T.N. Ukwatta (LANL), A. Melandri (INAF-OAB) and M.H. Siegel (PSU) for the Swift team
At 15:22:22 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180924A (trigger=863186) (Ukwatta et al. GCN Circ. 23261). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 110° from the Sun (8.8 hours West) and 66° from the 100%-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 Stamatikos et al. (GCN Circ. 23264),
the BAT ground-calculated position is RA, Dec = 49.207, -58.524 deg, which is RA(J2000) = 0
The mask-weighted light curve (Figure 1) shows a couple peaks starting at ~T-90 s, peaking at ~T=50 and ~
The time-averaged spectrum from T-85.15 to T+26.35 s is best fit by a simple power-law model.
The power law index of the time-averaged spectrum is 1.94 ± 0.14.
The fluence in the 15-150 keV band is 1.4 ± 0.1 x 1
The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/863186/BA/.
Analysis of the initial XRT data was reported by Melandri et al. (GCN Circ. 23266). We have analysed 11 ks of XRT data for GRB 180924A, from 82 s to 189.9 ks after the BAT trigger. The data comprise 141 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 23262).
The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=1.75 (+0.28, -0.25). At T+166 s the decay steepens to an α of 2.81 (+1.48, -0.29). The light curve breaks again at T+476 s to a decay with α=-1.42 (+2.05, -0.08), before a final break at T+895 s s after which the decay index is 1.05 (+0.14, -0.12).
A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.96 (+0.14, -0.13). The best-fitting absorption column is 1.0 ± 0.4 x 1
A summary of the PC-mode spectrum is thus:
Total column: 6.9 (+5.2, -4.5) x 1
Galactic foreground: 2.0 x 1
Excess significance: 1.8 σ
Photon index: 1.88 (+0.20, -0.19)
The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00863186.
The Swift/UVOT began settled observations of the field of GRB 180924A 83 s after the BAT trigger
(Siegel and Ukwatta GCN Circ. 23277).
No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 23262) 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
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 |
---|---|---|---|---|
0 |
-58°31'57.1" | 1.5" | XRT-final | UKSSDC |
0 |
-58°31'57.3" | 1.7" | XRT-enhanced | Evans et al. GCN Circ. 23262 |
0 |
-58°31'27.6" | 1.4' | BAT-refined | Stamatikos et al. GCN Circ. 23264 |
Band | Authors | GCN Circ. | Subject | Observatory | Notes |
---|---|---|---|---|---|
Optical | Lipunov et al. | 23267 | MASTER inspection | MASTER |
Filter | Exp(s) | Mag | ||
---|---|---|---|---|
whit |
83 | 233 | 147 | >20.8 |
241 | 491 | 246 | >19.7 | |
white | 83 | 542 | 167 | >20.9 |
v | 572 | 592 | 19 | >17.1 |
b | 497 | 517 | 19 | >18.4 |
u | 241 | 491 | 246 | >19.7 |
Table 3. UVOT observations reported by Siegel and Ukwatta (GCN Circ. 23277). 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.
September 28, 2018