Swift Observations of GRB 140928A

C. Pagani (U. Leicester) for the Swift team

1. Introduction

Desiante et al. (GCN Circ. 16847) reported the GRB detection. At 10:29:53.55 on September 28, 2014, Fermi-GBM (trigger 433592996/140928437) triggered on GRB 140928A, which entered the Fermi-LAT field of view at T0+1550 s, and was observed until T0+3200 s, during which the LAT detected high-energy emission. Table 1 contains the best reported positions from Swift.

Varela et al. (GCN Circ. 16849) 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

BAT did not observe this burst.

3. XRT Observations and Analysis

We have analysed 7.4 ks of XRT data for the Fermi/LAT-detected burst: GRB 140928A, from 40.7 ks to 295.9 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=1.62 (+0.19, -0.15).

A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.83 (+0.26, -0.24). The best-fitting absorption column is 2.4 (+1.0, -0.9) x 10²¹ cm^-2, in excess of the Galactic value of 1.6 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 (4.4 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column: 2.4 (+1.0, -0.9) x 10²¹ cm^-2
Galactic foreground: 1.6 x 10²⁰ cm^-2
Excess significance: 4.2 σ
Photon index: 1.83 (+0.26, -0.24)

4. UVOT Observations and Analysis

UVOT results are not available.

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

02^h54^m47.71^s -55°55'43.8" 1.5" XRT-final UKSSDC

02^h54^m47.71^s -55°55'43.8" 1.5" XRT Pagani et al. GCN Circ. 16848

Table 1. Positions from the Swift instruments.

Band Authors GCN Circ. Subject Observatory Notes

Optical Varela et al. 16849 GROND Afterglow Candidate GROND detection

Optical Kopac et al. 16851 LCO-Cerro Tololo observations LCO detection

Optical Guidorzi et al. 16853 LCO-Cerro Tololo further observations LCO

Optical Cucchiara et al. 16856 Gemini-South observation Gemini

X-ray Morii et al. 16864 MAXI-GSC detection MAXI detection

Gamma-ray Desiante et al. 16847 Fermi-LAT detection Fermi LAT E_max=35 GeV

Gamma-ray von Kienlin and Meegan 16850 Fermi GBM detection Fermi GBM E_peak=732±72 keV

Gamma-ray Hurley et al. 16852 IPN Triangulation IPN

Gamma-ray Golenetskii et al. 16854 Konus-Wind observation Konus-Wind E_peak=304 (-66,+80) keV
Fluence=8.6(-1.4,+1.3)x10^-5erg cm^-2

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

RA (J2000)	Dec (J2000)	Error	Note	Reference
02^h54^m47.71^s	-55°55'43.8"	1.5"	XRT-final	UKSSDC
02^h54^m47.71^s	-55°55'43.8"	1.5"	XRT	Pagani et al. GCN Circ. 16848

Band	Authors	GCN Circ.	Subject	Observatory	Notes
Optical	Varela et al.	16849	GROND Afterglow Candidate	GROND	detection
Optical	Kopac et al.	16851	LCO-Cerro Tololo observations	LCO	detection
Optical	Guidorzi et al.	16853	LCO-Cerro Tololo further observations	LCO
Optical	Cucchiara et al.	16856	Gemini-South observation	Gemini
X-ray	Morii et al.	16864	MAXI-GSC detection	MAXI	detection
Gamma-ray	Desiante et al.	16847	Fermi-LAT detection	Fermi LAT	E_max=35 GeV
Gamma-ray	von Kienlin and Meegan	16850	Fermi GBM detection	Fermi GBM	E_peak=732±72 keV
Gamma-ray	Hurley et al.	16852	IPN Triangulation	IPN
Gamma-ray	Golenetskii et al.	16854	Konus-Wind observation	Konus-Wind	E_peak=304 (-66,+80) keV Fluence=8.6(-1.4,+1.3)x10^-5erg cm^-2

May 22, 2015