A. Melandri (INAF-OAB), S.L. Gibson (U. Leicester) and A.A. Breeveld (UCL-MSSL) for the Swift team
At 04:57:25 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180316A (trigger=814677) (Melandri et al. GCN Circ. 22500). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 90° from the Sun (6.0 hours West) and 77° from the 2%-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. 22503) reported the position from MASTER for the optical afterglow of this GRB. Breeveld and Melandri (GCN Circ. 22510) reported the detection 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 Cummings et al. (GCN Circ. 22512),
the BAT ground-calculated position is RA, Dec = 265.422, 0.737 deg which is RA(J2000) = 1
The mask-weighted light curve (Figure 1) shows a main structure with several overlapping pulses that starts at ~ T-15 s and ends at ~ T+30 s.
The major peak occurs at ~ T+1 s.
In addition, there are some weak emission before the main structure that starts at ~ T-80 s, and a weak tail afterward that lasts until ~ T+40 s.
The time-averaged spectrum from T-79.73 to T+37.27 s is best fit by a simple power-law model.
The power law index of the time-averaged spectrum is 1.46 ± 0.12.
The fluence in the 15-150 keV band is 4.9 ± 0.4 x 1
The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/814677/BA/.
Analysis of the initial XRT data was reported by Gibson et al. (GCN Circ. 22511). We have analysed 30 ks of XRT data for GRB 180316A, from 84 s to 528.9 ks after the BAT trigger. The data comprise 534 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 Evans et al. (GCN Circ. 22501).
The late-time light curve (Figure 2) (from T0+4.4 ks) can be modelled with a power-law decay with a decay index of α=1.33 ± 0.06.
A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.94 (+0.07, -0.04). The best-fitting absorption column is 2.038 (+0.286, -0.012) x 1
A summary of the PC-mode spectrum is thus:
Total column: 3.3 ± 0.7 x 1
Galactic foreground: 2.0 x 1
Excess significance: 3.1 σ
Photon index: 2.03 (+0.16, -0.15)
The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00814677.
The Swift/UVOT began settled observations of the field of GRB 180316A 105 s after the BAT trigger
(Breeveld and Melandri GCN Circ. 22510).
A fading source consistent with the enhanced XRT position (Evans et al. GCN Circ. 22508) and the optical transient reported by Lipunov et al. (GCN Circ. 22503), Shady & Steinmassl (GCN Circ. 22504) and Martone et al. (GCN Circ. 22507), Butler et al. (GCN Circ. 22509) 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 |
+00°44'53.7" | 0.42" | UVOT-refined | Breeveld and Melandri GCN Circ. 22510 |
| 1 |
+00°44'53.2" | 1.4" | XRT-final | UKSSDC |
| 1 |
+00°44'53.3" | 1.4" | XRT-enhanced | Evans et al. GCN Circ. 22508 |
| 1 |
+00°44'14.3" | 1.5' | BAT-refined | Cummings et al. GCN Circ. 22512 |
| Band | Authors | GCN Circ. | Subject | Observatory | Notes |
|---|---|---|---|---|---|
| Optical | Lipunov et al. | 22502 | MASTER-IAC optical detection | MASTER | detection |
| Optical | Lipunov et al. | 22503 | MASTER-IAC OT light curve | MASTER | light curve |
| Optical | Schady and Steinmassl | 22504 | GROND observations | GROND | detection |
| Optical | Martone et al. | 22507 | LCO Cerro Tololo observations | LCO | detection |
| Optical | Butler et al. | 22509 | RATIR Optical Observations | RATIR | detection |
| Optical | Butler et al. | 22514 | RATIR Optical Observations, Possible Re-brightening |
RATIR | |
| Optical | Volnova et al. | 22516 | TSHAO optical observations | Zeiss-1000 | marginal detection |
| Optical | Murata et al. | 22517 | MITSuME Okayama optical upper limits | MITSuME Okayama | upper limits |
| Optical | Murata et al. | 22518 | MITSuME Akeno optical upper limits | MITSuME Akeno | upper limits |
| Optical | Watson et al. | 22519 | RATIR Optical Observations | RATIR | upper limits |
| Gamma-ray | Tsvetkova et al. | 22548 | Konus-Wind observation | Konus-Wind | Duration=~29 seconds Fluence=1.43(-0.21,+0.38)x1 |
| Filter | Exp(s) | Mag | ||
|---|---|---|---|---|
| white_fc | 105 | 254 | 147 | 16.27 ± 0.03 |
| white | 596 | 616 | 20 | 17.40 ± 0.11 |
| v | 646 | 839 | 39 | 17.15 ± 0.18 |
| b | 572 | 592 | 20 | 17.40 ± 0.15 |
| u | 317 | 566 | 246 | 16.44 ± 0.04 |
| w1 | 695 | 22410 | 1175 | >20.4 |
| m2 | 12527 | 12578 | 50 | >19.2 |
| w2 | 622 | 6753 | 255 | >19.5 |
Table 3. UVOT observations reported by Breeveld and Melandri (GCN Circ. 22510). 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.
March 27, 2018