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Radio-loud AGN as sources for ultra-high-energy cosmic rays. Martin Hardcastle Hertfordshire SOCoR , Trondheim, 17 th June 2009

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radio loud agn as sources for ultra high energy cosmic rays

Radio-loud AGN as sources for ultra-high-energy cosmic rays

Martin Hardcastle


SOCoR, Trondheim, 17th June 2009

Thanks to Teddy Cheung, ŁukaszStawarz, IlanaFeain for work on Cen A lobes; Ralph Kraft, Joanna Goodger, Diana Worrall, Judith Croston et al for X-ray aspects of Cen A and other jet work.

  • Introduction
    • Types and extended structures of radio galaxies
  • Acceleration in FRI jets
    • The Cen A jet as a model
  • Acceleration in lobes:
    • Basic model
    • Application to Cen A
    • Implications for UHECR source population
  • Summary

Radio galaxy morphology








what radio galaxies can accelerate uhe cosmic rays
What radio galaxies can accelerate UHE cosmic rays?
  • Here we consider extended (>pc scale) components only (but see other talks)
  • Hotspots of FRIIs are well known to be physically capable of accelerating UHECRs (e.g. Hillas 1984)
  • Modern studies confirm that high-energy particle acceleration takes place at hotspots
  • However RGs are rare and luminosity function of FRIIs is quite steep; none within 100 Mpc (~GZK)
  • Hence we concentrate on the low-luminosity FRIs.
fri morphology
FRI morphology

Left: lobed twin-jet. Right: plumed twin-jet.

Scales are a few hundred kpc (cf elliptical hosts)

Images courtesy of Alan Bridle, NRAO

fri dynamics
FRI dynamics
  • Jets decelerate smoothly (as seen at low spatial resolution) from relativistic to sub-relativistic speeds; no jet-wide shocks in general
  • For momentum conservation, deceleration must take place by entrainment of external material (e.g. stellar winds or external hot phase)
  • If no jet-wide shocks, how and where are particles accelerated?

Models from Laing & Bridle 2001, 2002

particle acceleration probes
Particle acceleration probes
  • dE/dt = 4/3(q4/6πε0m02c4)c γ2 (Uphoton+B2/2μ0)
  • Higher energies have shorter loss timescales (E/(dE/dt) goes as 1/γ or 1/E).
  • Thus we can use observations of synchrotron radiation at the highest frequencies to locate the sites of particle acceleration.
  • Imaging and spectroscopy at these locations may tell us the processes implicated.
particle acceleration in fri jets
Particle acceleration in FRI jets
  • X-ray counterparts of jets inFRIs are common, possiblyuniversal (Worrall+ 01)
  • X-ray emission is clearlysynchrotron (MJH+ 01)
  • X-ray emission comes fromdeceleration region =>bulk k.e. is being translated intointernal energy of particle pop’n(and fields?)
  • We should be able to use this X-rayemission to study the nature of particle acceleration.
  • To do this we need the highest spatial resolution since at X-rays the loss spatial scale is of the order of parsecs.
case study centaurus a jet
Case study: Centaurus A jet
  • D = 3.7 Mpc; the closest radio galaxy
  • 1 arcsec is 18 pc, so Chandra has ~ 9-pc resolution, 5-20 times higher than in other well-studied X-ray jets
  • Should be able to generalize from this to other systems.
cen a in the radio
Cen A in the radio
  • Cen A’s outer radio lobes are physically & in terms of angles on the sky very large (500 kpc).
  • Evidence for multiple phases of AGN activity.

10 kpc

the inner jet

MJH+ 2003

The (inner) jet
  • Strong point-to-point radio/X-ray ratio variation – particle acceleration efficiency varies spatially
  • Compact X-ray emitting ‘knots’ are stationary in multi-epoch radio imaging – could be shocks?


the jet
The jet

3) Diffuse X-ray emission comes to dominate at large distances from the nucleus (out to 4 kpc).

4) X-ray spectra of knots are flat: X-ray spectrum of diffuse emission gets progressively steeper ending at very high values: X-ray surface brightness falls off faster than radio.

