Black Holes in Binaries and Galactic Nuclei Abstracts of the Posters
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Effects of Anisotropy on the Central Dark Mass in NGC 3115 - New Results from Integral Field Spectroscopy

S. Anders
Max-Planck-Institut für Extraterrestrische Physik, Garching bei München, Germany

We present new results on the stellar kinematics and the mass distribution in the nuclear region the galaxy NGC 3115.
Previous investigations based on long slit spectroscopic data postulated the presence of a massive dark object of ~10⁹ solar masses in its center, thus making NGC 3115 one of the most prominent black hole candidates. We will show that using integral field spectroscopy one can much better constrain the rotation and the dispersion field in this galaxy by sampling both spatial dimensions. Including this additional information in our stellar kinematical analysis yields revised and also more secure scientific results, demonstrating the absolute necessity to adress the field of galactic stellar dymanics and mass distribution with integral field spectroscopic techniques.
We obtained our NGC 3115 data using MPE's NIR-field integral spectrometer 3D. The near-IR data are fully consistent with the measured velocity dispersions, LOSVDs, h3 and h4 parameters previously measured using HST/CFHT-SIS. Based on that data we modelled the kinematics of that galaxy and could therefore build a mass model. We will show that a model with spatially anisotropic velocity dispersions can much better fit the observations than such with isotropic assumptions. While the latter one produces central masses of ~10⁹ solar masses, the anisotropic models reduce it to roughly 2 · 10⁷ Msun, resulting in a much lower estimate than previously obtained. Our anisotropic models best reproduce the observed two dimensional forms of the rotation and dispersion fields, and are also consistent with measured Gauss-Hermite coefficients ot the LOSVD. In addition, they lead to self-consistent values of the anisotropy parameter and to plausible distribution functions.
This result forces one to rethink the current mass models and the black hole distribution in galaxies and proves the necessity to involve integral field spectroscopy in this area of research.

Multiwavelength Observations of Cygnus X-1 - Results of a Two Year Monitoring Campaign

C. Brocksopp, R.P.Fender, V.Larionov, V.M.Lyuty, A.E.Tarasov, G.G.Pooley, W.S.Paciesas and P.Roche
Astronomy Centre, University of Sussex, UK

We present the results of a 2.5-year multiwavelength monitoring programme of Cygnus X-1, making use of hard and soft X-ray data, optical spectroscopy, UBVJ HK photometry and radio data. In particular we confirm that the 5.6-day orbital period is apparent in all wavebands and note the existence of a wavelength-dependence to the modulation, in the sense that higher energies reach minimum first.
We also find a strong modulation at a period of 142 ± 7 days, which we suggest is due to precession and/or radiative warping of the accretion disc. Strong modulation of the hard and soft X-ray flux at this long period may not be compatible with simple models of an optically thin accretion flow and corona in the low state. We present the basic components required for more detailed future modelling of the system - including a partially optically thick jet, quasi-continuous in the low state, the base of which acts as the Comptonising corona. In addition, we find that there are a number of flares which appear to be correlated in at least two wavebands and generally in more. We choose two of these flares to study in further detail and find that the hard and soft X-rays are well-correlated in the first and that the soft X-rays and radio are correlated in the second. In general, the optical and infrared show similar behaviour to each other but are not correlated with the X-rays or radio.

The Behavior of the Molecular Gas in the Vicinity of an AGN

A. Eckart, E. Schinnerer, L.J. Tacconi
Max-Planck-Institut für Extraterrestrische Physik, Garching bei München, Germany

A detailed analysis of the distribution and kinematics of the molecular gas in the inner 300 pc of the two nearby Seyfert galaxies NGC 3227 and NGC 1068 indicates that the molecular gas disk is probably warped. The position-velocity diagrams show rising rotation curves at r <= 13 pc and an indication for large enclosed masses of >= 2 · 10⁷ Msun for NGC 3227 and >= 10⁸ Msun for NGC 1068 within the central 25 pc. This is the first time that such a compact central mass concentration has been interferometrically detected using the emission of the cold molecular gas in its vicinity.
The sub-arcsecond mm-interferometric observations of the ¹²CO (2-1) line emission in both nearby Seyfert galaxies revealed complex molecular gas kinematics in the inner 100 pc to 300 pc not consistent with pure circular motion of the gas in the inclined host galaxies. Modeling of the kinematics with elliptical orbits in the plane of the host (representing gas motion in a bar potential) does not reproduce all observed features in a satisfying way. A better match of the data is achieved by circular orbits which are tilted relative to the plane of the host galaxy. This indicates the presence of thin circumnuclear warped gas disks in both sources. In the case of the Seyfert 2 NGC 1068 the varying inclination of the thin molecular gas disk leads to the fact that at a radius of ~ 70 pc the disk is oriented edge-on providing material for the obscuration of the AGN nucleus. The most likely cause for the observed warps is the gas pressure of the ionization cone acting on the gas disk. This mechanism represents an important link between the host galaxy and the AGN itself.

The 1996 Outburst of GRO J1655-40: Disc Irradiation and Enhanced Mass Transfer

A. Esin
Theoretical Astrophysics, Caltech, Pasadena, USA

We show that the 1996 outburst of the X-ray binary transient system GRO 1655-40 can be explained by the standard dwarf-nova type disc instability, followed by an episode of enhanced mass transfer from the secondary. Since the quiescent mass transfer rate in GRO 1655-40 might be close to the stability limit, we argue that irradiation of the secondary during the onset of the outburst driven by the thermal instability in the outer disc can be sufficient to increase the mass transfer rate above the minimum value required for stable accretion. This will then produce a period of constant X-ray emission seen in this system. This scenario can also explain the observed anti-correlation between the optical and X-ray fluxes. We assume that optical emission is produced by irradiation of the warped outer disc by X-rays. A burst of mass from the secondary is likely to decrease the warping of the outer disc, thereby causing the decrease in the observed optical flux while X-ray flux remains constant or even increases, exactly as observed in GRO 1655-40. Finally, the decrease of the disc warping and, therefore, irradiation will cause the disc to become unstable once again, terminating the outburst.

