arXiv subjects

R. Narayan

Publications and source records attributed to R. Narayan.

31 records · Page 2Linked to original sources

arXiv2001

The Coalescence Rate of Double Neutron Star Systems

We estimate the coalescence rate of close binaries with two neutron stars (NS) and discuss the prospects for the detection of NS-NS inspiral events by ground-based gravitational-wave observatories, such as LIGO. We derive the Galactic coalescence rate using the observed sample of close NS-NS binaries (PSR B1913+16 and PSR B1534+12) and examine in detail each of the sources of uncertainty associated with the estimate. Specifically, we investigate (i) the dynamical evolution of NS-NS binaries in the Galactic potential and the vertical scale height of the population, (ii) the pulsar lifetimes, (iii) the effects of the faint end of the radio pulsar luminosity function and their dependence on the small number of observed objects, (iv) the beaming fraction, and (v) the extrapolation of the Galactic rate to extragalactic distances expected to be reachable by LIGO. We find that the dominant source of uncertainty is the correction factor (up to about 200) for faint (undetectable) pulsars. All other sources are much less important, each with uncertainty factors smaller than 2. Despite the relatively large uncertainty, the derived coalescence rate is approximately consistent with previously derived upper limits, and is more accurate than rates obtained from population studies. We obtain a most conservative lower limit for the LIGO II detection rate of 2 events per year. Our upper limit on the detection rate lies between 300 to more than 1000 events per year.

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arXiv1999

Low radiative efficiency accretion in the nuclei of elliptical galaxies

The discovery of hard, power-law X-ray emission from a sample of six nearby elliptical galaxies, including the dominant galaxies of the Virgo,Fornax and Centaurus clusters (M87, NGC 1399 and NGC 4696, respectively), and NGC 4472, 4636 and 4649 in the Virgo cluster, has important implications for the study of quiescent supermassive black holes. We describe how the broad band spectral energy distributions of these galaxies, which accrete from their hot gaseous halos at rates comparable to their Bondi rates, can be explained by low-radiative efficiency accretion flows in which a significant fraction of the mass, angular momentum and energy is removed from the flows by winds. The observed suppression of the synchrotron component in the radio band and the systematically hard X-ray spectra, which are interpreted as thermal bremsstrahlung emission, support the conjecture that significant mass outflow is a natural consequence of systems accreting at low-radiative efficiencies. We briefly discuss an alternative model for the X-ray emission, namely that it is due to nonthermal synchrotron-self Compton processes in the accretion flow, or wind. This appears to require implausibly weak magnetic fields. Emission from a collimated jet viewed off axis should be distinguishable from the bremsstrahlung model by variability and thermal line emission studies. We argue that the difference in radiative efficiency between the nuclei of spiral and elliptical galaxies arises from the different manner in which interstellar gas is fed into the nuclei. In ellipticals, matter fed from the hot (slowly cooling) ISM is likely to be highly magnetized and with low specific angular momentum, both of which favor low-radiative efficiency accretion solutions and possibly the formation of the observed jets.

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arXiv1998

On the Origin of the Hard X-Ray Background

Several models for the hard X-ray Background (XRB) suggest that it is due to the emission from heavily obscured AGN. Recent studies have revealed the presence of a new population of hard X-ray sources which must contribute significantly to the XRB. However, whether the majority of these sources are obscured AGN or some other type of object at present remains unclear. Here, we further examine the possibility that a significant fraction of the XRB comes from a population of galaxies undergoing advection-dominated accretion in the high-\mdot regime and thus produce intrinsically hard spectra. When the accretion rate is close to its critical value, above which an advection-dominated accretion flow (ADAF) no longer exists, the major contribution to X-ray emission is due to inverse Compton scattering of the soft seed photons produced by cyclo-synchrotron radiation. In this regime, the resulting ADAF spectra are relatively hard with a fairly constant X-ray spectral index α\sim 0.2-0.4 and a spectral cut-off at \sim 200 keV. We show that the integrated emission from such sources can provide a good fit to the hard (> 2 keV) X-ray background, provided that the spectrum is dominated by the contribution from objects located at redshifts z \sim 2-3. The model requires most of the contribution to the XRB to be due to objects accreting at the critical rate.

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arXiv1997

Astrophysical Evidence for Black Hole Event Horizons

Astronomers have discovered many potential black holes in X-ray binaries and galactic nuclei. These black holes are usually identified by the fact that they are too massive to be neutron stars. Until recently, however, there was no convincing evidence that the objects identified as black hole candidates actually have event horizons. This has changed with extensive applications of a class of accretion models for describing the flow of gas onto compact objects; for these solutions, called advection-dominated accretion flows (ADAFs), the black hole nature of the accreting star, specifically its event horizon, plays an important role. We review the evidence that, at low luminosities, accreting black holes in both X-ray binaries and galactic nuclei contain ADAFs rather than the standard thin accretion disk.

