Swift Observations of GRB 200224A

T.N. Ukwatta (LANL), J.D. Gropp (PSU) and F.E. Marshall (NASA/GSFC) for the Swift team

1. Introduction

At 03:24:49 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200224A (trigger=958141) (Ukwatta et al. GCN Circ. 27173). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 95° from the Sun (5.9 hours West) and 101° from the 0%-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.

Stecklum et al. (GCN Circ. 27177) reported the position from Tautenburg for the optical afterglow of this GRB. Marshall and Ukwatta (GCN Circ. 27182) 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.

2. BAT Observations and Analysis

As reported by Lien et al. (GCN Circ. 27194), the BAT ground-calculated position is RA, Dec = 248.743, 41.666 deg which is RA(J2000) = 16h34m58.4s Dec(J2000) = +41°39'57.2" with an uncertainty of 3.5 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.

The mask-weighted light curve (Figure 1) shows a weak pulse from ~T0 to ~T+5 s, followed by some weaker emission that lasts till ~T+50 s. T90 (15-350 keV) is 45.0 ± 9.2 s (estimated error including systematics).

The time-averaged spectrum from T-0.79 to T+49.21 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.69 ± 0.51. The fluence in the 15-150 keV band is 2.9 ± 0.9 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+2.21 s in the 15-150 keV band is 0.4 ± 0.1 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/958141/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Gropp et al. (GCN Circ. 27179). We have analysed 24 ks of XRT data for the Swift-detected burst GRB 200224A, from 105 s to 69.5 ks after the Swift trigger. The data are entirely in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=0.87 (+0.09, -0.08).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.1 (+0.4, -0.3). The best-fitting absorption column is 3.7 (+7.7, -2.8) x 1020 cm-2, consistent with the Galactic value of 8.9 x 1019 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.0 x 10-11 (3.3 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 3.7 (+7.7, -2.8) x 1020 cm-2
Galactic foreground: 8.9 x 1019 cm-2
Excess significance: <1.6 σ
Photon index: 2.1 (+0.4, -0.3)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00958141.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200224A 101 s after the BAT trigger (Marshall and Ukwatta GCN Circ. 27182). A source consistent with the enhanced XRT position (Gropp et al., GCN Circ. 27179) and the optical positions reported by Sota et al. (GCN Circ. 27176) and Stecklum et al. (GCN Circ. 27177) 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 0.005 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
16h35m04.14s +41°36'42.3" 0.43" UVOT-refined Marshall and Ukwatta GCN Circ. 27182
16h35m03.99s +41°36'40.8" 1.7" XRT-final UKSSDC
16h35m03.97s +41°36'40.8" 1.7" XRT-refined Gropp et al. GCN Circ. 27179
16h34m58.4s +41°39'57.2" 3.5' BAT-refined Lien et al. GCN Circ. 27194

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 27172 Swift GRB200224.14: Global MASTER-Net
observations report 		MASTER
Optical Sota et al. 27176 BOOTES-1 and 1.5m OSN optical
observations		 BOOTES-1
Optical Stecklum et al. 27177 Tautenburg observations Tautenburg
Optical Strausbaugh and Cucchiara 27191 LCO Upper Limits LCO upper limits
Optical Fernandez et al. 27222 Calar Alto 2.2m optical afterglow
detection		 Calar Alto detection

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
u 259 509 246 18.75 ± 0.10
white 861 1011 147 20.57 ± 0.20
w1 5978 19265 1098 >20.7
m2 5772 18542 1279 >21.4
w2 5363 30092 1953 >21.9

Table 3. UVOT observations reported by Marshall and Ukwatta (GCN Circ. 27182). 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 27, 2020