Where are the WR + BH binaries?

C. Foellmi and A.F.J. Moffat
Université de Montréal, Montréal (Québec), Canada

In the framework of current knowledge on massive single star evolution we know that massive single stars, after passing through a Wolf-Rayet (WR) stage, will possibly end their lives as black holes (BH), depending on a few important parameters. Such parameters are initial mass, metallicity (Maeder & Meynet 1994) and rotation (which is currently being included in the models and probably induces enhanced internal mixing). These parameters influence the exact final stage of the single star evolution.
But for massive close binaries our knowledge is less precise. The question is: How often do BHs form and survive in massive binary systems? Three key parameters have to be added to the scenario for single stars: the initial mass ratio, the orbital period of the system, and (but maybe less important) the eccentricity. These new parametres are crucial to understanding the impact of wind-wind collisions and mass transfer via Roche-lobe overflow (RLOF) in massive binaries. According to De Donder et al. (1997) only 1 - 3 % of all WR stars may hide a compact companion (CC = neutron star or BH). Their theory also predicts the existence of ``weird'' WR stars, i.e. WR stars with a CC at their centers, resulting from binary evolution in which the primary exploded as a SN to a CC and spiraled in to the secondary. These predictions hold for our Galaxy. But what is the case for the Magellanic Clouds, where the binary frequency is expected to be much larger (alone from RLOF: 50% for the LMC and 100% for the SMC, Maeder & Meynet 1994)? Can one also expect significantly more WR + BH systems there?
We propose here to search for and study WNE binaries among the WR Magellanic population, to probe the question of the formation of black holes (and more generally compact companions) in massive stellar binary systems.

The Formation of Broad Line Clouds in the Accretion Shocks of Active Galactic Nuclei

M. Fromerth
University of Arizona, Tuscon, USA

Recent work on the gas dynamics in the Galactic Center has improved our understanding of the accretion processes in galactic nuclei, particularly with regard to properties such as the specific angular momentum distribution, density and temperature of the inflowing plasma. With the appropriate extrapolation of the physical conditions, this information can be valuable in trying to determine the origin of the Broad Line Region (BLR) in Active Galactic Nuclei (AGNs).
Winds accreting onto a central black hole are subjected to several disturbances capable of producing shocks, including a Bondi-Hoyle flow, stellar wind-wind collisions, and turbulence. Shocked gas is initially compressed and heated out of thermal equilibrium with the ambient radiation field; a cooling instability sets in as the gas is cooled via inverse-Compton and bremsstrahlung processes. If the cooling time is less than the dynamical flow time through the shock region, the gas may clump to form the clouds responsible for broad line emission seen in many AGN spectra.
The most viable model incorporates local production of clouds in turbulent shocks throughout the accretion flow region. Clouds produced by this process display the correct range of physical parameters and velocity fields seen in broad emission lines. Very importantly, this distribution agrees with the results of reverberation studies, in which it seen that the central line peak (due to infalling gas at large radii) responds slower to continuum changes than the line wings, which originate in the faster moving, circularized clouds at smaller radii.

Black Hole Formation in Dark Matter Halos

J.J. Gracia Calvo and M. Camenzind
Landessternwarte Königstuhl, Heidelberg, Germany

The cores of active galaxies are the most luminous objects in the universe. We do not only see them in our cosmic neighborhood, they can be detected at high redshifts. Today we believe, that a supermassive black hole can be found in the very center of AGN. Matter accretion onto a black hole is believed to be the most effective energy production mechanism.
The problem how supermassive black holes are formed is not solved yet. Some early attempts have been done, but the proposed mechanism would only lead to the formation of these in very recent times. We study a new scenario, in which the black holes form in a CDM cosmology. While baryonic matter is smoothly distributed until after the recombination era, cold dark matter decouples very early from background expansion and DM inhomogeneities can grow long before recombination. After recombination baryonic matter finally decouples from photons and falls into the deep gravitational potential wells of the dark matter.
We numerically investigate the spherical accretion of dense baryonic matter into the potential well of dark matter, both adiabatic and Compton-cooled. Though these are only preliminary results, and further investigation is necessary, our simulations suggest that massive cores with masses of 10⁷ Msun - 10⁸ Msun might form as early as z=50-25. If these cores stay optically thick, they might cool, contract further, eventually become gravitationally unstable and collapse to a massive black hole. If angular momentum of the baryonic gas is sufficiently high, a self-gravitating disk would be formed.

On the donor in GRS 1915+105

J. Greiner, M.J McCaughrean and A.J. Castro-Tirado
Astrophysical Institute Potsdam, Potsdam, Germany

We will present the results of our upcoming (mid-July) high-resolution IR spectroscopy of GRS 1915+105 with the VLT. These observations are expected to identify (or at least strongly constrain) the luminosity class and spectral type of the donor in this intriguing X-ray binary. Based on X-ray and radio measurements (X-ray luminosity, quasi-periodic oscillations, superluminal jets) GRS 1915+105 is expected to harbour the most massive stellar black hole in our Galaxy - 10-80 Msun. We aim to discuss the implications of our IR spectroscopy results concerning the possible orbital period (assuming the donor to fill its Roche lobe) and mass of the putative black hole in GRS 1915+105.

