Swift Observations of GRB 200306A

K.K. Simpson (PSU), J.P. Osborne (U. Leicester) and J.D. Gropp (PSU) for the Swift team

1. Introduction

At 00:19:40 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200306A (trigger= 959917) (Simpson et al. GCN Circ. 27320). Swift slewed immediately to the burst. 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.

Strausbaugh and Cucchiara (GCN Circ. 27321) reported the position from LCO for the optical afterglow of this GRB. 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 Sakamoto et al. (GCN Circ. 27346), the BAT ground-calculated position is RA, Dec = 101.114, -62.010 deg which is RA(J2000) = 06h44m27.3s Dec(J2000) = -62°00'34.4" with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 75%.

The mask-weighted light curve (Figure 1) shows a few overlapping pulses that starts at ~T-4 s and ends at ~T+50 s. The main peak occurs at ~T+20 s. T90 (15-350 keV) is 32.87 ± 1.13 s (estimated error including systematics).

The time-averaged spectrum from T-3.67 to T+49.95 s is best fit by a power law with an exponential cutoff. This fit gives a photon index 1.06 ± 0.15, and Epeak of 81.6 ± 6.9 keV (χ2 32.91 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 1.0 ± 0.02 x 10-5 erg cm-2 and the 1-s peak flux measured from T+20.11 s in the 15-150 keV band is 7.0 ± 0.3 ph cm-2 s-1. This fluence is larger than that of 92.5% 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 1.70 ± 0.03 (χ2 98.65 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/959917/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Osborne et al. (GCN Circ. 27331). We have analysed 12 ks of XRT data for GRB 200306A, from 66 s to 166.7 ks after the BAT trigger. The data comprise 619 s in Windowed Timing (WT) mode (the first 9 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode.

The light curve (Figure 2) can be modelled with an initial power-law decay with an index of α=4.12 (+0.24, -0.23), followed by a break at T+133 s to an α of 0.83 (+0.09, -0.10).

A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.93 ± 0.14. The best-fitting absorption column is 5.3 ± 0.5 x 1021 cm-2, in excess of the Galactic value of 8.6 x 1020 cm-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.04 ± 0.14 and a best-fitting absorption column of 4.7 (+0.8, -0.7) x 1021 cm-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.9 x 10-11 (6.5 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.7 (+0.8, -0.7) x 1021 cm-2
Galactic foreground: 8.6 x 1020 cm-2
Excess significance: 8.7 σ
Photon index: 2.04 ± 0.14

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00959917.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200306A 85 s after the BAT trigger (Gropp and Simpson GCN Circ. 27351). No optical afterglow consistent with the optical position (Strausbaugh et al., GCN Circ. 27321) 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 0.10 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
06h44m24.65s -62°00'23.8" 1.4" XRT-final UKSSDC
06h44m24.56s -62°00'23.5" 1.5" XRT-refined Osborne et al. GCN Circ. 27331
06h44m27.3s -62°00'34.4" 1.0' BAT-refined Sakamoto et al. GCN Circ. 27346

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 27319 Swift GRB200306.01: Global MASTER-Net
observations report 		MASTER
Optical Strausbaugh and Cucchiara 27321 Potential LCO Optical Afterglow
Detection		 LCO detection
Optical Lipunov et al. 27324 Swift GRB200306.95: Global MASTER-Net
OT detection		 MASTER detection
Optical Lipunov et al. 27325 Swift GRB200306.95: Global MASTER-Net
OT detection		 MASTER detection
Optical Lipunov et al. 27327 Swift GRB200306.95: Global MASTER-Net
observations report 		MASTER
Optical Watson et al. 27343 RATIR Optical Observations RATIR upper limits
Gamma-ray Gupta et al. 27340 AstroSat CZTI detection CZTI

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 85 235 147 >21.0
white 85 6191 459 >20.7
v 5049 11928 1011 >20.1
b 4434 16845 1278 >20.5
u 297 5864 351 >20.1
w1 5459 5659 197 >19.7
m2 5254 5454 197 >19.1
w2 4844 11087 1082 >20.3

Table 3. UVOT observations reported by Gropp and Simpson (GCN Circ. 27351). 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 9, 2020