Julian H. Krolik

Johns Hopkins University, Baltimore, Maryland, United States

Are you Julian H. Krolik?

Claim your profile

Publications (171)714.86 Total impact

  • Source
    Julian H. Krolik, John F. Hawley
    [Show abstract] [Hide abstract]
    ABSTRACT: Using only physical mechanisms, i.e., 3D MHD with no phenomenological viscosity, we have simulated the dynamics of a moderately thin accretion disk subject to torques whose radial scaling mimics those produced by lowest-order post-Newtonian gravitomagnetism. In this simulation, we have shown how, in the presence of MHD turbulence, a time-steady transition can be achieved between an inner disk region aligned with the equatorial plane of the central mass's spin and an outer region orbiting in a different plane. The position of the equilibrium orientation transition is determined by a balance between gravitomagnetic torque and warp-induced inward mixing of misaligned angular momentum from the outer disk. If the mixing is interpreted in terms of diffusive transport, the implied diffusion coefficient is ~(0.6--0.8)c_s^2/Omega for sound speed c_s and orbital frequency Omega. This calibration permits estimation of the orientation transition's equilibrium location given the central mass, its spin parameter, and the disk's surface density and scaleheight profiles. However, the alignment front overshoots before settling into an equilibrium, signaling that a diffusive model does not fully represent the time-dependent properties of alignment fronts under these conditions. Because the precessional torque on the disk at the alignment front is always comparable to the rate at which misaligned angular momentum is brought inward to the front by warp-driven radial motions, no break forms between the inner and outer portions of the disk in our simulation. Our results also raise questions about the applicability to MHD warped disks of the traditional distinction between "bending wave" and "diffusive" regimes.
    The Astrophysical Journal 05/2015; 806(1). DOI:10.1088/0004-637X/806/1/141 · 6.28 Impact Factor
  • Source
    Jeremy D. Schnittman, Julian H. Krolik
    [Show abstract] [Hide abstract]
    ABSTRACT: We consider the evolution of a supermassive black hole binary (SMBHB) surrounded by a retrograde accretion disk. Assuming the disk is exactly in the binary plane and transfers energy and angular momentum to the binary via direct gas accretion, we calculate the time evolution of the binary's semi-major axis $a$ and eccentricity $e$. Because the gas is predominantly transferred when the binary is at apocenter, we find the eccentricity grows rapidly while maintaining constant $a(1+e)$. After accreting only a fraction of the secondary's mass, the eccentricity grows to nearly unity; from then on, gravitational wave emission dominates the evolution, preserving constant $a(1-e)$. The high-eccentricity waveforms redistribute the peak gravitational wave power from the nHz to $\mu$Hz bands, substantially affecting the signal that might be detected with pulsar timing arrays. We also estimate the torque coupling binaries of arbitrary eccentricity with obliquely aligned circumbinary disks. If the outer edge of the disk is not an extremely large multiple of the binary separation, retrograde accretion can drive the binary into the gravitational wave-dominated state before these torques align the binary with the angular momentum of the mass supply.
    The Astrophysical Journal 04/2015; 806(1). DOI:10.1088/0004-637X/806/1/88 · 6.28 Impact Factor
  • Source
    Ji-Ming Shi, Julian H. Krolik
    [Show abstract] [Hide abstract]
    ABSTRACT: When an accretion disk surrounds a binary rotating in the same sense, the binary exerts strong torques on the gas. Analytic work in the 1D approximation indicated that these torques sharply diminish or even eliminate accretion from the disk onto the binary. However, recent 2D and 3D simulational work has shown at most modest diminution. We present new MHD simulations demonstrating that for binaries with mass ratios of 1 and 0.1 there is essentially no difference between the accretion rate at large radius in the disk and the accretion rate onto the binary. To resolve the discrepancy with earlier analytic estimates, we identify the small subset of gas trajectories traveling from the inner edge of the disk to the binary and show how the full accretion rate is concentrated onto them.
