VHE emission from radio galaxies

Transcription

1 VHE emission from radio galaxies Martin Hardcastle The future of gamma-ray astronomy and the CTA Leicester, October 2010

2 Outline Recap: radio galaxy physics Inverse-Compton emission Locations of high-energy electrons Leptonic vs hadronic processes VHE emission from lobes Hotspots? Jets in nearby radio galaxies The case of Cen A What the CTA can do for us

3 1. RADIO GALAXY PHYSICS 3C66B, MJH et al 97 / A. Bridle

4 X-ray inverse Compton emission from radio lobes Inverse-Compton scattering mainly of the CMB. Now routinely detected from FRII radio galaxies by Chandra & XMM Allows direct measurement of electron density, since CMB photon energy density is well known. X-ray IC + radio synchrotron from same electron populations provide direct measurement of B. 300 kpc Colour: XMM IC Contours: radio Croston et al Croston+ 04

5 High-energy electrons Presumably associated with local, ongoing particle acceleration (since lifetimes are short) If present, should be visible via highfrequency synchrotron radiation. Observations at lower frequencies are mandatory to figure out where they are...

6 Where are the highest-energy electrons? 1) Sub-pc jets of all classes of object (but physics hard to constrain observationally or model) 2) FRII hotspots 3) Jets of FRI radio galaxies, and 4) Shocks around the large-scale lobes In the last two cases nearby objects give us exquisitely detailed pictures of the electron distribution...

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8 Leptons vs. baryons We know leptons are present so inverse- Compton is a required process. If radio galaxies are the sources of UHECR => very high energy protons, nuclei etc. (Big if.) Protons not required energetically. Density of thermal material poorly known, but must be low. => Consider mostly leptonic processes from here on.

9 2. VHE EMISSION FROM LOBES

10 The giant lobes of Cen A Work on this the result of an argument (I was wrong...) 10 kpc

11 Cen A with WMAP MJH, Cheung, Stawarz, Feain 2009

12 Cen A IC predictions Based on the WMAP detection we knew there were high-energy electrons in the lobes. Still required some extrapolation from what we could see to what might be present (~ TeV electrons)...

13 Cen A results

14 Cen A results LAT >200 MeV WMAP 22 GHz Background (isotropic and diffuse) and field point sources subtracted Cheung Science

15 Cen A results Model fitting from the Science paper. Photon fields include the CMB, EBL and galactic light. Good fits with B ~ 0.1 nt; giant lobes close to equipartition!

16 Lobes at higher energies? Cen A is too big, but most RGs are pointlike to TeV instruments. TeV inverse-compton would require ~TeV electrons in the lobes => efficient in situ leptonic particle acceleration. (Cf. what is required for cosmic rays.) Already required in Cen A? Interesting possibility for the CTA...

17 Lobes at TeV Assume lobe spectrum extends to highest energies possible (cutoff before X-ray; not clear what sets this energy). X-ray IC tells us normalization of electron spectrum. Then detectable with CTA w/o much difficulty. Would greatly change our ideas about particle acceleration in lobes! Lobe inverse-compton in 3C452

18 3. HOTSPOTS

19 Hotspots? Bright in X-ray synchrotron in some cases would be great to be able to do this...

20 Brightest nearby X-ray synchrotron hotspot. Not readily detectable in SSC for fields close to equipartition. (Consistent with Zhang+ 09 they assume B << B eq.) Tough to detect hotspots even with CTA, but still an interesting experiment... Pictor A

21 4. LOW-POWER JETS

22 Existing TeV sources Many blazars, plus a total of 2.5 radio galaxies: M87: long-standing detection; recent timing analysis shows at least some TeV associated with inner jet (Acciari+ 09)...

23 Existing TeV sources Many blazars, plus a total of 2.5 radio galaxies: M87: long-standing detection; recent timing analysis shows at least some TeV associated with inner jet (Acciari+ 09) Cen A: recent HESS detection (Aharonian+ 09) 3C66B? Confused with blazar 3C66A, but a possible detection (e.g. Tavecchio + Ghisellini 09) All 3 RGs have bright X-ray jets (TeV electrons on kpc scales).

24 Existing TeV sources Many blazars, plus a total of 2.5 radio galaxies: M87: long-standing detection; recent timing analysis shows at least some TeV associated with inner jet (Acciari+ 09) Cen A: recent HESS detection (Aharonian+ 09) 3C66B? Confused with blazar 3C66A, but a possible detection (e.g. Tavecchio + Ghisellini 09) All 3 RGs have bright X-ray jets (TeV electrons on kpc scales).

25 Models for RG TeV emission Three general classes of IC models: 1) From close to accretion flow e.g. Rieger + Aharonian 09 for Cen A. 2) From pc-scale jet e.g. Ghisellini+ 05. Requires assumptions about electron distributions that are not directly testable, but consistent with variability observations in M87 & with many detections of blazars; probably true at some level. 3) From kpc-scale structures (e.g. Stawarz+ 03) constrained by, and constraining of, reasonably well-understood physics.

26 Extended IC modelling Key advantage: electron energy distribution constrained via synchrotron observations But various photon fields must be considered: Synchrotron photons (SSC) CMB Extragalactic background light (EBL) Starlight (inside host galaxy) Hidden quasar/blazar Crucial to take Klein-Nishina effects and anisotropy of photon fields, IC emissivity into account. Work in progress...

27 Cen A jet: one zone One-zone model of X-ray jet. Starlight dominates. Klein- Nishina corrections crucial. Beaming has significant effect.

28 Extended IC modelling New IC code under development adapted from MJH+ 02. Full anisotropic IC from Brunetti 1999; Klein- Nishina effects treated properly. Deals with non-uniform electron and, optionally, B-field distribution in jet Illumination by galaxy-frame isotropic photon fields (CMB, EBL) and also anisotropic (point sources, starlight). Special relativity treated consistently throughout.

29 Extended IC modelling Still require some simplifying assumptions about the jet & host galaxy...

30 Angle & beaming constraints Biggest uncertainty is jet angle to the line of sight. Big effects on both beaming and geometry. From observed proper motions we can constrain apparent speed & so combination of,. MJH+ 03, Goodger+10

31 Results

32 Results (B = 1/3 B eq )

33 Next steps Very encouraging results on Cen A existing TeV detection already constrains B. Next up: M87! Paper in prep. will give more details.

34 5. WHAT CAN THE CTA DO FOR US?

35 What can the CTA do for us? Improved sensitivity But we are going to struggle to detect new non-nuclear IC jet sources, see next slide Improved spatial resolution Helps us separate nuclear and off-nuclear components, important for emission mechanism constraints.

36 Representative nearby (D = 100 Mpc) FRI radio galaxy not M87 or Cen A! Only marginally detectable in this onezone model. (Electron distribution gives IC peak in between CTA and Fermi sensitivity maxima.) MAGIC detection, if real, is probably not jet IC? 3C66B

37 CTA resolution Peak resolution ~ couple of arcmin Capable of resolving nearby FRII sources if any are detectable; might see hotspots or lobes Marginally resolves Cen A jet and inner lobes! if jet is not detected strong constraints are placed on B-field strength in jet.

38 Summary VHE gamma-ray studies of (lobe-dominated) radio-loud AGN provide us with the opportunity to extend successful use of inverse-compton diagnostics to systems in which X-ray studies are not possible. TeV IC is mandatory for X-ray synchrotron sources. Detailed inverse-compton studies taking into account all the physics have not yet been done, but existing constraints are already interesting for a few famous objects. CTA sensitivity and resolution will improve things, though there will still be a lot that we can t see!