Presentation on theme: "Radio-loud AGN as sources for ultra-high-energy cosmic rays Martin Hardcastle Hertfordshire SOCoR, Trondheim, 17 th June 2009 Thanks to Teddy Cheung, Łukasz."— Presentation transcript:
Radio-loud AGN as sources for ultra-high-energy cosmic rays Martin Hardcastle Hertfordshire SOCoR, Trondheim, 17 th June 2009 Thanks to Teddy Cheung, Łukasz Stawarz, Ilana Feain 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.
Overview 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
Hotspot Core Jet Lobe FRIFRII Plume Radio galaxy morphology
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 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 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 dE/dt = 4/3(q 4 /6πε 0 m 0 2 c 4 ) c γ 2 (U photon +B 2 /2μ 0 ) 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 X-ray counterparts of jets in FRIs are common, possibly universal (Worrall+ 01) X-ray emission is clearly synchrotron (MJH+ 01) X-ray emission comes from deceleration region => bulk k.e. is being translated into internal energy of particle pop’n (and fields?) We should be able to use this X-ray emission 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 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’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 1)Strong point-to-point radio/X-ray ratio variation – particle acceleration efficiency varies spatially 2)Compact X-ray emitting ‘knots’ are stationary in multi-epoch radio imaging – could be shocks? MJH+ 2003 ‘Knots’
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 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 TeV by first-order Fermi. – The diffuse emission surrounding them is probably something else! Possibly 2 nd -order Fermi at turbulence, or mag. reconnection...
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 B eq for 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 > B eq to 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 Cen A jet falls short of UHECR energies unless B > B eq (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 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 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 lobes Lobes satisfy Hillas criterion for 10 20 eV protons, for B = B eq 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 t acc << t propagation 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 2 nd -order Fermi in lobes can only be efficient if ‘speeds of scatterers’ are ~ c – for magnetic turbulence, require v A >~ 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 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 what we learn about such models from the case of Cen A. Nagar & Matulich 08
UHECR from nearby radio galaxy lobes Require that the lobes satisfy the Hillas criterion for protons, i.e. R > 100 E 20 B -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 ~ B eq in those systems; not clear whether this holds in FRIs.)
Luminosity constraint Consider a spherical lobe with radius R and a constant electron density and B-field strength, such that U e = εU B. Equipartition corresponds to ε = 1. Let electron energy spectrum be a power law with index p (back-of-envelope only) Then we can show that H.C. corresponds to L(ν) > K(p) ε ν (1-p)/2 E (5+p)/2 R (1-p)/2 where 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 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 L 408 >~ 2 x 10 24 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 10 25 W Hz -1. (Sanity check; Cen A just satisfies this limit, at around 3 x 10 24 W Hz -1 at 408 MHz, as it should.)
Luminosity function constraints Using 408-MHz luminosity function from MJH+03 we find that such powerful RGs are rare; we expect only 15-20 RGs of this luminosity in the southern sky out to 100 Mpc, not all of which will have physical size capable of accelerating UHECR. These objects will have 408-MHz flux > 1.7 Jy. (Thus all in southern sky are in Molonglo Reference Catalogue.) Because of flat luminosity function of FRIs 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 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 www.atlasoftheuniverse. com
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 sources of UHECR via this mechanism and they will correlate with large-scale structure. Accelerated particles will be predominantly protons (but nuclei are easier to confine so will be disproportionately present at highest energies). Proton spectrum will cut off at ~ 100 EeV since required radio luminosity is strong function of E; no RGs available with required B-field strengths. ‘Cen B’: Jones + McAdam 01
Summary 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.