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

ATel #3368; Sara Cutini (ASDC) on behalf of Fermi LAT collaboration
on 22 May 2011; 16:48 UT
Credential Certification: Dario Gasparrini (dario.gasparrini@asdc.asi.it)

Tweet

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 (1FGL J2202.8+4216; Abdo et al. 2010, ApJS, 188, 405; more information in: Abdo et al. 2011, ApJ, 730, 101; VLBI position: R.A.=22:02:43.29137 Dec.=+42:16:39.9799, J2000; z=0.0686 Vermeulen et al., 1995, ApJ, 452, L5). The VLBI position is from Beasley et al. 2002, ApJS, 141, 13.

Our preliminary analysis indicates that BL Lacertae was in a high gamma-ray state for several days reaching a flux (E > 100MeV) of (1.4 ± 0.4)*10^-6 photons/cm^2/s on 2011-05-20. Most of the photons were detected in the six-hour interval from 2011-05-20 06:00:00 to 2011-05-20 12:00:00. During this interval we estimated source flux (E > 100MeV) as (2.7 +/-0.9)*10^-6 photons/cm^2/s. This six-hours flux is ~ 2.5 times higher than the peak reached by the source on 2010-01-21 (ATel #2402).

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 BL Lacertae the LAT contact person is Davide Donato (donato at milkyway.gsfc.nasa.gov). This source 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 gamma-ray flux observed by Fermi LAT is publicly available (http://fermi.gsfc.nasa.gov/ssc/data/access/).

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