S. Laha (GSFC/UMBC/CRESST), K.L. Page (U. Leicester) and N.P.M. Kuin (UCL-MSSL) for the Swift team
At 11:14:41 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200228B (trigger=958733) (Laha et al. GCN Circ. 27254). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 91° from the Sun (5.9 hours West) and 133° from the 20%-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.
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 Markwardt et al. (GCN Circ. 27273),
the BAT ground-calculated position is RA, Dec = 252.007, 16.964 deg which is RA(J2000) = 1
The mask-weighted light curve (Figure 1) shows a weak pulse that starts at ~T-1 s, peaks at ~T+2 s, and ends at ~T+7 s.
The time-averaged spectrum from T-0.84 to T+6.97 s is best fit by a simple power-law model.
The power law index of the time-averaged spectrum is 2.27 ± 0.36.
The fluence in the 15-150 keV band is 3.0 ± 0.6 x 1
The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/958733/BA/.
Analysis of the initial XRT data was reported by Page and Laha (GCN Circ. 27270).
We have analysed 4.6 ks of XRT data for GRB 200228B, from 143 s to 18.2 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 27256).
The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.65 (+0.10, -0.12).
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 3.0 (+0.8, -0.7). The best-fitting absorption column is 1.6 (+0.8, -0.6) x 1
A summary of the PC-mode spectrum is thus:
Total column: 1.6 (+0.8, -0.6) x 1
Galactic foreground: 6.4 x 1
Excess significance: 4.3 σ
Photon index: 3.0 (+0.8, -0.7)
The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00958733.
The Swift/UVOT began settled observations of the field of GRB 200228B 137 s after the BAT trigger
(Kuin and Laha GCN Circ. 27268).
No optical afterglow consistent with the XRT position (Evans et al., GCN Circ. 27256 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 |
+16°59'04.3" | 2.1" | XRT-final | UKSSDC |
1 |
+16°59'06.8" | 3.5" | XRT-enhanced | Evans et al. GCN Circ. 27256 |
1 |
+16°57'50.3" | 2.5' | BAT-refined | Markwardt et al. GCN Circ. 27273 |
Band | Authors | GCN Circ. | Subject | Observatory | Notes |
---|---|---|---|---|---|
Optical | Zheng and Filippenko | 27263 | KAIT Optical Upper Limit | KAIT | upper limits |
Optical | Pereyra et al. | 27267 | RATIR Optical Observations | RATIR | upper limits |
Optical | Lipunov et al. | 27269 | Swift GRB 200228B: Global MASTER-Net observations report |
MASTER | |
Optical | Perley and Cockeram | 27271 | Liverpool Telescope imaging | Liverpool Telescope | |
Optical | Becerra et al. | 27272 | RATIR Optical Observations | RATIR | upper limits |
Optical | Strausbaugh and Cucchiara | 27274 | LCO Optical Upper Limit | LCO | upper limits |
Optical | Pozanenko et al. | 27276 | Mondy, TSHAO, AbAO and CrAO optical observations, afterglow candidate |
Abastumani Astro. Obs. | detection |
Optical | Pozanenko et al. | 27318 | continued optical observations | Mondy | light curve |
Gamma-ray | Fermi | 27253 | Fermi GBM Final Real-time Localization | Fermi GBM | |
Gamma-ray | Bissaldi et al. | 27266 | Fermi GBM observation | Fermi GBM | Fluence=1.30±0.18x1 (0.1 percentile for long GRBs) |
Filter | Exp(s) | Mag | ||
---|---|---|---|---|
137 | 387 | 246 | >20.1 | |
v | 4992 | 10685 | 676 | >20.1 |
b | 4580 | 6012 | 393 | >20.6 |
u | 137 | 5807 | 442 | >20.4 |
w1 | 5402 | 5602 | 197 | >19.5 |
m2 | 5197 | 5396 | 197 | >19.8 |
w2 | 4786 | 6218 | 393 | >19.4 |
Table 3. UVOT observations reported by Kuin and Laha (GCN Circ. 27268). 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.
March 6, 2020