02h32m28.28s +60°20'56.8" 1.8" XRT-enhanced Beardmore et al. GCN Circ. 22933
02h32m20.7s +60°19'59.4" 1.6' BAT-refined Stamatikos et al. GCN Circ. 22938

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Butler et al. 22934 COATLI Optical Observations COATLI
Optical Halpern 22935 Optical limits MDM upper limits
Optical Carrasco et al. 22939 BOOTES-5/JGT optical observations BOOTES

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 115 264 147 >20.5
uFC 273 523 246 >19.6
v 604 797 39 >18.0
b 529 723 39 >19.0
uvw1 654 846 39 >17.7

Table 3. UVOT observations reported by Breeveld et al. (GCN Circ. 22937). 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.

July 11, 2018