arXiv subjects

Ramesh Narayan

Publications and source records attributed to Ramesh Narayan.

At least 235 records · Page 13Linked to original sources

arXiv2004

An Accretion-Jet Model for Black Hole Binaries: Interpreting the Spectral and Timing Features of XTE J1118+480

Multi-wavelength observations of the black hole X-ray binary XTE J1118+480 have offered abundant spectral and timing information about the source, and have thus provided serious challenges to theoretical models. We propose a coupled accretion-jet model to interpret the observations. We model the accretion flow as an outer standard thin accretion disk truncated at a transition radius by an inner hot accretion flow. The accretion flow accounts for the observed UV and X-ray emission, but it substantially under-predicts the radio and infrared fluxes, even after we allow for nonthermal electrons in the hot flow. We attribute the latter components to a jet. We model the jet emission by means of the internal shock scenario which is widely employed for gamma-ray bursts. In our accretion-jet model of XTE J1118+480, the jet dominates the radio and infrared emission, the thin disk dominates the UV emission, and the hot flow produces most of the X-ray emission. The optical emission has contributions from all three components: jet, thin disk, and hot flow. The model qualitatively accounts for timing features, such as the intriguing positive and negative time lags between the optical and X-ray emission, and the wavelength-dependent variability amplitude.

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arXiv2004

Low-Luminosity Accretion in Black Hole X-ray Binaries and Active Galactic Nuclei

At luminosities below a few percent of Eddington, accreting black holes switch to a hard spectral state which is very different from the soft blackbody-like spectral state that is found at higher luminosities. The hard state is well-described by a two-temperature, optically thin, geometrically thick, advection-dominated accretion flow (ADAF) in which the ions are extremely hot (up to $10^{12}$ K near the black hole), the electrons are also hot ($\sim10^{9-10.5}$ K), and thermal Comptonization dominates the X-ray emission. The radiative efficiency of an ADAF decreases rapidly with decreasing mass accretion rate, becoming extremely low when a source reaches quiescence. ADAFs are expected to have strong outflows, which may explain why relativistic jets are often inferred from the radio emission of these sources. It has been suggested that most of the X-ray emission also comes from a jet, but this is less well established.

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arXiv2004

Origin of the Soft Excess in X-ray Pulsars

The spectra of many X-ray pulsars show, in addition to a power law, a low-energy component that has often been modeled as a blackbody with kT ~ 0.1 keV. However the physical origin of this soft excess has remained a mystery. We examine a sample of well-studied, bright X-ray pulsars, which have been observed using ROSAT, ASCA, Ginga, RXTE, BeppoSAX, Chandra, and XMM-Newton. In particular we consider the Magellanic Cloud pulsars SMC X-1, LMC X-4, XTE J0111.2-7317, and RX J0059.2-7138 and the Galactic sources Her X-1, 4U 1626-67, Cen X-3, and Vela X-1. We show that the soft excess is a very common if not ubiquitous feature intrinsic to X-ray pulsars. We evaluate several possible mechanisms for the soft emission, using theoretical arguments as well as observational clues such as spectral shapes, eclipses, pulsations of the soft component, and superorbital modulation of the source flux. We find that reprocessing of hard X-rays from the neutron star by the inner region of the accretion disk is the only process that can explain the soft excess in all the pulsars with Lx > 10^38 ergs/s. Other mechanisms, such as emission from diffuse gas in the system, are important in less luminous objects.

