arXiv subjects

Mitchell C. Begelman

Publications and source records attributed to Mitchell C. Begelman.

At least 127 records · Page 7Linked to original sources

arXiv2008

X-ray flares and their relation to the inner engine activity of GRBs

Flares overlaid on the smooth power-law decay of Swift X-ray afterglows are rather common, appearing in roughly half the observed light curves. They are a manifestation of the late time activity of the inner engine, since their temporal evolution is too fast to be linked to activity taking place in the external shock blastwave. In this paper we show that the energy emitted in the form of flares decreases with time as a power-law. We discuss several possibilities in which the flares can be powered and the source of the observed variability. We show that late time accretion from a disk can provide the necessary energy input in both classes of short duration and long duration gamma-ray bursts.

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arXiv2008

Quasistars: Accreting black holes inside massive envelopes

We study the structure and evolution of "quasistars", accreting black holes embedded within massive hydrostatic gaseous envelopes. These configurations may model the early growth of supermassive black hole seeds. The accretion rate onto the black hole adjusts so that the luminosity carried by the convective envelope equals the Eddington limit for the total mass. This greatly exceeds the Eddington limit for the black hole mass alone, leading to rapid growth of the black hole. We use analytic models and numerical stellar structure calculations to study the structure and evolution of quasistars. We derive analytically the scaling of the photospheric temperature with the black hole mass and envelope mass, and show that it decreases with time as the black hole mass increases. Once the photospheric temperature becomes lower than 10,000 K, the photospheric opacity drops precipitously and the photospheric temperature hits a limiting value, analogous to the Hayashi track for red giants and protostars, below which no hydrostatic solution for the convective envelope exists. For metal-free (Population III) opacities this limiting temperature is approximately 4000 K. After a quasistar reaches this limiting temperature, the envelope is rapidly dispersed by radiation pressure. We find that black hole seeds with masses between 1000 and 10000 solar masses could form via this mechanism in less than a few Myr.

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arXiv2008

Hot self-similar relativistic MHD flows

We consider axisymmetric relativistic jets with a toroidal magnetic field and an ultrarelativistic equation of state, with the goal of studying the lateral structure of jets whose pressure is matched to the pressure of the medium through which they propagate. We find all self-similar steady-state solutions of the relativistic MHD equations for this setup. One of the solutions is the case of a parabolic jet being accelerated by the pressure gradient as it propagates through a medium with pressure declining as p(z)\propto z^{-2}. As the jet material expands due to internal pressure gradients, it runs into the ambient medium resulting in a pile-up of material along the jet boundary, while the magnetic field acts to produce a magnetic pinch along the axis of the jet. Such jets can be in a lateral pressure equilibrium only if their opening angle θ_j at distance z is smaller than about 1/γ, where γis the characteristic bulk Lorentz-factor at this distance; otherwise, different parts of the jet cannot maintain causal contact. We construct maps of optically thin synchrotron emission from our self-similar models. We suggest that the boundary pile-up may be the reason for the limb-brightening of the sub-parsec jet of M87. We find that if the synchrotron emissivity falls with the distance from the jet axis, the polarization fraction rises toward the edge, as seen in 3C273 and Mkn501. Projection effects and the emissivity pattern of the jet have a strong effect on the observed polarization signal, so the interpretation of the polarization data in terms of the geometry of magnetic fields is rather uncertain. For example, jets with toroidal magnetic fields display the `spine-sheath' polarization angle pattern seen in some BL Lac objects.

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arXiv2007

Implications of very rapid TeV variability in blazars

We discuss the implications of rapid (few-minute) variability in the TeV flux of blazars, which has been observed recently with the HESS and MAGIC telescopes. The variability timescales seen in PKS 2155-304 and Mrk 501 are much shorter than inferred light-crossing times at the black hole horizon, suggesting that the variability involves enhanced emission in a small region within an outflowing jet. The enhancement could be triggered by dissipation in part of the black hole's magnetosphere at the base of the outflow, or else by instabilities in the jet itself. By considering the energetics of the observed flares, along with the requirement that TeV photons escape without producing pairs, we deduce that the bulk Lorentz factors in the jets must be >50. The distance of the emission region from the central black hole is less well-constrained. We discuss possible consequences for multi-wavelength observations.

