Numerical Simulations of FRI jets

Manel Perucho PlaMax-Planck-Institut für Radioastronomie

andJ.M. Martí (Universitat de València)

Introduction: Laing & Bridle 2002a,b

Assumptions• Axisymmetric, time-stationary, relativistic jet• Symmetric jet/counterjet system• Parameterized distributions of velocity, magnetic field and synchrotron emissivity.

Comparison with VLA total intensity and polarization data allows to fix parameterizations.

Dynamical model based on conservation laws. • External gas density and pressure distributions are taken from Hardcastle et al. (2002)• Pressure equilibrium with the external medium assumed in the outermost studied region.

Results:• Jet axial structure: inner, flaring and outer regions• Spine velocity decreases (from 0.9 to 0.25 c) due to entrainment in the flaring region•Transversal structure: spine + shear layer

Jet dynamics:• The jet is overpressured at the inlet: expansion and acceleration.• Recollimation occurs when the jet becomes underpressured (wrt ambient).• Entrainment: peak in the entrainment rate at the recollimation site (stellar mass loss?); outwards the jet is slowly entrained and decelerated.

Evolution of FRI jets: setupPerucho & Martí 2007, MNRAS

• Jet injected according to Laing & Bridle (2002a,b) model at 500 pc from the core

rm = 7.8 kpc

Axisymmetric simulation of a purely leptonic jet with Lj ~ 1044 erg/s.

Physical domain: 18 kpc x 6 kpc [Resolution: 8 cells/Rj (axial) x 16 cells/Rj (radial)]

• ambient medium conditions from Hardcastle et al. 2002

Evolution of FRI jets: picture at 7 106 yearsPerucho & Martí 2007, MNRAS

Last snapshot (T = 7 106 yrs ~ 10 % lifetime of 3C31)

beam

cavity/cocoon

shocked ambient

bow shock

bow shock

shocked ambient

Evolution of FRI jets: dynamics Perucho & Martí 2007, MNRAS

Extended B&C model:

~ 0.1, ~ 1

Cocoon evolution:

t 1.3

t 1

~ constant

for negligible pollution with ambient particles (Nc b ~ 20 - 200 Nc a ), and assuming self-similar transversal expansion

Nc b

Nc a

Ps

Tc

Rs

vbs

Pc c

Evolution of FRI jets: dynamics in the beamPerucho & Martí 2007, MNRAS

As in Laing & Bridle’s model, the evolution is governed by adiabatic expansion of the jet, recollimation, oscillations around pressure equilibrium, mass entrainment and deceleration.However, in the simulation there are more shocks and all the entrainment is due to a destabilization of the jet as a result of those shocks.

recollimation shockand jet expansion

jet disruption and mass load

jet deceleration

L&B model

adiabatic expansion

pressure density Mach number

Simulation

Evolution of FRI jets: some thoughtsPerucho & Martí 2007, MNRAS

jet disruption and mass load

jet deceleration

Comparison with L&BSimulations confirm the FRI paradigm qualitatively, but

• jet flare occurs in a series of shocks• the presence of the cocoon is crucial: the jet is not interacting directly with the ambient, but with the cocoon.

• comparison wth L&B model is difficult as the jet has not reached a steady state

Strong vs mild shockHardcastle et al. 2002 found X-ray emission from the flaring region of the northern jet, where the jet decollimates and shows brighter radio emission.

• this is the region which we identify with the post-recollimation shock region, where particles could gain enough energy to emit in the X-ray• this fact favors the presence of a strong shock, as seen in the simulation

The simulated jet is very young if compared to 3C31, but• how does it compare to younger FRI jets like CenA or NGC3801?

Comparison with young FRI’sPerucho & Martí 2007, MNRAS

jet disruption and mass load

jet deceleration

Mach number of the bow shock by the end of the simulation M ~ 2.5, consistent with X-ray observations by Kraft et al. 2003 (Cen A) and Croston et al. 2007 (NGC3081):

• M ~ 3 - 8 for the bow shock of those sources with ages ~ 106 yrs (NGC3801).

Pressure, number density and Temperature in the shocked ambient gas (shell) and the ambient medium of the galactic gas are comparable too.

The high temperature in the shell compared to observations could be due to:• Initial jet power of the simulated jet is 1044 erg/s (cf. 3 1042 erg/s in NGC 3801, Croston et al. 2007).• Lack of thermal cooling in the simulation. • The ambient medium in 3C 31 is modelled as hotter (107 K) and denser (104 m-3) than those in NGC3801 or Cen A (106 K, 103 m-3).• X-ray observing energies in those sources are low for these temperatures.

Thermal emission from young FRI’s

jet disruption and mass load

jet deceleration

t 106 yrs (LS 3 kpc)Tsh 109 K 100 keVne,sh 6 x 10-2 cm-3

Vsh 1065 cm3

t 7 106 yrs (LS 16 kpc)Tsh 109 K 100 keV ne,sh 6 x 10-3 cm-3

Vsh 1067 cm3

3C31 – NGC3801: DL 50 Mpc

νSV 10-12 erg cm-2 s-1

3C31 – NGC3801: DL 50 Mpc

νSV 10-13 erg cm-2 s-1

CenA: DL 3 Mpc

νSV 10-10 erg cm-2 s-1

CenA: DL 3 Mpc

νSV 10-11 erg cm-2 s-1

Kraft et al. 2003Cen A (t ?)νSV 10-12 erg cm-2 s-1

LX 1039 erg s-1 (0.1-10 keV)

Croston et al. 2007NGC3801 (t 2 x 106 yrs)νSV 6 – 7 x 10-15 erg cm-2 s-1

LX 1039 erg s-1 (0.4-2 keV)

Our pre/post-diction is that young (t<107 yrs) FRI sources present bow-shocks in general and these should be observable in X-rays to gamma-rays, depending on the jet power.

The evolution of theluminosity with time is as predicted by Kino et al. (2007): t-1

L(100 keV) 8 1040 erg s-1

L(100 keV) 1040 erg s-1

Siemiginowska et al. (2008) have found X-ray luminosities (0.5-10 keV) in GPS sources within 1042 - 1046 erg/s.

• Suggested to be related to the accretion power.• A significant fraction of this flux could come from thermal emission if the sources are very young (t ≤ 103 yrs), depending also on their power (FRI’s or FRII’s).

• Long term simulations (up to 100 kpc).

– Supercomputation.• RATPENAT: a 3D RHD code

parallelised for the use of supercomputers.

• The parallelisation of the numerical grid has been performed in the direction of propagation of the flow.

– Physics.• Relativistic EoS.• Mass load from stars.• Bremsstrahlung cooling. ...(couple of years)...• Magnetic fields.

FRI jets: next steps

RATPENAT