Swift Observations of GRB 181213A

P.A. Evans (U. Leicester), P. D'Avanzo (INAF-OAB) and M.H. Siegel (PSU) for the Swift team

1. Introduction

At 12:57:38 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 181213A (trigger=876016) (Evans et al. GCN Circ. 23525). Swift will slew at ~T+33 min due to an observing constraint. At the time of the trigger, the initial BAT position was 102° from the Sun (0.8 hours West) and 103° from the 32%-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.

Lipunov et al. (GCN Circ. 23526) reported the position from MASTER for the optical afterglow of this GRB. Siegel and Evans (GCN Circ. 23529) 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 Cummings et al. (GCN Circ. 23533), the BAT ground-calculated position is RA, Dec = 248.210, 78.500 deg which is RA(J2000) = 16h32m50.5s Dec(J2000) = +78°30'00.7" with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment). The partial coding was 21%.

The mask-weighted light curve (Figure 1) shows a multi-peaked structure that starts at ~T-1 s and ends at ~T+16 s. Note that the burst went out of the BAT field of view at ~T+100 s. T90 (15-350 keV) is 15.31 ± 1.24 s (estimated error including systematics).

The time-averaged spectrum from T-0.70 to T+15.94 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.57 ± 0.20. The fluence in the 15-150 keV band is 1.5 ± 0.2 x 10-6 erg cm-2. This fluence is larger than that of 52% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The 1-s peak photon flux measured from T+1.16 s in the 15-150 keV band is 2.1 ± 0.6 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/876016/BA/.

3. XRT Observations and Analysis

Analysis of the initial XRT data was reported by D'Avanzo et al. (GCN Circ. 23536). We have analysed 15 ks of XRT data for GRB 181213A, from 2.4 ks to 83.9 ks after the BAT trigger. The data comprise 218 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 23535).

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.45 ± 0.09.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.06 (+0.22, -0.20). The best-fitting absorption column is 7.2 (+5.4, -2.8) x 1020 cm-2, consistent with the Galactic value of 4.5 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 3.2 x 10-11 (3.8 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 7.2 (+5.4, -2.8) x 1020 cm-2
Galactic foreground: 4.5 x 1020 cm-2
Excess significance: <1.6 σ
Photon index: 2.06 (+0.22, -0.20)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00876016.

4. UVOT Observations and Analysis

Analysis of the UVOT data was reported by Siegel and Evans (GCN Circ. 23529). UVOT took a finding chart exposure of 150 seconds with the White filter starting 2352 seconds after the BAT trigger. 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 0.04 mag. in the direction of the GRB (Schlegel et al. 1998).

The Swift/UVOT began settled observations of the field of GRB 181213A 2352 s after the BAT trigger (Siegel and Evans GCN Circ. 23545). A fading optical afterglow consistent with initial reports (Lipunov et al., GCN Circ. 23526, Hu et al., GCN Circ. 23529) was detected in four of UVOT’s filters. 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 0.04 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
16h33m04.86s +78°29'48.7" 0.43" UVOT-refined Siegel and Evans GCN Circ. 23545
16h33m04.83s +78°29'49.1" 1.6" XRT-final UKSSDC
16h33m04.82s +78°29'49.1" 1.6" XRT-enhanced Osborne et al. GCN Circ. 23535
16h32m50.5s +78°30'00.7" 1.9' BAT-refined Cummings et al. GCN Circ. 23533

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Lipunov et al. 23526 SWIFT GRB181213A Global MASTER-Net
bright OT discovery		 MASTER
Optical Hu et al. 23527 BOOTES-5/JGT optical observations BOOTES
Optical Belkin et al. 23530 TSHAO optical observations Zeiss-1000 detection
Optical Han et al. 23532 GWAC-F60A upper limit Xinglong Obs. upper limits
Optical Heintz et al. 23537 NOT optical observations NOT
Optical Zhu et al. 23538 HMM-0.5m, NEXT-0.6m, CNEOST-1m optical
observations		 detection
Optical Belkin et al. 23539 AbAO optical observations Abastumani Astro. Obs. detection
Optical Belkin et al. 23543 continued TSHAO optical observations Zeiss-1000 detection
Optical Belkin et al. 23544 Mondy optical observations Mondy marginal detection
Gamma-ray Xiao et al. 23542 Insight-HXMT/HE detection Insight-HXMT T90=15.67 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
white (fc) 2353 2503 294 17.33±0.04
white 2633 2653 19 17.15±0.10
white 8535 9061 513 19.03±0.07
white 66263 67021 738 >21.71
v 2683 2703 19 >17.35
v 26217 37929 1168 >20.45
b 2608 3393 87 17.82±0.10
b 8330 15947 230 19.18±0.23
b 65351 82801 1115 >20.39
u 2584 3318 216 17.25±0.06
u 14999 33141 1327 19.69±0.10
u 71983 72421 426 >20.32
uvw1 2559 3113 216 17.50±0.11
uvw1 14093 32991 2656 20.07±0.16
uvw1 71077 71977 885 >19.97
uvm2 2708 2908 196 >19.26
uvm2 19555 32084 2047 >20.84
uvw2 2659 2679 19 >17.50
uvw2 25310 37634 1771 >20.67

Table 3. UVOT observations reported by Siegel and Evans (GCN Circ. 23545). 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.

December 19, 2018