arXiv subjects

Fulvio Melia

Publications and source records attributed to Fulvio Melia.

179 records · Page 10Linked to original sources

arXiv2001

Polarized mm And sub-mm Emission From Sgr A* At The Galactic Center

The recent detection of significant linear polarization at mm and sub-mm wavelengths in the spectrum of Sgr A* (if confirmed) will be a useful probe of the conditions within several Schwarzschild radii ($r_S$) of the event horizon at the Galactic Center. Hydrodynamic simulations of gas flowing in the vicinity of this object suggest that the infalling gas circularizes when it approaches within $5-25 r_S$ of the black hole. We suggest that the sub-mm ``excess'' of emission seen in the spectrum of Sgr A* may be associated with radiation produced within the inner Keplerian region and that the observed polarization characteristics provide direct evidence for this phenomenon. The overall spectrum from this region, including the high-energy component due to bremsstrahlung and inverse Compton scattering processes, is at or below the recent {\it Chandra} measurement, and may account for the X-ray source if it turns out to be the actual counterpart to Sgr A*.

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arXiv2000

Electron Positron Annihilation Radiation from SgrA East at the Galactic Center

Maps of the Galactic electron-positron annihilation radiation show evidence for three distinct and significant features: (1) a central bulge source, (2) emission in the Galactic plane, and (3) an enhancement of emission at positive latitudes above the Galactic Center. In this paper, we explore the possibility that Sgr A East, a very prominent radio structure surrounding the Galactic nucleus, may be a significant contributer to the central bulge feature. The motivation for doing so stems from a recently proposed link between this radio object and the EGRET gamma-ray source 2EG J1746-2852. If this association is correct, then Sgr A East is also expected to be a source of copious positron production. The results presented here show that indeed Sgr A East must have produced a numerically significant population of positrons, but also that most of them have not yet had sufficient time to thermalize and annihilate. As such, Sgr A East by itself does not appear to be the dominant current source of annihilation radiation, but it will be when the positrons have cooled sufficiently and they have become thermalized. This raises the interesting possibility that the bulge component may be due to the relics of earlier explosive events like the one that produced Sgr A East.

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arXiv1999

Viewing the Shadow of the Black Hole at the Galactic Center

In recent years, the evidence for the existence of an ultra-compact concentration of dark mass associated with the radio source Sgr A* in the Galactic Center has become very strong. However, an unambiguous proof that this object is indeed a black hole is still lacking. A defining characteristic of a black hole is the event horizon. To a distant observer, the event horizon casts a relatively large ``shadow'' with an apparent diameter of ~10 gravitational radii due to bending of light by the black hole, nearly independent of the black hole spin or orientation. The predicted size (~30 micro-arcseconds) of this shadow for Sgr A* approaches the resolution of current radio-interferometers. If the black hole is maximally spinning and viewed edge-on, then the shadow will be offset by ~8 micro-arcseconds from the center of mass, and will be slightly flattened on one side. Taking into account scatter-broadening of the image in the interstellar medium and the finite achievable telescope resolution, we show that the shadow of Sgr A* may be observable with very long-baseline interferometry at sub-millimeter wavelengths, assuming that the accretion flow is optically thin in this region of the spectrum. Hence, there exists a realistic expectation of imaging the event horizon of a black hole within the next few years.

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arXiv1999

The Role of Magnetic Field Dissipation in the Black Hole Candidate Sgr A*

The compact, nonthermal radio source Sgr A* at the Galactic Center appears to be coincident with a 2.6 million solar mass point-like object. Its energy source may be the release of gravitational energy as gas from the interstellar medium descends into its potential well. Simple attempts at calculating the spectrum and flux based on this picture have come close to the observations, yet have had difficulty in accounting for the low efficiency in this source. There now appear to be two reasons for this low conversion rate: (1) the plasma separates into two temperatures, with the protons attaining a significantly higher temperature than that of the radiating electrons, and (2) the magnetic field, B, is sub-equipartition, which reduces the magnetic bremsstrahlung emissivity, and therefore the overall power of Sgr A*. We investigate the latter with improvement over what has been attempted before: rather than calculating B based on a presumed model, we instead infer its distribution with radius empirically with the requirement that the resulting spectrum matches the observations. Our ansatz for B(r) is motivated in part by earlier calculations of the expected magnetic dissipation rate due to reconnection in a compressed flow. We find reasonable agreement with the observed spectrum of Sgr A* as long as its distribution consists of 3 primary components: an outer equipartition field, a roughly constant field at intermediate radii (~1000 Schwarzschild radii), and an inner dynamo (more or less within the last stable orbit for a non-rotating black hole) which increases B to about 100 Gauss. The latter component accounts for the observed sub-millimiter hump in this source.

