Fermi-LAT detection of a new transient gamma-ray source Fermi J2007-2518

ATel #6553; D. Kocevski (GSFC/NASA), M. Ajello (Clemson University), R. Buehler (DESY/Zeuthen), S. Buson (University of Padova/INFN), S. Ciprini (ASDC & INAF), C. C. Cheung (NRL) on behalf of the Fermi LAT Collaboration
on 7 Oct 2014; 22:21 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Marco Ajello (majello@clemson.edu)

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During the period between September 25th and October 3rd, 2014, 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. This source is not in any published Fermi LAT catalog and there is no previously reported EGRET gamma-ray detection at this location. The significance of the source detection above 100 MeV for the entire LAT exposure for this period is approximately 8 sigma. The source was detected previously with the LAT at a lower significance during the week of April 25th. The preliminary best-fit location of the gamma-ray source is RA = 302.371 deg, Dec = -25.411 deg, J2000, with a 90% confidence that the source is within 0.22 deg of this position (statistical errors only). A preliminary analysis of the source indicates that the source brightened in gamma rays to a flux (E > 100 MeV) of (1.5 +/- 0.1) x 10^-6 ph cm^-2 s^-1 on September 29th, 2014. A power-law fit to the spectrum between the period of September 25th and October 3rd yields a spectral index of 3.07 +/- 0.04 (errors are statistical only).

Analysis of archival Swift exposures reveals, within the LAT error radius, the presence of a bright, variable, X-ray source at RA = 302.58218 deg, Dec= -25.39980 deg, J2000, with a 68% error of 3.9 arcsec. This source is also spatially coincident with the radio source NVSS J201019-252358. Because Fermi provides all-sky coverage, regular gamma-ray monitoring of this source will continue. Multi-wavelength observations during the ongoing activity of this source are strongly encouraged. The Fermi LAT contact person for Fermi J2007-2518 is Daniel Kocevski (e-mail: daniel.kocevski@nasa.gov).

The transient was identified by the `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.