Swift Observations of GRB 180512A

A. Deich (PSU), D.N. Burrows (PSU) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 22:01:47 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180512A (trigger=832119) (Deich et al. GCN Circ. 22710). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 132° from the Sun (10.2 hours East) and 156° from the 8%-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 Cummings et al. (GCN Circ. 22720), the BAT ground-calculated position is RA, Dec = 201.940, 21.404 deg which is RA(J2000) = 13h27m45.5s Dec(J2000) = +21°24'15.7" with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 67%.

The mask-weighted light curve (Figure 1) shows a weak pulse that starts at ~T+30 s, peaks at ~T+40 s, and ends at ~T+55 s. The short spike at T+54.4 s is due to detector noise. T90 (15-350 keV) is 24.0 ± 7.1 s (estimated error including systematics). Note that the burst went out of the BAT FOV at T+469 s. The large pulse seen in the BAT raw light curve from ~T+400 s to ~T+550 s was from a bright background source that came in and out of the BAT FOV during this time interval.

The time-averaged spectrum from T+29.00 to T+55.00 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 2.10 ± 0.27. The fluence in the 15-150 keV band is 4.2 ± 0.7 x 10-7 erg cm-2. This fluence is larger than that of 18% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+39.00 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/832119/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Burrows et al. (GCN Circ. 22721). We have analysed 8.4 ks of XRT data for GRB 180512A, from 230 s to 101.5 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 22715).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.87 (+0.12, -0.11).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.5 (+0.9, -0.8). The best-fitting absorption column is 2.3 (+1.5, -1.1) x 1022 cm-2, in excess of the Galactic value of 1.7 x 1020 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.2 x 10-11 (1.9 x 10-10) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.3 (+1.5, -1.1) x 1022 cm-2
Galactic foreground: 1.7 x 1020 cm-2
Excess significance: 3.6 σ
Photon index: 2.5 (+0.9, -0.8)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00832119.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180512A 218 s after the BAT trigger (Kuin and Deich GCN Circ. 22717). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 22715) 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 0.02 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
13h27m45.13s +21°24'12.8" 1.9" XRT-final UKSSDC
13h27m45.13s +21°24'12.8" 1.9" XRT-enhanced Evans et al. GCN Circ. 22715
13h27m45.5s +21°24'15.7" 1.9' BAT-refined Cummings et al. GCN Circ. 22720

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical de Ugarte Postigo et al. 22711 Upper limits from OSN Obs.de Sierra Nevada upper limits
Optical Izzo 22712 OASDG optical observations S.Di Giacomo Obs.
Optical Heintz et al. 22713 NOT upper limits NOT upper limits
Optical Bolmer 22714 GROND upper limits GROND upper limits
Optical Guidorzi et al. 22716 LT prompt observations Liverpool Telescope
Optical Rossi and Palazzi 22718 LBT optical detection LBT detection
Optical Watson et al. 22719 RATIR Optical Observations RATIR upper limits
Optical Rossi et al. 22724 VLT/HAWK-I NIR Observations VLT

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 218 368 147 >21.1
white 218 4358 344 >21.2
v 5986 6120 132 >19.0
b 3953 4153 197 >20.7
u 3748 16254 422 >20.7
w1 3543 16019 1107 >20.3
m2 9380 10280 886 >20.9
w2 5781 5981 197 >20.1

Table 3. UVOT observations reported by Kuin and Deich (GCN Circ. 22717). 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 15, 2018