Swift Observations of GRB 180204A

K.L. Page (U. Leicester), J.P. Osborne (U. Leicester) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

At 02:36:16 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180204A (trigger=808483) (Page et al. GCN Circ. 22375). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 49° from the Sun (0.8 hours East) and 134° from the 84%-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. 22380), the BAT ground-calculated position is RA, Dec = 330.123, 30.852 deg which is RA(J2000) = 22h00m29.5s Dec(J2000) = +30°51'07.5" with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 70%.

The mask-weighted light curve (Figure 1) shows two distict pulses. The first pulse starts and peaks at ~T0, and ends at ~T+0.2. The second pulse starts at ~T+0.6 s, peaks at ~T+1 s, and ends at ~T+1.3 s. T90 (15-350 keV) is 1.16 ± 0.12 s (estimated error including systematics).

The time-averaged spectrum from T-0.05 to T+1.29 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.09 ± 0.10. The fluence in the 15-150 keV band is 4.8 ± 0.3 x 10-7 erg cm-2. This fluence is larger than that of 95% of the short GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T-0.05 s in the 15-150 keV band is 3.6 ± 0.3 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/808483/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Osborne et al. (GCN Circ. 22376). We have analysed 36 ks of XRT data for GRB 180204A, from 90 s to 269.5 ks after the BAT trigger. The data comprise 32 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. 22377).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.114 ± 0.030.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.99 (+0.18, -0.17). The best-fitting absorption column is 7.6 (+1.4, -1.3) x 1021 cm-2, in excess of the Galactic value of 1.0 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 4.5 x 10-11 (8.0 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 7.6 (+1.4, -1.3) x 1021 cm-2
Galactic foreground: 1.0 x 1021 cm-2
Excess significance: 8.4 σ
Photon index: 1.99 (+0.18, -0.17)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00808483.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 180204A 92 s after the BAT trigger (Kuin and Page GCN Circ. 22379). A star is present in the GSC whos position is consistent with the enhanced XRT position (Osborne et al. GCN Circ. 22377). 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 0.10 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
22h00m31.98s +30°50'16.2" 1.4" XRT-final UKSSDC
22h00m31.99s +30°50'16.2" 1.4" XRT-enhanced Osborne et al. GCN Circ. 22377
22h00m29.5s +30°51'07.5" 1.1' BAT-refined Cummings et al. GCN Circ. 22380

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Morita et al. 22399 MITSuME Okayama optical upper limits MITSuME Okayama upper limits
Radio Bright et al. 22422 15 GHz upper limits from AMI AMI upper limits
Gamma-ray Veres and Meegan 22378 Fermi GBM detection Fermi GBM Epeak=781±161 keV
T90=1.2 seconds
Fluence=8.48±0.32x10-7erg cm-2
(77th percentile for short GRBs)
Gamma-ray Kozlova et al. 22385 Konus-Wind observation Konus-Wind Epeak=843 (-177,+263) keV
Fluence=3.81(-0.57,+0.66)x10-6erg cm-2

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 92 242 147 >20.3
uFC 304 554 246 >19.9
white 92 17884 1210 >20.8
v 633 10587 1107 >19.0
b 560 23622 1840 >20.7
u 304 23062 2409 >21.1
w1 3943 22148 1917 >21.0
m2 658 11492 1095 >20.8
w2 609 6398 413 >20.1

Table 3. UVOT observations reported by Kuin and Page (GCN Circ. 22379). 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.

February 20, 2018