  • Justin Bankert, Julian H. Krolik, Jiming Shi
    [Show abstract] [Hide abstract]
    ABSTRACT: We evaluate the interactions between a central equal mass binary and a surrounding retrograde circumbinary accretion disk in a three-dimensional, MHD simulation. We find (as widely expected) that the net torque expressed on a retrograde circumbinary disk by the central binary is much (more than two orders of magnitude) smaller than when the disk orbits in the prograde direction. For this reason, unlike the prograde case, there is no "gap" carved out of the retrograde disk, and accreting fluid can travel directly from the disk to the domain of the binary. For the same reason, retrograde disks do not develop the large-amplitude "lumps" generically seen near the inner edges of prograde disks. Although not directly treated in this simulation, the finite extent of accretion disks around the individual masses of the binary can lead to shocks between their outer edges and the circumbinary disk, potentially creating a photon signal modulated at the binary orbital period. The spectra of these shocks should peak in the hard X-ray band as is expected for prograde systems, but retrograde systems would not be expected to display a "notch" in the optical/UV band. Because the disk's angular momentum is directed oppositely to that of the binary, accretion onto the binary causes a rapid increase in eccentricity. Lastly, we discuss the comparative rates of orbital evolution associated with accretion in prograde and retrograde circumbinary disks, finding that the principal contrast is that should be rather greater in the retrograde case.
    The Astrophysical Journal 03/2015; 801(2):114. DOI:10.1088/0004-637X/801/2/114 · 6.28 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A tidal disruption event (TDE) takes place when a star passes near enough to a massive black hole to be disrupted. About half the star's matter is given elliptical trajectories with large apocenter distances, the other half is unbound. To "circularize", i.e., to form an accretion flow, the bound matter must lose a significant amount of energy, with the actual amount depending on the characteristic scale of the flow measured in units of the black hole's gravitational radius (~ 10^{51} (R/1000R_g)^{-1} erg). Recent numerical simulations (Shiokawa et al., 2015) have revealed that the circularization scale is close to the scale of the most-bound initial orbits, ~ 10^3 M_{BH,6.5}^{-2/3} R_g ~ 10^{15} M_{BH,6.5}^{1/3} cm from the black hole, and the corresponding circularization energy dissipation rate is $\sim 10^{44} M_{BH,6.5}^{-1/6}$~erg/s. We suggest that the energy liberated during circularization, rather then energy liberated by accretion onto the black hole, powers the observed optical TDE candidates (e.g.Arcavi et al. 2014). The observed rise times, luminosities, temperatures, emission radii, and line widths seen in these TDEs are all more readily explained in terms of heating associated with circularization than in terms of accretion.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We study how the matter dispersed when a supermassive black hole tidally disrupts a star joins an accretion flow. Combining a relativistic hydrodynamic simulation of the stellar disruption with a relativistic hydrodynamics simulation of the tidal debris motion, we track such a system until ~80% of the stellar mass bound to the black hole has settled into an accretion flow. Shocks near the stellar pericenter and also near the apocenter of the most tightly-bound debris dissipate orbital energy, but only enough to make the characteristic radius comparable to the semi-major axis of the most-bound material, not the tidal radius as previously thought. The outer shocks are caused by post-Newtonian effects, both on the stellar orbit during its disruption and on the tidal forces. Accumulation of mass into the accretion flow is non-monotonic and slow, requiring ~3--10x the orbital period of the most tightly-bound tidal streams, while the inflow time for most of the mass may be comparable to or longer than the mass accumulation time. Deflection by shocks does, however, remove enough angular momentum and energy from some mass for it to move inward even before most of the mass is accumulated into the accretion flow. Although the accretion rate rises sharply and then decays roughly as a power-law, its maximum is ~0.1x the previous expectation, and the duration of the peak is ~5x longer than previously predicted. The geometric mean of the black hole mass and stellar mass inferred from a measured event timescale is therefore ~0.2x the value given by classical theory.
