S.L. Gibson (U. Leicester), B. Sbarufatti (INAF-OAB/PSU) and N.P.M. Kuin (UCL-MSSL) for the Swift team
At 13:49:40 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180624A (trigger=844192) (Gibson et al. GCN Circ. 22832). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 132° from the Sun (9.0 hours West) and 87° from the 88%-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.
Rossi et al. (GCN Circ. 22845) reported the position from LBT for the optical afterglow of this GRB. Rossi et al. (GCN Circ. 22845) determined a redshift of 2.86 from LBT. 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 Cummings et al. (GCN Circ. 22848),
the BAT ground-calculated position is RA, Dec = 318.096, -2.356 deg which is RA(J2000) = 2
The mask-weighted light curve (Figure 1) shows a multi-peaked light curve that starts at ~T-60 s and ends at ~T+540 s.
The largest peak occurs at ~T+7 s.
The burst went out of the BAT FOV at ~T+720 s.
The time-averaged spectrum from T-60.61 to T+541.80 s is best fit by a simple power-law model.
The power law index of the time-averaged spectrum is 1.91 ± 0.10.
The fluence in the 15-150 keV band is 5.8 ± 0.3 x 1
The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/844192/BA/.
Analysis of the initial XRT data was reported by Sbarufatti et al. (GCN Circ. 22837). We have analysed 18 ks of XRT data for GRB 180624A, from 100 s to 269.2 ks after the BAT trigger. The data comprise 569 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Goad et al. (GCN Circ. 22835).
The late-time light curve (Figure 2) (from T0+3.9 ks) can be modelled with a power-law decay with a decay index of α=0.95 ± 0.07.
A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.427 (+0.022, -0.021). The best-fitting absorption column is 1.28 (+0.15, -0.14) x 1
A summary of the PC-mode spectrum is thus:
Galactic foreground: 4.6 x 1
Intrinsic column: 9.2 (+6.0, -5.4) x 1
Photon index: 2.07 (+0.16, -0.15)
The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00844192.
The Swift/UVOT began settled observations of the field of GRB 180624A 118 s after the BAT trigger
(Kuin and Gibson GCN Circ. 22836).
No optical afterglow consistent with the XRT position 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 |
-02°20'17.0" | 1.4" | XRT-final | UKSSDC |
2 |
-02°20'17.7" | 1.5" | XRT-enhanced | Goad et al. GCN Circ. 22835 |
2 |
-02°21'21.0" | 1.3' | BAT-refined | Cummings et al. GCN Circ. 22848 |
Band | Authors | GCN Circ. | Subject | Observatory | Notes |
---|---|---|---|---|---|
Optical | Itoh et al. | 22838 | MITSuME Okayama optical upper limits | MITSuME Okayama | upper limits |
Optical | Guidorzi et al. | 22839 | LCO Siding Springs afterglow candidate | LCO | detection |
Optical | Schady | 22840 | GROND observations | GROND | detection |
Optical | Rossi et al. | 22845 | LBT/MODS redshift measurement | LBT | redshift |
Optical | Watson et al. | 22846 | RATIR Optical Observations | RATIR | |
Optical | Guidorzi et al. | 22858 | LCO Haleakala observations | LCO | |
Optical | Selsing et al. | 22867 | NOT optical counterpart | NOT | detection |
Filter | Exp(s) | Mag | ||
---|---|---|---|---|
whit |
118 | 268 | 147 | >20.7 |
276 | 526 | 246 | >19.4 | |
white | 118 | 6151 | 541 | >21.3 |
v | 4927 | 6395 | 229 | >20.0 |
b | 4312 | 5947 | 393 | >21.2 |
u | 276 | 5742 | 639 | >20.6 |
w1 | 3902 | 5537 | 393 | >20.2 |
m2 | 5132 | 5332 | 197 | >19.7 |
w2 | 4723 | 6357 | 393 | >20.8 |
Table 3. UVOT observations reported by Kuin and Gibson (GCN Circ. 22836). 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.
June 28, 2018