Swift Observations of GRB 140716A

J.R. Cummings (NASA/UMBC), K.L. Page (U. Leicester) and L.M.Z. Hagen (PSU) for the Swift team

1. Introduction

At2014-07-16T10:27:57.3 Swift-BAT rate-triggered on the first episode of GRB 140716 (Cummings GCN Circ. 16596). Table 1 contains the best reported positions from Swift.

Knust et al. (GCN Circ. 16601) reported the position from GROND for the optical afterglow of this GRB. Table 2 is a summary of GCN Circulars about this GRB from observatories other than Swift.

2. BAT Observations and Analysis

The burst had three peaks with similar maximum flux, the first 2 seconds long, the second 4 seconds long at T+90, and the third about 8 seconds long at T+100. The emission was undetectable between the first and second peaks, while there was a slight overlap between peaks 2 and 3. The total T90 was about 104 ± 2 seconds. At T+112 seconds, Swift executed a preplanned slew, putting the source completely out of the BAT coded field of view, but we can say that there was no bright emission above about 50 keV after this time.

The average spectrum from 15 to 150 keV is best fit by a simple power law function with a photon index of 1.61 ± 0.33. The total fluence was (5.9 ± 1.2) x 10-6 ergs/cm2. This fluence is larger than that of 86% of the long GRBs in the Second BAT GRB Catalog (Sakamoto et al. 2011). The peak 1-second flux was 5.2 ± 1.5 photons s-1/cm2 from T+0.2 to T+1.2 seconds. All the quoted errors are at the 90% confidence level.

3. XRT Observations and Analysis

We have analysed 10 ks of XRT data for the Swift/BAT-detected burst: GRB 140716A, from 34.1 ks to 478.7 ks after the Swift/BAT trigger. The data are entirely in Photon Counting (PC) mode. An X-ray source is detected within the Swift/BAT error circle.

The light curve (Figure 2) can be modelled with a power-law decay with a decay index of α=1.19 (+0.36, -0.25).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.58 (+0.58, -0.22). The best-fitting absorption column is consistent with the Galactic value of 1.3 x 1021 cm-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.3 x 10-11 (4.8 x 10-11) erg cm-2 count-1.

A summary of the PC-mode spectrum is thus:
Total column: 1.3 ± 1.6 x 1021 cm-2
Galactic foreground: 1.3 x 1021 cm-2
Excess significance: <1.6 σ
Photon index: 1.58 (+0.58, -0.22)

The results of the XRT team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00020390.

4. UVOT Observations and Analysis

The Swift/UVOT began settled observations of the field of GRB 140716A 34072 s after the BAT trigger (Hagen and Page GCN Circ. 16604). No optical afterglow consistent with the XRT candidate (Page, GCN Circ. 16600) or GROND candidate (Knust et al., GCN Circ. 16601) 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 0.131 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
07h12m41.06s -60°10'31.3" 2.4" XRT-final UKSSDC
07h12m41.53s -60°10'32.7" 4.7" XRT Page GCN Circ. 16600
07h12m31.9s -60°08'59" 1.5' BAT-refined Cummings et al. GCN Circ. 16598

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes
Optical Knust et al. 16601 GROND afterglow candidate GROND detection
Optical Bolmer et al. 16614 GROND confirmation of the afterglow GROND detection
Gamma-ray Jenke and Yu 16599 A Fermi GBM Detection Fermi GBM Epeak=92±13 keV
T90=106 seconds
Fluence=1.03±0.04x10-5erg cm-2
(75th percentile for long GRBs)
Gamma-ray Golenetskii et al. 16605 Konus-Wind observation Konus-Wind Epeak=157 (-41,+84) keV
Fluence=9.4(-1.6,+2.2)x10-6erg 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
u (fc) 34072 34367 291 >19.17
u 40452 56904 1860 >20.20
v 40559 57641 1125 >19.52

Table 3. UVOT observations reported by Hagen and Page (GCN Circ. 16604). The start and stop times of the exposures are given in seconds since the BAT trigger. The preliminary 3-σ upper limits are given. No correction has been made for extinction in the Milky Way.

July 22, 2014