1. 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

2. 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.

3. Why should we expect measurable VHE  -ray emission from 0.1-1 kpc-scale FR I jets ? They are confirmed sources of the synchrotron radio-to-X-ray emission, with the observed luminosities L syn ~ 10 39 -10 42 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 ≥ 10 -10 erg/cm 3 (Stawarz et al. 2003). They are at least mildly relativistic, with bulk Lorentz factors  ≥ 2 - 3 (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

4. The expected  -ray spectra of FR I kpc-scale jets Template  -ray spectra at different z, for a total IC jet luminosity L ic = 10 41 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 = 0.004360 Cen A  z = 0.001825 (applying a „universal” broken-power-law electron spectrum)

5. 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

6. 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)

7. 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:

8. Inverse-comptonisation of the starlight emission in M 87 jet (the brightest knot A, placed ~1 kpc from the nucleus) Stawarz et al. 2005 IACT array 100h sensitivity

9. 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)

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

11. 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)

12. 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.