Fallback in Supernovae and Black Hole Formation

A. Heger, A. MacFadyen and S.E. Woosley
UCO/Lick Observatory, Santa Crux, USA

When the iron core of a (non-rotating) massive star collapses to a neutron star there are three possible outcomes. First, if a neutrino powered explosion of sufficient intensity develops, almost all the material outside the neutron star is ejected and a typical supernova of about 10⁵¹erg produced. If the neutrino powered explosion does not develop, then a black hole forms in the time it takes for the proto-neutron star to accrete a few tenths of a solar mass - a second or so. But there is a third intermediate possibility that we want to explore in some detail here. A shock may be launched initially, but the shock may lack adequate energy to eject all the overlying matter. Then, after a time that is highly sensitive to the initial shock energy, mass will fall back to the origin, accrete, and possibly produce a black hole. In SN 1987A, an explosion of roughly 1.2 · 10⁵¹erg in a 20 Msun star, it was previously estimated that roughly 0.1 Msun re-imploded in this way.
Here we explore fallback as a function of shock energy in a presupernova star of 25 Msun. A shock of variable energy is initiated artificially using a piston and the subsequent hydrodynamical evolution of the star is followed using two one-dimensional codes - an implicit Lagrangian hydrodynamics code with a reflecting inner boundary condition (KEPLER) and an explicit, Eulerian code with absorbing inner boundary (PROMETHEUS). The results of these two calculations should bracket the actual fall back behavior. We find a variable range of accreted masses and time scales. For shock energies of 0.3, 0.7, 1.2, and 1.5 · 10⁵¹ erg, 1.8, 1.4, 0.5, and 0.3 Msun (half of the total fallback) fall back in approximately 2, 7, 5, and 3 minutes. This results in accretion rates of about 10^-4 to 10^-2 Msun Sec^-1. For the lower energies explored (energies <= 10⁵¹erg), an appreciable fraction of the helium core imploded and a black hole surely formed. Still a bright, somewhat sub-luminous optical supernova would have resulted. At late times, our numerically derived accretion rates verify the predicted scaling of Chevalier (1988), an energy dependent constant times t^-5/3. Delayed black hole formation does not alter the neutrino signal, e.g., as observed from SN~1987A, but if the material falling back has sufficient angular momentum, an accretion disk may form and launch a jet (see contribution by A. MacFadyen). This jet may increase the power of the explosion, affect its symmetry, or even lead to a X-ray/gamma-ray burst as it penetrates through the stellar surface. The issue of fallback is also important for the nucleosynthesis yields of supernovae and, in particular, on the amount of iron and other heavy elements ejected. Iron yields for all our models will be presented.

A Search for the Presence of Bow Shocks around High-mass X-ray Binaries

F. Huthoff and L. Kaper
Astronomical Institute, University of Amsterdam, Amsterdam, The Netherlands

The measured radial velocities, and the recently derived tangential velocities based on Hipparcos observations, have demonstrated that several high-mass X-ray binaries with OB-supergiant companion travel through interstellar space with velocities on the order of 50 km s^-1. The origin of this high space velocity is the supernova explosion that formed the compact companion. Also Cyg~X-1 has a large space velocity (49 km s^-1), indicating that during the supernova explosion a substantial amount of material was lost from the system.
When a massive star is running through space with a supersonic velocity, the interaction of its stellar wind with the interstellar medium will result in the formation of a bow shock. The detection of a wind bow shock around a high-mass X-ray binary would be an indication that the system is moving with a high space velocity. Its shape shows the direction of motion. Especially for the systems at larger distances, for which no Hipparcos measurements are available, the detection of a bow shock would be a valuable diagnostic. We searched for the presence of bow shocks around high-mass X-ray binaries using infrared (IRAS, ISO) and optical (narrow-band filters) observations. Up to now, we have only one firm detection, while for (apparently single) OB runaway stars the detection rate is about 30%.

The Magnetic Field Configuration of Accretion Disks around Black Holes

N. Ikhsanov
Pulkovo Observatory, St. Petersburg, Russia

The configuration of the magnetic field generated in a turbulent differentially rotating accretion disk is discussed. We explain the resulting configuration of the magnetic field in the disk corona in terms of Z-pinch. We find that the energy release in Z-pinch leads to the formation of relativistic particle beams and plasma outflows. The energies of particles accelerated in Z-pinch are comparable with that evaluated form the X-ray and Gamma ray observations of AGN.

Outflowing Coronae above Accretion Discs in AGN and GBH

A. Janiuk, P.T. Zycki, B. Czerny
N. Copernicus Astronomical Centre, Warsaw, Poland

One of the basic phenomena connected with central regions of active galaxies are either collimated or uncollimated outflows of material, however the origin of this effect is not well understood. Also the models proposed to explain the observed spectra of galactic black hole systems require the corona, or 'active regions', moving with mildly relativistic velocities rather than static. We discuss the role of the coronal outflow perpendicular to the disc surface. The problem of its influence on spectral shape and on the amplitude of reflected component in hard X-ray spectrum is studied. We take into account thermal motions within the outflowing plasma. We compare our results with the predictions of model of X-ray emission from the central spherical source, surrounded by the cold disc which inner part is disrupted and we try to find diffrences in the spectra that could help to distinguish between the two scenarios.