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arXiv2007

Did supermassive black holes form by direct collapse?

Rapid infall of gas in the nuclei of galaxies could lead to the formation of black holes by direct collapse, without first forming stars. Black holes formed in this way would have initial masses of a few solar masses, but would be embedded in massive envelopes that would allow them to grow at a highly super-Eddington rate. Thus, seed black holes as large as 10^3-10^4 solar masses could form very rapidly. I will sketch the basic physics of the direct collapse process and the properties of the accreting envelopes.

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arXiv2007

Temporal and Angular Properties of GRB Jets Emerging from Massive Stars

We study the long-term evolution of relativistic jets in collapsars and examine the effects of viewing angle on the subsequent gamma ray bursts. We carry out a series of high-resolution simulations of a jet propagating through a stellar envelope in 2D cylindrical coordinates using the FLASH relativistic hydrodynamics module. For the first time, simulations are carried out using an adaptive mesh that allows for a large dynamic range inside the star while still being efficient enough to follow the evolution of the jet long after it breaks out from the star. Our simulations allow us to single out three phases in the jet evolution: a precursor phase in which relativistic material turbulently shed from the head of the jet first emerges from the star, a shocked jet phase where a fully shocked jet of material is emerging, and an unshocked jet phase where the jet consists of a free-streaming, unshocked core surrounded by a thin boundary layer of shocked jet material. The appearance of these phases will be different to observers at different angles. The precursor has a wide opening angle and would be visible far off axis. The shocked phase has a relatively narrow opening angle that is constant in time. During the unshocked jet phase the opening angle increases logarithmically with time. As a consequence, some observers see prolonged dead times of emission even for constant properties of the jet injected in the stellar core. We also present an analytic model that is able to reproduce the overall properties of the jet and its evolution. We finally discuss the observational implications of our results, emphasizing the possible ways to test progenitor models through the effects of jet propagation in the star. In an appendix, we present 1D and 2D tests of the FLASH relativistic hydrodynamics module.

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arXiv2006

The physics of GRB jets and their interaction with the progenitor star

It is now generally accepted that long gamma-ray bursts are associated with the final evolutionary stages of massive stars. As a consequence, their jets must propagate through the stellar progenitor and break out on their surface, before they can reach the photospheric radius and produce the gamma-ray photons. We investigate the role of the progenitor star in shaping the jet properties. We show that even a jet powered by a steady engine can develop a rich phenomenology at the stellar surface. We present special-relativistic simulations and compare the results to analytic considerations. We show that the jet is complex in the time as well as in the angular domain, so that observers located along different lines of sight detect significantly different bursts.

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arXiv2006

Gamma-Ray Burst jet dynamics and their interaction with the progenitor star

The association of at least some long gamma-ray bursts with type Ic supernova explosions has been established beyond reasonable doubt. Theoretically, the challenge is to explain the presence of a light hyper-relativistic flow propagating through a massive stellar core without losing those properties. We discuss the role of the jet-star interaction in shaping the properties of the outflow emerging on the surface of the star. We show that the nature of the inner engine is hidden from the observer for most of the evolution, well beyond the time of the jet breakout on the stellar surface. The discussion is based on analytical considerations as well as high resolution numerical simulations. Finally, the observational consequences of the scenario are addressed in light of the present capabilities.