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arXiv1997

Black Hole Event Horizons and X-ray Nova Luminosities: Update

Recently, Narayan, Garcia and McClintock showed that there is a difference in the quiescent luminosities of black hole and neutron star x-ray nova: when the quiescent, Eddington scaled luminosities are compared, every black hole x-ray nova (BHXN) is fainter than every the neutron star x-ray nova (NSXN). To this earlier work we add three more BHXN, GRO J1655-40, GRO J0422+32, and 4U1543-47. This increases the significance of the difference between the quiescent luminosities to the >99% level. Given the reasonable assumption that the (Eddington scaled) mass transfer swings in BHXN and NSXN are similar, this difference provides direct evidence for the existence of event horizons in the BHXN: they have larger luminosity swings because the quiescent mass transfer energy is being hidden behind the event horizon.

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arXiv1997

Two-phase spectral modelling of 1E1740.7-2942

Combined ASCA and SIGMA data of 1E1740.7-2942 during the standard state (September 1993 and 1994) were fitted with two-phase models (ISMBB/Poutanen at al. and ADAF/Narayan et al.). In ISMBB's, the radius of the spherical hot (Te = 150 - 200 KeV) corona lies between 200 - 250 km where it joins the classical inner disc. The disc radiates 40 per cent of the total luminosity with 1.7 per cent of the Eddington accretion rate of 10 solar mass black hole. ADAF's need an extra component to reproduce the soft part of the spectrum. However, the origin of the soft excess remains somewhat uncertain, although special care was taken in the background elimination.

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arXiv1997

Advection-Dominated Accretion and the Spectral States of Black Hole X-Ray Binaries: Application to Nova Muscae 1991

We present a self-consistent model of accretion flows which unifies four distinct spectral states observed in black hole X-ray binaries: quiescent, low, intermediate and high states. In the quiescent, low and intermediate states, the flow consists of an inner hot advection-dominated part extending from the black hole horizon to a transition radius and an outer thin disk. In the high state the thin disk is present at all radii. The model is essentially parameter-free and treats consistently the dynamics of the accretion flow, the thermal balance of the ions and electrons, and the radiation processes in the accreting gas. With increasing mass accretion rate, the model goes through a sequence of stages for which the computed spectra resemble very well observations of the four spectral states; in particular, the low-to-high state transition observed in black hole binaries is naturally explained as resulting from a decrease in the transition radius. We also make a tentative proposal for the very high state, but this aspect of the model is less secure. We test the model against observations of the soft X-ray transient Nova Muscae during its 1991 outburst. The model reproduces the observed lightcurves and spectra surprisingly well, and makes a number of predictions which can be tested with future observations.

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arXiv1997

Advection-Dominated Flows and the X-ray Delay in the Outburst of GRO J1655-40

We show that the time delay between the optical and X-ray outbursts of the black-hole soft X-ray transient source GRO J1655-40, observed in April 1996, requires that the accretion flow in this object must consist of two components: a cold outer accretion disk and an extremely hot inner advection-dominated accretion flow (ADAF). In quiescence, the model predicts a spectrum which is in good agreement with observations, with most of the observed radiation coming from the ADAF. By fitting the observed spectrum, we estimate the mass accretion rate of the quiescent system and the transition radius between the disk and the ADAF. We present a detailed numerical simulation of a dwarf-nova type instability in the outer disk. The resulting heat front reaches the ADAF cavity promptly; however, it must then propagate inward slowly on a viscous time scale, thereby delaying the onset of the X-ray flux. The model reproduces the observed optical and X-ray light curves of the April 1996 outburst, as well as the 6-day X-ray delay. Further, the model gives an independent estimate of the quiescent mass accretion rate which is in very good agreement with the rate estimated from fitting the quiescent spectrum. We show that a pure thin disk model without an ADAF zone requires significant tuning to explain the X-ray delay; moreover, such a model does not explain the quiescent X-ray emission of GRO J1655-40.

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arXiv1997

Advection-Dominated Accretion Model of X-Ray Nova Muscae in Outburst

We present a model for the high-low state transition of the X-Ray Nova GS 1124-68 (Nova Muscae 1991) observed by Ginga. The model consists of an advection-dominated accretion flow (ADAF) near the central black hole surrounded by a thin accretion disk. During the rise phase of the outburst, as the mass accretion rate increases, the transition radius between the thin disk and the ADAF moves closer to the center, until the thin disk extends all the way in to the last stable orbit. The transition radius increases again during decline. We reproduce the basic features of the spectra taken during and after outburst, and the light curves in the soft and hard X-ray bands. We estimate that the accretion rate in Nova Muscae decreased exponentially with a time scale $\sim$ 95 days during decline.