    The Astrophysical Journal 01/2015; 804(2). DOI:10.1088/0004-637X/804/2/85 · 6.28 Impact Factor
  • Julian H Krolik, Kareem Sorathia, John F Hawley
    [Show abstract] [Hide abstract]
    ABSTRACT: Accretion disks occur in a wide variety of astrophysical contexts, from planet formation to accretion onto black holes. For simplicity, they are generally imagined as thin and flat. However, whenever the disks angular momentum is oblique to the angular momentum of the central object(s), a torque causes rings within the disk to precess, twisting and warping it. Because the torque weakens rapidly with increasing radius, it has long been thought that some unspecified ‘friction’ brings the inner portions of such disks into alignment, while the outer parts remain in their original orientation. Nearly all previous work on this topic has assumed that such a disks internal stresses can be described by an isotropic viscosity, even though it has been known for more than four decades that fluid viscosity is far too weak to be significant in accretion disks, and for two decades that accretion stresses are actually due to anisotropic MHD turbulence. This paper reviews recent numerical simulation work showing how twisted disks align when their mechanics are described only in terms of real forces, including MHD turbulence. The detailed mechanisms of alignment are identified, the rate at which it occurs is quantified, and the isotropic viscosity model is shown to be in drastic disagreement with the simulation data.
    Classical and Quantum Gravity 12/2014; 31(24). DOI:10.1088/0264-9381/31/24/244004 · 3.10 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present the results of local, vertically stratified, radiation MHD shearing box simulations of MRI turbulence appropriate for the hydrogen ionizing regime of dwarf nova and soft X-ray transient outbursts. We incorporate the frequency-integrated opacities and equation of state for this regime, but neglect non-ideal MHD effects and surface irradiation, and do not impose net vertical magnetic flux. We find two stable thermal equilibrium tracks in the effective temperature versus surface mass density plane, in qualitative agreement with the S-curve picture of the standard disk instability model. We find that the large opacity at temperatures near $10^4$K, a corollary of the hydrogen ionization transition, triggers strong, intermittent thermal convection on the upper stable branch. This convection strengthens the magnetic turbulent dynamo and greatly enhances the time-averaged value of the stress to thermal pressure ratio $\alpha$, possibly by generating vertical magnetic field that may seed the axisymmetric magnetorotational instability, and by increasing cooling so that the pressure does not rise in proportion to the turbulent dissipation. These enhanced stress to pressure ratios may alleviate the order of magnitude discrepancy between the $\alpha$-values observationally inferred in the outburst state and those that have been measured from previous local numerical simulations of magnetorotational turbulence that lack net vertical magnetic flux.
    The Astrophysical Journal 03/2014; 787(1). DOI:10.1088/0004-637X/787/1/1 · 6.28 Impact Factor
  • Source
    Constanze Roedig, Julian H. Krolik, M. Coleman Miller
    [Show abstract] [Hide abstract]
    ABSTRACT: Observations indicate that most massive galaxies contain a supermassive black hole, and theoretical studies suggest that when such galaxies have a major merger, the central black holes will form a binary and eventually coalesce. Here we discuss two spectral signatures of such binaries that may help distinguish them from ordinary AGN. These signatures are expected when the mass ratio between the holes is not extreme and the system is fed by a circumbinary disk. One such signature is a notch in the thermal continuum that has been predicted by other authors; we point out that it should be accompanied by a spectral revival at shorter wavelengths and also discuss its dependence on binary properties such as mass, mass ratio, and separation. In particular, we note that the wavelength $\lambda_n$ at which the notch occurs depends on these three parameters in such a way as to make the number of systems displaying these notches $\propto \lambda_n^{16/3}$; longer wavelength searches are therefore strongly favored. A second signature, first discussed here, is hard X-ray emission with a Wien-like spectrum at a characteristic temperature $\sim 100$ keV produced by Compton cooling of the shock generated when streams from the circumbinary disk hit the accretion disks around the individual black holes. We investigate the observability of both signatures. The hard X-ray signal may be particularly valuable as it can provide an indicator of black hole merger a few decades in advance of the event.