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arXiv2004

On the Lack of Thermal Emission from the Quiescent Black Hole XTE J1118+480: Evidence for the Event Horizon

A soft component of thermal emission is very commonly observed from the surfaces of quiescent, accreting neutron stars. We searched with Chandra for such a surface component of emission from the dynamical black-hole candidate XTE J1118+480 (= J1118), which has a primary mass of about 8 solar masses. None was found, as one would expect if the compact X-ray source is a bona fide black hole that possesses an event horizon. The X-ray spectrum of J1118 is well-fitted by a simple power-law model. In our search for a thermal component, we fitted our Chandra data to a power-law model (with slope and column density fixed) plus a series of nine hydrogen-atmosphere models with radii ranging from 9/8 to 2.8 Schwarzschild radii. For the more compact models, we included the important effect of self-irradiation of the atmosphere. Because of the extremely low column density to J1118, we obtained very strong limits on a hypothetical thermal source: kT(infinity) < 0.011 keV and L(infinity) < 9.4E30 erg/s (99% confidence level). In analogy with neutron stars, we examine two possible sources of thermal radiation from a hypothetical surface of J1118: deep crustal heating and accretion in quiescence. We conclude that if J1118 possesses a surface composed of normal nuclear matter, we would have detected thermal radiation from this surface. The absence of thermal radiation indicates that J1118 is a true black hole with an event horizon.

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arXiv2004

The Nature of X-ray Bright Optically Normal Galaxies

Recent X-ray surveys by {\it Chandra} and {\it XMM-Newton} have revealed a population of X-ray bright, optically normal galaxies (XBONGs) at moderate redshifts. We propose that many XBONGs are powered by an inner radiatively inefficient accretion flow (RIAF) plus an outer radiatively efficient thin accretion disk. The absence of optical/UV activity in XBONGs is explained by the truncation of the thin disk near the black hole, while the relatively strong X-ray emission is explained as inverse Compton emission from the hot RIAF. As an example, we show that the spectra of two XBONGs can be fit fairly well with such a model. By comparing these two sources to other accreting black holes, we argue that XBONGs are intermediate in their characteristics between distant luminous active galactic nuclei and nearby low-luminosity nuclei.

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arXiv2004

Conduction and Turbulent Mixing in Galaxy Clusters

We discuss hydrostatic models of galaxy clusters in which heat diffusion balances radiative cooling. We consider two different sources of diffusion, thermal conduction and turbulent mixing, parameterized by dimensionless coefficients, f and alpha_mix, respectively. Models with thermal conduction give reasonably good fits to the density and temperature profiles of several cooling flow clusters, but some clusters require unphysically large values of f>1. Models with turbulent mixing give good fits to all clusters, with reasonable values of alpha_mix ~ 0.01-0.03. Both types of models are found to be essentially stable to thermal perturbations. The mixing model reproduces the observed scalings of various cluster properties with temperature, and also explains the entropy floor seen in galaxy groups.

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arXiv2004

Constraining Alternate Models of Black Holes: Type I X-ray Bursts on Accreting Fermion-Fermion and Boson-Fermion Stars

The existence of black holes remains open to doubt until other conceivable options are excluded. With this motivation, we consider a model of a compact star in which most of the mass consists of dark particles of some kind, and a small fraction of the mass is in the form of ordinary nucleonic gas. The gas does not interact with the dark matter other than via gravity, but collects at the center as a separate fermionic fluid component. Depending on whether the dark mass is made of fermions or bosons, the objects may be called fermion-fermion stars or boson-fermion stars, respectively. For appropriate choices of the mass of the dark matter particles, these objects are viable models of black hole candidates in X-ray binaries. We consider models with a dark mass of 10 solar masses and a range of gas mass from 10^{-6} to nearly one solar mass, and analyse the bursting properties of the models when they accrete gas. We show that all the models would experience thermonuclear Type I X-ray bursts at appropriate mass accretion rates. Since no Type I bursts have been reported from black hole candidates, the models are ruled out. The case for identifying black hole candidates in X-ray binaries as true black holes is thus strengthened.