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arXiv1996

Mechanisms for the Outbursts of Soft X-ray Transients

We show that the Keplerian thin disk in quiescent Soft X-ray Transients cannot extend down to the last stable orbit around the central black hole. We analyse the properties of the Narayan, McClintock & Yi (1996) model of quiescent Soft X-ray Transients in which the cold Keplerian disk has its inner edge at a large transition radius and transforms to a hot, advection-dominated flow on the inside. We show that outbursts of transient sources could be triggered in this model either by a pure thermal accretion disk instability or by a disk instability generated by an enhanced mass transfer from the stellar companion. Both mechanisms operate in the outer thin disk and could be at work, either in different systems or in the same system at different epochs, depending on the mass transfer rate and the value of the viscosity parameter $α_t$ of the thin disk. We show that the recurrence time between outbursts in SXTs can be explained with values of $α_t$ similar to these required by the dwarf nova disk instability model instead of the unreasonably low values needed in the model in which the thin disks extends down to the last stable orbit. We extend the Narayan, McClintock & Yi (1996) model to the case when the outer disk is non-stationary. We show that such disk is too cold to account for the observed UV flux. This difficulty is common to all models in which the outer disk is assumed to be optically thick.

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arXiv1996

Hot One-Temperature Accretion Flows Around Black Holes

We describe hot, optically-thin solutions for one-temperature accretion disks around black holes. We include cooling by synchrotron, bremsstrahlung, and Comptonization. Our solutions are thermally and viscously stable, with gas temperatures on the order of $T \sim 10^9-10^{10.7}$K. The thermal stability is a direct result of the inclusion of synchrotron cooling. The new solution branch is related to the advection-dominated solution for a two-temperature gas described by Narayan \& Yi (1995b). It is present only for mass accretion rates less than some critical $\dot{M}_{crit}$ which depends on the radius $R$ and viscosity parameter $α$. The solutions are advection-dominated for extremely low values of $\dot{M}$. However, for a range of intermediate accretion rates, the new solutions are both hot ($T \sim 10^{10}$K) and cooling-dominated. Because of this new feature, one-temperature solutions are significantly more luminous than the corresponding two temperature solutions. The radial profile of the new solutions is unusual. The inner parts of the flow are cooling-dominated and have a disk-like geometry, while the outer parts are fully advection-dominated and nearly quasi-spherical.

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arXiv1995

LensMEM: A Gravitational Lens Inversion Algorithm Using the Maximum Entropy Method

We present a new algorithm for inverting poorly resolved gravitational lens systems using the maximum entropy method (MEM). We test the method with simulations and then apply it to an 8 GHz VLA map of the radio ring lens MG1654+134. We model the lens as a singular isothermal sphere embedded in an external shear field and find the critical radius of the lens is $b=0\parcs9820$, the dimensionless shear is $γ=0.0771$, and the position angle of the shear is $θ=100\pdeg8$. These results are consistent with the results obtained by Kochanek (1995) using a complementary inversion algorithm based on Clean.

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arXiv1995

Is the Accretion Flow in NGC 4258 Advection-dominated?

The mass of the central black hole in the active galaxy NGC 4258 (M106) has been measured to be $M=3.6\times10^7\Msun$ (Miyoshi et al. 1995). The Eddington luminosity corresponding to this mass is $L_E=4.5\times10^{45}$ erg s$^{-1}$. By contrast the X-ray luminosity of the nucleus of NGC 4258 between $2-10$ keV is $(4\pm 1)\times10^{40}~{\rm erg\,s^{-1}}$ while the optical/UV luminosity is less than $1.5\times10^{42} ~{\rm erg\,s^{-1}}$. The luminosity of NGC 4258 is therefore extremely sub-Eddington, $L\sim10^{-5}L_E$ in X-rays and $L\sim3\times10^{-4} L_E$ even if we take the maximum optical/UV luminosity. Assuming the usual accretion efficiency of 0.1 would imply accretion rates orders of magnitude lower than in Seyfert galaxies and quasars. We show that the properties of the AGN in NGC 4258 can be explained by an accretion flow in the form of a very hot, optically-thin plasma which advects most of the viscously generated thermal energy into the central black hole and radiates only a small fraction of the energy. In this case the accretion rate in Eddington units could be as high as $\sim 0.16α$, where $α$ is the standard viscosity parameter; and the size of the hot disk should be larger than $\sim 10$ times the Schwarzschild radius. We compare the predictions of this model with observations and discuss its application to other low luminosity AGN.

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