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Fermi LAT detection of gamma-ray spectrum hardening in BL Lacertae

ATel #3462; D. Gasparrini (ASDC/INAF) on behalf of the Fermi Large Area Telescope Collaboration
on 30 Jun 2011; 19:07 UT
Credential Certification: Dario Gasparrini (dario.gasparrini@asdc.asi.it)

Subjects: Gamma Ray, >GeV, AGN, Blazar, Quasar

Referred to by ATel #: 3468, 4028

The Large Area Telescope (LAT), one of the instruments on board the Fermi Gamma-ray Space Telescope, has observed a significant change of the gamma-ray spectrum from a source positionally coincident with BL Lacertae (1FGL J2202.8+4216: Abdo et al. 2010, ApJS, 188, 405; more information in: Abdo et al. 2011, ApJ, 730, 101; the VLBI position is R.A.=22:02:43.29137, Dec.=+42:16:39.9799, J2000: Beasley et al. 2002, ApJS, 141, 13; z=0.0686: Vermeulen et al., 1995, ApJ, 452, L5).

On June 28 2011, our preliminary analysis indicates that BL Lacertae shows an unusual gamma-ray spectrum with a photon number power-law index of 1.70 +/-0.15, substantially harder than the average spectrum reported in the 1FGL catalog (Abdo et al. 2010, ApJS, 188, 405) of 2.37 +/-0.04 . This is in agreement with the TeV flare detection from VERITAS on the same day reported in ATel #3459. The source was in a high gamma-ray state for several days as reported in ATel # 3368 and #3387 and was detected on June 28 with a flux of (0.55 +/- 0.16) 10^-6 photons/cm^2/s (errors are statistical only).

Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of these sources will continue. We encourage multi-wavelength observations. The LAT contact person for BL Lacertae 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/ssc/data/access/lat/msl_lc/ ).

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