Observations of Cygnus X-3 in Quiescence in the 2.4-12 Microns Range

L. Koch Miramond, J.M. Bonnet-Bidaud and P. Abraham
DAPNIA/Astrophysics/CEA Saclay, France

We present infrared spectroscopic and multi-filter photometric results obtained with ISO on the peculiar X-ray binary Cygnus X-3. The observations cover orbital phases 0.02 to 0.23 during quiescence. The flux observed at 3.6 microns in the ISOPHOT multi-filter mode is 8.1 mJy with a 1 sigma statistical error of 3.3 mJy, i.e. a flux of 22 mJy (dereddened), in good agreement with that observed in the K band and radio mm ranges during quiescence (Fender et al, 1999; Ogley et al, 1999). Spectro-photometry with ISOPHOT/S in the range 2.4-12 microns shows a line at about 4 microns which we interpret as HeII at 4.05 microns. It peaks at about 100 mJy (dereddened), the line width being limited by the instrumental resolution at the width observed for the HeII lines seen in the K band (van Kerkwijk, 1992). Orbitally phase-resolved spectra do not show the expected modulation at the 4.8 h period seen in the K and radio ranges (Fender, 1995, Fender et al, 1996), that being possibly due to the large statistical errors in the ISOPHOT/S spectra when they are orbitally phase-resolved. The dereddened flux measured at 7.5 microns in a 1 micron bandwidth is 60 mJy with a one sigma error of 10 mJy. We will discuss the dynamical interpretations of the above results and of results obtained at the same epoch with ISOCAM at 10 microns.

Determination of Mass limits around Pulsars at 10 and 90 Microns

L. Koch Miramond, M. Haas, P. Podsiadlowski, T. Naylor and M. Sauvage
DAPNIA/Astrophysics/CEA Saclay, France

We present infrared photometric results obtained with ISOCAM and ISOPHOT on 3 recycled millisecond Pulsars B1534+12 (binary), J2322+2057 (isolated), J2019+2425 (binary) and 3 ordinary radio Pulsars B0149-16, B1604-00 and J0108-1431. No detections have been obtained on the 3 recycled ms PSRs nor on the 2 first radio PSRs located at 680, 780, 850 and 790, 590 pc respectively. Upper limits on the mass of dust emitting at 10 and 90 microns are inferred. A faint enhancement in the brightness map is seen at about 5 arcseconds from the radio position of PSR J0108-1431, the nearest radio PSR, located at 85 pc from us. No faint object appears in the Palomar Sky Survey at this position. The corresponding flux observed at 90 microns is 12 mJy with a 1 sigma statistical error of 3 mJy. The IRAS all-sky survey gives a flux of 12 mJy with a one sigma error of 1 mJy at this position, for the same 46 arcsec square aperture. We conclude that the 90 microns emission observed by ISOPHOT probably originates in cirrus and we deduce the upper limit of mass orbiting this PSR.

The Giant X-ray Outburst of NGC 5905 - a Tidal Disruption Event?

S. Komossa
Max-Planck-Institut für Extraterrestrische Physik, Garching bei München, Germany

NGC 5905 underwent a giant X-ray outburst, with a change in ROSAT photon countrate of a factor ~200. Remarkably, the optical spectrum of the galaxy does not show any signs of Seyfert activity. A number of outburst scenarios is carefully examined. A particularly tight constraint is the huge outburst luminosity of at least nearly 10⁴³erg/s in the soft X-ray band (0.1-2.4 keV). An exciting explanation of the X-ray outburst, and among those investigated the only one that is able to account for the high outburst luminosity despite the absence of optical Seyfert activity, is the tidal disruption of a star by a supermassive black hole (SMBH) residing in the nucleus of this galaxy. The expected flare of electromagnetic radiation that is produced when the stellar debris is swallowed by the black hole was proposed by Rees (1988, 1990) as a means of detecting SMBHs in nearby galaxies.
Presently, and among the X-ray outbursts, NGC 5905 seems to be the best candidate for such a tidal disruption event in an otherwise non-active galaxy. A comparison with the similar, less luminous case - the UV flare in the elliptical galaxy NGC 4552 (Renzini et al. 1995, Nat 378, 39) - is performed.

Discovery of a Giant, Ultra-soft, and Luminous X-ray Outburst from the Optically Inactive Galaxy Pair RX J1242-11

S. Komossa and J. Greiner
Max-Planck-Institut für Extraterrestrische Physik, Garching bei München, Germany

We report the discovery of a large X-ray outburst from the direction of the previously unknown, optically inactive galaxy pair RX J1242-11. The X-ray source is variable by a factor > 20 between the ROSAT all-sky survey and the later pointed observation separated by ~1.5 yr. The outburst spectrum is extremely soft (powerlaw photon index Gamma_x =~ -5), among the steepest ever observed among galaxies. Further, the source's intrinsic luminosity is huge, L_x > 1 · 10⁴³erg/s. Surprisingly, the optical spectra of both galaxies are characterized by absorption lines and do not show signs of (Seyfert) activity. This makes RX J1242-11 the third candidate for giant variability in an otherwise non-active galaxy, the first two being NGC 4552 (in the UV) and NGC 5905 (in X-rays). Several mechanisms to explain this unexpected and peculiar behavior are investigated. The most likely one seems to be an accretion event onto an otherwise dormant supermassive black hole (SMBH), by the tidal disruption of a star.

Measuring the Mass of the Black Hole in GS 2000+25 Using Infrared Ellipsoidal Variations

D.M. Leeber, T.E. Harrison and B.J. McNamara
New Mexico State University, New Mexico, USA

Soft X-ray Transients are binary systems that are believed to consist of a black hole, and a normal late type dwarf star which fills its Roche Lobe. As the distorted dwarf companion orbits the black hole, the observed flux rises and falls in a predictable manner, giving rise to ``ellipsoidal variations''. We observe these systems in the near infrared since the secondary star dominates the systemic luminosity and the contamination from the accretion disk is minimized. Observations in the near infrared are therefore better suited for detecting true ellipsoidal variations. By modeling these variations, we can determine the orbital period and inclination of the system. The inclination can be combined with the observed mass function to determine the mass of the primary object. We present results on GS 2000+25 (QZ Vul). We have obtained data on this system with the Apache Point Observatory 3.5 meter telescope over several nights spanning from November 1998 to July 1999. We have modeled the data with the WD95 light curve modeling program.

