R.L.C. Starling (U. Leicester), A. Tohuvavohu (PSU) and S.J. LaPorte (PSU) for the Swift team
At 17:29:00 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180728A (trigger=850471) (Starling et al. GCN Circ. 23046). Swift did not slew immediately due to the Earth limb constraint. At the time of the trigger, the initial BAT position was 126° from the Sun (8.4 hours East) and 61° from the 99%-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.
Lipunov et al. (GCN Circ. 23050) reported the position from MASTER for the optical afterglow of this GRB. LaPorte and Starling (GCN Circ. 23064) reported the detection with UVOT of an optical afterglow. Rossi et al. (GCN Circ. 23055) determined a redshift of 0.117 from VLT. 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 Markwardt et al. (GCN Circ. 23054),
the BAT ground-calculated position is RA, Dec = 253.574, -54.037 deg which is RA(J2000) = 1
The mask-weighted light curve (Figure 1) shows a relatively faint, soft precursor near the trigger time, lasting about 3 seconds.
This was followed by a single, much brighter pulse, which consisted of two overlapping peaks.
This pulse began at T+11 s, peaked at T+13 s, and faded to background by T+40 s.
The time-averaged spectrum from T-0.24 to T+36.51 s is best fit by a simple power-law model.
The power law index of the time-averaged spectrum is 1.97 ± 0.03.
The fluence in the 15-150 keV band is 2.96 ± 0.06 x 1
The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/850471/BA/.
Analysis of the initial XRT data was reported by Tohuvavohu et al. (GCN Circ. 23052). We have analysed 113 ks of XRT data for GRB 180728A, from 1.7 ks to 2562.1 ks after the BAT trigger. The data comprise 2.0 ks 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. 23051).
The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=0.56 ± 0.05. At T+5370 s the decay steepens to an α of 1.45 (+0.20, -0.15). The light curve breaks again at T+14.1 ks to a decay with α=0.43 (+0.26, -0.57), before a final break at T+23.5 ks s after which the decay index is 1.299 ± 0.020.
A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.072 (+0.018, -0.016). The best-fitting absorption column is consistent with the Galactic value of 3.2 x 1
A summary of the PC-mode spectrum is thus:
Galactic foreground: 3.2 x 1
Intrinsic column: 3.16 (+0.20, -0.00) x 1
Photon index: 1.769 (+0.044, -0.027)
The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00850471.
The Swift/UVOT began settled observations of the field of GRB 180728A 1740 s after the BAT trigger
(LaPorte and Starling GCN Circ. 23064).
A source consistent with the enhanced XRT position (Osborne et al. GCN Circ. 23051) 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 |
|---|---|---|---|---|
| 1 |
-54°02'40.2" | 0.43" | UVOT-refined | LaPorte and Starling GCN Circ. 23064 |
| 1 |
-54°02'40.2" | 1.4" | XRT-final | UKSSDC |
| 1 |
-54°02'40.3" | 1.5" | XRT-enhanced | Osborne et al. GCN Circ. 23051 |
| 1 |
-54°02'13.7" | 1.0' | BAT-refined | Markwardt et al. GCN Circ. 23054 |
| Band | Authors | GCN Circ. | Subject | Observatory | Notes |
|---|---|---|---|---|---|
| Optical | Lipunov et al. | 23048 | MASTER OT (= new object) detection | MASTER | detection |
| Optical | Lipunov et al. | 23050 | MASTER OT J165415.75-540239.27 is the Optical Counterpart of GRB |
MASTER | detection |
| Optical | Rossi et al. | 23055 | VLT/X-shooter redshift of a nearby energetic GRB |
VLT | redshift |
| Optical | Heintz et al. | 23067 | No evidence of SN in early VLT/X-shooter spectra |
VLT | |
| Optical | Lipunov et al. | 23143 | MASTER early polarization and 19 days monitoring |
MASTER | |
| X-ray | Ruffini et al. | 23066 | A long GRB of the X-ray flash (XRF) subclass, expecting supernova appearance |
||
| Gamma-ray | Veres et al. | 23053 | Fermi GBM observation | Fermi GBM | Fluence=5.65±0.02x1 (95.8 percentile for long GRBs) |
| Gamma-ray | Frederiks et al. | 23061 | Konus-Wind observation of the bright nearby GRB 180728A |
Konus-Wind | |
| Gamma-ray | Sharma et al. | 23102 | AstroSat CZTI detection | CZTI | |
| Other | Izzo et al. | 23142 | discovery of the associated supernova | ||
| Other | Selsing et al. | 23181 | classification of the associated SN 2018fip |
| Filter | Exp(s) | Mag | ||
|---|---|---|---|---|
| white | 1740 | 1890 | 147 | 16.25 ± 0.03 |
| white | 7713 | 15148 | 465 | 17.28 ± 0.03 |
| v | 1897 | 1917 | 20 | 16.07 ± 0.14 |
| b | 1995 | 2013 | 18 | 16.58 ± 0.10 |
| u | 1970 | 1990 | 20 | 15.93 ± 0.10 |
| w1 | 1946 | 1966 | 20 | 16.20 ± 0.17 |
| m2 | 1921 | 2114 | 39 | 16.60 ± 0.18 |
| w2 | 2045 | 2065 | 19 | 16.37 ± 0.19 |
Table 3. UVOT observations reported by LaPorte and Starling (GCN Circ. 23064). 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.
August 28, 2018