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Fermi-LAT observation of increased gamma-ray activity from PKS 2155-304

ATel #4755; Sara Cutini (ASI ASDC & INAF OAR, Rome), on behalf of the Fermi Large Area Telescope Collaboration
on 23 Jan 2013; 00:15 UT
Credential Certification: Stefano Ciprini (stefano.ciprini@asdc.asi.it)

Subjects: Gamma Ray, >GeV, Request for Observations, AGN, Blazar

Referred to by ATel #: 6148

The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed an increasing gamma-ray flux from a source positionally consistent with well-known blazar PKS 2155-304, R.A. = 21h58m52.0651s Dec. =-30d13m32.118s (J2000, Fey et al. 2004, AJ, 127, 3587), classified as a TeV BL Lac object with redshift z=0.116 (Falomo et al. 1993, ApJ, 411, L63).

Preliminary analysis indicates that the source on January 21, 2013 was in a high state with a gamma-ray flux (E>100MeV) of (1.4+/-0.4) x 10^-6 photons cm^-2 s^-1, more than a factor of 8 greater than reported in the Fermi-LAT 2-year catalog (2FGL J2158.8-3013; Nolan et al. 2012, ApJS, 199, 31). During the first six hours of the day the source reached a flux (E>100MeV) of (2.5+/-0.8) x 10^-6 photons cm^-2 s^-1 showing a fast flaring activity (errors are statistical only).

Fermi-LAT already detected enhanced gamma-ray activity from this object in October 2010 (ATel#2944). Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. PKS 2155-304 is one of the "LAT Monitored Sources", and consequently, a preliminary, uncalibrated estimation of the daily gamma-ray flux observed by Fermi LAT is publicly available at: http://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/source/PKS_2155-304

The Fermi-LAT contact people for this source are B. Giebels (berrie@llr.in2p3.fr) and D. Sanchez (dsanchez@llr.in2p3.fr). Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. In consideration of the activity of this source we strongly encourage multiwavelength observations.

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.