Fermi LAT detection and Swift follow up observation of Fermi J0617-4026: a new transient gamma-ray source

ATel #6912; D. Kocevski (GSFC/NASA), M. Ajello (Clemson University), R. Buehler (DESY/Zeuthen) on behalf of the Fermi Large Area Telescope Collaboration
on 13 Jan 2015; 19:40 UT
Credential Certification: Roopesh Ojha (Roopesh.Ojha@gmail.com)

Tweet

During the week between Dec 29th, 2014 and Jan 5th, 2015, the Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, observed gamma-ray activity from a previously unidentified transient source Fermi J0617-4026. The significance of the source detection above 100 MeV for the entire LAT exposure from Dec 29th, 2014 to Jan 5th, 2015 is approximately 8 sigma. The preliminary best-fit location of the gamma-ray source is RA = 94.28 deg, Dec = -40.44 deg, J2000, with 90% confidence that the source is within 0.29 deg of this position (statistical errors only). A preliminary spectral analysis of the source indicates that the source brightened in gamma-rays to a flux (E > 100 MeV) of (2.0 +/- 0.6) x10^-7 ph cm^-2 s^-1, with a spectral index of 2.47 +/- 0.26 (errors are statistical only).

A Swift target of opportunity observation was performed on Jan 9th, 2015. Swift/XRT detected a bright X-ray source at RA =94.195, Dec =-40.361, J2000, (with a 90% error of 3.6 arcseconds) within the LAT error circle. The XRT spectrum of the source can be fit in the 0.3-10 keV range with an absorbed power law model with a column density consistent with the Galactic one in the direction of the source (n_H = 6.60 x10^20 cm^-2, Kalberla et al. 2005, A&A, 440, 775) and a photon index Gamma =1.35 +/- 0.07. The corresponding absorbed 0.3-10 keV flux is (3.12 +/- 0.26) x10^-11 erg cm-2 s-1. We additionally note that there is a WISE source coincident with the XRT position, J061647.01-402142.8, whose infrared colors are compatible to those of blazars (Massaro et al. 2011, ApJ, 740, 48).

Because Fermi provides all-sky coverage, regular gamma-ray monitoring of this source will continue. Multiwavelength observations during the ongoing activity of this source are strongly encouraged. The Fermi LAT contact person for Fermi J0617-4026 is Daniel Kocevski (e-mail:daniel.kocevski@nasa.gov).

This transient was identified thanks to a new method implemented within the Fermi-LAT Collaboration known as `Fermi All-sky Variability Analysis' that searches the sky for high-energy transients on weekly time scales (Ackermann et al. 2013, ApJ, 771, 57).

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.