    The Astrophysical Journal 02/2014; 785(2). DOI:10.1088/0004-637X/785/2/115 · 6.28 Impact Factor
  • Source
    Kareem A. Sorathia, Julian H. Krolik, John F. Hawley
    [Show abstract] [Hide abstract]
    ABSTRACT: When matter orbits around a central mass obliquely with respect to the mass's spin axis, the Lense-Thirring effect causes it to precess at a rate declining sharply with radius. Ever since the work of Bardeen & Petterson (1975), it has been expected that when a fluid fills an orbiting disk, the orbital angular momentum at small radii should then align with the mass's spin. Nearly all previous work has studied this alignment under the assumption that a phenomenological "viscosity" isotropically degrades fluid shears in accretion disks, even though it is now understood that internal stress in flat disks is due to anisotropic MHD turbulence. In this paper we report a pair of matched simulations, one in MHD and one in pure (non-viscous) HD in order to clarify the specific mechanisms of alignment. As in the previous work, we find that disk warps induce radial flows that mix angular momentum of different orientation; however, we also show that the speeds of these flows are generically transonic and are only very weakly influenced by internal stresses other than pressure. In particular, MHD turbulence does not act in a manner consistent with an isotropic viscosity. When MHD effects are present, the disk aligns, first at small radii and then at large; alignment is only partial in the HD case. We identify the specific angular momentum transport mechanisms causing alignment and show how MHD effects permit them to operate more efficiently. Lastly, we relate the speed at which an alignment front propagates outward (in the MHD case) to the rate at which Lense-Thirring torques deliver angular momentum at smaller radii.
    The Astrophysical Journal 09/2013; 777(1). DOI:10.1088/0004-637X/777/1/21 · 6.28 Impact Factor
  • Source
    M. Coleman Miller, Julian H. Krolik
    [Show abstract] [Hide abstract]
    ABSTRACT: Recent studies of accretion onto supermassive black hole binaries suggest that much, perhaps most, of the matter eventually accretes onto one hole or the other. If so, then for binaries whose inspiral from ~1 pc to 0.001 - 0.01 pc is driven by interaction with external gas, both the binary orbital axis and the individual black hole spins can be reoriented by angular momentum exchange with this gas. Here we show that, unless the binary mass ratio is far from unity, the spins of the individual holes align with the binary orbital axis in a time few-100 times shorter than the binary orbital axis aligns with the angular momentum direction of the incoming circumbinary gas; the spin of the secondary aligns more rapidly than that of the primary by a factor ~(m_1/m_2)^{1/2}>1. Thus the binary acts as a stabilizing agent, so that for gas-driven systems, the black hole spins are highly likely to be aligned (or counteraligned if retrograde accretion is common) with each other and with the binary orbital axis. This alignment can significantly reduce the recoil speed resulting from subsequent black hole merger.
    The Astrophysical Journal 07/2013; 774(1). DOI:10.1088/0004-637X/774/1/43 · 6.28 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Global disk simulations provide a powerful tool for investigating accretion and the underlying magnetohydrodynamic turbulence driven by the magneto-rotational instability (MRI). Using them to predict accurately quantities such as stress, accretion rate, and surface brightness profile requires that purely numerical effects, arising from both resolution and algorithm, be understood and controlled. We use the flux-conservative Athena code to conduct a series of experiments on disks having a variety of magnetic topologies to determine what constitutes adequate resolution. We develop and apply several resolution metrics: Qz and Qphi, the ratio of the grid zone size to the characteristic MRI wavelength, alpha_mag, the ratio of the Maxwell stress to the magnetic pressure, and the ratio of radial to toroidal magnetic field energy. For the initial conditions considered here, adequate resolution is characterized by Qz > 15, Qphi > 20, alpha_mag = 0.45, and a field energy ratio of 0.2. These values are associated with > 35 zones per scaleheight, a result consistent with shearing box simulations. Numerical algorithm is also important. Use of the HLLE flux solver or second-order interpolation can significantly degrade the effective resolution compared to the HLLD flux solver and third-order interpolation. Resolution at this standard can be achieved only with large numbers of grid zones, arranged in a fashion that matches the symmetries of the problem and the scientific goals of the simulation.