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arXiv2006

Trapping of magnetic flux by the plunge region of a black hole accretion disk

The existence of the radius of marginal stability means that accretion flows around black holes invariably undergo a transition from a MHD turbulent disk-like flow to an inward plunging flow. We argue that the plunging inflow can greatly enhance the trapping of large scale magnetic field on the black hole, and therefore may increase the importance of the Blandford-Znajek (BZ) effect relative to previous estimates that ignore the plunge region. We support this hypothesis by constructing and analyzing a toy-model of the dragging and trapping of a large scale field by a black hole disk, revealing a strong dependence of this effect on the effective magnetic Prandtl number of the MHD turbulent disk. Furthermore, we show that the enhancement of the BZ effect depends on the geometric thickness of the accretion disk. This may be, at least in part, the physical underpinnings of the empirical relation between the inferred geometric thickness of a black hole disk and the presence of a radio jet.

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arXiv2006

Formation of Supermassive Black Holes by Direct Collapse in Pregalactic Halos

We describe a mechanism by which supermassive black holes can form directly in the nuclei of protogalaxies, without the need for seed black holes left over from early star formation. Self-gravitating gas in dark matter halos can lose angular momentum rapidly via runaway, global dynamical instabilities, the so-called "bars within bars" mechanism. This leads to the rapid buildup of a dense, self-gravitating core supported by gas pressure - surrounded by a radiation pressure-dominated envelope - which gradually contracts and is compressed further by subsequent infall. These conditions lead to such high temperatures in the central region that the gas cools catastrophically by thermal neutrino emission, leading to the formation and rapid growth of a central black hole. We estimate the initial mass and growth rate of the black hole for typical conditions in metal-free halos with T_vir ~ 10^4 K, which are the most likely to be susceptible to runaway infall. The initial black hole should have a mass of <~20 solar masses, but in principle could grow at a super-Eddington rate until it reaches ~ 10^4-10^6 solar masses. Rapid growth may be limited by feedback from the accretion process and/or disruption of the mass supply by star formation or halo mergers. Even if super-Eddington growth stops at \~10^3-10^4 solar masses, this process would give black holes ample time to attain quasar-size masses by a redshift of 6, and could also provide the seeds for all supermassive black holes seen in the present universe.

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arXiv2006

Photon Bubbles and the Vertical Structure of Accretion Disks

We consider the effects of "photon bubble" shock trains on the vertical structure of radiation pressure-dominated accretion disks. These density inhomogeneities are expected to develop spontaneously in radiation-dominated accretion disks where magnetic pressure exceeds gas pressure, even in the presence of magnetorotational instability. They increase the rate at which radiation escapes from the disk, and may allow disks to exceed the Eddington limit by a substantial factor. We first generalize the theory of photon bubbles to include the effects of finite optical depths and radiation damping. Modifications to the diffusion law at low optical depth tend to fill in the low-density regions of photon bubbles, while radiation damping inhibits the formation of photon bubbles at large radii, small accretion rates, and small heights above the equatorial plane. Accretion disks dominated by photon bubble transport may reach luminosities of 10 to >100 times the Eddington limit (L_E), depending on the mass of the central object, while remaining geometrically thin. However, photon bubble-dominated disks with alpha-viscosity are subject to the same thermal and viscous instabilities that plague standard radiation pressure-dominated disks, suggesting that they may be intrinsically unsteady. Photon bubbles can lead to a "core-halo" vertical disk structure. In super-Eddington disks the halo forms the base of a wind, which carries away substantial energy and mass, but not enough to prevent the luminosity from exceeding L_E. Photon bubble-dominated disks may have smaller color corrections than standard accretion disks of the same luminosity. They remain viable contenders for some ultraluminous X-ray sources and may play a role in the rapid growth of supermassive black holes at high redshift.

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arXiv2005

Thick fireballs and the steep decay in the early X-ray afterglow of gamma-ray bursts

We study the early afterglows of gamma-ray bursts produced by geometrically thick fireballs, following the development of the external shock as energy is continually supplied to the shocked material. We study the dependence of the early afterglow slope on the luminosity history of the central engine. The resulting light curves are modeled with power-law functions and the importance of a correct choice of the reference time t_0 is investigated. We find that deviations from a simple power-law are observed only if a large majority of the energy is released at late times. The light curve in this case can be described as a simple power-law if the reference time is set to be close to the end of the burst. We applied our analysis to the cases of GRB 050219a and GRB 050315. We show that the early steep decay of the afterglow cannot result from the interaction of the fireball with the ambient medium. We conclude that the early X-ray afterglow emission is associated with the prompt phase and we derive limits on the radius at which the prompt radiation is produced.

