arXiv subjects

Brooks E. Kinch

Publications and source records attributed to Brooks E. Kinch.

7 recordsLinked to original sources

arXiv2026

Thermal and non-thermal emission from supermassive black hole circumbinary disks: Disks, Coronae, Streams, and Cavities

The search for electromagnetic signals from supermassive black hole (SMBH) binary systems is one of the cornerstones of multi-messenger astrophysics, complementing gravitational wave observations of such systems by pulsar timing arrays and LISA. Although extensive simulations have been run to understand the time variability of the bolometric luminosity from accreting binary SMBH systems, comparatively few spectral predictions have been made, and none that go beyond simple emission models. In this paper, we post-process a \texttt{HARM3D} simulation snapshot of a binary at $20 M$ separation accreting at 0.01 Eddington. For black hole masses $10^6$, $10^7$, and $10^8\, M_\odot$, we self-consistently solve for the radiated spectrum on the basis of time-steady radiation transfer, thermal balance, and ionization equilibrium, including all relevant relativistic effects as well as emission and absorption processes. Although most of the bolometric luminosity is radiated thermally by the disk, the low density regions evacuated by the binary's quadrupole moment produce copious X-rays, with $\sim40\%$ of the observed luminosity in a soft X-ray power law ($Γ= 2.3$). We identify two modes of observed azimuthal variation. The X-ray continuum varies by $\sim10\%$ due to an underlying asymmetry in the gas temperature of spiral shocks in the disk. The Fe~K$α$ equivalent width dips by $\sim25\%$ when the line of sight to the inner disk is partially obscured by the lump. These two effects share the same period and are $\sim π/2$ out of phase; the period is order days to weeks for typical AGN masses and a 20$M$ separation.

astro-ph.HE↗

arXiv2026

Simulation-Based Prediction of Black Hole Fe K$α$ Line Profiles

One of the most useful spectral diagnostics of accreting black hole systems is the Fe K$α$ fluorescence line. Detected in many systems, it is often used to estimate the black hole spin, as its breadth is attributed to relativistic kinematics near the spin-dependent innermost stable circular orbit (ISCO). In a companion paper, we showed how continuum spectra emitted by accreting black holes can be derived from snapshots of general relativistic magnetohydrodynamics simulations by combining radiation transfer solutions for the disk body and the corona. In this paper, we focus on the Fe K$α$ line, solving its transfer problem on the basis of local ionization and thermal balance. Its equivalent width is $\sim 25-225$ eV, depending mainly on viewing angle, for an accretion rate of 1$\%$ Eddington. Contrary to common assumptions, the illuminating X-ray spectrum and ionization parameter $ξ$ can be strong functions of radius; e.g. $ξ\propto r^{-1.5}$ in this simulation. Consequently, the region of the disk near the ISCO is completely ionized and contributes almost no Fe K$α$ photons; most of the flux is made at radii $\gtrsim 10 r_g$. The lines are broadened by a combination of relativistic Doppler shifts, Compton broadening in the disk atmosphere, and the differing line energies emitted by different Fe ions. These new mechanisms expand the parameter space of acceptable models, including the possibility of broad line profiles without large black hole spin; physical trends revealed by the simulations can refocus fitting efforts on the most relevant sections of the parameter space.

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arXiv2026

Simulation-Based Prediction of Black Hole Spectra: From $10M_\odot$ to $10^8 M_\odot$

It has long been thought that black hole accretion flows are driven by magnetohydrodynamic (MHD) turbulence, and there are now many general relativistic global simulations illustrating the dynamics of this process. However, many challenges must be overcome in order to predict observed spectra from luminous systems. Ensuring energy conservation, local thermal balance, and local ionization equilibrium, our post-processing method incorporates all the most relevant radiation mechanisms: relativistic Compton scattering, bremsstrahlung, and lines and edges for 30 elements and all their ions. Previous work with this method was restricted to black holes of $10 M_\odot$; here, for the first time, we extend it to $10^8 M_\odot$ and present results for two sub-Eddington accretion rates and black hole spin parameter 0.9. The spectral shape predicted for stellar-mass black holes matches the low-hard state for the lower accretion rate and the steep power law state for the higher accretion rate. For high black hole mass, both accretion rates yield power-law continua from $\sim 0.5 - 50$~keV whose X-ray slopes agree well with observations. For intermediate mass black holes, we find a soft X-ray excess created by inverse Compton scattering of low-energy photons produced in the thermal part of the disk; this mechanism may be relevant to the soft X-ray excess commonly seen in massive black holes. Thus, our results show that standard radiation physics applied to GRMHD simulation data can yield spectra reproducing a number of the observed properties of accreting black holes across the mass spectrum.

