Swift Observations of GRB 180614A

J.D. Gropp (PSU) and A.P. Beardmore (U. Leicester) for the Swift team

1. Introduction

At 19:16:13 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180614A (trigger=841548) (Gropp et al. GCN Circ. 22780). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 66° from the Sun (5.3 hours West) and 78° from the 1%-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 Palmer et al. (GCN Circ. 22786), the BAT ground-calculated position is RA, Dec = 3.073, 46.969 deg which is RA(J2000) = 00h12m17.6s Dec(J2000) = +46°58'07.7" with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T-4 s and ends at ~T+3 s. The main peak occurs at ~T+2 s. T90 (15-350 keV) is 7.04 ± 0.92 s (estimated error including systematics).

The time-averaged spectrum from T-4.35 to T+3.45 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.53 ± 0.26. The fluence in the 15-150 keV band is 2.1 ± 0.3 x 10-7 erg cm-2. This fluence is larger than that of 7.3% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+1.56 s in the 15-150 keV band is 0.7 ± 0.1 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/841548/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Beardmore et al. (GCN Circ. 22782). We have analysed 14 ks of XRT data for GRB 180614A, from 50 s to 166.4 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. 22781).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.73 ± 0.06.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.7 ± 0.4. The best-fitting absorption column is 1.7 (+0.5, -0.4) x 1022 cm-2, in excess of the Galactic value of 1.1 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 4.0 x 10-11 (1.9 x 10-10) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.7 (+0.5, -0.4) x 1022 cm-2
Galactic foreground: 1.1 x 1021 cm-2
Excess significance: 6.8 σ
Photon index: 2.7 ± 0.4

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00841548.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180614A 65s after the BAT trigger (Gropp GCN Circ. 22787). No optical afterglow consistent with the enhanced 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 0.14 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
00h12m18.82s +46°57'10.9" 1.5" XRT-final UKSSDC
00h12m18.83s +46°57'10.8" 1.8" XRT-enhanced Evans et al. GCN Circ. 22781
00h12m17.6s +46°58'07.7" 1.9' BAT-refined Palmer et al. GCN Circ. 22786

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 22783 MASTER observation MASTER
Optical Heintz et al. 22784 NOT optical upper limits NOT upper limits
Optical Hu et al. 22788 1.5m OSN I-band observation Obs.de Sierra Nevada
Optical Mazaeva et al. 22789 AbAO optical observations Abastumani Astro. Obs.

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

Filter Tstart(s) Tstop(s) Exp(s) Mag
b 556 1872 136 >19.63
uvm2 654 1797 136 >18.91
u 300 1847 362 >20.06
v 630 1773 136 >18.75
uvw1 679 1823 136 >19.03
uvw2 605 1922 155 >19.20
white 87 1897 430 >21.15
whiteFC 87 237 147 >20.75

Table 3. UVOT observations reported by Gropp (GCN Circ. 22787). 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.

June 17, 2018