Swift Observations of GRB 141028A

J.A. Kennea (PSU) and M.H. Siegel (PSU) for the Swift team

1. Introduction

Bissaldi et al. (GCN Circ. 16969) reported the GRB detection. At 10:54:46.78 on October 28, 2014, Fermi-LAT detected high-energy emission from GRB 141028A, which was also detected by Fermi-GBM (trigger 436186489/141028455) and initiated an autonomous repoint of the spacecraft. Table 1 contains the best reported positions from Swift.

Graham et al. (GCN Circ. 16977) reported the position from GROND for the optical afterglow of this GRB. Siegel and Pagani (GCN Circ. 16979) reported the detection with UVOT of an optical afterglow. Cucchiara (GCN Circ. 16982) determined a redshift of 1.82 from Gemini, and Xu et al. (GCN Circ. 16983) determined a redshift of 2.33 from VLT. 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 19 ks of XRT data for the Fermi/LAT-detected burst: GRB 141028A, from 31.0 ks to 676.2 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) is consistent with a constant source of mean count rate 1.8 x 10-2 ct s-1. A power-law fit gives an index of 0.5 (+0.7, -0.8).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.4 ± 0.4. The best-fitting absorption column is 1.6 (+1.6, -1.1) x 1021 cm-2, in excess of the Galactic value of 4.1 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 3.0 x 10-11 (4.3 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.6 (+1.6, -1.1) x 1021 cm-2
Galactic foreground: 4.1 x 1020 cm-2
Excess significance: 1.8 σ
Photon index: 2.4 ± 0.4

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00020420.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations (Siegel and Pagani GCN Circ. 16979) of the field of GRB 141028A 31037 s after the LAT trigger (Bissaldi et al., GCN Circ. 16969). We detect an optical afterglow consistent with the afterglow reported by MASTER (Gorbovsky et a., GCN, Circ 16972), GROND (Graham et al., GCN Circ. 16977) and the XRT (Kennea, et al., GCN Circ. 16978). 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 0.05 mag. in the direction of the GRB (Schlegel et al. 1998).

The Swift/UVOT began settled observations (Breeveld and Pagani GCN Circ. 16990) of the field of GRB 141028A 31037 s after the LAT trigger (Bissaldi et al., GCN Circ. 16969). A fading source is detected in the initial exposures (Siegel and Pagani, GCN Circ. 16979) in the position given in that circular, also consistent with MASTER (Gorbovsky et a., GCN, Circ 16972), GROND (Graham et al., GCN Circ. 16977) and the XRT (Pagani, et al., GCN Circ. 16986). 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 0.05 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 crosshatched region are not included in the fit.

RA (J2000) Dec (J2000) Error Note Reference
21h30m24.54s -00°13'52.4" 0.4" UVOT-refined Siegel and Pagani GCN Circ. 16979
21h30m24.40s -00°13'53.2" 4.0" XRT-final UKSSDC
21h30m24.40s -00°13'53.2" 4.0" XRT Pagani et al. GCN Circ. 16986

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Gorbovskoy et al. 16972 MASTER OT candidate detection MASTER detection
Optical Graham et al. 16977 GROND Afterglow confirmation (MASTER
source)		 GROND detection
Optical Troja et al. 16980 RATIR Optical and NIR Observations RATIR detection
Optical Cucchiara 16982 Gemini-North redshift Gemini redshift
Optical Xu et al. 16983 VLT/X-shooter redshift of z=2.33 VLT redshift
Optical Kopac et al. 16985 LCOGTN observations LCO
Optical Cenko and Perley 16989 P60 Observations and Decay Index Palomar 60-inch
Optical Troja et al. 16991 Continued RATIR Observations RATIR
Optical Moskvitin and Komarova 16993 SAO RAS Zeiss-1000 photometry SAO RAS detection
Optical Mazaeva et al. 17030 TShAO optical observation Tien Shan Astro. Obs. detection
Optical Mazaeva et al. 17035 Mondy optical observations Mondy detection
Gamma-ray Bissaldi et al. 16969 Fermi-LAT detection Fermi LAT Emax=3.9 GeV
Gamma-ray Roberts 16971 Fermi GBM detection Fermi GBM Epeak=249.9 ± 12.6 keV
T90=31.5 seconds
Fluence=3.478±0.009x10-5erg cm-2
(92nd percentile for long GRBs)
Gamma-ray Hurley et al. 16975 IPN triangulation IPN

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 31037 37759 233 19.14± 0.19

Table 3. UVOT observation reported by Siegel and Pagani (GCN Circ. 16979). The start and stop time of the exposure are given in seconds since the BAT trigger. The preliminary detection is given. No correction has been made for extinction in the Milky Way.

November 8, 2014