Obscured Nuclei in Radio Galaxies

J. McAllister, J. Peacock, P. Hewett and R. Jimenez
Institute for Astronomy, University of Edinburgh, Edinburgh, UK

There is an accumulating amount of evidence from imaging and polarization studies that the central engines of radio galaxies are obscured from view by a large amount of extinction along the line-of-sight. It has been shown (Djorgovski et al. 1991) that at infrared K and L' wavelengths it is possible to observe the unresolved AGN component directly, due to reduced extinction. This analysis is unfortunately fraught with difficulties, as the normal `scale and subtract' method of estimating the nuclear component at K will always leave a red nucleus (Simpson 1994), which could be attributed to one or all of three things: unresolved quasar nucleus, stellar population gradients and dust.
We have obtained optical and infrared images and optical spectra of a sample of low redshift (z < 0.1) radio galaxies. We fit elliptical surface brightness profiles to the infrared images to better estimate the excess nuclear flux. In a preliminary analysis we find excess nuclear fluxes in over half of our sample of 36 galaxies. We use the optical spectra and stellar population modelling to predict the L' nuclear flux due to the intrinsic stellar populations of the galaxies and hence any excess which could be attributed to an AGN. Line-of-sight extinction estimates are then computed from the K-L' colour of the nuclear point source.

The Importance of Rapid Black Hole Spin in Relativistic Jet Formation

D.L. Meier and S. Koide
Jet Propulsion Laboratory, Pasadena, USA

We present the results of several studies that investigate the possible importance of black hole spin in determining the strength and speed of a relativistic, magnetohydrodynamic (MHD) jet. Fully general relativistic MHD simulations in Kerr geometry indicate that the power and speed of the jet produced increase substantially when the hole rotates rapidly and the infall of the accreting magnetized material is very fast. Pseudo-relativistic simulations of MHD jet production in a much larger region of parameter space imply the existence of a jet luminosity ``limit'' which plays the same role in MHD acceleration as the Eddington luminosity plays in radiative acceleration. Above this ``magnetic switch'' luminosity, the jet velocity increases dramatically -- a condition that can be triggered by increasing the spin rate or strength of a rotating magnetic field. These results have been used to develop a model for extragalactic radio sources and the Fanaroff & Riley class division. This model has implications for the evolution of rotating black holes in both galactic nuclei and binaries.

Black Hole X-ray Transients: A New View on Soft-hard Spectral Transitions

F. Meyer, B.F. Liu and E. Meyer-Hofmeister
Max-Planck-Institut für Astrophysik, Garching bei München, Germany

The theory of coronal evaporation predicts the formation of an inner hole in the cool thin accretion disk for mass accretion rates below a certain threshold and the sudden disappearance of this hole when the mass accretion rate rises above that threshold. This appears to quantitatively account for the observed transitions between hard and soft spectral states at critical luminosities.

Limits on Accretion Rates in BHXTs

E. Meyer-Hofmeister and F. Meyer
Max-Planck-Institut für Astrophysik, Garching bei München, Germany

Several black hole X-ray transients have a very long recurrence time, decades of years. We interprete this as a marginal occurrence of a dwarf nova type disk instability in the cool outer accretion disk. We compute how the recurrence time varies with the mass overflow rate from the companion star. The evaporation of the inner accretion disk to a coronal flow depends on the black hole mass and is an essential feature of the disk evolution. For accretion rates only slightly lower than those in the known black hole X-ray transients the disk does not reach the instability. We argue that many of those stationary, optically faint black hole X-ray binaries exist.

Constraints on Mass Loss in Black Hole Formation Derived from Black Hole X-ray Binaries

G. Nelemans, T.M. Tauris and E.P.J. van den Heuvel
Astronomical Institute, University of Amsterdam, Amsterdam, The Netherlands

Both the recently observed high runaway velocities of Cyg X-1 (49 km s^-1) and X-ray Nova Sco 1994 (114 km s^-1) and the relatively low radial velocities of the low-mass black hole X-ray binaries, can be explained by symmetric mass ejection in the supernovae which formed the black holes in these systems. This removes the need in the case of Nova Sco to invoke an indirect black hole formation, in which the intermediate neutron star receives a kick that produces the space velocity for this system as proposed by Brandt et al. (1995)
Assuming symmetric mass ejection in black hole formation, we estimate the amount of mass that must have been ejected in the stellar core collapse in order to explain the velocities of the above X-ray binaries. We find that at least 2.5 Msun and 3.0 Msun must have been ejected in the formation of Cyg X-1 and Nova Sco, respectively. A similar mass loss fraction (f = 0.35) for the remaining black hole binaries gives system velocities sufficiently low to be in agreement with the observations.
We discuss other scenarios for producing the observed runaway velocities of Cyg X-1 and Nova Sco 1994, including delayed black hole formation, but conclude that the black holes must have formed in a direct core collapse in which mass was ejected.

YALO Observations of 4U 1543-47

J.A. Orosz, C.D. Bailyn, R.K. Jain and J. Greene
University of Utrecht, Utrecht, the Netherlands

The black hole binary 4U 1543-47 has been observed extensively (using B, V, and I) during the 1998 and 1999 seasons by the YALO (Yale - AURA - Lisbon - Ohio State) 1 meter telescope at Cerro Tololo. The folded light curves show clean amplitudes of 0.05 mag in B, 0.03 mag in V, and < 0.02 mag in I. We discuss models of the light curves and present updated and refined system parameters.