MJH+ 2008

the jet14
The jet
  • We conclude:
    • The spatial and spectral differences between the compact ‘knots’ and diffuse emission means that there are two acceleration processes going on.
    • The compact knots may be shocks where jet flow interacts with obstacles (Blandford & Königl 1979), producing electrons up to at least 10 TeVby first-order Fermi.
    • The diffuse emission surrounding them is probably something else! Possibly 2nd-order Fermi at turbulence, or mag. reconnection...
uhecr acceleration in the jet
UHECR acceleration in the jet
  • Compact knots are too small to accelerate UHECR if mag. fields are close to equipartition; for R ~ 10 pc we require fields 2 orders of magnitude higher than Beqfor 100 EeV.
  • But observations of diffuse X-ray synchrotron emission allow us to consider the whole jet to be an acceleration region, with R ~ 400 pc.
  • Still require B > Beqto reach 100 EeV for protons.
  • Particles accelerated in jet (or nucleus) will be scattered by interactions with B-field in lobes (see later); need not be a ‘point source’!
consequences for uhecr models
Consequences for UHECR models
  • Cen A jet falls short of UHECR energies unless B > Beq (but no independent constraints on B) or primaries are nuclei (possible, since entrained thermal material will be enriched).
  • Radio galaxies accelerating particles by this process should have dissipative inner jets; we expect a diffuse p.a. process throughout the jet to dominate leptonic acceleration.
  • Expect X-ray synchrotron from inner jet and UHECR production rate to be associated; testable in principle.
  • Brighter jet => higher B => more UHECR.
the giant lobes of cen a
The giant lobes of Cen A
  • We used WMAP to study the high-frequency radio behaviour of the giant lobes.
  • Detection at high radio frequencies implies recent, possibly ongoing particle injection.
  • Magnetic field constrained from inverse-Compton limits using X-ray and gamma-ray properties (may make measurement with Fermi)

Junkes+ 1993 MJH, Cheung, Stawarz, Feain 2009

uhecr acceleration in the giant lobes
UHECR acceleration in the giant lobes
  • Long-standing possibility that UHECR may be accelerated in the giant lobes of Cen A (e.g. Cavallo 1978, Romero+ 1996, Gureev & Troitsky 08)
  • Current discussion stimulated by Pierre Auger result
  • Our contribution is detailed information on the properties of the giant lobes.
uhecr acceleration in the giant lobes19
UHECR acceleration in the giant lobes
  • Lobes satisfy Hillas criterion for 1020 eV protons, for B = Beq
  • We propose a second-order Fermi process involving scattering from turbulent B-field structures in lobes.
  • We show that the acceleration timescale is compatible with lobe lifetimes (but see next slide), the power in UHECR is a small fraction of the jet power assuming PAO detection, and loss processes are not important.
    • Acceleration in lobes has nice feature that dominant photon fields are CMB and EBL – so we just require that tacc<< tpropagation to avoid photopion/photodisintegration losses.
  • Possible – but unlikely – that we will see gamma rays from p-p interactions with future Cerenkov instruments.
  • Note the possibility of ‘hybrid’ processes in which UHECR are first accelerated in AGN or jet and then in lobe.
lobe acceleration a caveat
Lobe acceleration – a caveat
  • 2nd-order Fermi in lobes can only be efficient if ‘speeds of scatterers’ are ~ c – for magnetic turbulence, require vA >~ c/3.
  • This requires lobe contents to be relativistic plasma; will not work if the lobe is energetically dominated by thermal material.
  • Some evidence that FRI lobe energetics are dominated by non-radiating particles, but no firm evidence that they has a non-relativistic equation of state; model is not ruled out for now.
  • Only existing constraints are very conservative upper limits that assume no thermal emission from external environment of Cen A (known untrue on smaller scales).
uhecr from nearby radio galaxy lobes
UHECR from nearby radio galaxy lobes
  • Some evidence that the early PAO events may be as well, or better, correlated with nearby radio galaxies as with RQ AGN (e.g. Nagar & Matulich 08, Hillas 09).
  • Let’s now consider whatwe learn about suchmodels from the case ofCen A.