    The Astrophysical Journal 06/2013; 772(2). DOI:10.1088/0004-637X/772/2/102 · 6.28 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present the results of a new global radiation transport code coupled to a general relativistic magnetohydrodynamic simulation of an accreting, non-rotating black hole. For the first time, we are able to explain from first principles in a self-consistent way all the components seen in the X-ray spectra of stellar-mass black holes, including a thermal peak and all the features associated with strong hard X-ray emission: a power law extending to high energies, a Compton reflection hump, and a broad iron line. Varying only the mass accretion rate, we are able to reproduce a wide range of X-ray states seen in most galactic black hole sources. The temperature in the corona is Te ~ 10 keV in a boundary layer near the disk and rises smoothly to Te 100 keV in low-density regions far above the disk. Even as the disk's reflection edge varies from the horizon out to 6M as the accretion rate decreases, we find that the shape of the Fe Kα line is remarkably constant. This is because photons emitted from the plunging region are strongly beamed into the horizon and never reach the observer. We have also carried out a basic timing analysis of the spectra and find that the fractional variability increases with photon energy and viewer inclination angle, consistent with the coronal hot spot model for X-ray fluctuations.
    The Astrophysical Journal 05/2013; 769(2):156. DOI:10.1088/0004-637X/769/2/156 · 6.28 Impact Factor
  • Source
    Kareem A. Sorathia, Julian H. Krolik, John F. Hawley
    [Show abstract] [Hide abstract]
    ABSTRACT: Orbiting disks may exhibit bends due to a misalignment between the angular momentum of the inner and outer regions of the disk. We begin a systematic simulational inquiry into the physics of warped disks with the simplest case: the relaxation of an unforced warp under pure fluid dynamics, i.e., with no internal stresses other than Reynolds stress. We focus on the nonlinear regime in which the bend rate is large compared to the disk aspect ratio. When warps are nonlinear, strong radial pressure gradients drive transonic radial motions along the disk's top and bottom surfaces that efficiently mix angular momentum. The resulting nonlinear decay rate of the warp increases with the warp rate and the warp width, but, at least in the parameter regime studied here, is independent of the sound speed. The characteristic magnitude of the associated angular momentum fluxes likewise increases with both the local warp rate and the radial range over which the warp extends; it also increases with increasing sound speed, but more slowly than linearly. The angular momentum fluxes respond to the warp rate after a delay that scales with the square root of the time for sound waves to cross the radial extent of the warp. These behaviors are at variance with a number of the assumptions commonly used in analytic models to describe linear warp dynamics.
    The Astrophysical Journal 04/2013; 768(2):133. DOI:10.1088/0004-637X/768/2/133 · 6.28 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: In this document, we describe the scientific potential of blazar observations with a X-ray polarimetry mission like GEMS (Gravity and Extreme Magnetism SMEX). We describe five blazar science investigations that such a mission would enable: (i) the structure and the role of magnetic fields in AGN jets, (ii) analysis of the polarization of the synchrotron X-ray emission from AGN jets, (iii) discrimination between synchrotron self-Compton and external Compton models for blazars with inverse Compton emission in the X-ray band, (iv) a precision study of the polarization properties of the X-ray emission from Cen-A, (v) tests of Lorentz Invariance based on X-ray polarimetric observations of blazars. We conclude with a discussion of a straw man observation program and recommended accompanying multiwavelength observations.
  • Source
    Jeremy D. Schnittman, Julian H. Krolik
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a new code for radiation transport around Kerr black holes, including arbitrary emission and absorption terms, as well as electron scattering and polarization. The code is particularly useful for analyzing accretion flows made up of optically thick disks and optically thin coronae. We give a detailed description of the methods employed in the code, and also present results from a number of numerical tests to assess its accuracy and convergence.