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arXiv2005

Are Quasar Jets Matter or Poynting Flux Dominated?

If quasar jets are accelerated by magnetic fields but terminate as matter dominated, where and how does the transition occur between the Poynting-dominated and matter-dominated regimes? To address this question, we study constraints which are imposed on the jet structure by observations at different spatial scales. We demonstrate that observational data are consistent with a scenario where the acceleration of a jet occurs within 10^{3-4} R_g. In this picture, the non-thermal flares -- important defining attributes of the blazar phenomenon - are produced by strong shocks formed in the region where the jet inertia becomes dominated by matter. Such shocks may be formed due to collisions between the portions of a jet accelerated to different velocities, and the acceleration differentiation is very likely to be related to global MHD instabilities.

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arXiv2005

Nonlinear Photon Bubbles Driven by Buoyancy

We derive an analytic model for nonlinear "photon bubble" wave trains driven by buoyancy forces in magnetized, radiation pressure-dominated atmospheres. Continuous, periodic wave solutions exist when radiative diffusion is slow compared to the dynamical timescale of the atmosphere. We identify these waves with the saturation of a linear instability discovered by Arons - therefore, these wave trains should develop spontaneously. The buoyancy-driven waves are physically distinct from photon bubbles in the presence of rapid diffusion, which evolve into trains of gas pressure-dominated shocks as they become nonlinear. Like the gas pressure-driven shock trains, buoyancy-driven photon bubbles can exhibit very large density contrasts, which greatly enhance the flow of radiation through the atmosphere. However, steady-state solutions for buoyancy-driven photon bubbles exist only when an extra source of radiation is added to the energy equation, in the form of a flux divergence. We argue that this term is required to compensate for the radiation flux lost via the bubbles, which increases with height. We speculate that an atmosphere subject to buoyancy-driven photon bubbles, but lacking this compensating energy source, would lose pressure support and collapse on a timescale much shorter than the radiative diffusion time in the equivalent homogeneous atmosphere.

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arXiv2005

Self-regulated black hole accretion, the M-sigma relation, and the growth of bulges in galaxies

We argue that the velocity dispersions and masses of galactic bulges and spheroids are byproducts of the feedback that regulates rapid black hole growth in protogalaxies. We suggest that the feedback energy liberated by accretion must pass through the accreting material, in an energy-conserving flux close-in and a momentum-conserving flux further out. If the inflowing gas dominates the gravitational potential outside the Bondi radius, feedback from Eddington-limited accretion drives the density profile of the gas to that of a singular isothermal sphere. We find that the velocity dispersion associated with the isothermal potential, sigma, increases with time as the black hole mass M grows, in such a way that M is proportional to sigma^4. The coefficient of this proportionality depends on the radius at which the flow switches from energy conserving to momentum conserving, and gives the observed M-sigma relation if the transition occurs at ~100 Schwarzschild radii. We associate this transition with radiative cooling and show that bremsstrahlung, strongly boosted by inverse Compton scattering in a two-temperature (T_p >> T_e) plasma, leads to a transition at the desired radius. According to this picture, bulge masses M_b are insensitive to the virial masses of their dark matter haloes, but correlate linearly with black hole mass. Our analytic model also explains the M_b-sigma (Faber-Jackson) relation as a relic of black hole accretion. The model naturally explains why the M-sigma relation has less scatter than either the M-M_b (Magorrian) or the Faber-Jackson relation. It suggests that the M-sigma relation could extend down to very low velocity dispersions, and predicts that the relation should not evolve with redshift.

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arXiv2005

Cyclotron Maser Emission from Blazar Jets?

