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VHE g -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.
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VHE g-ray Emission From Nearby FR I Radio Galaxies M. Ostrowski1 & L. Stawarz 1,2 1 Astronomical Observatory, Jagiellonian University 2 Landessternwarte Heidelberg & MPIfK Heidelberg
At present, all but one detected extragalactic sources of VHE g-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 g-rays, produced not only in their active nuclei („misaligned BL Lacs”), but also within their kpc-scale jets.
Why should we expect measurable VHE g-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 Lsyn ~ 1039-1042 erg/s. This implies energies of the emitting electrons up to Ee ~ 100 TeVfor the equipartition jet magnetic field Beq ~ 100 mG (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 Ustar ≥ 10-10 erg/cm3 (Stawarz et al. 2003). • They are at least mildly relativistic, with bulk Lorentz factors G ≥ 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
The expected g-ray spectra of FR I kpc-scale jets (applying a „universal” broken-power-law electron spectrum) Template g-ray spectra at differentz, for a total IC jetluminosity Lic = 1041 ergs/s and an equipartition jetBeq = 300 mG. 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
Low luminosities of FR I jets are compensated by their small distances M 87: dL = 16 Mpc Cen A: dL = 3.4 Mpc Kpc-scale Cen A jet in radio and X-rays (Kraft et al. 2001). Kpc-scale M 87 jet in radio, optical, and X-rays (Marshall et al. 2002).
Detection of nearby FR I sources by modern Cherenkov telescopes at VHE g-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)
For illustration: 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 J - a Lorentz factor G - 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)
Inverse-comptonisation of the starlight emission in M 87 jet (the brightest knot A, placed ~1 kpc from the nucleus) IACT array 100hsensitivity Stawarz et al. 2005
HEGRA and HESS detected variable TeV signal from M 87. Since the g-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. Beilicke et al. (2005) Aharonian et al. (2003)
The lower limit for the jet magnetic field approximately equals its equipartition value equipartition B for different J and G
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.