VHE  -ray Emission From Nearby FR I Radio Galaxies M. Ostrowski 1 & L. Stawarz 1,2 1 Astronomical Observatory, Jagiellonian University 2 Landessternwarte Heidelberg & MPIfK Heidelberg

At present, all but one detected extragalactic sources of VHE  - ray radiation belong to the class of low-luminosity blazars, i.e. BL Lac objects. FR I Radio Galaxies are believed to be a parent („unbeamed”) population of BL Lacs. As such, FR Is are more numerous in the local Universe than blazars. Till now, however, only one FR I galaxy - M 87 - has been firmly detected at TeV photon energies. With improved sensitivity (and the lower-energy threshold) of the future Cherenkov Telescopes, several FR Is should be detected at VHE  -rays, produced not only in their active nuclei („misaligned BL Lacs”), but also within their kpc-scale jets.

Why should we expect measurable VHE  -ray emission from kpc-scale FR I jets ? They are confirmed sources of the synchrotron radio-to-X-ray emission, with the observed luminosities L syn ~ erg/s. This implies energies of the emitting electrons up to E e ~ 100 TeV for the equipartition jet magnetic field B eq ~ 100   G (e.g., Kataoka et al. 2006, for the case of Centaurs A jet). They are surrounded by relatively intense starlight photon field of host elliptical galaxies, with the energy density U star ≥ erg/cm 3 (Stawarz et al. 2003). They are at least mildly relativistic, with bulk Lorentz factors  ≥ (e.g., Biretta et al. 1999, for the case of M 87 jet). Therefore, we expect relatively intensive GeV-TeV emission produced by the synchrotron-emitting jet electrons through IC scattering of the starlight photons

The expected  -ray spectra of FR I kpc-scale jets Template  -ray spectra at different z, for a total IC jet luminosity L ic = ergs/s and an equipartition jet B eq = 300  G. Dashed lines - emission intrinsic to the source thick solid lines - emission which would be measured by the observer located at z = 0 (with absorption/reemission effects included) dotted lines - emission from the source's halo (Stawarz et al. 2006a) present IACT array 100h sensitivity z = 0.03  distance ~150 Mpc M 87  z = Cen A  z = (applying a „universal” broken-power-law electron spectrum)

Low luminosities of FR I jets are compensated by their small distances Kpc-scale M 87 jet in radio, optical, and X-rays (Marshall et al. 2002). Kpc-scale Cen A jet in radio and X-rays (Kraft et al. 2001). M 87: d L = 16 MpcCen A: d L = 3.4 Mpc

Detection of nearby FR I sources by modern Cherenkov telescopes at VHE  -ray photon energy range is already possible, and likely. Even upper limits are meaningful, since they allow to constrain some unknown (or hardly known) parameters of FR I jets. See below: jet magnetic field in M 87 (Stawarz et al. 2005)

A special case of M 87 radio galaxy One can relatively precisely constrain a spectral shape of the synchrotron- emitting electrons and different target radiation fields. It enables to compute the expected IC emission (including relativistic and Klein-Nishina effects) as a function of jet parameters: - a viewing angle - a Lorentz factor  - a magnetic field B Energy densities of different radiation fields, as functions of the distance from the active nucleus of M 87. Stawarz et al. (2005, 2006b) For illustration:

Inverse-comptonisation of the starlight emission in M 87 jet (the brightest knot A, placed ~1 kpc from the nucleus) Stawarz et al IACT array 100h sensitivity

HEGRA and HESS detected variable TeV signal from M 87. Since the  - ray emission of kpc-scale knot A is not expected to vary on the time scale of months/years, we consider the detected flux as the upper limit. Aharonian et al. (2003) Beilicke et al. (2005)

The lower limit for the jet magnetic field approximately equals its equipartition value equipartition B for different and 

So where is the variable TeV emission of the M 87 produced ? Is it necessarily the active nucleus? Not necessarily! Emission of the HST-1 knot (placed at ~100 pc from the active nucleus and revealing superluminal motions), when modelled as a reconfinement shock, can explain varying TeV fluxes detected by HEGRA and HESS With increased CTA sensitivity possibly a number of different TeV-components can be studied through its spectral and temporal signatures. Harris et al. (2006): variable radio, optical, and X-ray emission of HST-1 knot. Stawarz et al. (2006b)

Summary: FR I kiloparsec-scale jets are viable sources of ~TeV gamma rays in the nearby universe. The expected IC-emissions can be ~precisely evaluated for such sources. Even upper limits for the source can provide valuable constraints for its physical parameters Increasing sensitivity of CTA by a factor ~10 can increase the number of studied sources (jets) from the present 1 up to several.