A Causal Model for Relativistic Accretion Flow

J. Peitz
Harvard-Smithsonian Center for Astrophysics, Cambridge, USA

Accretion onto compact objects is a prominent astrophysical problem involving dissipative fluid flow in strong gravitational fields. The generic approach for modeling is by the relativistic Navier-Stokes-Fourier equations, which are of non-hyperbolic type. As a consequence, fluctuations in the dissipative variables (shear stress and heat flux) propagate at causality violating infinite speeds, and thermodynamic equilibrium states are unstable. The description is therefore problematic in particular for modeling systems that undergo time-variablity on shorter time-scales, such as the kilohertz quasi periodic oscillations (QPOs) observed in low-mass X-ray binaries (LMXBs). To overcome these difficulties we proposed to model relativistic accretion flows and other dissipative astrophysical systems within the framework of causal theories for dissipative fluid dynamics (Peitz & Appl 1998, MNRAS 296, 231; Peitz & Appl 1999, CQG 16, 979).
I will briefly discuss two types of causal fluid theories and the corresponding representation within the 3+1 formalism appropriate for numerical simulation. I will discuss a possible adaptation of this framework towards the relativistic accretion problem, and propose a new causal disc model that is based on an alpha-type parametrization for the transport coefficients.

The X-ray Emission and the Central Properties of Early-type Galaxies

S. Pellegrini
Dipartimento di Astronomia, Università di Bologna, Bologna, Italy

The Hubble Space Telescope observed the surface brightness profiles of the central regions of nearby ellipticals and spiral bulges (the so called ``hot systems'') with a resolution of a few parsecs (Jaffe et al. 1994, Lauer et al. 1995). It turned out that luminous galaxies (M_V < -22) have cuspy cores with steep outer power law profiles that break internally to shallow inner profiles proportional to r^-gamma, with 'gamma' <= 0.3; faint galaxies (M_V > -20.5) show steep featureless power law profiles that lack cores down to the resolution limit (Faber et al. 1997). At intermediate magnitudes cuspy core and power law galaxies coexist. Here I investigate whether, in the range of optical luminosities where they overlap, these two galaxy families differ in their global X-ray luminosity L_X. I find that power law galaxies are confined at low L_X values (log L_X(erg s^-1) < 40.5), while cuspy core galaxies can reach L_X values at least one order of magnitude higher. This prompts that the central properties of hot stellar systems have profound effects on the total X-ray emission. In fact, it has been suggested that nuclear massive black holes are important for explaining the dicothomy of the inner light profiles, as they should have substantial influence on the dynamics and evolution of the surrounding gas and stars. Similarly, massive black holes could be at the origin of the differences in L_X.

Monitoring the Short-term Variability of Cyg X-1 with RXTE

K. Pottschmidt, J. Wilms, M.A. Nowak, J.B. Dove, W.A. Heindl, D. Smith and R. Staubert
Institut für Astronomie und Astrophysik, Tübingen, Germany

For stellar black hole candidates several distinct states can be identified that differ in their general spectral and temporal properties. Based mainly on spectral arguments the states have been associated with different accretion rates and geometries. With broad band instruments like RXTE it is now possible to study the canonical black hole states with high time resolution over a time base of several years.
We analyzed the hard, i.e., most common, state of Cyg X-1, using RXTE observations that were performed from 1996 to 1999. The focus of our work lies on parameters and functions characterizing the short-term variability (< 1000 sec), like:
a) Power Specral Densities (PSDs).
b) Linear State Space Models. According to these stochastic models, one typical relaxation timescale can be attributed to the hard state lightcurves of Cyg X-1.
c) Fourier frequency dependent quantities describing the relationship between different energy bands, namely coherence functions and timelags.
We find that the timing properties show a considerable variability, even though the state does not change. This has already been noted for PSDs, but is probably even more pronounced for the timelags. We also show how the changes relate to spectral parameters such as power law indices, coronal temperatures or reflection hump strengths.

Detection of an X-Ray Outburst in a Non-Active Galaxy

T.H. Reiprich, and J. Greiner
Max-Planck-Institut für Extraterrestrische Physik, Garching bei München, Germany

A soft X-ray outburst has been detected in ROSAT PSPC archival data. The measured countrate is < 0.01 cts/s half a year before and after the burst. At the burst the countrate rises from < 0.02 cts/s to ~ 0.09 ± 0.02 cts/s within one week. Assuming the anonymous galaxy to belong to the nearby galaxy cluster (distance to the cluster center ~ one Abell radius) a rough estimate of the peak X-ray luminosity in the ROSAT band yields L_X > 10⁴³erg/s. The optical spectrum of the candidate source galaxy (taken 6 years after the burst) shows no obvious signs of activity.
We present the detailed analysis of the X-ray and optical observations and discuss possible outburst scenarios for this unusual source.

On the Influence of Resonant Fe Kalpha Absorption on the Iron Emission Line Profiles near Black Holes

M. Ruszkowski, and A.C. Fabian
Institute of Astronomy, Cambridge, UK

The fluorescent iron Kalpha emission line profile provides an excellent probe of the innermost regions of active galactic nuclei. It is conceivable that diffuse plasma exists above the accretion disc and can affect the X-ray spectra by iron Kalpha resonance absorption. This in turn can influence the interpretation of the data and estimation of the accretion disc and black hole parameters. We embark on the computation of this effect and calculate the iron line profile in the framework of a specific model in which rotating, highly ionized and resonantly-absorbing plasma occurs close to the black hole. The Sobolev approximation is used and we take into account all general relativistic effects. Iron Kalpha line profiles of high signal-to-noise ratio have recently been obtained by Nandra et al. (1999) for the Seyfert 1 galaxy NGC 3516. These profiles suggest redshifted iron Kalpha absorption features which can be explained by our model for the case of a Kerr black hole. The forthcoming X-ray missions such as XMM, ASTRO-E and AXAF-Chandra provide excellent opportunities to test the model against high quality observational data.

