Swift Observations of GRB 140508A

F.E. Marshall (NASA/GSFC) for the Swift team

1. Introduction

Yu and Goldstein (GCN Circ. 16224) reported the GRB detection. At 03:03:54.60 UT on 08 May 2014, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 140508A (trigger 421211037 / 140508128). Table 1 contains the best reported positions from Swift.

Singer et al. (GCN Circ. 16226) reported the position from iPTF for the optical afterglow of this GRB. Marshall and Amarel-Rogers (GCN Circ. 16243) reported the detection with UVOT of an optical afterglow. Malesani et al. (GCN Circ. 16229) determined a redshift of 1.03 from NOT, and Wiersema et al. (GCN Circ. 16231) determined a redshift of 1.027 from WHT. 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 17 ks of XRT data for the Fermi/GBM-detected burst: GRB 140508A, from 69.0 ks to 1020.7 ks after the Fermi/GBM trigger. The data are entirely in Photon Counting (PC) mode. An X-ray source is detected within the Fermi/GBM error circle.

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.41 ± 0.10.

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.98 (+0.23, -0.22). The best-fitting absorption column is 1.4 (+1.5, -1.4) x 1021 cm-2, at a redshift of 1.027, in addition to the Galactic value of 2.1 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 3.3 x 10-11 (3.9 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Galactic foreground: 2.1 x 1020 cm-2
Intrinsic column: 1.4 (+1.5, -1.4) x 1021 cm-2 at z=1.027
Photon index: 1.98 (+0.23, -0.22)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00020376.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 140508A 68965 s after the GBM trigger (Yu and Goldstein, GCN Circ. 16224) (Marshall and Amarel-Rogers GCN Circ. 16243). A source consistent with the optical position (Singer et al., GCN Circ. 16226) and the X-ray position (Amarel-Rogers, GCN Circ. 16232) 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 0.031 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
17h01m51.95s +46°46'49.5" 0.46" UVOT-refined Marshall and Amarel-Rogers GCN Circ. 16243
17h01m51.96s +46°46'49.2" 1.8" XRT-final UKSSDC
17h01m51.96s +46°46'49.2" 1.8" XRT GCN Circ. 16232

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Singer et al. 16226 iPTF optical transient candidate iPTF detection
Optical Gorosabel et al. 16227 1.23m CAHA BVRI-band observations CAHA detection
Optical Moskvitin et al. 16228 SAO RAS and MASTER photometric and
spectroscopic observations		 MASTER
Optical Malesani et al. 16229 NOT photometry and spectroscopy NOT spectroscopy
Optical Wiersema et al. 16231 WHT spectroscopy WHT spectroscopy
Optical Masi 16235 Optical Observations via Virtual
Telescope		 Virtual Telescope
Optical Butler et al. 16236 RATIR Optical and NIR Observations RATIR detection
Optical Bhalerao 16244 ​HCT spectroscopy Himalayan Chandra Telescope spectroscopy
Optical Butler et al. 16246 Continued RATIR Optical and NIR
Observations		 RATIR detection
Optical Fujiwara et al. 16259 MITSuME Akeno Optical observation MITSuME Akeno detection
Optical Volnova et al. 16260 Mondy upper limit Mondy upper limits
Radio Horesh et al. 16266 VLA detection VLA detection
Gamma-ray Yu and Goldstein 16224 Fermi GBM detection Fermi GBM Epeak=256.2±11.8 keV
T90=44.3 seconds
Fluence=6.24±0.07x10-5erg cm-2
(96.1 percentile for long GRBs)
Gamma-ray Hurley et al. 16225 IPN triangulation IPN
Gamma-ray Golenetskii et al. 16234 Konus-Wind observation Konus-Wind Epeak=213 (-20,+23) keV
Fluence=7.4(-0.7,+0.8)x10-5erg cm-2

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

Filter Tstart(s) Tstop(s) Exp(s) Mag
v 68965 69792 814 19.38 ± 0.10
b 70632 71427 782 19.59 ± 0.06
u 69799 70626 814 18.81 ± 0.05

Table 3. UVOT observations reported by Marshall and Amarel-Rogers (GCN Circ. 16243). 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.

May 20, 2014