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Future prospects for probing strong gravity with X-ray spectroscopy

Future prospects for probing strong gravity with X-ray spectroscopy . Chris Reynolds Department of Astronomy University of Maryland, College Park & Joint Space Scence Institute. Outline. What are we doing here? The near future for strong gravity studies

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Future prospects for probing strong gravity with X-ray spectroscopy

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  1. Future prospects for probing strong gravity with X-ray spectroscopy Chris Reynolds Department of Astronomy University of Maryland, College Park & Joint Space Scence Institute

  2. Outline • What are we doing here? • The near future for strong gravity studies • Prying open the window on black hole spin • First results from Suzaku AGN Key Project • The next generation • Bringing astrophysics of spin into main-stream • Testing accretion models close to the horizon • Conclusions

  3. What does it mean to “probe strong gravity” ? • Physicist’s answer… want to test GR • But… this is a difficult and subtle business! • GR is remarkably robust theory • Vast majority of alternatives are ill-founded, unstable, or highly un-natural • Known failure points of GR are when curvature scale ~ Planck scale • Isolated black holes may not be a good lab… • Kerr metric is the stationary, vacuum solution of field equations; hence it is not unique to General Relativity (see papers by Psaltis) • Deep potential wells does not mean strong curvature; in that sense, our black holes may not even have particularly strong gravity

  4. X-ray probes of strong gravity • X-ray signatures of strong gravity need to be understood in context of the (incomplete) theory of accretion! • The fundamental physics community (at least in US) does not believe that X-ray astrophysics can provide a useful test of the foundations of GR for the foreseeable future • But we get to do something just as cool… • We get to play with black holes!!!! • We have the ultimate window into nature’s most powerful engines...

  5. The near future Dana Berry / NASA • Two big stories of the past decade: • First believable determinations of black hole spin • First observational investigations of innermost disk • Still much more to do with current X-ray observatories! • GBHBs; possibility of assessing concordance in spin measured by iron lines, continuum fitting & HFQPO (+ sociological flexibility) • AGN; need more, deep, X-ray campaigns

  6. Suzaku AGN Spin Survey C.Reynolds (PI), L.Brenneman, A.Fabian, K.Iwasawa, J.Lee, J.Miller, R.Mushotzky, K.Nandra, M.Nowak, R.Reis, M.Trippe, M.Volonteri Fairall 9 NGC3516 NGC3783 3C120 Mrk766 Mrk841 Maryland workshop

  7. Suzaku AGN Spin Survey C.Reynolds (PI), L.Brenneman, A.Fabian, K.Iwasawa, J.Lee, J.Miller, R.Mushotzky, K.Nandra, M.Nowak, R.Reis, M.Trippe, M.Volonteri Fairall 9 NGC3516 NGC3783 3C120 Mrk766 Mrk841 Maryland workshop

  8. NGC3783: well known warm absorber Kaspi et al. (2002) 900ks Chandra/HETG observation

  9. Model independent approach… convolve HETG data to XIS resolution and drop on top of XIS data (refit only for normalization). Only significant residuals are <0.9keV and 5.5-6.5keV. M.Nowak

  10. Modeling approach: fit 3-zone warm absorber model to XIS+PIN spectrum. Need to include distant reflection (green) and relativistic disk (blue). NGC 3783 (210ks with Suzaku)

  11. Deleting relativistic disk and refitting leaves clear residuals in fit indicating broad iron line and unmodeled reflection

  12. Variable fraction in NGC3783 (XMM; G.Ponti)

  13. See significant short-timescale variability; XIS color tracks flux very well (apart from distinct anomalies) 0.3-1keV 2-10 keV Conclude : spectral variability is mostly broad band, not emergence of distinct absorption/emission components only affecting one band M.Trippe

  14. The Next Generation • New era: NuSTAR, GEMS, Astro-H, IXO • Astro-H and eventually IXO will allow simultaneous, high-throughput, high-resolution and broad-band pass X-ray spectroscopy • Simple/robust separation of broadened disk features from other emission/absorption features • Bring study of black hole spin to maturity • Ability to measure SMBH and GBHC spin distribution functions • Explore role of spin as a power source in the Universe • New window on inner disk; rapid iron line variability • Very direct probe of “coronal” geometry and flaring properties • X-ray astronomy’s best bet for catching any hints of problems with GR / Kerr metric

