B. Sbarufatti (PSU), A.P. Beardmore (U. Leicester) and N.P.M. Kuin (UCL-MSSL) for the Swift team
At 22:41:15 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200131A (trigger=953689) (Sbarufatti et al. GCN Circ. 26953). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 81° from the Sun (3.3 hours East) and 49° from the 39%-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.
Sbarufatti et al. (GCN Circ. 26953) 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 Sakamoto et al. (GCN Circ. 26967),
the BAT ground-calculated position is RA, Dec = 3.067, 51.121 deg which is RA(J2000) = 0
The mask-weighted light curve (Figure 1) shows several bright overlapping pulses that starts from ~T-3.5 s and ends at ~T+3.
It is followed by another weaker pulse at ~T+30 s and some weak emission that lasts till ~T+60 s.
The burst went out of the BAT FOV at T+472 s.
The time-averaged spectrum from T-3.45 to T+46.56 s is best fit by a simple power-law model.
The power law index of the time-averaged spectrum is 1.32 ± 0.04.
The fluence in the 15-150 keV band is 7.0 ± 0.1 x 1
The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/953689/BA/.
Analysis of the initial XRT data was reported by Beardmore et al. (GCN Circ. 26961). We have analysed 19 ks of XRT data for GRB 200131A, from 61 s to 492.4 ks after the BAT trigger. The data comprise 249 s in Windowed Timing (WT) mode (the first 5 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. 26956).
The light curve (Figure 2) can be modelled with a series of power-law decays. The initial decay index is α=4.5 (+0.8, -0.5). At T+87.7 s the decay flattens to an α of 0.79 (+0.19, -0.13). The light curve breaks again at T+197 s to a decay with α=1.147 (+0.155, -0.018), before a final break at T+10.1 ks s after which the decay index is 0.94 (+0.06, -0.05).
A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.83 (+0.04, -0.03). The best-fitting absorption column is consistent with the Galactic value of 1.8 x 1
A summary of the PC-mode spectrum is thus:
Total column: 2.3 ± 0.5 x 1
Galactic foreground: 1.8 x 1
Excess significance: <1.6 σ
Photon index: 1.78 (+0.13, -0.12)
The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00953689.
The Swift/UVOT began settled observations of the field of GRB 200131A 78 s after the BAT trigger
(Kuin and Sbarufatti GCN Circ. 26962).
A source consistent with the XRT position (Goad et al. GCN Circ. 26956) 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 |
+51°07'00.3" | 0.42" | UVOT-refined | Kuin and Sbarufatti GCN Circ. 26962 |
| 0 |
+51°07'00.2" | 1.4" | XRT-final | UKSSDC |
| 0 |
+51°07'00.4" | 1.5" | XRT-enhanced | Goad et al. GCN Circ. 26956 |
| 0 |
+51°07'15.6" | 1.2' | BAT-refined | Sakamoto et al. GCN Circ. 26967 |
| Band | Authors | GCN Circ. | Subject | Observatory | Notes |
|---|---|---|---|---|---|
| Optical | Lipunov et al. | 26952 | Swift GRB200131.95: Global MASTER-Net observations report |
MASTER | |
| Optical | Lipunov et al. | 26954 | MASTER optical transient detection | MASTER | detection |
| Optical | Butler et al. | 26957 | RATIR Optical Observations | RATIR | detection |
| Optical | Zheng and Filippenko | 26958 | KAIT Optical Upper Limit | KAIT | upper limits |
| Optical | Strausbaugh and Cucchiara | 26959 | LCO Optical Detection | LCO | detection |
| Optical | Gokuldass et al. | 26960 | VIRT optical transient detection | Virgin Island Robotic Telescope | detection |
| Optical | Pozanenko et al. | 26964 | CDK700 optical observations | Assy-Turgen Obs. | marginal detection |
| Optical | Pozanenko et al. | 26965 | ISON-Castelgrande optical observations | Castelgrande Obs. | |
| Optical | Watson et al. | 26977 | Further RATIR Optical Observations | RATIR | detection |
| Optical | Stecklum et al. | 27035 | Tautenburg observations | Tautenburg | |
| Gamma-ray | Ricciarini et al. | 26972 | CALET Gamma-Ray Burst Monitor detection | CALET | |
| Gamma-ray | Svinkin et al. | 26975 | Konus-Wind observation | Konus-Wind | Fluence=1.60(-0.08,+0.09)x1 |
| Filter | Exp(s) | Mag | ||
|---|---|---|---|---|
| white | 78 | 227 | 147 | 15.47 ± 0.02 |
| v | 4281 | 5919 | 393 | 19.57 ± 0.28 |
| b | 5103 | 5303 | 197 | 19.53 ± 0.16 |
| u | 290 | 323 | 32 | 16.09 ± 0.08 |
| w1 | 4692 | 4892 | 197 | 18.48 ± 0.18 |
| m2 | 4486 | 6124 | 393 | 18.98 ± 0.20 |
| w2 | 4076 | 5714 | 393 | 19.06 ± 0.19 |
Table 3. UVOT observations reported by Kuin and Sbarufatti (GCN Circ. 26962). 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.
February 12, 2020