P.A. Evans (U. Leicester), A. D'Ai (INAF-IASFPA) and A.A. Breeveld (UCL-MSSL) for the Swift team
At 08:34:58 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180620A (trigger=843122) (Evans et al. GCN Circ. 22798). Swift slewed to the burst with a 3 min delay. At the time of the trigger, the initial BAT position was 132° from the Sun (11.3 hours West) and 98° from the 49%-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.
Evans et al. (GCN Circ. 22798) reported the discovery with UVOT of an optical afterglow. Elenin et al. (GCN Circ. 22800) reported the position for the optical afterglow of this GRB. 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 Stamatikos et al. (GCN Circ. 22815),
the BAT ground-calculated position is RA, Dec = 279.890, 23.239 deg which is RA(J2000) = 1
The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T-7 s and ends at ~T+37 s.
The two main peaks occur at ~T+5 s and ~T+12 s, respectively.
The time-averaged spectrum from T-6.92 to T+37.07 s is best fit by a simple power-law model.
The power law index of the time-averaged spectrum is 1.72 ± 0.05.
The fluence in the 15-150 keV band is 5.8 ± 0.2 x 1
The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/843122/BA/.
Analysis of the initial XRT data was reported by D'Ai et al. (GCN Circ. 22808). We have analysed 20 ks of XRT data for GRB 180620A, from 274 s to 283.3 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 22802).
The late-time light curve (Figure 2) (from T0+6.1 ks) can be modelled with a power-law decay with a decay index of α=2.21 (+0.11, -0.10).
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.46 (+0.10, -0.08). The best-fitting absorption column is 1.76 (+0.42, -0.16) x 1
A summary of the PC-mode spectrum is thus:
Total column: 1.76 (+0.42, -0.16) x 1
Galactic foreground: 1.6 x 1
Excess significance: <1.6 σ
Photon index: 1.46 (+0.10, -0.08)
The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00843122.
The Swift/UVOT began settled observations of the field of GRB 180620A 270 s after the BAT trigger
(Breeveld and Evans GCN Circ. 22805).
A fading source consistent with the XRT position (Osborne et al. GCN Circ. 22802), and also detected by Butler et al. (GCN Circ. 22799), Elenin et al. (GCN Circ. 22800), Tyurina et al. (GCN Circ. 22801) and Guidorzi et al. (GCN Circ. 22803), 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 |
+23°14'39.1" | 0.43" | UVOT-refined | Breeveld and Evans GCN Circ. 22805 |
| 1 |
+23°14'38.9" | 1.4" | XRT-final | UKSSDC |
| 1 |
+23°14'39.0" | 1.8" | XRT-enhanced | Osborne et al. GCN Circ. 22802 |
| 1 |
+23°14'18.6" | 1.0' | BAT-refined | Stamatikos et al. GCN Circ. 22815 |
| Band | Authors | GCN Circ. | Subject | Observatory | Notes |
|---|---|---|---|---|---|
| Optical | Butler et al. | 22799 | RATIR Optical Afterglow Detection | RATIR | detection |
| Optical | Tyurina et al. | 22801 | MASTER OT observation | MASTER | |
| Optical | Guidorzi et al. | 22803 | LCO McDonald observations | LCO | detection |
| Optical | Watson et al. | 22806 | COATLI Detection and Light Curve | COATLI | light curve |
| Optical | Zhu et al. | 22811 | Xinglong-2.16m optical observations | Xinglong | detection |
| Optical | Zheng and Filippenko | 22812 | KAIT Optical Observations | KAIT | detection |
| Optical | Schweyer and Schady | 22818 | GROND observations | GROND | detection |
| Optical | Nakamura and Sakamoto | 22826 | iTelescope observation | iTelescope | |
| Optical | Cunningham and Neill | 25671 | SEDM Observations | Spectral Energy Distribution Machine at Palomar | |
| Gamma-ray | Svinkin et al. | 22824 | Konus-Wind observation | Konus-Wind | Duration=~16 seconds Fluence=9.80(-1.02,+1.74)x1 |
| Gamma-ray | Sharma et al. | 22843 | AstroSat CZTI detection | CZTI | |
| Other | Elenin et al. | 22800 | ISON-MN afterglow confirmation | detection |
| Filter | Exp(s) | Mag | ||
|---|---|---|---|---|
| white | 270 | 420 | 147 | 17.68 ± 0.04 |
| v | 427 | 1420 | 117 | 18.43 ± 0.19 |
| b | 525 | 717 | 39 | 18.77 ± 0.20 |
| u | 500 | 692 | 39 | 17.92 ± 0.17 |
| w1 | 476 | 2158 | 194 | 18.86 ± 0.20 |
| m2 | 623 | 7111 | 391 | 19.65 ± 0.27 |
| w2 | 574 | 8136 | 588 | 20.00 ± 0.23 |
Table 3. UVOT observations reported by Breeveld and Evans (GCN Circ. 22805). 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.
September 6, 2019