Swift Observations of GRB 200228B

S. Laha (GSFC/UMBC/CRESST), K.L. Page (U. Leicester) and N.P.M. Kuin (UCL-MSSL) for the Swift team

1. Introduction

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.

2. BAT Observations and Analysis

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) = 16h48m01.8s Dec(J2000) = +16°57'50.3" with an uncertainty of 2.5 arcmin, (radius, sys+stat, 90% containment). The partial coding was 31%.

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. T90 (15-350 keV) is 7.37 ± 1.58 s (estimated error including systematics).

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 10-7 erg cm-2. This fluence is larger than that of 12% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+1.32 s in the 15-150 keV band is 1.0 ± 0.4 ph cm-2 s-1. All the quoted errors are at the 90% confidence level.

The results of the batgrbproduct analysis are available at https://gcn.gsfc.nasa.gov/notices_s/958733/BA/.

3. XRT Observations and Analysis

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 1022 cm-2, in excess of the Galactic value of 6.4 x 1020 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10-11 (2.3 x 10-10) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.6 (+0.8, -0.6) x 1022 cm-2
Galactic foreground: 6.4 x 1020 cm-2
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.

4. UVOT Observations and Analysis

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 EB-V of 0.092 mag. in the direction of the GRB (Schlegel et al. 1998).

BAT light curve

Figure 1. The BAT mask-weighted light curve in the four individual and total energy bands. The units are counts s-1 illuminated-detector-1. The vertical green dash-dotted lines show the T50 interval, the vertical black dashed lines show the T90 interval, and vertical blue (orange) solid lines show the start (stop) of slews.

XRT light curve

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
16h48m10.96s +16°59'04.3" 2.1" XRT-final UKSSDC
16h48m11.14s +16°59'06.8" 3.5" XRT-enhanced Evans et al. GCN Circ. 27256
16h48m01.8s +16°57'50.3" 2.5' BAT-refined Markwardt et al. GCN Circ. 27273

Table 1. Positions from the Swift instruments.

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 Epeak=76±16 keV
T90=14 seconds
Fluence=1.30±0.18x10-7erg cm-2
(0.1 percentile for long GRBs)

Table 2. Summary of GCN Circulars from other observatories sorted by band and then circular number.

Filter Tstart(s) Tstop(s) Exp(s) Mag
uFC 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