X-ray Reflection and Reprocessing:Probing Black HolesThrough Study of Fluorescent Iron LinesLaura A. LopezAstro 59720 September 2004OverviewL.A. Lopez -- Astro 597 09/2004X-ray Reflection and ReprocessingIntroductionLine ProductionComputing and ModelingLine ProfilesIron Lines in AGNProbing Spin via Iron Line ObservationsFuture Studies of Iron LinesConclusionsIntroductcionL.A. Lopez -- Astro 597 09/2004X-ray Reflection and ReprocessingIn the immediate vicinities of black holes, high-speed matter, electromagnetic fields, and strong gravity effects come together. Direct study of phenomena near black holes is limited because apparent angular scales of even the largest hole event horizons are ∼ 10 arcsec. Medium- and high-resolution X-ray spectroscopy offers a means to indirectly observe these regions.Fluorescent emission lines resulting from an irradiated cold disk produces Kα iron lines from 6.40-6.97 keV-6Intrinsically narrow, the iron lines are broadened by Doppler shifts and strong gravity effects. By investigating spectral features in X-ray luminous black holes, scientists can better understand the physics in the immediate vicinity of these systems.Line Productioneee---Assume a cold accretion disk, with uniform density gas.Soft UV/optical photons from the disk will be inverse Compton scattered from high-energy, coronal electrons. Thermal Comptonization of these photons produces a power-law X-ray spectrumThese photons gain energy as they are repeatedly IC scattered in the coronaL.A. Lopez -- Astro 597 09/2004ν’ννν’ν’νX-ray Reflection and ReprocessingResulting high-energy photons re-enter the accretion diskMany possible fates of the incident photons:Fate #1: Photon can be Compton scattered by free-electrons from the ionized H and He or by outer electrons of metals. Line ProductionL.A. Lopez -- Astro 597 09/2004X-ray Reflection and ReprocessingResulting high-energy photons re-enter the accretion diskMany possible fates of the incident photons:Fate #2: Photon can be photoelectrically absorbed by a neutral atom.As a result, one K-shell electron will escape, and an L-shell electron will drop into the K-shellLine ProductionL.A. Lopez -- Astro 597 09/2004X-ray Reflection and ReprocessingThis transition creates excess energy which can be released in two ways:Line ProductionL.A. Lopez -- Astro 597 09/2004Fluorescence: Excess energy can be radiated away as a Kα line photonAuger Effect: Excess energy is carried away via the ejection of a second L-shell electron. The fluorescent yield of a species determines the probability that an excited ion will de-excite via fluorescence versus the Auger effect. X-ray Reflection and ReprocessingAt soft X-ray energies, reflection is small because of photoabsorption by the metals in the slab.Resulting SpectrumL.A. Lopez -- Astro 597 09/2004At hard X-ray energies, incident X-rays are Compton back-scattered from the slab. A spectrum of fluorescent emission lines arises from photoionization of metals in the slab. Iron Kα line at 6.40 keV is the most prominent because of its high fluorescent yield and its large cosmic abundance. X-ray Reflection and ReprocessingEffect of Ionization on Spectrum L.A. Lopez -- Astro 597 09/2004The strength of the iron line depends on - the ionization state of the surface of the accretion disk - accretion disk geometry - elemental abundances of the reflecting matter - the inclination angle of the reflecting materialAmount of ionization is characterized by the ionization parameter, ξ, where:ξ (r) = and F (r) is received X-ray flux per unit area of the disk at radius r and n(r) is the comoving electron number density. 4π F (r) n (r) xxAssuming uniform density across the disk, iron line emission depends on ξ and breaks down into four regimes:X-ray Reflection and ReprocessingReflection Features for Varying ξL.A. Lopez -- Astro 597 09/2004Case 1: ξ < 100 ergs cm s -- cold neutral reflection, weak ionization - X-ray reflection resembles that from cold gas containing neutral metals - Features include 6.4 keV iron line, a weak iron K-shell edge at 7.1 keV, and a weak Compton back-scattered continuum. -1X-ray Reflection and ReprocessingL.A. Lopez -- Astro 597 09/2004Case 2: 100 ergs cm s < ξ < 500 ergs cm s -- intermediate ionization - Iron is in the form Fe XVII - Fe XIII with a vacancy in the L-shell - These ions can absorb the Kα line photons. Following absorption, excess energy is released via fluorescence or the Auger effect. - Only a few line photons escape the disk, generating a very weak iron line. - Spectrum has a moderate iron absorption edge below the iron edge-1-1Reflection Features for Varying ξX-ray Reflection and ReprocessingL.A. Lopez -- Astro 597 09/2004Case 3: 500 ergs cm s < ξ < 5000 ergs cm s -- high ionization - Ions are too ionized to permit the Auger effect (AE requires 2 L-shell e ) - Photons can escape the disk, creating a Kα emission of Fe XXV and Fe XVI - Compton backscattered continuum significantly contributes to the observed emission at 6 keV, creating a large iron absorption edge. -1-1-Reflection Features for Varying ξX-ray Reflection and ReprocessingL.A. Lopez -- Astro 597 09/2004Case 4: ξ > 5000 ergs cm s -- full ionization - The disk is too highly ionized to produce any atomic signatures. - No iron emission line or edge is observed. -1Reflection Features for Varying ξX-ray Reflection and ReprocessingReal disk atmospheres are much more complex. To determine their structure, we must model their radiative transfer, calculate thermal and ionization balance of each fluid element, and take into account MHD and effects from radiation pressure. Realistic ModelsL.A. Lopez -- Astro 597 09/2004Next step beyond fixed density is to assume hydrostatic equilibrium (where density, temperature, and pressure are constant). X-ray Reflection and ReprocessingIn the hydrostatic equilibrium case, thermal ionization instability arises. TII is the phenomenon that happens when pressure is specified and multiple thermal and ionization equilibriums exist. Thus, very hot regions can coexist with cool regions while in pressure equilibrium. When modeling real reflection spectra, constant density models can be used with different inferred reflection fractions and ionization parameters.Relativistic EffectsL.A. Lopez -- Astro 597 09/2004X-ray Reflection and ReprocessingProfile Dependence on