Swift Observations of GRB 180514A

S.J. LaPorte (PSU), A. Tohuvavohu (PSU) and A.A. Breeveld (UCL-MSSL) for the Swift team

1. Introduction

At 13:25:33 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180514A (trigger=832368) (LaPorte et al. GCN Circ. 22723). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 116° from the Sun (9.8 hours East) and 128° 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 Krimm et al. (GCN Circ. 22727), the BAT ground-calculated position is RA, Dec = 197.382, 36.970 deg which is RA(J2000) = 13h09m31.7s Dec(J2000) = +36°58'13.0" with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 91%.

The mask-weighted light curve (Figure 1) shows a weak pulse that starts at ~T-8 s, peaks at ~T0, and ends at ~T+55 s. T90 (15-350 keV) is 46.9 ± 14.5 s (estimated error including systematics).

The time-averaged spectrum from T-8.33 to T+54.71 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.77 ± 0.14. The fluence in the 15-150 keV band is 1.0 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 38% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+0.23 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/832368/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Tohuvavohu et al. (GCN Circ. 22726). We have analysed 12 ks of XRT data for GRB 180514A, from 84 s to 126.0 ks after the BAT trigger. The data comprise 88 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 22725).

The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=2.4 (+0.5, -0.7). At T+136 s the decay steepens to an α of 5.0 (+1.3, -0.5) before breaking again at T+374 s to a final decay with index α=0.53 (+0.07, -0.08).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 3.7 (+0.4, -0.3). The best-fitting absorption column is 2.9 (+0.7, -0.6) x 1021 cm-2, in excess of the Galactic value of 1.2 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.84 (+0.29, -0.27) and a best-fitting absorption column of 6.6 (+7.4, -5.5) x 1020 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10-11 (4.0 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 6.6 (+7.4, -5.5) x 1020 cm-2
Galactic foreground: 1.2 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.84 (+0.29, -0.27)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00832368.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180514A 87 s after the BAT trigger (Breeveld and LaPorte GCN Circ. 22733). No optical afterglow consistent with the enhanced XRT position (Goad et al. GCN Circ. 22725) 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 0.01 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
13h09m28.94s +36°58'20.5" 1.7" XRT-final UKSSDC
13h09m29.07s +36°58'20.3" 2.4" XRT-enhanced Goad et al. GCN Circ. 22725
13h09m31.7s +36°58'13.0" 1.3' BAT-refined Krimm et al. GCN Circ. 22727

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Gompertz et al. 22728 GOTO Optical Observations
Optical de Ugarte Postigo et al. 22729 1.5m OSN optical observations Obs.de Sierra Nevada
Optical de Ugarte Postigo et al. 22731 Detection limits from 2.5m NOT NOT upper limits
Optical Itoh et al. 22732 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Volnova et al. 22735 CrAO/ZTSH optical observations and
afterglow candidate		 CrAO detection
Optical Horiuchi et al. 22736 MITSuME Ishigakijima optical upper
limits		 MITSuME Ishigakijima 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
whiteFC 87 237 147 >20.8
uFC 300 549 246 >20.2
white 87 7353 688 >22.8
v 6129 6329 197 >20.1
b 5513 7148 393 >20.7
u 300 6943 639 >21.6
w1 5104 6739 393 >20.8
m2 6334 6533 197 >20.1
w2 5924 7540 374 >20.3

Table 3. UVOT observations reported by Breeveld and LaPorte (GCN Circ. 22733). 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.

May 17, 2018