X-ray Iron Line Variability for the Model of an Orbiting Flare Above a Black Hole Accretion Disc

M. Ruszkowski
Institute of Astronomy, Cambridge, UK

The broad X-ray iron line, detected in many active galactic nuclei, is likely to be produced by fluorescence from the X-ray illuminated central parts of an accretion disc close to a supermassive black hole. The time-averaged shape of the line can be explained most naturally by a combination of special and general relativistic effects. Such line profiles contain information about the black hole spin and the accretion disc as well as the geometry of the emitting region and may help to test general relativity in the strong gravity regime. In this paper we embark on the computation of the temporal response of the line to the illuminating flux. Previous studies concentrated on the calculation of reverberation signatures from static sources illuminating the disc. In this paper we focus on the more physically justified case of flares located above the accretion disc and corotating with it. We compute the time dependent iron line taking into account all general relativistic effects and show that its shape is of very complex nature, and also present light curves accompanying the iron line variability. We suggest that future X-ray satellites like XMM or Constellation-X may be capable of detecting features present in the computed reverberation maps.

New Evidences for the Supermassive Binary Black Holes in the Blazar OJ 287

A. Sillanpää
Tuorla Observatory, Piikkiv, Finland

There have been many suggestions about the existence of the supermassive binary black hole systems in the nuclei of Active Galactic Nuclei (AGN). One of the best candidates has been a blazar called OJ287 where we have found a possible periodicity of about 12 year in the optical major outbursts. However, there has been a confusion if the periodicity is exactly strict or not. In this new work we will show clear evidences that the periodicity is really amazingly strict and this gives strong support to the binary black hole model developed for OJ287.

Non-stationary Accretion onto Black Holes

E. Szuszkiewicz and J. Miller
Nicolaus Copernicus University, Torún, Poland
SISSA, Trieste, Italy

We are carrying out a programme of non-linear, time-dependent numerical calculations to study non-stationary accretion onto black holes using the ``slim-disc'' approach (Abramowicz et al. 1988) with vertically-integrated equations. Our strategy has been to start from the original version of the slim-disc model, with all of its associated approximations and assumptions, and then to add improvements to this systematically, one at a time, so that the effect of each can be clearly understood. In the first phase of the work, we used the original alpha p viscosity prescription and investigated the behaviour of thermally-unstable models. For alpha = 10^-1 we observed limit-cycle behaviour with successive evacuation and refilling of the inner parts of the disc, the evacuation being accompanied by a swelling up of the region of lowered surface density. For alpha = 10^-3, on the other hand, a violent instability was observed just outside the sonic point which terminated the computation with no evidence for limit-cycle behaviour being seen. The alpha p viscosity prescription brings great simplifications but it is certainly very idealized and, while it may well represent a reasonable approximation for stationary models, its use for time-varying solutions is highly suspect. Our next step in the work was then to introduce a more physical viscosity prescription with the phi r component of the viscous stress tensor being proportional to the radial gradient of the angular velocity. Calculations with this prescription have now been carried out and have given some interesting new insights into the behaviour of thermally unstable discs. A progress report will be presented.

X-Ray Evidence of an AGN in the Starburst Galaxy M82

T. Tsuru
Department of Physics, Kyoto University, Kyoto, Japan

The X-ray spectrum of the famous starburst galaxy M82 consists of three components: soft, medium, and hard (Tsuru et al. 1997). The spectrum of the hard component, which is spatially unresolved, is well represented by an absorbed thermal bremsstrahlung, or an absorbed power-law model. However, the origin of the hard component is unclear. Thus, we conducted a monitoring observation with ASCA in 1996. Although the X-ray flux of the soft and medium components remained constant, a significant time variability of the hard component was found between 3 · 10⁴⁰ erg s^-1 and 1 · 10⁴¹ erg s^-1 at various time scales from 10 ks to one month. The temperature or photon index of the hard component also changed. We proved that the spatial position of the hard component is the center of M82. The spectrum of the variable source obtained by subtracting the spectrum of the lowest state from the highest state suggests strong absorption of N_H ~ 10²² cm², which means a variable source is embedded in the center of M82. All of these observations suggest that a low-luminosity AGN exists in M82.

Model for Flares in Accretion Flow around a Black Hole

S. Tsuruta
Physics Department, Montana State University, Bozeman, Montana, USA

We tested a model where individual flare-like events occurring in the X-ray emissions from an accretion disk around a black hole follow the similar linear increase and exponential decrease characteristic of solar and stellar flares. Adopting the ASCA data for NGC 3227 as an example, we find that this model works very well for this Seyfert 1 galaxy.
From our results a scaling law is derived which relates the loop length of an individual magnetic flux tube to the rise and decay times of the flare using plasma parameters suitable for an accretion disk corona near a supermassiv black hole. The predicted loop length is consistent with the observed luminosity and duration of the flare-like events. Our results suggest that the similar mechanism which explains solar flares may explain variability in accretion powered AGN.

Modelling synchrotron outbursts in the jet of 3C 273

M. Türler
Observatoire de Genève, Sauverny, Switzerland
and INTEGRAL Science Data Centre, Versoix, Switzerland

We present a new approach to derive the observed properties of synchrotron outbursts in relativistic jets. The idea is to use the submillimetre-to-radio long-term light curves of 3C 273 to extract the spectral and temporal evolution of a typical outburst. The method consists in a decomposition of these light curves into a series of self-similar outbursts. We show that it is possible to understand the very different shapes of the submillimetre-to-radio light curves of 3C 273 with only about one outburst every 1.5 year starting simultaneously at all frequencies. The outbursts that we identify do well correspond to the VLBI components observed in the jet. The spectral and temporal evolution of the outbursts is found to be in very good agreement with the evolution expected by the shock model of Marscher & Gear (1985). We can therefore derive the values of the model parameters which apply to 3C 273.

