Swift Observations of GRB 131231A

V. Mangano (PSU) and S.T. Holland (STScI) for the Swift team

1. Introduction

Sonbas et al. (GCN Circ. 15640) reported the GRB detection. At 04:45:16.08 on Dec 31, 2013, Fermi LAT detected high energy emission from GRB 131231A, which was also detected by Fermi-GBM (trigger 410157919/131231198). Table 1 contains the best reported positions from Swift.

Malesani et al. (GCN Circ. 15642) reported the position from NOT for the optical afterglow of this GRB. Xu et al. (GCN Circ. 15645) determined a redshift of 0.642 from VLT, and Cucchiara (GCN Circ. 15652) determined a redshift of 0.6439 from Gemini. Table 2 is a summary of GCN Circulars about this GRB from observatories other than Swift.

2. BAT Observations and Analysis

BAT did not observe this burst.

3. XRT Observations and Analysis

We have analysed 23 ks of XRT data for the Fermi/LAT-detected burst: GRB 131231A, from 52.1 ks to 431.2 ks after the Fermi/LAT trigger. The data are entirely in Photon Counting (PC) mode. An X-ray source is detected within the Fermi/LAT error circle.

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.47 ± 0.06.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.86 ± 0.13. The best-fitting absorption column is 1.4 (+0.7, -0.6) x 1021 cm-2, at a redshift of 0.642, in addition to the Galactic value of 2.6 x 1020 cm-2 (Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.1 x 10-11 (3.7 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 2.6 x 1020 cm-2
Intrinsic column: 1.4 (+0.7, -0.6) x 1021 cm-2 at z=0.642
Photon index: 1.86 ± 0.13

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00020336.

4. UVOT Observations and Analysis

The Swift/UVOT began observations of the field of GRB 131231A 136.5 ks after the Fermi/LAT trigger (Sonbas et al., 2013, GCN Circ. 15640) (Holland and Mangano GCN Circ. 15673). The preliminary UVOT position is 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).

Figure 1. The BAT light curve is not available.

XRT light curve

Figure 2. The XRT light curve. Any data from a cross-hatched region are not included in the fit.

RA (J2000) Dec (J2000) Error Note Reference
00h42m21.66s -01°39'10.6" 0.43" UVOT-refined Holland and Mangano GCN Circ. 15673
00h42m21.70s -01°39'06.7" 3.5" XRT-final UKSSDC
00h42m21.70s -01°39'06.7" 3.5" XRT Mangano et al. GCN Circ. 15648

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Xu et al. 15641 Nanshan afterglow candidate Xinjiang Astro.Obs. detection
Optical Malesani et al. 15642 afterglow confirmation from the NOT NOT detection
Optical Singer et al. 15643 iPTF optical afterglow candidate coincident
with Nanshan detection		 Xinjiang Astro.Obs. detection
Optical Xu et al. 15645 VLT/X-shooter redshift VLT redshift
Optical Halpern 15646 MDM afterglow observation MDM
Optical Xin et al. 15647 xinglong TNT optical observation TNT
Optical Perley 15650 P60 observations Palomar 60-inch detection
Optical Cucchiara 15652 Gemini-South Redshift Gemini redshift
Optical Pandey et al. 15676 Optical Observations Nainital detection
Optical Huang and Urata 15681 Lulin Optical Observations Lulin
Optical Volnova et al. 15711 AAO optical observations Abastumani detection
Radio Perley 15680 CARMA 3mm detection CARMA detection
Gamma-ray Sonbas et al. 15640 Fermi-LAT detection of a burst Fermi LAT detection
Gamma-ray Jenke and Xiong 15644 Fermi GBM detection Fermi GBM Epeak = 146 ± 3 keV
Gamma-ray Golenetskii et al. 15670 Konus-Wind observation Konus-Wind Epeak = 163 ± 6 keV
Gamma-ray Jenke 15672 Fermi GBM correction to GCN Circ. 15644 Fermi GBM

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

Filter Tstart Tstop Exposure Mag Err
u 246,875 248,554 1652 19.57 0.10
white 136,762 138,942 1650 19.37 0.05
white 228,985 231,172 1639 20.07 0.07

Table 3. UVOT observations reported by Holland and Mangano (GCN Circ. 15673). 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.

January 15, 2014