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arXiv1999

Time-Dependent Disk Models for the Microquasar GRS1915+105

We investigate different accretion disk models and viscosity prescriptions in order to provide a basic explanation for the exotic temporal behavior in GRS 1915+105. Based on the fact that the overall cycle times are very much longer than the rise/fall time scales in GRS 1915, we rule out the geometry of ADAF or a hot quasi-spherical region plus a cold outer disk for this source. We thus concentrate on geometrically thin Shakura-Sunyaev type disks (Shakura & Sunyaev 1973; hereafter SS73). We have devised a modified viscosity law that has a quasi-stable upper branch. Via numerical simulations, we show that the model does account for several gross observational features of GRS 1915+105. On the other hand, the rise/fall time scales are not short enough, and no rapid oscillations on time scales $\simlt$ 10 s emerge naturally from the model. We then consider and numerically test a more elaborate model that includes the cold disk, a corona, and plasma ejections from the inner disk region and show that this model allows us to reproduce several additional observed features of GRS 1915+105. We conclude that the most likely structure of the accretion flow in this source is that of a cold disk with a modified viscosity prescription, plus a corona that accounts for much of the X-ray emission, and unsteady plasma ejections that occur when the luminosity of the source is high.

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arXiv1999

Accretion Disk Evolution with Wind Infall II. Results of 3D Hydrodynamical Simulations with an Illustrative Application to Sgr A*

In the first paper of this series, using analytic tools, we examined how the evolution and structure of a massive accretion disk may be influenced by the deposition of mass and angular momentum by an infalling Bondi-Hoyle wind. Such a mass influx impacts the long-term behavior of the disk by providing additional sources of viscosity and heating. Here, we make improvements over this earlier work by incorporating the results of 3D hydrodynamical simulations of the large scale accretion from an ambient medium into the disk evolution equations developed previously. We discuss two models, one with the axis of the disk parallel to, and the second with the axis oriented perpendicular to the large scale Bondi-Hoyle flow. We find that the mass inflow rate onto the disk within logarithmic annuli is roughly constant with radius and that the impacting wind carries much less specific angular momentum than Keplerian. We also find, in general, that the infrared spectrum of a wind-fed disk system is steeper than that of a Shakura-Sunyaev configuration, due mainly to the dissipation of the wind's kinetic energy at the disk's surface. In applying our results to the Galactic Center black hole candidate, Sgr A*, we demonstrate that a high wind inflow rate of order 10^-4 solar masses per year cannot be incorporated into a fossil disk without a significant dissipation of kinetic energy at all radii. Such a high dissipation would violate current infrared and near-infrared limits on the observed spectrum of Sgr A*.

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arXiv1998

The Hard X-Ray to Gamma-ray Spectrum in the EGRET AGNs

EGRET (20 MeV to 30 GeV) on board the Compton GRO has observed high-energy emission from about 40-50 Active Galactic Nuclei. Theoretical models of this emission based on the upscattering of thermal disk photons by cooling, relativistic electrons can successfully account for the EGRET observations, but they predict a considerably greater X-ray flux than that actually observed in a majority of these sources. This inconsistency may be an indication that the particles are energized during the Compton scattering process, since the X-ray emission is produced by the lowest energy electrons, whose density may be relatively small due to the acceleration. Such a situation may arise as a result of resistive field generation in electromagnetic acceleration schemes, which we here explore. A key feature of this model is the assumed existence of a current associated with the azimuthal component B_phi of the underlying magnetic field by a slight imbalance in the energy distributions of outwardly moving, relativistic electrons and protons produced at the disk surface via shock acceleration. The generation of an electric field (via magnetic field line reconnection) is thus required to maintain the current in the presence of a resistivity induced by the radiative drag on the relativistic electrons. We show that the resulting spectrum can exhibit a significant deficit of X-rays compared with gamma-rays. In addition, due to the uni-directional flow of the current associated with B_phi, this model would predict that the electrons are energized relative to the protons in the outflow only on one side of the disk. They should be decelerated on the reverse side. As such, we would anticipate that any given blazar should have a ~50% probability of being gamma-bright, which appears to be consistent with the observed ratio.