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arXiv2004

On The Nature of the Variable Infrared Emission from Sgr A*

Recent infrared (IR) observations of the center of our Galaxy indicate that the supermassive black hole source Sgr A* is strongly variable in the IR. The timescale for the variability, $\sim 30$ min, is comparable to that of the X-ray flares observed by {\em Chandra} and {\em XMM}, suggesting a common physical origin. In this paper, we investigate the nature of the IR emission in the context of models recently proposed to interpret the X-ray flares. We show that the IR emission in Sgr A* can be well explained by nonthermal synchrotron emission if a small fraction of the electrons in the innermost region of the accretion flow around the black hole are accelerated {into a broken power-law distribution}, perhaps due to transient events such as magnetic reconnection. The model predicts differences in the variability amplitudes of flares in the IR and X-rays, in general agreement with observations. It also predicts that the IR emission should be linearly polarized, as has indeed been observed during one epoch. IR and X-ray flares analogous to those observed in Sgr A* may be detectable from other accreting SMBHs, provided $L \lsim 10^{-8} L_{EDD}$; at higher luminosities the flaring emission is dominated by thermal synchrotron-self Compton emission, which is likely to be less variable.

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arXiv2003

Evidence for the Black Hole Event Horizon

Astronomers have discovered many candidate black holes in X-ray binaries and in the nuclei of galaxies. The candidate objects are too massive to be neutron stars, and for this reason they are considered to be black holes. While the evidence based on mass is certainly strong, there is no proof yet that any of the objects possesses the defining characteristic of a black hole, namely an event horizon. Type I X-ray bursts, which are the result of thermonuclear explosions when gas accretes onto the surface of a compact star, may provide important evidence in this regard. Type I bursts are commonly observed in accreting neutron stars, which have surfaces, but have never been seen in accreting black hole candidates. It is argued that the lack of bursts in black hole candidates is compelling evidence that these objects do not have surfaces. The objects must therefore possess event horizons.

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arXiv2003

Quasi-Periodic Oscillations from Rayleigh-Taylor and Kelvin-Helmholtz Instability at a Disk-Magnetosphere Interface

We consider the interface between an accretion disk and a magnetosphere surrounding the accreting mass. We argue that such an interface can occur not only with a magnetized neutron star but also sometimes with an unmagnetized neutron star or a black hole. The gas at the magnetospheric interface is generally Rayleigh-Taylor unstable and may also be Kelvin-Helmholtz unstable. Because of these instabilities, modes with low azimuthal wavenumbers m are expected to grow to large amplitude. It is proposed that the resulting nonaxisymmetric structures contribute to the high frequency quasi-periodic oscillations that have been seen in neutron-star and black-hole X-ray binaries. The mode oscillation frequencies are calculated to be approximately equal to m Ω_m, where Ω_m is the angular velocity of the accreting gas at the magnetospheric radius. Thus, mode frequencies should often be in the approximate ratio 1:2:3, etc. If the pressure of the gas in the disk is not large, then the m = 1 mode will be stable. In this case, the mode frequencies should be in the approximate ratio 2:3, etc. There is some observational evidence for such simple frequency ratios.

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arXiv2003

Magnetically Arrested Disk: An Energetically Efficient Accretion Flow

We consider an accretion flow model originally proposed by Bisnovatyi-Kogan & Ruzmaikin (1974), which has been confirmed in recent 3D MHD simulations. In the model, the accreting gas drags in a strong poloidal magnetic field to the center such that the accumulated field disrupts the axisymmetric accretion flow at a relatively large radius. Inside the disruption radius, the gas accretes as discrete blobs or streams with a velocity much less than the free-fall velocity. Almost the entire rest mass energy of the gas is released as heat, radiation and mechanical/magnetic energy. Even for a non-rotating black hole, the efficiency of converting mass to energy is of order 50% or higher. The model is thus a practical analog of an idealized engine proposed by Geroch and Bekenstein.

