VHE -ray Emission From NearbyFR 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 -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 inthe local Universe than blazars. Till now, however, only oneFR I galaxy - M 87 - has been firmly detected at TeV photonenergies.
•With improved sensitivity (and the lower-energy threshold)of the future Cherenkov Telescopes, several FR Is should bedetected 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 -rayemission from 0.1-1 kpc-scale FR I jets ?
•They are confirmed sources of the synchrotron radio-to-X-rayemission, 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 G(e.g., Kataoka et al. 2006, for the case of Centaurs A jet).
•They are surrounded by relatively intense starlight photon fieldof host elliptical galaxies, with the energy densityUstar ≥ 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 emissionproduced by the synchrotron-emitting jet electrons throughIC 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 thesource
thick solid lines - emission which wouldbe measured by the observer locatedat z = 0 (with absorption/reemissioneffects included)
dotted lines - emission from the source'shalo (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 bytheir 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 modernCherenkov telescopes at VHE -ray photonenergy range is already possible, and likely.
•Even upper limits are meaningful, since theyallow to constrain some unknown (or hardlyknown) 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 emissionin M 87 jet(the brightest knot A, placed ~1 kpc from the nucleus)
Stawarz et al. 2005
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 scaleof 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 fieldapproximately equals its equipartition value
equipartition B for different and
So where is the variable TeV emission of the M 87produced ? Is it necessarily the active nucleus?
Not necessarily!
Emission of the HST-1 knot (placedat ~100 pc from the active nucleusand revealing superluminalmotions), when modelled as areconfinement shock, can explainvarying TeV fluxes detected byHEGRA and HESS
With increased CTA sensitivitypossibly a number of different
TeV-components can be studied
through its spectral and temporalsignatures.
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 expectedIC-emissions can be ~precisely evaluated for such
sources.
Even upper limits for the source can provide valuableconstraints 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.