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arXiv1998

A Model of the EGRET Source at the Galactic Center: Inverse Compton Scattering Within Sgr A East and its Halo

Continuum low-frequency radio observations of the Galactic Center reveal the presence of two prominent radio sources, Sgr A East and its surrounding Halo, containing non-thermal particle distributions with power-law indices around 2.5-3.3 and 2.4, respectively. The central 1-2 pc region is also a source of intense (stellar) UV and (dust-reprocessed) far-IR radiation that bathes these extended synchrotron-emitting structures. A recent detection of gamma-rays (2EGJ1746-2852) from within around 1 degree of the Galactic Center by EGRET onboard the Compton GRO shows that the emission from this environment extends to very high energies. We suggest that inverse Compton scatterings between the power-law electrons inferred from the radio properties of Sgr A East and its Halo, and the UV and IR photons from the nucleus, may account for the possibly diffuse gamma-ray source as well. We show that both particle distributions may be contributing to the gamma-ray emission, though their relevant strength depends on the actual physical properties (such as the magnetic field intensity) in each source. If this picture is correct, the high-energy source at the Galactic Center is extended over several arcminutes, which can be tested with thenext generation of gamma-ray and hard X-ray missions.

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arXiv1998

Stellar Gas Flows Into A Dark Cluster Potential At The Galactic Center

The evidence for the presence of a concentration of dark matter at the Galactic center is now very compelling. There is no question that the stellar and gas kinematics within 0.01 pc is dominated by under-luminous matter in the form of either a massive black hole, a highly condensed distribution of stellar remnants, or a more exotic source of gravity. The unique, compact radio source Sgr A* appears to be coincident with the center of this region, but its size (less than 3x10^14 cm at lambda=1.35cm) is still significantly smaller than the current limiting volume enclosing this mass. Sgr A* may be the black hole, if the dark matter distribution is point-like. If not, we are left with a puzzle regarding its nature, and a question of why this source should be so unique and lie only at the Galactic center. Here, we examine an alternative to the black hole paradigm---that the gravitating matter is a condensed cluster of stellar remnants---and study the properties of the Galactic center wind flowing through this region. Some of this gas is trapped in the cluster potential, and we study in detail whether this hot, magnetized gas is in the proper physical state to produce Sgr A*'s spectrum. We find that at least for the Galactic center environment, the temperature of the trapped gas never attains the value required for significant GHz emission. In addition, continuum (mostly bremsstrahlung) emission at higher frequencies is below the current measurements and upper limits for this source. We conclude that the cluster potential is too shallow for the trapped Galactic center wind to account for Sgr A*'s spectrum, which instead appears to be produced only within an environment that has a steep-gradient potential like that generated by a black hole.