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arXiv2003

Turbulent Mixing in Clusters of Galaxies

We present a spherically-symmetric, steady-state model of galaxy clusters in which radiative cooling from the hot gas is balanced by heat transport through turbulent mixing. We assume that the gas is in hydrostatic equilibrium, and describe the turbulent heat diffusion by means of a mixing length prescription with a dimensionless parameter alpha_mix. Models with alpha_mix ~ 0.01-0.03 yield reasonably good fits to the observed density and temperature profiles of cooling core clusters. Making the strong simplification that alpha_mix is time-independent and that it is roughly the same in all clusters, the model reproduces remarkably well the observed scalings of X-ray luminosity, gas mass fraction and entropy with temperature. The break in the scaling relations at kT \~ 1-2 keV is explained by the break in the cooling function at around this temperature, and the entropy floor observed in galaxy groups is reproduced naturally.

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arXiv2003

Thermonuclear Stability of Material Accreting onto a Neutron Star

We present a global linear stability analysis of nuclear fuel accumulating on the surface of an accreting neutron star and we identify the conditions under which thermonuclear bursts are triggered. The analysis reproduces all the recognized regimes of hydrogen and helium bursts, and in addition shows that at high accretion rates, near the limit of stable burning, there is a regime of ``delayed mixed bursts'' which is distinct from the more usual ``prompt mixed bursts.'' In delayed mixed bursts, a large fraction of the fuel is burned stably before the burst is triggered. Bursts thus have longer recurrence times, but at the same time have somewhat smaller fluences. Therefore, the parameter alpha, which measures the ratio of the energy released via accretion to that generated through nuclear reactions in the burst, is up to an order of magnitude larger than for prompt bursts. This increase in alpha near the threshold of stable burning has been seen in observations. We explore a wide range of mass accretion rates, neutron star radii and core temperatures, and calculate a variety of burst properties. From a preliminary comparison with data, we suggest that bursting neutron stars may have hot cores, with T_{core} >~ 10^{7.5} K, consistent with interior cooling via the modified URCA or similar low-efficiency process, rather than T_{core} ~ 10^7 K, as expected for the direct URCA process. There is also an indication that neutron star radii are somewhat small <~ 10 km. Both of these conclusions need to be confirmed by comparing more careful calculations with better data.

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arXiv2003

Nonthermal Electrons in Radiatively Inefficient Accretion Flow Models of Sagittarius A*

We investigate radiatively inefficient accretion flow models for Sgr A*, the supermassive black hole in our Galactic Center, in light of new observational constraints. Confirmation of linear polarization in the submm emission argues for accretion rates much less than the canonical Bondi rate. We consider models with low accretion rates and calculate the spectra produced by a hybrid electron population, consisting of both thermal and nonthermal particles. The thermal electrons produce the submm emission and can account for its linear polarization properties. As noted in previous work, the observed low-frequency radio spectrum can be explained if a small fraction ($\approx 1.5$%) of the electron thermal energy resides in a soft power-law tail. In the innermost region of the accretion flow, turbulence and/or magnetic reconnection events may occasionally accelerate a fraction of the electrons into a harder power-law tail. We show that the synchrotron emission from these electrons, or the Compton up-scattering of synchrotron photons by the same electrons, may account for the X-ray flares observed by {\it Chandra}.

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arXiv2003

Thermal Instability in Clusters of Galaxies with Conduction

We consider a model of galaxy clusters in which the hot gas is in hydrostatic equilibrium and maintains energy balance between radiative cooling and heating by thermal conduction. We analyze the thermal stability of the gas using a Lagrangian perturbation analysis. For thermal conductivity at the level of 20-40 % of Spitzer conductivity, consistent with previous estimates for cluster gas, we find that the growth rate of the most unstable global radial mode is ~ 6-9 times lower than the growth rate of local isobaric modes at the cluster center in the absence of conduction. The growth time in typical clusters is ~ 2-5 Gyr, which is comparable to the time since the last major merger episode, when the gas was presumably well mixed. Thus, we suggest that thermal instability is not dynamically significant in clusters, provided there is an adequate level of thermal conduction. On the other hand, if the heating of the gas is not the result of thermal conduction or any other diffusive process such as turbulent mixing, then the thermal instability has a growth time under a Gyr in the central regions of the cluster and is a serious threat to equilibrium. We also analyze local nonradial modes and show that the Lagrangian technique leads to the same dispersion relation as the Eulerian approach, provided that clusters are initially in strict thermal equilibrium. Because cluster gas is convectively stable, nonradial modes always have a smaller growth rate than equivalent radial modes.

