Swift Observations of GRB 200227A

S. Laha (GSFC/UMBC/CRESST), J.A. Kennea (PSU) and S.R. Oates (U. Birmingham) for the Swift team

1. Introduction

At 07:20:08 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200227A (trigger=958592) (Laha et al. GCN Circ. 27234). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 79° from the Sun (5.1 hours East) and 39° from the 12%-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 Cummings et al. (GCN Circ. 27246), the BAT ground-calculated position is RA, Dec = 56.433, 9.489 deg, which is RA(J2000) = 03h45m43.9s Dec(J2000) = +09°29'22.0" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 46%.

The mask-weighted light curve (Figure 1) a couple overlapping peaks starting at T-10 s, peaking at T+8 s, and ending at T+90 s. T90 (15-350 keV) is 32.8 ± 9.7 s (estimated error including systematics).

The time-averaged spectrum from T-0.50 to T+87.64 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.15 ± 0.19, and Epeak of 156.4 ± 88.9 keV (χ2 55.3 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 7.1 ± 0.2 x 10-6 erg cm-2 and the 1-s peak flux measured from T+8.54 s in the 15-150 keV band is 5.7 ± 0.4 ph cm-2 s-1. This fluence is larger than that of 88% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). A fit to a simple power law gives a photon index of 1.46 ± 0.05 (χ2 63.6 for 57 d.o.f.). 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/958592/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Kennea et al. (GCN Circ. 27249). We have analysed 8.6 ks of XRT data for GRB 200227A, from 72 s to 127.3 ks after the BAT trigger. The data comprise 288 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 Beardmore et al. (GCN Circ. 27243).

The late-time light curve (Figure 2) (from T0+4.8 ks) can be modelled with a power-law decay with a decay index of α=1.11 (+0.18, -0.16).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.88 (+0.08, -0.07). The best-fitting absorption column is 5.9 ± 0.5 x 1021 cm-2, in excess of the Galactic value of 2.1 x 1021 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.83 (+0.17, -0.16) and a best-fitting absorption column of 5.6 (+1.1, -1.0) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.6 x 10-11 (6.9 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 5.6 (+1.1, -1.0) x 1021 cm-2
Galactic foreground: 2.1 x 1021 cm-2
Excess significance: 5.7 σ
Photon index: 1.83 (+0.17, -0.16)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00958592.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200227A 93 s after the BAT trigger (Oates and Laha GCN Circ. 27244). No optical afterglow consistent with the XRT position (Beardmore et al. GCN Circ. 27243) 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 0.28 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
03h45m43.91s +09°28'57.1" 1.4" XRT-final UKSSDC
03h45m43.86s +09°28'56.3" 2.5" XRT-enhanced Beardmore et al. GCN Circ. 27243
03h45m43.9s +09°29'22.0" 1.0' BAT-refined Cummings et al. GCN Circ. 27246

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Oeda et al. 27236 MITSuME Akeno optical upper limit MITSuME Akeno upper limits
Optical Belkin et al. 27257 Mondy optical upper limit Mondy upper limits
Optical Watson et al. 27265 RATIR Optical Observations RATIR upper limits
Gamma-ray Fermi 27233 Fermi GBM Final Real-time Localization Fermi GBM
Gamma-ray Bissaldi 27239 Fermi GBM observation Fermi GBM Epeak=139±10 keV
T90=24 seconds
Fluence=1.41±0.04x10-5erg cm-2
(82nd percentile for long GRBs)
Gamma-ray Luo et al. 27251 Insight-HXMT/HE detection Insight-HXMT T90=37.71 seconds

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
whiteFC 93 243 147 >20.3
uFC 305 555 246 >19.7
white 93 1185 353 >20.8
v 634 1235 78 >18.3
b 560 1160 58 >19.2
u 305 1135 285 >19.8
w1 685 1284 58 >18.4
m2 659 1259 78 >18.4
w2 610 1210 78 >18.6

Table 3. UVOT observations reported by Oates and Laha (GCN Circ. 27244). 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 2, 2020