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arXiv1998

X-rays From Magnetic Flares In Cygnus X-1: A Unified Model With Seyfert Galaxies

Some recent work has shown that the spectrum of Seyfert 1 Galaxies is very similar to that of several Galactic Black Hole Candidates (GBHCs) in their hard state. However, the smallness of the observed reflection component in Cygnus X-1, in addition to other constraints, seem to rule out the two-phase model (otherwise successful in the case of Seyfert Galaxies) for GBHCs. Here, we show that the latter conclusion is based on a number of key assumptions that probably are not valid when the overlying corona is patchy, e.g., when it is comprised of localized magnetic flares above the disk. We show that in GBHCs the energy deposited by the X-rays cannot be re-radiated fast enough to maintain equilibrium, unless the X-ray skin heats up to the Compton temperature, at which point the gas is mostly ionized. This leads to a substantially reduced cooling rate for the active regions due to the correspondingly smaller number of re-injected low-energy photons. We model this effect by introducing a transition layer situated between the corona and the cold disk, and find that the resulting spectrum is harder than that obtained with the standard (and unrealistic) two-phase model. We apply this model to Cygnus X-1 and show that it can account for its observed spectrum. This analysis therefore seems to provide a consistentpicture for both Seyfert Galaxies and GBHCs within the same framework, with differences arising due to the changing physical conditionsin the two categories of sources, rather than due to an ad hoc variation of the model parameters.

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arXiv1997

Magnetic Flares and State Transitions in Galactic Black Hole and Neutron Star Systems

We here examine the conditions of the two-phase disk model under which magnetic flares arise above the cold accretion disk due to magnetic buoyancy and produce X-rays via Comptonization of the disk's soft radiation. We find that the disk's ability to produce strong magnetic flares is substantially diminished in its radiation dominated regions due to the diffusion of radiation into the magnetic flux tubes. Using a simplified, yet physically self-consistent, model that takes this effect into account, we show that the hard X-ray spectrum of some GBHCs can be explained as the X-ray emission by magnetic flares only when the disk's bolometric luminosity is a relatively small fraction ($\sim$ 5%) of the Eddington value, $L_{Edd}$. Further, we compute the hard ($20-200$ keV) and soft ($1-20$ keV) X-ray power as a function of the disk's luminosity, and find an excellent agreement with the available data for GBHC transient and persistent sources. We conclude that the observed high-energy spectrum of stellar-sized accretion disk systems can be explained by Comptonization of the disk's soft radiation by the hot gas trapped inside the magnetic flares when the luminosity falls in the range $\sim 10^{-3}-10^{-1}\times L_{Edd}$. For higher luminosities, another emission mechanism must be at work. For lower luminosities, the X-ray emissivity may still be dominated by magnetic flares, but this process is more likely to be thermal or non-thermal bremstrahlung, so that the X-ray spectrum below $\sim 10^{-3}L_{Edd}$ may be quite distinct from the typical hard spectrum for higher luminosities.

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arXiv1997

Physical Constraints on, and a Model for, the Active Regions in Seyfert Galaxies

We discuss several physical constraints on the nature of the Active Regions (AR) in Seyfert 1 Galaxies, and show that a plausible model consistent with these constraints is one in which the ARs are magnetically confined and ``fed''. The unique X-ray index of these sources points to a large compactness parameter ($l\gg 1$). This, together with the conditions required to account for the observed optical depth being close to unity, suggests that the magnetic energy density in the AR should be comparable to the equipartition value in the accretion disk, and that it should be released in a flare-like event above the surface of the cold accretion disk. We consider the various issues pertaining to magnetic flares and attempt to construct a coherent picture, including a reason for the optical depth in the AR being $\sim 1$, and an understanding of the characteristics of the X-ray reflection component and the power density spectra associated with this high-energy emission.

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arXiv1997

Magnetic Flares and the Observed Optical Depth in Seyfert Galaxies

We here consider the pressure equilibrium during an intense magnetic flare above the surface of a cold accretion disk. Under the assumption that the heating source for the plasma trapped within the flaring region is an influx of energy transported inwards with a group velocity close to $c$, e.g., by magnetohydrodynamic waves, this pressure equilibrium can constrain the Thomson optical depth $τ_T$ to be of order unity. We suggest that this may be the reason why $τ_T\sim 1$ in Seyfert Galaxies. We also consider whether current data can distinguish between the spectrum produced by a single X-ray emitting region with $τ_T\sim 1$ and that formed by many different flares spanning a range of $τ_T$. We find that the current observations do not yet have the required energy resolution to permit such a differentiation. Thus, it is possible that the entire X-ray/$γ$-ray spectrum of Seyfert Galaxies is produced by many independent magnetic flares with an optical depth $0.5<τ_T<2$.

