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Fermi LAT detection of increased gamma-ray activity of two blazars PKS 0420-01 and BL Lacertae

ATEL # 2402; K. V. Sokolovsky (MPIfR/ASC Lebedev), F. K. Schinzel (MPIfR) and E. Wallace (Uni. Washington); on behalf of the Fermi Large Area Telescope Collaboration
on 25 Jan 2010; 16:22 UT
Password Certification: Frank Schinzel (

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

The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope has observed an increasing gamma-ray flux from sources positionally consistent with PKS 0420-01 (VLBI position: R.A.=04:23:15.80073 Dec.=-01:20:33.0655, J2000; z=0.914, Wills & Lynds 1978, ApJS, 36, 317) and BL Lacertae (VLBI position: R.A.=22:02:43.29137 Dec.=+42:16:39.9799, J2000; z=0.0686 Vermeulen et al., 1995, ApJ, 452L, 5). The VLBI positions are from Beasley et al. 2002, ApJS, 141, 13.

Preliminary analysis indicates that PKS 0420-01 on January 21, 2010, was in a high state with a gamma-ray flux (E>100MeV) of (0.8 +/-0.2)*10^-6 photons/cm^2/s, which represents an increase of a factor of six with respect to the average source flux level in the first eleven months of Fermi observations (see The current gamma-ray flare has been preceded by a gradual flux rise lasting for ~40 days. A recent, rapid optical and radio brightening of this source was reported by Bach et al. (2010) in ATEL #2395.

Our preliminary analysis indicates that BL Lacertae was in a high gamma-ray state on January 21, 2010, with a flux (E>100MeV) of (0.8 +/-0.2)*10^-6 photons/cm^2/s, five times above the average level. Most of the photons were detected in a six-hour interval centered on 2010 January 21.375 (UT). The estimated source flux during this six-hour interval was (1.1 +/-0.5)*10^-6 photons/cm^2/s. BL Lacertae was also marginally detected recently on a six-hour timescale around 2010 January 10.875 (UT) with a flux of (0.8 +/-0.5)*10^-6 photons/cm^2/s.

All uncertainties mentioned in this telegram 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. For PKS 0420-01 the Fermi LAT contact person is Frank Schinzel (schinzel at, for BL Lacertae the LAT contact person is Davide Donato (donato at

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.

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