VHE -ray Emission From Nearby

FR I Radio Galaxies

M. Ostrowski1 & L. Stawarz 1,2

1 Astronomical Observatory, Jagiellonian University2 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 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 TeV for the equipartition jet magnetic field Beq ~ 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 Ustar ≥ 10-10 erg/cm3 (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

The expected -ray spectra of FR I kpc-scale jets

Template -ray spectra at different z, for a total IC jet luminosity Lic = 1041 ergs/s and an equipartition jet Beq = 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)

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: dL = 16 Mpc Cen A: dL = 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 constraina 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 BEnergy 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. 2005IACT 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, andX-ray emissionof 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.