P. D'Avanzo (INAF-OAB), P.A. Evans (U. Leicester) and S.J. LaPorte (PSU) for the Swift team
At 11:11:51 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180630A (trigger=845443) (D'Avanzo et al. GCN Circ. 22872). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 112° from the Sun (3.5 hours West) and 71° from the 95%-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.
D'Avanzo et al. (GCN Circ. 22872) 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 Lien et al. (GCN Circ. 22877),
the BAT ground-calculated position is RA, Dec = 48.885, -87.476 deg which is RA(J2000) = 0
The mask-weighted light curve (Figure 1) shows a double-peaked structure.
The first pulse starts at
The time-averaged spectrum from T-0.36 to T+30.02 s is best fit by a simple power-law model.
The power law index of the time-averaged spectrum is 2.07 ± 0.10.
The fluence in the 15-150 keV band is 1.8 ± 0.1 x 1
The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/845443/BA/.
Analysis of the initial XRT data was reported by Evans et al. (GCN Circ. 22876). We have analysed 9.1 ks of XRT data for GRB 180630A, from 87 s to 121.5 ks after the BAT trigger. The data comprise 69 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 22873).
The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=3.0 (+1.1, -0.5). At T+146 s the decay flattens to an α of 0.48 (+0.12, -0.26) before breaking again at T+2147 s to a final decay with index α=0.99 (+0.08, -0.10).
A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.91 (+0.21, -0.17). The best-fitting absorption column is consistent with the Galactic value of 1.3 x 1
A summary of the PC-mode spectrum is thus:
Total column: 1.7 (+0.5, -0.4) x 1
Galactic foreground: 1.3 x 1
Excess significance: <1.6 σ
Photon index: 1.66 ± 0.12
The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00845443.
The Swift/UVOT began settled observations of the field of GRB 180630A 88 s after the BAT trigger
(LaPorte and D'Avanzo GCN Circ. 22881).
A fading source consistent with the XRT position (Beardmore et al. GCN Circ. 22873) 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 |
-87°28'42.3" | 0.43" | UVOT-refined | LaPorte and D'Avanzo GCN Circ. 22881 |
0 |
-87°28'43.5" | 1.4" | XRT-final | UKSSDC |
0 |
-87°28'43.7" | 1.7" | XRT-enhanced | Beardmore et al. GCN Circ. 22873 |
0 |
-87°28'33.4" | 1.0' | BAT-refined | Lien et al. GCN Circ. 22877 |
Band | Authors | GCN Circ. | Subject | Observatory | Notes |
---|---|---|---|---|---|
Gamma-ray | Hamburg et al. | 22875 | Fermi GBM detection | Fermi GBM | Fluence=2.5±0.1x1 (3 |
Filter | Exp(s) | Mag | ||
---|---|---|---|---|
white | 88 | 237 | 148 | 17.49 ± 0.04 |
white | 6384 | 6584 | 196 | 19.50 ± 0.11 |
v | 629 | 1597 | 117 | 18.56 ± 0.20 |
b | 555 | 748 | 39 | 18.76 ± 0.19 |
u | 300 | 549 | 246 | 17.76 ± 0.06 |
w1 | 678 | 7392 | 510 | 19.42 ± 0.17 |
m2 | 654 | 7187 | 452 | 20.16 ± 0.33 |
w2 | 605 | 6777 | 255 | >19.9 |
Table 3. UVOT observations reported by LaPorte and D'Avanzo (GCN Circ. 22881). 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.
July 2, 2018