Swift Observations of GRB 180602A

S.B. Cenko (GSFC), J.P. Osborne (U. Leicester) and J. Gropp (PSU) for the Swift team

1. Introduction

At 08:14:50 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180602A (trigger=835200) (Cenko et al. GCN Circ. 22752). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 64° from the Sun (4.8 hours East) and 152° from the 88%-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 Lien et al. (GCN Circ. 22755), the BAT ground-calculated position is RA, Dec = 142.343, 32.080 deg which is RA(J2000) = 09h29m22.4s Dec(J2000) = +32°04'48.1" with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment). The partial coding was 78%.

The mask-weighted light curve (Figure 1) shows a single-peaked structure that starts at ~T-30 s, peaks at ~T-20 s, and ends at ~T+25 s. We note that the burst emission indeed started and peaked during a pre-planned spacecraft slew, and the trigger occurred near the end of the pulse when the slew settled. T90 (15-350 keV) is 45.4 ± 7.1 s (estimated error including systematics).

The time-averaged spectrum from T-29.32 to T+25.73 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.72 ± 0.11. The fluence in the 15-150 keV band is 1.6 ± 0.1 x 10-6 erg cm-2. This fluence is larger than that of 53% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-20.12 s in the 15-150 keV band is 0.9 ± 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/835200/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Osborne et al. (GCN Circ. 22756). We have analysed 9.1 ks of XRT data for GRB 180602A, from 130 s to 63.7 ks after the BAT trigger. The data comprise 85 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 22753).

The late-time light curve (Figure 2) (from T0+5.6 ks) can be modelled with a power-law decay with a decay index of α=1.45 (+0.17, -0.13).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.36 (+0.28, -0.25). The best-fitting absorption column is 4.0 (+1.3, -1.1) x 1021 cm-2, in excess of the Galactic value of 1.6 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.04 (+0.16, -0.15) and a best-fitting absorption column of 4.2 (+0.8, -0.7) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.8 x 10-11 (6.2 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.2 (+0.8, -0.7) x 1021 cm-2
Galactic foreground: 1.6 x 1020 cm-2
Excess significance: 9.1 σ
Photon index: 2.04 (+0.16, -0.15)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00835200.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180602A 133 s after the BAT trigger (Gropp and Cenko GCN Circ. 22758). No optical afterglow consistent with the enhanced XRT position (Osborne et al. GCN Circ. 22753) 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
09h29m19.65s +32°03'60.0" 1.7" XRT-final UKSSDC
09h29m19.63s +32°04'01.6" 1.8" XRT-enhanced Osborne et al. GCN Circ. 22753
09h29m22.4s +32°04'48.1" 1.5' BAT-refined Lien et al. GCN Circ. 22755

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Tachibana et al. 22754 MITSuME Okayama optical upper limits MITSuME Okayama upper limits
Optical Murata et al. 22757 MITSuME Akeno optical upper limits MITSuME Akeno upper limits
Optical Horiuchi et al. 22760 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 133 283 147 >19.9
uFC 291 541 246 >19.8
white 133 1715 411 >20.8
v 621 2283 194 >19.0
b 547 2208 175 >20.0
u 291 2183 401 >20.3
w1 670 2159 175 >20.0
m2 645 665 19 >18.7
w2 597 2086 136 >20.4

Table 3. UVOT observations reported by Gropp and Cenko (GCN Circ. 22758). 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 5, 2018