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X-Ray Emission From The Nuclei of Radio Galaxies

X-Ray Emission From The Nuclei of Radio Galaxies. Daniel Evans University of Bristol -with- Diana Worrall, Ralph Kraft, Martin Hardcastle, Mark Birkinshaw, Judith Croston, Bill Forman, Christine Jones, Steve Murray. Contents. An introduction to AGN physics

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X-Ray Emission From The Nuclei of Radio Galaxies

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  1. X-Ray Emission From The Nuclei of Radio Galaxies Daniel Evans University of Bristol -with- Diana Worrall, Ralph Kraft, Martin Hardcastle, Mark Birkinshaw, Judith Croston, Bill Forman, Christine Jones, Steve Murray

  2. Contents • An introduction to AGN physics • X-ray emission from radio galaxies: the issues • Centaurus A: the nearest AGN • NGC 6251: a counterexample? • Nuclear emission from a sample of 3CRR radio galaxies • Conclusions

  3. What are AGN? • Unresolved nuclear component • Nuclear luminosity > sum of all stars • MBH ~ 106–1010 M๏ • Accompanied by strong X-ray emission NGC 3277 NGC 5548 NGC 6251 - XMM

  4. The Central Engine • Accretion flow surrounded by dusty torus

  5. The Central Engine • Accretion flow surrounded by dusty torus • BB radiation from disk  ‘big blue bump’

  6. The Central Engine • Accretion flow surrounded by dusty torus • BB radiation from disk  ‘big blue bump’ • B-field loops  optically thin corona

  7. The Central Engine • Accretion flow surrounded by dusty torus • BB radiation from disk  ‘big blue bump’ • B-field loops  optically thin corona • Isotropic X-rays from Comptonization of disk photons in hot corona • Power law spectrum

  8. Fe Ka Production • Reflection of X-rays from an optically thick accretion disk

  9. Fe Ka Production • Reflection of X-rays from an optically thick accretion disk

  10. Fe Ka Production • Reflection of X-rays from an optically thick accretion disk

  11. Fe Ka Production • Reflection of X-rays from an optically thick accretion disk

  12. Fe Ka Production • Reflection of X-rays from an optically thick accretion disk

  13. Fe Ka Production • Reflection of X-rays from an optically thick accretion disk Fe Kα

  14. Astrophysical Jets • Anisotropic emission, power law spectrum • Relativistic Doppler beaming, dependent on bulk speed (Γ), angle to line of sight 10 pc NGC 6251 5 GHz VLBI Jones et al. (1986)

  15. Radio Galaxy Radio Galaxy Nuclei – Two Competing Models • Is the X-ray emission dominated by: • The parsec-scale jet? -or- • The accretion flow?

  16. Jet-Related Emission Three Discoveries: • For a given optical luminosity, radio-loud quasars are brighter X-ray sources than radio-quiet quasars  An additional component of emission is present in RLQs e.g., Zamorani et al. (1981)

  17. Jet-Related Emission Shastri et al. (1993) • Dependence of X-ray luminosity and spectrum on beaming angle  A component is anisotropic • Correlation between ROSAT soft X-ray and radio fluxes and luminosities  Soft X-ray emission likely jet-related Canosa et al. (1999)

  18. Accretion-Related Emission • Have seen that soft (0.5-2.4 keV) X-ray emission likely jet related • What about the rest of the X-ray spectrum? Jet or accretion flow? • E.g. Gliozzi et al. (2004) claim broadened Fe Ka emission and variability on short timescales in NGC 6251 • Would imply accretion-dominated emission d < ct

  19. Radio Galaxy Summary of Introduction • Emission in the nuclei of AGN consists of: • “Radio-quiet” accretion-related component • “Radio-loud” jet-related component • Which dominates the X-ray emission? • Matter of considerable debate…

  20. Cen A – The Nearest AGN • Brightest extragalactic object in the hard X-ray sky • Closest radio galaxy (d = 3.4 Mpc) • Complex emission • Ideal object to study • Much-studied by earlier X-ray missions • Rich gallery of radio features (jet, lobes, etc.) Chandra0.5-2 keV X-ray

  21. Continuum Spectrum • Attempt to fit a heavily-absorbed (NH 1023 atoms cm-2) power-law (Γ  1.7) • Significant residuals below  2.5 keV XMM-Newton MOS2 1st observation

  22. Continuum Spectrum • Significant improvement with the addition of a second power-law component Key parameters:

  23. Core 0.7 pc Possible Origin of 2nd PL • VLBI jet? Flux density  5 Jy at 4.8 GHz • X-ray to radio ratio for 2nd PL and VLBI jet consistent with that of kpc-scale jet and VLA jet • Mildly absorbed soft power law seen in other FRI galaxies with ROSAT Tingay et al. (1998) Canosa et al. (1999)

  24. Fe Ka Fe Kb Fluorescent Line Emission • Chandra HETGS instrument of choice due to its high spectral resolution • Fe Ka centroid = 6.404±0.002 keV (90% c.l.))  fluorescence from cold, neutral material • Fe Kα is broadened (σ = 20±10 eV (90% c.l.))  v ~ 1000 km s-1  r ~ 0.1 pc (MBH = 2 x 108 M๏) Joint HEG+1 and HEG-1 spectrum

  25. Geometry Of Emission Region • Fe Ka line parameters consistent with fluorescence from NH  1023 atoms cm-2 with torus geometry • Also consistent with fluorescence from NH~ 1024 atoms cm-2 outside line of sight (Woźniak et al. 1998)

