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Fermi LAT detection of increasing gamma-ray activity and clear spectral hardening from blazar PMN J2345-1555

ATel #4735; Yasuyuki Tanaka (Hiroshima University) on behalf of the Fermi Large Area Telescope Collaboration
on 15 Jan 2013; 01:51 UT
Credential Certification: Yasuyuki T. Tanaka (tanaka@astro.isas.jaxa.jp)

Subjects: Gamma Ray, >GeV, Blazar, Transient

Referred to by ATel #: 4736, 4742

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 PMN J2345-1555 (RA: 356.3019263, Dec: -15.9188428, J2000; Petrov et al. 2006, AJ, 131, 1872), a flat spectrum radio quasar at redshift z=0.631 (Healey et al. 2008, ApJS, 175, 97). Preliminary analysis indicates that the source on January 13, 2013 was in a high state with a gamma-ray flux (E>100MeV) of (1.1+/-0.2) x 10^-6 photons cm^-2 s^-1, which represents an increase of a factor of 17 with respect to the average source flux reported in 2FGL catalog (2FGL J2345.0-1553; Nolan et al. 2012, ApJS, 199, 31). Interestingly, the spectral index measured by LAT showed clear hardening on the same day (1.78+/-0.13 with respect to 2.17+/-0.04 in the 2FGL catalog for assuming power-law shape, uncertainties are statistical only), and hence we strongly encourage multi-wavelength follow-up, especially in TeV range.

This is the second time that the Fermi-LAT Collaboration has reported a similar GeV flare from this blazar (January 2010, ATel #2408). In addition, a recent near-infrared flare from this source was reported in November 2012 (ATel #4608).

PMN J2345-1555 is one of the "LAT Monitored Sources" (http://fermi.gsfc.nasa.gov/ssc/data/policy/LAT_Monitored_Sources.html), and consequently, a preliminary, uncalibrated estimation of the daily gamma-ray flux observed by Fermi LAT is publicly available (http://fermi.gsfc.nasa.gov/ssc/data/access/). Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. For this source the Fermi LAT contact persons are S. Ciprini (stefano.ciprini@asdc.asi.it) and Y. Tanaka (ytanaka@hep01.hepl.hiroshima-u.ac.jp).

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.