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arXiv1997

Self-Consistent Fokker-Planck Treatment Of Particle Distributions in Astrophysical Plasmas

High-energy, multi-component plasmas in which pair creation and annihilation, lepton-lepton scattering, lepton-proton scattering, and Comptonization all contribute to establishing the particle and photon distributions, are present in a broad range of compact astrophysical objects. Earlier work has included much of the microphysics needed to account for electron-photon and electron-proton interactions, but little has been done to handle the redistribution of the particles as a result of their Coulomb interaction with themselves in an arbitrary case. Our goal here is to use a Fokker-Planck approach in order to develop a fully self-consistent theory for the interaction of arbitrarily distributed particles and radiation to arrive at an accurate representation of the high-energy plasma in these sources. We conduct several tests representative of two dominant segments of parameter space and discuss physical implications of the non-Maxwellian distribution function. Approximate analytical forms for the electron distribution in the case of strongly non-Maxwellian plasmas are given.

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arXiv1996

Accretion Disk Evolution With Wind Infall I. General Solution and Application to Sgr A*

The evolution of an accretion disk can be influenced significantly by the deposition of mass and angular momentum by an infalling Bondi-Hoyle wind. Such a mass influx impacts the long-term behavior of the disk by providing additional sources of viscosity and heating. In this paper, we derive and solve the disk equations when these effects are taken into account. We present a survey of models with various wind configurations and demonstrate that the disk spectrum may then differ substantially from that of a standard alpha-disk. In particular, it is likely that a wind-fed disk has a significant infrared bump due to the deposition of energy in its outer region. We apply some of the results of our calculations to the Galactic Center black hole candidate Sgr A* and show that if a fossil disk is present in this source, it must have a very low viscosity parameter (alpha<10^-4) and the Bondi-Hoyle wind must be accreting with a very high specific angular momentum to prevent it from circularizing in the inner disk region where its impact would be most noticeable.

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arXiv1993

Reconciliation of the Disparate Gamma-Ray Burst Catalogs in the Context of a Cosmological Source Distribution

It is well known that Gamma-ray Burst spectra often display a break at energies $\simless 400$ keV, with some exceptions extending to several MeV.\ \ Modeling of a cosmological source population is thus non-trivial when comparing the catalogs from instruments with different energy windows since this spectral structure is redshifted across the trigger channels at varying levels of sensitivity. We here include this important effect in an attempt to reconcile all the available data sets and show that a model in which bursts have a ``standard'' spectral break at $300$ keV and occur in a population uniformly distributed in a \qo $=1/2$ universe with no evolution can account very well for the combined set of observations. We show that the source population cannot be truncated at a minimum redshift $z_{min}$ beyond $\sim 0.1$, and suggest that a simple follow-on instrument to BATSE, with the same trigger window, no directionality and 18 times better sensitivity might be able to distinguish between a \qo$=0.1$ and a \qo$=0.5$ universe in $3$ years of full sky coverage, provided the source population has no luminosity evolution.

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arXiv1993

A Self-Consistent Model For The Long-Term Gamma-Ray Spectral Variability of Cygnus X-1

The long-term transitions of the black hole candidate Cygnus X-1 (between the states gamma_1, gamma_2, and gamma_3) include the occasional appearance of a strong ~ MeV bump (gamma_1), whose strength appears to be anti-correlated with the continuum flux (~ 400 keV) due to the Compton upscattering of cold disk photons by the inner, hot corona. We develop a self-consistent disk picture that accounts naturally for these transitions and their corresponding spectral variations. We argue that the bump is due to the self-Comptonization of bremsstrahlung photons emitted predominantly near the plane of the corona itself. Our results suggest that a decrease by a factor of approx 2 in the viscosity parameter alpha is responsible for quenching this bump and driving the system to the gamma_2 state, whereas a transition from gamma_2 to gamma_3 appears to be induced by an increase of about 25 % in the accretion rate Mdot. In view of the fact that most of the transitions observed in this source seem to be of the gamma_2 to gamma_3 variety, we conclude that much of the long term gamma-ray spectral variability in Cygnus X-1 is due to these small fluctuations in Mdot. The unusual appearance of the gamma_1 state apparently reflects a change in the dissipative processes within the disk.

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