Future prospects for probing strong gravity

with X-ray spectroscopy

Chris ReynoldsDepartment of Astronomy

University of Maryland, College Park& Joint Space Scence Institute

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

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

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...

The near future 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

Dana Berry / NASA

Maryland workshop 625th August 2009

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

NGC3516 NGC3783 Fairall 9

3C120 Mrk766 Mrk841

Maryland workshop 725th August 2009

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

NGC3516 NGC3783 Fairall 9

3C120 Mrk766 Mrk841

NGC3783: well known warm absorber

Kaspi et al. (2002)

900ks Chandra/HETG observation

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

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)

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

Variable fraction in NGC3783 (XMM; G.Ponti)

See significant short-timescale variability; XIS color tracks flux very well (apart from distinct anomalies)

Conclude : spectral variability is mostly broad band, not emergence of distinct absorption/emission components only affecting one band

0.3-1keV

2-10 keV

M.Trippe

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

NW=1022 cm2, ξ=20NW=3×1022 cm2, ξ=100=NW=1023 cm2, ξ=1000Low-velocity reflectionRelativistic disk model

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

Volonteri et al. (2005)

3C 120 / SuzakuRin~10rg, i9o, EW~30eV

(Kataoka et al. (2007)

Iron lines in radio-loud AGN

Flux-trapping model for jetsMost powerful jets will be from

retrograde accretion onto rapidly spinning BHs(Reynolds, Garofalo, Begelman 2006 Garofalo 2009ab)

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

Reynolds et al. (1999) Young & Reynolds (2000)

Iron line reverberation

Transfer function encodes flare-position as well as geometry of space-time

Arcs trace orbits of disk material around black hole… encode hot spot motion, velocity field, and space-time geometry.

Iron line intensity as function of energy and time.

TheoreticalIXO simulation(assuming 3x107 Msun black hole)

Armitage & Reynolds (2003)

a=0.98i=30o

R=2rg

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=3107 Msun

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

Backup slides

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)…

Courtesy K. Iwasawa

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.

Keplerian orbit of a single “hot spot”

a=0.98i=30o

R=30R=3R=2.5R=6R=15

X1H0707-495 (XMM-Newton; Fabian et al. 2009)Relativistic K- and L-shell lines of iron[Spectrum divided by underlying continuum]

IXO simulation with 1 million photons in 2-10keV bandConstrains a>0.90 for amodel=0.95

15ks observation for F2-10keV=510-11 erg/s/cm2

15ks for F2-10keV=510-11 erg/s/cm2

EW=180eV, r-3

Need fewer counts if…• Iron is super-solar (e.g., as in MCG-6-30-15)• Emission is centrally concentrated • z>>0

CSR & Miller (2008)CSR & Fabian (2008)

3-D MHD simulation of 60o wedge of diskPseudo-Newtonian potentialPerformed using ZEUS-MPConstant h ; h/r=0.05 at ISCOVertical resolution ~26 zones per scaleheight

CSR & Fabian (2008)

Ballistic plunge from r5.7rg

Radial scale length of ISCO transition r0.2-0.5rg

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=0a=0.7a=0.998

Reverberation?

Iron-L line vs continuum in 1H0707; Zoghbi et al. (2009)

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]

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