  26. Nature of Accretion Flow Hybrid inefficient flow/thin disk

  27. Cen A Summary • Emission characterized by a heavily-absorbed power law • Second power-law component necessary, consistent with VLBIjet • Fluorescent lines from cold, neutral material • Molecular torus? • Accretion flow: Hybrid? • More info: Evans et al. (2004), ApJ, 612, 786

  28. NGC 6251 – A Counterexample 1.6 GHz VLA Jones et al. (1986) • z=0.0244 (d~100 Mpc) FRI-type radio galaxy • Spectacular radio jet extending hundreds of kpc • Opening angle of 7.4o • X-ray emission from nucleus and three regions of kpc-scale jet • Synchrotron interpretation for kpc-scale jet (Evans et al. 2005) 1 arcmin 30 kpc 0.5-5 keV Chandra image with 1.6 GHz VLA contours (Evans et al. 2005)

  29. NGC 6251 - Results • XMM-Newton observation of NGC 6251. Gliozzi et al. (2004) claimed broadened Fe Ka line-emission • Would imply accretion-dominated X-ray emission • New Chandra data + reanalysis of XMM data • Nuclear spectrum well fitted with a featureless, single, unabsorbed power law Chandra 1 2 4 7 Energy (keV) XMM

  30. NGC 6251 – Fe Ka line • No evidence of Fe Ka emission in Chandra spectrum • No significant evidence for Fe Ka emission in XMM spectrum • Already evidence to disfavour accretion-dominated X-ray emission

  31. NGC 6251 - Interpretation • 1-keV X-ray flux density consistent with ROSAT soft X-ray results • Also consistent with soft power law observed in Cen A • SED double-peaked and modelled by SSC emission • X-ray emission dominated by a jet

  32. Intermediate Summary • Dissimilar spectra for two seemingly similar FRI-type radio galaxies: • Cen A: X-ray emission hard and heavily absorbed (likely accretion-related), accompanied by soft emission • NGC 6251: X-ray emission soft and unabsorbed • Soft emission of Cen A may have the same origin as that of NGC 6251 • Why might they be dissimilar? • Are both accretion-related and jet-related components present in all radio galaxies at varying levels? • Need to study a sample Cen A NGC 6251 1 2 4 7 Energy (keV)

  33. The 3CRR Sample • Criteria: • 178-MHz luminosity density > 10.9 Jy • Declination > 10o • |b| > 10o • Advantages: • No orientation bias • Spectroscopic identification • High-resolution radio observations • Select sources with z<0.1 • Unambiguously spatially separate unresolved nuclear emission from contaminating emission • Rich variety (FRI/FRII, broad/narrow lines, large luminosity range) • 19/38 X-ray observations of low-z 3CRRs, 16 of them with Chandra • Complete spectral analysis of each

  34. The 3CRR Sample VLA (Leahy, Bridle, & Strom) Chandra 0.5-5 keV

  35. Dominant X-ray emission mechanism: Accretion Flow: Fe Ka, variability -or- Jet: SED, radio-optical-X-ray luminosity correlations Nature of accretion flow: thin disk? RIAF? Is the torus ubiquitous? FRI-FRII dichotomy? Radio Galaxy The 3CRR Sample: Aims Unified AGN scheme:

  36. Luminosity-Luminosity Correlations • Consider LX and LR • Considerable scatter

  37. Luminosity-Luminosity Correlations • Consider LX and LR • Considerable scatter • Much better correlation between components with NH ≤ 5 x 1022 • Most components have NH consistent with 0 • Any intrinsic absorption consistent with dust in host galaxy • Origin of X-ray emission in pc-scale jet (outside any torus) Jet NH≤ 5 x 1022

  38. Luminosity-Luminosity Correlations • Components with NH ~ 1023 lie above trendline • As does 3C 390.3, unobscured BLRG • All have Fe Ka lines • Accretion-dominated and surrounded by a torus? • Soft components of these sources consistent with jet-related trendline • Accretion and jet components present in all radio galaxies at varying levels? • 7/8 are FRIIs, other is Cen A Jet Accretion NH≤ 5 x 1022 NH~ 1023 NH≤ 5 x 1022

  39. A Nuclear FRI/FRII Dichotomy? • So far: • X-ray emission of FRIIs is heavily absorbed and accompanied by Fe Ka lines  accretion-related and viewed through a torus • X-ray emission of FRIs much less absorbed. LX and LRcorrelations  jet-related emission • Where is the torus in FRIs? Problems with unified models, etc. • Find upper limits to accretion-related emission in FRIs • Data don’t exclude luminosities of ~ 1039-1041 ergs/s • Some comparable to, e.g., Cen A (5x1041 ergs/s) but lower than FRIIs (1043-1044 ergs/s) e.g. 3C 274 (M87)

  40. A Nuclear FRI/FRII Dichotomy? Implications: • Data do not exclude the presence of a torus in FRIs  supports AGN-unification models • FRI nuclei are not simply scaled versions of FRIIs • For a given jet power, FRI nuclei have less efficient accretion flows • Dichotomy in accretion-flow structure of FRIs and FRIIs • The FRI/FRII dichotomy is not just related to differences in the medium into which jet propagates • First evidence of dichotomy from X-ray studies MHD ADAF Simulation Armitage (2004)

  41. Summary • Unobscured (jet-related), and obscured (accretion-related) components present in all radio galaxies at varying levels • Data do not exclude the presence of a torus in FRI-type sources • An FRI/FRII dichotomy exists on nuclear scales

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