Fermi-LAT detection of increasing of gamma-ray activity of BL Lacertae

ATel #4028; S. Cutini (ASI Science Data Center) on behalf of Fermi-LAT Collaboration
on 10 Apr 2012; 20:24 UT
Credential Certification: Dario Gasparrini (dario.gasparrini@asdc.asi.it)

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The Large Area Telescope (LAT), one of the instruments on board the Fermi Gamma-ray Space Telescope, has observed an increasing gamma-ray flux from a source positionally coincident with BL Lacertae (VLBI position: R.A.=22:02:43.29137 Dec.=+42:16:39.9799, J2000 from Beasley et al. 2002, ApJS, 141, 13; with a z=0.0686, Vermeulen et al., 1995, ApJ, 452, L5; more information in Abdo et al. 2011, ApJ, 730, 101).

Preliminary analysis indicates that BL Lacertae was in a high gamma-ray state during April 9, 2012, reaching a daily flux (E > 100MeV) of (2.3 +/- 0.3)x10^-6 photons/cm^2/s (errors are statistical only). Unlike June 2011 (ATel #3462) no spectral hardening was observed this time, but the flux has increased by a factor of 13 over the average flux reported in the second Fermi-LAT Catalog (2FGL J2202.8+4216; Nolan et al. 2012, ApJS, 199, 31). The flux is also higher than the GeV flare reported in May 2011 (ATel #3368).

Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. We encourage multi-wavelength observations. For BL Lacertae the LAT contact person is Davide Donato (donato at milkyway.gsfc.nasa.gov). This source is one of the "LAT Monitored Sources" and consequently a preliminary estimation of the daily gamma-ray flux observed by Fermi/LAT is publicly available (link:http://fermi.gsfc.nasa.gov/FTP/glast/data/lat/catalogs/asp/current/lightcurves/BLLac_86400.png).

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