  15. NW=1022 cm2, ξ=20 NW=3×1022 cm2, ξ=100= NW=1023 cm2, ξ=1000 Low-velocity reflection Relativistic disk model

  16. IXO simulation with 1.3 million photons in 2-10keV band 15ks for F2-10=5x10-11 EW=180eV; ε~r-3 Constrains Δa~0.1

  17. Volonteri et al. (2005)

  18. Iron lines in radio-loud AGN 3C 120 / Suzaku Rin~10rg, i9o, EW~30eV (Kataoka et al. (2007) • Flux-trapping model for jets • Most powerful jets will be from retrograde accretion onto rapidly spinning BHs(Reynolds, Garofalo, Begelman 2006 Garofalo 2009ab)

  19. The Next Generation • New era: NuSTAR, GEMS, Astro-H, IXO • Astro-H and eventually IXO will allow simultaneous, high-throughput, high-resolution and broad-band pass X-ray spectroscopy • Simple/robust separation of broadened disk features from other emission/absorption features • Bring study of black hole spin to maturity • Ability to measure SMBH and GBHC spin distribution functions • Explore role of spin as a power source in the Universe • New window on inner disk; rapid iron line variability • Very direct probe of “coronal” geometry and flaring properties • X-ray astornomy’s best bet for catching any hints of problems with Kerr metric

  20. Iron line reverberation Transfer function encodes flare-position as well as geometry of space-time Reynolds et al. (1999) Young & Reynolds (2000)

  21. Armitage & Reynolds (2003) Iron line intensity as function of energy and time. Arcs trace orbits of disk material around black hole… encode hot spot motion, velocity field, and space-time geometry. IXO simulation (assuming 3x107Msun black hole) Theoretical

  22. a=0.98 i=30o R=2rg

  23. Powerful probes of the coronal physics… • Very existence of Keplerian arcs suggests co-rotating corona • Properties of arcs height of corona and nature of flaring • Possible probe of metric • Can fit each track for (r,a) assuming Kerr metric • Kerr metric a(r)=constant • Need more exploration of uncertainties & degeneracies r=3rg; a=0.95; M=3107Msun

  24. Conclusions • Still a lot we can do with current missions • Need more deep AGN campaigns • Attempt concordance modeling of GBHBs • And then… • High-throughout high-resolution spectrometers will make crucial contribution • Simple separation of absorption and broad emission features • Distribution of SMBH spin  grow history • SMBH spin and jets  testing jet theories • Rapid iron line variability  coronal geometry and gravity

  25. Backup slides

  26. Observing strategy… • One possible strategy : target known AGN on the basis of flux and the presence of a broad iron line… “run down log N - log S curve” • Using HEAO-A1 LogN-LogS… • f is fraction of sources with broad lines • nph is number of 2-10keV photons needed for measurement of spin in an individual spectrum • Need to refine/re-assess observing strategy as we learn more about AGN populations and X-ray spectra (e.g., from follow-up to the BAT survey)…

  27. Courtesy K. Iwasawa

  28. MCG-6-30-15 (AGN) with Suzaku (Miniutti et al. 2006) Can count how many iron line photons are expected given size of hard X-ray bump… require extreme broadening to fit those photons in the spectrum! True whether bump is due to reflection OR absorption.

  29. Keplerian orbit of a single “hot spot” a=0.98 i=30o R=30 R=3 R=2.5 R=6 R=15

  30. 1H0707-495 (XMM-Newton; Fabian et al. 2009) Relativistic K- and L-shell lines of iron [Spectrum divided by underlying continuum] X

  31. IXO simulation with 1 million photons in 2-10keV band Constrains a>0.90 for amodel=0.95 • Need fewer counts if… • Iron is super-solar (e.g., as in MCG-6-30-15) • Emission is centrally concentrated • z>>0 15ks for F2-10keV=510-11 erg/s/cm2 EW=180eV, r-3 15ks observation for F2-10keV=510-11 erg/s/cm2

  32. 3-D MHD simulation of 60o wedge of disk Pseudo-Newtonian potential Performed using ZEUS-MP Constant h ; h/r=0.05 at ISCO Vertical resolution ~26 zones per scaleheight CSR & Miller (2008) CSR & Fabian (2008)

  33. Ballistic plunge from r5.7rg Radial scale length of ISCO transition r0.2-0.5rg CSR & Fabian (2008)

  34. Illustrative calculation of ionization parameter at the “X-ray photosphere”…Assume X-ray source on symmetry axis of accretion disk at r=6rg Brenneman & Reynolds (2006) a=0 a=0.7 a=0.998

  35. Reverberation? Iron-L line vs continuum in 1H0707; Zoghbiet al. (2009)

  36. Midplane azimuthal velocity Comparison of disk velocity derived from a 3-d MHD simulation (dotted) with simple test-particle velocity (solid)… confirms analytic result that deviations are O[(h/r)2] Radius (GM/c2)

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