We consider the production of electron cyclotron maser emission by low-density, highly magnetized plasmas in relativistic jets. The population inversion required to drive cyclotron maser instability could occur in localized, transient sites where hydromagnetic instabilities, shocks, and/or turbulence lead to magnetic mirroring along current-carrying flux tubes. The maser is pumped as electrons are accelerated by the parallel electric field that develops as a result of the mirror. We estimate the maximum brightness temperatures that can be obtained in a single maser site and in an array of many masers operating simultaneously, under conditions likely to apply in blazar jets. Synchrotron absorption, by relativistic electrons within the jet, presents the largest obstacle to the escape of the maser radiation, and may render most of it invisible. However, we argue that a high brightness temperature could be produced in a thin boundary layer outside the synchrotron photosphere, perhaps in the shear layer along the wall of the jet. Induced Compton scattering provides additional constraints on the maximum brightness temperature of a masing jet. We suggest that recent observations of diffractive scintillation in the blazar J1819+3845, indicating intrinsic brightness temperatures greater than 10^{14} K at 5 GHz, may be explained in terms of cyclotron maser emission. High brightness temperature maser emission from blazar jets may extend to frequencies as high as ~100 GHz, with the maximum possible T_B scaling roughly as 1/frequency. Less massive relativistic jet sources, such as microquasars, are even better candidates for producing cyclotron maser emission, primarily in the infrared and optical bands.

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arXiv2005

Are quasar jets dominated by Poynting flux?

The formation of relativistic astrophysical jets is presumably mediated by magnetic fields threading accretion disks and central, rapidly rotating objects. As it is accelerated by magnetic stresses, the jet's kinetic energy flux grows at the expense of its Poynting flux. However, it is unclear how efficient is the conversion from magnetic to kinetic energy and whether there are any observational signatures of this process. We address this issue in the context of jets in quasars. Using data from all spatial scales, we demonstrate that in these objects the conversion from Poynting-flux-dominated to matter-dominated jets is very likely to take place closer to the black hole than the region where most of the Doppler boosted radiation observed in blazars is produced. We briefly discuss the possibility that blazar activity can be induced by global MHD instabilities, e.g., via the production of localized velocity gradients that lead to dissipative events such as shocks or magnetic reconnection, where acceleration of relativistic particles and production of non-thermal flares is taking place.

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arXiv2005

Constraining the Structure of GRB Jets Through the log(N)-log(S) Distribution

A general formalism is developed for calculating the luminosity function and the expected number $N$ of observed GRBs above a peak photon flux $S$ for any GRB jet structure. This new formalism directly provides the true GRB rate without the need for a `correction factor'. We apply it to the uniform jet (UJ) and universal structured jet (USJ) models for the structure of GRB jets and perform fits to the observed log(N)-log(S) distribution from the GUSBAD catalog which contains 2204 BATSE bursts. A core angle $θ_c$ and an outer edge at $θ_{max}$ are introduced for the structured jet, and a finite range of half-opening angles $θ_{min}\leqθ_j\leqθ_{max}$ is assumed for the uniform jets. The efficiency $ε_γ$ for producing gamma-rays, and the energy per solid angle $ε$ in the jet are allowed to vary with $θ_j$ (the viewing angle $θ_{obs}$) in the UJ (USJ) model, $ε_γ\proptoθ^{-b}$ and $ε\proptoθ^{-a}$. We find that a single power-law luminosity function provides a good fit to the data. Such a luminosity function arises naturally in the USJ model, while in the UJ model it implies a power-law probability distribution for $θ_j$, $P(θ_j)\proptoθ_j^{-q}$. The value of $q$ cannot be directly determined from the fit to the observed log(N)-log(S) distribution, and an additional assumption on the value of $a$ or $b$ is required. Alternatively, an independent estimate of the true GRB rate would enable one to determine $a$, $b$ and $q$. The implied values of $θ_c$ (or $θ_{min}$) and $θ_{max}$ are close to the current observational limits. The true GRB rate for the USJ model is found to be $R_{GRB}(z=0)=0.86^{+0.14}_{-0.05} Gpc^{-3} yr^{-1}$.

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