Nagar & Matulich 08

uhecr from nearby radio galaxy lobes22
UHECR from nearby radio galaxy lobes
  • Require that the lobes satisfy the Hillas criterion for protons, i.e. R > 100 E20B-6 kpc
  • This can be used to give a constraint on luminosity if we impose the additional requirement of an equipartition field or a fixed departure from equipartition.
  • (We know from inverse-Compton studies of FRIIs that B ~ Beq in those systems; not clear whether this holds in FRIs.)
luminosity constraint
Luminosity constraint
  • Consider a spherical lobe with radius R and a constant electron density and B-field strength, such that Ue= εUB. Equipartition corresponds toε = 1.
  • Let electron energy spectrum be a power law with index p (back-of-envelope only)
  • Then we can show that H.C. corresponds toL(ν) > K(p) ε ν(1-p)/2 E(5+p)/2 R(1-p)/2where E is max energy and K contains constants plus a weak p dependence.
  • p=2 is a plausible value giving L ~ R-1/2.
luminosity constraint24
Luminosity constraint
  • L depends on R, so we minimize L by maximising R; max plausible value is ~ 250 kpc (must be smallest dimension of lobes).
  • For Cen A we know ε <~ 1.
  • If ε ~ 1 then, substituting in numbers, we find L408 >~ 2 x 1024 W Hz-1 to satisfy H.C.
  • Only relatively powerful FRI radio galaxies satisfy this, and of course the luminosity increases for smaller lobes. FRI/FRII break at 3 x 1025 W Hz-1.
  • (Sanity check; Cen A just satisfies this limit, at around 3 x 1024 W Hz-1 at 408 MHz, as it should.)
luminosity function constraints
Luminosity function constraints
  • Using 408-MHz luminosity functionfrom MJH+03 we find that suchpowerful RGs are rare; we expect only15-20 RGs of this luminosity in thesouthern sky out to 100 Mpc, not all ofwhich will have physical size capable ofaccelerating UHECR.
  • These objects will have 408-MHz flux> 1.7 Jy. (Thus all in southern sky are inMolonglo Reference Catalogue.)
  • Because of flat luminosity function ofFRIs this is not vastly changed (~50) if ε ~ 0.1 instead of 1 (but lower flux cutoff)
  • Only a rough estimate because there is strong cosmic variance of RG density; may be lower in southern sky since we do not see the RG-rich Perseus-Pisces supercluster...
density of nearby galaxies
Density of nearby galaxies

Density of nearby galaxies out to ~ 170 Mpc. FRI radio galaxies are biased towards dense regions and so trace high-density large-scale structure. Many well-known RGs in the northern sky at ~ 100 Mpc lie in the Perseus-Pisces supercluster.

Image from Richard Powell via

testable predictions
Testable predictions
  • Lobe UHECR requires the most luminous radio galaxies within the GZK radius, thanks to B-field requirement.
  • They must have lobes (not plumes) and the lobes must be physically large (e.g. Cen A, Fornax A, ‘Cen B’; cf ‘FRII-like’ class of Nagar & Matulich).
  • Visible jets are not required, but pre-injection of high-energy protons from jet may help efficiency.
  • There must be relatively few discrete sourcesof UHECR via this mechanism and they willcorrelate with large-scale structure.
  • Accelerated particles will be predominantlyprotons (but nuclei are easier to confine sowill be disproportionately present at highestenergies).
  • Proton spectrum will cut off at ~ 100 EeV sincerequired radio luminosity is strong function ofE; no RGs available with required B-field strengths.

‘Cen B’:Jones + McAdam 01

  • If the sources of UHECR are radio-loud AGN and the GZK cutoff operates, they must be nearby, low-power objects.
  • Observations at other wavebands give new constraints on the locations of particle acceleration in these systems.
  • Acceleration of UHECR in the jet (where we know high-energy leptons are accelerated) requires high B-fields or nuclei as primaries, but cannot be ruled out.
  • Lobe acceleration of protons is possible but requires large, luminous objects of which there are relatively few in the local universe; predictions are testable in principle.