Swift Observations of GRB 200306C

J.D. Gropp (PSU) and K.L. Page (U. Leicester) for the Swift team

1. Introduction

At 22:50:39 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 200306C (trigger=960102) (Gropp et al. GCN Circ. 27326). Swift slewed immediately to the burst. At the time of the trigger, the initial BAT position was 149° from the Sun (9.9 hours West) and 65° from the 89%-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.

Gropp et al. (GCN Circ. 27326) reported the discovery 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 Stamatikos et al. (GCN Circ. 27353), the BAT ground-calculated position is RA, Dec = 198.577, 11.263 deg which is RA(J2000) = 13h14m18.6s Dec(J2000) = +11°15'47.7" with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 42%.

The mask-weighted light curve (Figure 1) shows several overlapping pulses that start at ~T-12 s and end at ~T+50 s. T90 (15-350 keV) is 53.5 ± 12.8 s (estimated error including systematics).

The time-averaged spectrum from T-12.33 to T+50.70 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 0.83 ± 0.12. The fluence in the 15-150 keV band is 2.8 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 70% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+3.45 s in the 15-150 keV band is 1.3 ± 0.3 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/960102/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by Page et al. (GCN Circ. 27345). We have analysed 19 ks of XRT data for GRB 200306C, from 101 s to 258.0 ks after the BAT trigger. The data comprise 36 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 enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 27342).

The late-time light curve (Figure 2) (from T0+5.5 ks) can be modelled with a power-law decay with a decay index of α=2.61 (+0.24, -0.20).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.56 (+0.11, -0.10). The best-fitting absorption column is 4.2 (+3.1, -2.3) x 1020 cm-2, consistent with the Galactic value of 1.9 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 4.2 x 10-11 (4.4 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 4.2 (+3.1, -2.3) x 1020 cm-2
Galactic foreground: 1.9 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 1.56 (+0.11, -0.10)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00960102.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 200306C 118 s after the BAT trigger (Gropp GCN Circ. 27352). 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 0.02 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
13h14m13.46s +11°16'11.7" 0.43" UVOT-refined Gropp GCN Circ. 27352
13h14m13.44s +11°16'13.0" 1.6" XRT-final UKSSDC
13h14m13.41s +11°16'10.3" 2.2" XRT-enhanced Osborne et al. GCN Circ. 27342
13h14m18.6s +11°15'47.7" 1.3' BAT-refined Stamatikos et al. GCN Circ. 27353

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Jelinek et al. 27328 FRAM-ORM afterglow detection FRAM detection
Optical Hu et al. 27329 BOOTES-1 detection of the optical
afterglow		 BOOTES-1 detection
Optical Ackley et al. 27332 GOTO upper limits Gravitational-wave Optical Transient Observer upper limits
Optical Moskvitin 27333 SAO RAS observations of OT SAO RAS detection
Optical Izzo and Perley 27334 Liverpool Telescope observations Liverpool Telescope detection
Optical Kann et al. 27335 OSN 1.5m afterglow detection Obs.de Sierra Nevada detection
Optical Strausbaugh and Cucchiara 27337 LCO Optical Upper Limits LCO upper limits
Optical Steele 27338 First Hour Liverpool Telescope
Observations		 Liverpool Telescope detection
Optical Lim et al. 27339 KCT upper limit of the optical afterglow upper limits
Optical Gokuldass et al. 27341 VIRT optical upper limits Virgin Island Robotic Telescope upper limits
Optical Kann et al. 27348 Calar Alto 2.2m upper limit Calar Alto upper limits
Radio Laskar et al. 27354 ALMA 3mm upper limit ALMA upper limits

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_fc 141 390 245
u 899 1049 147

Table 3. UVOT observations reported by Gropp (GCN Circ. 27352). 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.

March 10, 2020