S.J. LaPorte (PSU), V. D'Elia (ASDC) and M.H. Siegel (PSU) for the Swift team
At 00:43:13 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180618A (trigger=842475) (LaPorte et al. GCN Circ. 22790). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 66° from the Sun (5.6 hours East) and 61° from the 24%-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.
LaPorte et al. (GCN Circ. 22790) reported the discovery with UVOT of an optical afterglow. 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 Sakamoto et al. (GCN Circ. 22796),
the BAT ground-calculated position is RA, Dec = 169.948, 73.825 deg which is RA(J2000) = 1
The mask-weighted light curve (Figure 1) shows a short multi-peaked structure from ~
The time-averaged spectrum from T-0.06 to T+56.33 s is best fit by a simple power-law model.
The power law index of the time-averaged spectrum is 1.39 ± 0.27.
The fluence in the 15-150 keV band is 6.7 ± 1.0 x 1
Using a 4-ms binned light curve (Figure 1) , the lag analysis of the first two tallest pulses (from ~
The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/842475/BA/.
Analysis of the initial XRT data was reported by D'Elia et al. (GCN Circ. 22793). We have analysed 25 ks of XRT data for GRB 180618A, from 83 s to 224.3 ks after the BAT trigger. The data comprise 293 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 22791).
The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=0.65 (+0.20, -0.29). At T+158 s the decay steepens to an α of 1.81 ± 0.05 before breaking again at T+12.2 ks to a final decay with index α=0.1 (+0.9, -1.0).
A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.91 ± 0.09. The best-fitting absorption column is 2.0 (+0.4, -0.3) x 1
A summary of the PC-mode spectrum is thus:
Total column: 3.0 (+0.9, -0.8) x 1
Galactic foreground: 4.4 x 1
Excess significance: 5.6 σ
Photon index: 2.25 (+0.22, -0.21)
The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00842475.
The Swift/UVOT began settled observations of the field of GRB 180618A 86 s after the BAT trigger
(Siegel and LaPorte GCN Circ. 22810).
We confirm the previously reported fading optical afterglow, also detected by LT (Guidorzi et al., GCN Circ. 22792) and MASTER (Tyurina et al., GCN Circ. 22797).
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 |
|---|---|---|---|---|
| 1 |
+73°50'13.5" | 0.43" | UVOT-refined | Siegel and LaPorte GCN Circ. 22810 |
| 1 |
+73°50'14.3" | 1.4" | XRT-final | UKSSDC |
| 1 |
+73°50'14.3" | 1.7" | XRT-enhanced | Goad et al. GCN Circ. 22791 |
| 1 |
+73°49'29.3" | 1.5' | BAT-refined | Sakamoto et al. GCN Circ. 22796 |
| Band | Authors | GCN Circ. | Subject | Observatory | Notes |
|---|---|---|---|---|---|
| Optical | Guidorzi et al. | 22792 | LT prompt observations | Liverpool Telescope | detection |
| Optical | Tyurina et al. | 22797 | MASTER OT observation (a short GRB with extended emission) |
MASTER | |
| Optical | Zhu et al. | 22804 | Xinglong-2.16m optical upper limit | Xinglong | upper limits |
| Optical | Mazaeva et al. | 22809 | TSHAO optical upper limit | Zeiss-1000 | upper limits |
| Gamma-ray | Hamburg and Bissaldi | 22794 | Fermi GBM detection | Fermi GBM | Fluence=2.12±0.12x1 (3 |
| Gamma-ray | Svinkin et al. | 22822 | Konus-Wind observation of GRB 180618A (a short GRB with extended emission) |
Konus-Wind | Duration=~0.250 seconds Fluence=6.24(-1.13,+1.20)x1 |
| Gamma-ray | Sharma et al. | 22842 | AstroSat CZTI detection | CZTI |
| Filter | Exp(s) | Mag | ||
|---|---|---|---|---|
| white (fc) | 86 | 236 | 147 | 17.46 ± 0.04 |
| white | 86 | 1544 | 390 | 17.95 ± 0.03 |
| white | 5680 | 7315 | 393 | >21.09 |
| v | 628 | 1596 | 117 | 18.71 ± 0.22 |
| v | 6090 | 6290 | 196 | >19.25 |
| b | 553 | 1175 | 58 | 18.84 ± 0.16 |
| b | 5475 | 7110 | 393 | >20.39 |
| u (fc) | 298 | 548 | 246 | 17.50 ± 0.06 |
| u | 298 | 1497 | 313 | 17.62 ± 0.06 |
| u | 5270 | 6905 | 393 | >19.94 |
| w1 | 676 | 1472 | 97 | 18.59 ± 0.24 |
| w1 | 5065 | 6700 | 393 | >19.63 |
| m2 | 1254 | 1621 | 58 | >17.6 |
| w2 | 604 | 1398 | 97 | 18.85 ± 0.28 |
Table 3. UVOT observations reported by Siegel and LaPorte (GCN Circ. 22810). The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary detections and 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.
June 26, 2018