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arXiv2003

Bondi Accretion and the Problem of the Missing Isolated Neutron Stars

A large number of neutron stars (NSs), ~10^9, populate the Galaxy, but only a tiny fraction of them is observable during the short radio pulsar lifetime. The majority of these isolated NSs, too cold to be detectable by their own thermal emission, should be visible in X-rays as a result of accretion from the interstellar medium. The ROSAT all sky survey has however shown that such accreting isolated NSs are very elusive: only a few tentative candidates have been identified, contrary to theoretical predictions that up to several thousands should be seen. We suggest that the fundamental reason for this discrepancy lies in the use of the standard Bondi formula to estimate the accretion rates. We compute the expected source counts using updated estimates of the pulsar velocity distribution, realistic hydrogen atmosphere spectra, and a modified expression for the Bondi accretion rate as suggested by recent MHD simulations, and supported by direct observations in the case of accretion around supermassive black holes in nearby galaxies and in our own. We find that, whereas the inclusion of atmospheric spectra partly compensates for the reduction in the counts due to the higher mean velocities of the new distribution, the modified Bondi formula dramatically suppresses the source counts. The new predictions are consistent with a null detection at the ROSAT sensitivity.

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arXiv2003

Self-Similar Hot Accretion onto a Spinning Neutron Star: Matching the Outer Boundary Conditions

Medvedev & Narayan have described a hot accretion flow onto a spinning neutron star in which the gas viscously brakes the spin of the star. Their self-similar solution has the surprising property that the density, temperature and angular velocity of the gas at any radius are completely independent of the outer boundary conditions. Hence, the solution cannot be matched to a general external medium. We resolve this paradoxical situation by showing that there is a second self-similar solution which bridges the gap between the original solution and the external medium. This new solution has an extra degree of freedom which permits it to match general outer boundary conditions. We confirm the main features of the analytical results with a full numerical solution.

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arXiv2003

Multiwavelength Spectrum of the Black Hole XTE J1118+480 in Quiescence

We present an X-ray/UV/optical spectrum of the black hole primary in the X-ray nova XTE J1118+480 in quiescence at L_x = 4 x 10^{-9} of the Eddington luminosity. The Chandra, HST and MMT spectroscopic observations were performed simultaneously on 2002 January 12 UT. Because this 4.1-hr binary is located at b = 62 deg, the transmission of the ISM is very high (e.g., 70% at 0.3 keV). We present many new results for the quiescent state, such as the first far-UV spectrum and evidence for an 0.35 mag orbital modulation in the near-UV flux. However, the centerpiece of our work is the multiwavelength spectrum of XTE J1118+480, which we argue represents the canonical spectrum of a stellar-mass black hole radiating at L_x = 4 x 10^{-8.5} of the Eddington luminosity. This spectrum is comprised of two apparently disjoint components: a hard X-ray spectrum with a photon index Gamma = 2.02 +/- 0.16, and an optical/UV continuum that resembles a 13,000 K disk blackbody spectrum punctuated by several strong emission lines. We present a model of the source in which the accretion flow has two components: (1) an X-ray-emitting interior region where the flow is advection-dominated, and (2) a thin, exterior accretion disk with a truncated inner edge (R_tr ~ 10^4 Schwarzschild radii) that is responsible for the optical/UV spectrum. For D = 1.8 kpc, the luminosity of the X-ray component is L_x = 3.5 x 10^{30} erg/s (0.3-7 keV); the bolometric luminosity of the optical/UV component is 20 times greater.

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