A.P. Beardmore (U. Leicester), V. D'Elia (ASDC) and F.E. Marshall (NASA/GSFC) for the Swift team
At 14:15:28 UT, the Swift Burst Alert Telescope triggered and located GRB180727A (Trigger #850231) (Beardmore et al. GCN Circ. 23043). 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 Lien et al. (GCN Circ. 23047),
the BAT ground-calculated position is RA, Dec = 346.639, -63.066 deg which is RA(J2000) = 2
The mask-weighted light curve (Figure 1) shows a single peak of duration ~1 second, peaking at T+0 s.
The time-averaged spectrum from T-0.1 to T+1.3 s is best fit by a power law with an exponential cutoff.
This fit gives a photon index 0.59 ± 0.51, and
The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/850231/BA/.
Analysis of the initial XRT data was reported by D'Elia et al. (GCN Circ. 23045). We have analysed 9.0 ks of XRT data for GRB 180727A, from 58 s to 96.9 ks after the BAT trigger. The data comprise 24 s in Windowed Timing (WT) mode (the first 7 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode.
The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=2.4 (+0.3, -0.4).
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.9 ± 0.4. The best-fitting absorption column is 2.1 (+1.8, -1.4) x 1
A summary of the PC-mode spectrum is thus:
Total column: 2.1 (+1.8, -1.4) x 1
Galactic foreground: 2.1 x 1
Excess significance: 2.2 σ
Photon index: 1.9 ± 0.4
The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00850231.
The Swift/UVOT began settled observations of the field of GRB 180727A 77 s after the BAT trigger
(Marshall and Beardmore GCN Circ. 23062).
No optical afterglow consistent with the enhanced XRT position (D'Elia et al., GCN Circ. 23045) 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 |
---|---|---|---|---|
2 |
-63°03'06.6" | 1.7" | XRT-final | UKSSDC |
2 |
-63°03'06.6" | 1.7" | XRT-refined | D'Elia et al. GCN Circ. 23045 |
2 |
-63°03'57.9" | 1.1' | BAT-refined | Lien et al. GCN Circ. 23047 |
Band | Authors | GCN Circ. | Subject | Observatory | Notes |
---|---|---|---|---|---|
Gamma-ray | Veres | 23044 | Fermi GBM observation | Fermi GBM | Fluence=3.32±0.18x1 (5 |
Filter | Exp(s) | Mag | ||
---|---|---|---|---|
whit |
77 | 227 | 147 | >20.9 |
290 | 303 | 12 | >18.4 | |
white | 4280 | 5915 | 393 | >21.7 |
v | 4690 | 6312 | 379 | >19.6 |
b | 4075 | 5710 | 393 | >21.2 |
u | 5305 | 5505 | 197 | >20.3 |
uvw1 | 5101 | 5301 | 197 | >20.0 |
uvm2 | 4895 | 5095 | 197 | >21.0 |
uvw2 | 4485 | 6122 | 393 | >21.4 |
Table 3. UVOT observations reported by Marshall and Beardmore (GCN Circ. 23062). 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.
August 3, 2018