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arXiv2021

Spin and Accretion Rate Dependence of Black Hole X-Ray Spectra

We present a survey of how the spectral features of black hole X-ray binary systems depend on spin, accretion rate, viewing angle, and Fe abundance when predicted on the basis of first principles physical calculations. The power law component hardens with increasing spin. The thermal component strengthens with increasing accretion rate. The Compton bump is enhanced by higher accretion rate and lower spin. The Fe K$α$ equivalent width grows sub-linearly with Fe abundance. Strikingly, the K$α$ profile is more sensitive to accretion rate than to spin because its radial surface brightness profile is relatively flat, and higher accretion rate extends the production region to smaller radii. The overall radiative efficiency is at least 30--100% greater than as predicted by the Novikov-Thorne model.

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arXiv2020

Inverse Compton Cooling in the Coronae of Simulated Black Hole Accretion Flows

We present a formulation for a local cooling function to be employed in the diffuse, hot corona region of 3D GRMHD simulations of accreting black holes. This new cooling function calculates the cooling rate due to inverse Compton scattering by considering the relevant microphysics in each cell in the corona and approximating the radiation energy density and Compton temperature there by integrating over the thermal seed photon flux from the disk surface. The method either assumes ion and electron temperatures are equal (1T), or calculates them separately (2T) using an instantaneous equilibrium approach predicated on the actual relevant rate equations (Coulomb and Compton). The method is shown to be consistent with a more detailed ray-tracing calculation where the bulk of the cooling occurs, but is substantially less costly to perform. As an example, we apply these methods to a \textsc{harm3d} simulation of a $10 M_\odot$, non-spinning black hole, accreting at nominally 1\% the Eddington value. Both 1T and 2T approaches lead to increased radiative efficiency and a larger fraction of total cooling in the corona as compared to the original target-temperature cooling function used by \textsc{harm3d}, especially in the 1T case. Time-averaged post-processing reveals that the continuum spectral observations predicted from these simulations are qualitatively similar to actual X-ray binary data, especially so for the 1T approach which yields a harder power-law component ($Γ= 2.25$) compared to the 2T version ($Γ= 2.53$)

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arXiv2018

Predicting Stellar-Mass Black Hole X-ray Spectra from Simulations

We describe results from a new technique for the prediction of complete, self-consistent X-ray spectra from three-dimensional General Relativistic magnetohydrodynamic (GRMHD) simulations of black hole accretion flows. Density and cooling rate data from a HARM3D GRMHD simulation are processed by both an improved version of the Monte Carlo radiation transport code PANDURATA (in the corona) and the Feautrier solver PTRANSX (in the disk), with XSTAR subroutines. The codes are run in a sequential but iterative fashion to achieve globally energy-conserving and self-consistent radiation fields, temperature maps, and photoionization equilibria. The output is the X-ray spectrum as seen by a distant observer. For the example cases we consider here---a non-rotating $10 M_\odot$ black hole with solar abundances, accreting at 0.01, 0.03, 0.1, or 0.3 Eddington---we find spectra resembling actual observations of stellar-mass black holes in the soft or steep power-law state: broad thermal peaks (at 1-3 keV), steep power-laws extending to high energy ($Γ$ = 2.7-4.5), and prominent, asymmetric Fe K$α$ emission lines with equivalent widths in the range 40-400 eV (larger EW at lower accretion rates). By starting with simulation data, we obviate the need for parameterized descriptions of the accretion flow geometry---no a priori specification of the corona's shape or flux, or the disk temperature or density, etc., are needed. Instead, we apply the relevant physical principles to simulation output using appropriate numerical techniques; this procedure allows us to calculate inclination-dependent spectra after choosing only a small number of physically meaningful parameters: black hole mass and spin, accretion rate, and elemental abundances.

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arXiv2016

Fe K$α$ Profiles from Simulations of Accreting Black Holes

We present first results from a new technique for the prediction of Fe K$α$ profiles directly from general relativistic magnetohydrodynamic (GRMHD) simulations. Data from a GRMHD simulation are processed by a Monte Carlo global radiation transport code, which determines the X-ray flux irradiating the disk surface and the coronal electron temperature self-consistently. With that irradiating flux and the disk's density structure drawn from the simulation, we determine the reprocessed Fe K$α$ emission from photoionization equilibrium and solution of the radiation transfer equation. We produce maps of the surface brightness of Fe K$α$ emission over the disk surface, which---for our example of a $10 M_\odot$, Schwarzschild black hole accreting at $1\%$ the Eddington value---rises steeply one gravitational radius outside the radius of the innermost stable circular orbit and then falls $\propto r^{-2}$ at larger radii. We explain these features of the Fe K$α$ radial surface brightness profile as consequences of the disk's ionization structure and an extended coronal geometry, respectively. We also present the corresponding Fe K$α$ line profiles as would be seen by distant observers at several inclinations. Both the shapes of the line profiles and the equivalent widths of our predicted K$α$ lines are qualitatively similar to those typically observed from accreting black holes. Most importantly, this work represents a direct link between theory and observation: in a fully self-consistent way, we produce observable results---iron fluorescence line profiles---from the theory of black hole accretion with almost no phenomenological assumptions.

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