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Fermi LAT detection of a short High-Energy Galactic transient

ATel #5625; G. Vianello (Stanford University), N. Omodei (Stanford University), E. Bottacini (Stanford University), S. Cutini (ASI/ASDC) on behalf of the Fermi LAT Collaboration, and V. Connaughton (UAH) on behalf of the Fermi GBM Collaboration:
on 4 Dec 2013; 07:58 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Giacomo Vianello (giacomov@slac.stanford.edu)

Subjects: Gamma Ray, >GeV, Transient

The Large Area Telescope (LAT) on board Fermi detected on December 1, 2013 an outburst from a position close to the Galactic plane. The outburst started around 11:22 UTC and lasted for about 5 minutes. The location of the transient is:

(R.A., Dec) = (261.32 deg, -37.44 deg) (J2000),

corresponding to Galactic coordinates:

(L, B) = (350.55 deg, -1.02 deg),

with a statistical uncertainty of 0.40 deg (95 % c.l.).

There is no known LAT source compatible with that position. We designate the source as Fermi J1725-3726.

The detected signal extends up to 1 GeV, and it has a significance of more than 5 sigma in the time interval 11:22 - 11:27 UTC. The spectrum of Fermi J1725-3726 is well described by a power law. The best fit parameters are a photon index of 2.2 +/- 0.4 and a mean energy flux of 9 +/- 5 x 10^-9 erg cm^-2 s^-1 in the energy band 100 MeV - 100 GeV, which corresponds to a photon flux of 1.3 +/- 0.5 x 10^-5 ph cm^-2 s^-1 in the same energy band.

The only noteworthy source within the LAT error circle is the X-ray burster XTE J1723-376 (Galloway et al. 2008, ApJS 179, 360, and references therein), which is offset by 0.39 deg from the LAT centroid.

Fermi J1725-3726 is not detected in any 5-minute interval from the beginning of November up to December 2, excluding the time interval of the outburst. The upper limits we obtain in those time intervals are below the flux of the transient, indicating that the source was below the detection threshold before the outburst, and returned below it around 11:27 UTC on December 1.

An unrelated solar flare triggered the scintillators of the Gamma-ray Burst Monitor (GBM) on-board Fermi at 11:19:41.6 UTC. Solar activity visible in the GBM detectors during the LAT transient outburst masks any potential signal from the LAT transient source. The solar flare is also weakly detected by the LAT at the position of the Sun, ~20 deg away from Fermi J1725-3726.

Fermi will operate in an all-sky scanning mode up to December 5. After that, a pre-planned switch to a modified survey mode will further increase the exposure of the regions around the Galactic center, incidentally also enhancing the sensitivity at the position of Fermi J1725-3726. In consideration of the activity of this source we strongly encourage multi-wavelength observations.

The Fermi LAT contact person for this source is Giacomo Vianello (giacomov@stanford.edu).

All results reported above are preliminary.

The Fermi LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.