K.L. Page (U. Leicester), J.P. Osborne (U. Leicester) and N.P.M. Kuin (UCL-MSSL) for the Swift team
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.
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) = 2
The mask-weighted light curve (Figure 1) shows two distict pulses.
The first pulse starts and peaks at ~
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 1
The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/808483/BA/.
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 1
A summary of the PC-mode spectrum is thus:
Total column: 7.6 (+1.4, -1.3) x 1
Galactic foreground: 1.0 x 1
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.
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
Figure 1. The BAT
mask-weighted light curve in the four individual and total
energy bands. The units are counts
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 |
---|---|---|---|---|
2 |
+30°50'16.2" | 1.4" | XRT-final | UKSSDC |
2 |
+30°50'16.2" | 1.4" | XRT-enhanced | Osborne et al. GCN Circ. 22377 |
2 |
+30°51'07.5" | 1.1' | BAT-refined | Cummings et al. GCN Circ. 22380 |
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 | Fluence=8.48±0.32x1 (7 |
Gamma-ray | Kozlova et al. | 22385 | Konus-Wind observation | Konus-Wind | Fluence=3.81(-0.57,+0.66)x1 |
Filter | Exp(s) | Mag | ||
---|---|---|---|---|
whit |
92 | 242 | 147 | >20.3 |
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