Active Galactic Nuclei

The first thing that might cross your mind is, so what exactly is a blazar anyway? A blazar is specific kind of Active Galactic Nuclei (AGN). An AGN is a galaxy with a black hole at the center that is accreting matter. A blazar is an AGN that we happen to be looking at down the jet axis. Blazars are compact, highly variable sources. A subclass of blazars are objects known as BL Lacs, or BL Lacertae objects. These are blazars with intrisincally weak radio emission. All of our chosen sources fall into the category of being BL Lacs. BL Lacs have a characteristic two hump spectral energy distribution (SED). High frequency peaked BL Lacs (HBLs) have a higher frequency peaked synchrotron hump, and likewise with low frequency peaked BL Lacs (LBLs). All of our sources are HBLs. The SED of one of our sources, Markarian 421 shows this two hump distribution.

Another interesting question might be, why are HBLs neutrino point source candidates? As can be seen from the SED of Markarian 421 there are two distinct energy "humps". It is generally agreed upon that the low energy component of this SED is synchotron emission from relativistic electrons. (Mücke A., et al. 2003) The mechanism responsible for the high energy hump is still in dispute. There are both leptonic models and hadronic models for the source of these high energy X-rays and gamma rays. In leptonic models there is no neutrino production, but in hadronic models including the Synchrotron Proton Blazar Model proposed by Mücke A., et al. 2003, and the hadronic Synchrotron Mirror Model proposed by Reimer A., et al. 2005 gamma ray production by pion photoproduction would also lead to neutrino production through the decay of charged pions, and muons. These decays would result in the production of both electron and muon neutrinos.

The major difference between leptonic and hadronic models is that in leptonic models, sometimes known as synchrotron-self Compton (SSC) models, the gamma ray emission is correlated with the X-ray flares. There is one very clear example of an "orphan" TeV flare, that is a gamma ray flare with no corresponding X-ray flare, from the source 1ES 1959+650. This orphan flare was observed on June 4, 2002 approximately fifteen days after an initial TeV flare that was correlated with X-ray emission. Krawczynski et al. 2004 Several hadronic models use the prescence of protons to explain this orphan flare. Protons then create charged and neutral pions, which would result in both high energy gamma rays and neutrinos.

These hadronic models could be confirmed by the detection of neutrinos by IceCube in the right time window and from the right direction as a TeV flare.