    The Astrophysical Journal 02/2013; 777(1). DOI:10.1088/0004-637X/777/1/11 · 6.28 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We examine the expected X-ray polarization properties of neutron-star X-ray sources of various types, e.g., accretion and rotation powered pulsars, magnetars, and low-mass X-ray binaries. We summarize the model calculations leading to these expected properties. We describe how a comparison of these with their observed properties, as inferred from GEMS data, will probe the essential dynamical, electromagnetic, plasma, and emission processes in neutron-star binaries, discriminate between models of these processes, and constrain model parameters. An exciting goal is the first observational demonstration in this context of the existence of vacuum resonance, a fundamental quantum electrodynamical phenomenon first described in the 1930s.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present here a summary of the scientific goals behind the Gravity and Extreme Magnetism SMEX (GEMS) X-ray polarimetry mission's black hole (BH) observing program. The primary targets can be divided into two classes: stellar-mass galactic BHs in accreting binaries, and super-massive BHs in the centers of active galactic nuclei (AGN). The stellar-mass BHs can in turn be divided into various X-ray spectral states: thermal-dominant (disk), hard (radio jet), and steep power-law (hot corona). These different spectral states are thought to be generated by different accretion geometries and emission mechanisms. X-ray polarization is an ideal tool for probing the geometry around these BHs and revealing the specific properties of the accreting gas.
  • Source
    Norita Kawanaka, Tsvi Piran, Julian H. Krolik
    [Show abstract] [Hide abstract]
    ABSTRACT: A hyperaccretion disk formed around a stellar mass black hole is a plausible model for the central engine that powers gamma-ray bursts (GRBs). If the central black hole rotates and a poloidal magnetic field threads its horizon, a powerful relativistic jet may be driven by a process resembling the Blandford-Znajek mechanism. We estimate the luminosity of such a jet assuming that the poloidal magnetic field strength is comparable to the inner accretion disk pressure. We show that the jet efficiency attains its maximal value when the accretion flow is cooled via optically-thin neutrino emission. The jet luminosity is much larger than the energy deposition through neutrino-antineutrino annihilation provided that the black hole is spinning rapidly enough. When the accretion rate onto a rapidly spinning black hole is large enough (> 0.003-0.01M_sun/sec), the predicted jet luminosity is sufficient to drive a GRB.
    The Astrophysical Journal 11/2012; 766(1). DOI:10.1088/0004-637X/766/1/31 · 6.28 Impact Factor
  • Source
    Tsvi Piran, Julian Krolik
    [Show abstract] [Hide abstract]
    ABSTRACT: We explore the temporal structure of tidal disruption events pointing out the corresponding transitions in the lightcurves of the thermal accretion disk and of the jet emerging from such events. The hydrodynamic time scale of the disrupted star is the minimal time scale of building up the accretion disk and the jet and it sets a limit on the rise time. This suggest that Swift J1644+57, that shows several flares with a rise time as short as a few hundred seconds could not have arisen from a tidal disruption of a main sequence star whose hydrodynamic time is a few hours. The disrupted object must have been a white dwarf. A second important time scale is the Eddington time in which the accretion rate changes form super to sub Eddington. It is possible that such a transition was observed in the light curve of Swift J2058+05. If correct this provides intersting constraints on the parameters of the system.
    The European Physical Journal Conferences 10/2012; DOI:10.1051/epjconf/20123902006

Publication Stats

7k Citations
714.86 Total Impact Points

Institutions

  • 1992–2015
    • Johns Hopkins University
      • Department of Physics and Astronomy
      Baltimore, Maryland, United States
  • 2005–2010
    • Princeton University
      • Department of Astrophysical Sciences
      Princeton, New Jersey, United States
  • 2009
    • Space Telescope Science Institute
      Baltimore, Maryland, United States
  • 2005–2009
    • University of Virginia
      • Department of Astronomy
      Charlottesville, Virginia, United States
  • 2008
    • University of Cambridge
      • Institute of Astronomy
      Cambridge, England, United Kingdom
  • 2002
    • University of Maryland, College Park
      • Department of Astronomy
      Maryland, United States
    • The University of Tokyo
      • Institute for Cosmic Ray Research
      Edo, Tōkyō, Japan
  • 1994
    • Nicolaus Copernicus University
      Toruń, Kujawsko-Pomorskie, Poland
  • 1987
    • NASA
      • Goddard Space Flight Centre
      Вашингтон, West Virginia, United States