An Optical Spectroscopic Study of the Black-Hole Candidates GX 339-4 and XTE J1550-564

M. Buxton and S. Vennes
Astrophysical Theory Centre, Australian National University, ATC, Australia

The low-mass X-ray binary GX 339-4 exhibits four X-ray states: a rare phenomenon in X-ray binaries. From the X-ray properties, it is believed that the system contains a black hole, but as yet the spectral type of the companion star is unknown. No definite orbital period has been measured, although a 14.8hr period was found via R and I-band photometry (Callanan et al, 1992) which was interpreted as being the orbital period. For the last year we have monitored GX 339 via optical spectroscopy during high and low X-ray states.
XTE J1550-564 is a new black-hole candidate discovered in September last year. We have obtained optical spectra from March-May, 1999 when this system was decreasing in X-rays. The spectral type of the companion star was not found. However, Halpha was clearly in emission during this period and its equivalent-half-width was noticeably increasing due to the decrease of the continuum.
We will present our optical spectra on these low-mass X-ray binaries and make some interpretation on the various states of black-hole candidates. Work on other X-ray binaries such as LMC X-1, LMC X-3 and GRO J1655-40 will also be mentioned.

Long Term RXTE Monitoring of LMC X-3 and LMC X-1

J. Wilms, M.A. Nowak, K. Pottschmidt, W.A. Heindl, J. Dove, M.C. Begelman and R. Staubert
Institut für Astronomie und Astrophysik, Tübingen, Germany

The black hole candidate LMC X-3 is varying by a factor of four on a timescale of 200 or 100 days (Cowley et al., 1991). The variability has been interpreted as either due to variations in the mass transfer rate from the B3 V star companion, or by partial obscurations by a warped accretion disk. On the other hand, LMC X-1, which has almost the same luminosity, does not exhibit any long term variability. We have monitored the long term variability of both objects with RXTE in three to four weekly intervals starting in December 1996 obtaining a large observational database to shed light on the nature of the long term X-ray behavior of these sources.
In this contribution we present the results from this monitoring campaign as well as from two long quasi-continuous observations. We study the correlations between the flux in the soft and power-law spectral components and we model the broad band X-ray spectra using Monte Carlo Comptonization computations.

Luminosity and mass functions of Active Galactic Nuclei

L. Wisotzki
Hamburger Sternwarte, Universität Hamburg, Germany
and Institute for Astronomy, Royal Observatory Edinburgh, UK

The luminosities of AGN are closely linked to their central black hole masses, with the `Eddington ratio' as principal free parameter (assuming standard radiative efficiency). Observed luminosity functions can therefore be employed to set constraints on the underlying mass distributions as well as on the unknown details of the accretion flow.
I present a new `local' (z =~ 0) luminosity function of QSOs and Seyfert 1 nuclei, constructed from 174 low-redshift AGN found in the course of the Hamburg/ESO survey. Together with information from spectroscopic follow-up, this is used to estimate the AGN mass function under the hypothesis of gravitationally bound motion within the broad-line region. I also present an emission line luminosity function for the QSO sample, to connect this study to the demography of low-luminosity AGN in nearly normal galaxies, where nuclear activity is often detected only from weak broad Halpha components.
The derived AGN mass spectrum rises is almost a single power law over a wide range of masses, gradually flattening between 10⁸ and 10⁷ Msun. The similarity to the AGN luminosity function suggests that the average Eddington ratio does not depend strongly on nuclear mass, and the simple assumption M `proportional to' L^beta (beta = 1 meaning constant Eddington ratio) allows to approximately convert a luminosity function directly into a mass distribution.
Assuming that a similar relation for the Eddington ratio holds at higher redshifts, QSO luminosity functions can be used to estimate the mass spectrum of high-z quasars. While the mass density of BHs with M > 10⁹ Msun at z=2 exceeds the same value at z=0 by a factor of > 1000, the same ratio is only ~ 50 for M > 10⁷ Msun. A possible explanation is that present-day nuclear activity takes place preferentially in intermediate-mass BHs, leaving the most massive BHs found today as underrepresented in low-z AGN samples.

Spectral Variations of Black Hole X-ray Nova XTE J1550-564 during the Rising Phase of Its 1998 Major Outburst

X.-B. Wu
Beijing Astronomical Observatory, Beijing, China

We present the spectral analysis results of the RXTE/PCA observations during the 13 day rising phase of the 1998 major outburst of X-ray nova XTE J1550-564. The spectra are dominated by a hard component which can be fit by a power law with photon index varying from 1.4 at the initial outburst to 2.7 near the giant flare. The soft component is very weak during the whole rising phase and only becomes significant when the source approaches the giant flare. The correlations among 0.1-14Hz QPO frequency and the 2-20 keV X-ray flux, as well as the flux of its hard components, are confirmed. They can be robustly described as f_QPO ~ f_x^2.3 with correlation coefficient about 0.99. The spectral transition from low/very hard state to very high/hard state, together with the slow decline of its optical brightness, supports that XTE J1550-564 may experience an outside-in burst caused by accretion disk instability. Assuming typical disk parameters of black hole X-ray binaries, the 13 day rising time can be explained by the viscous timescale provided the disk instability was trigged at a distance of more than 100 times of grivational radius away from the central object.

Gravitational Lens Diagnosis of Quasar Accretion Disk

A. Yonehara, S. Mineshige and E.L. Turner
Department of Astronomy, Kyoto University, Kyoto, Japan

The origin of the huge amount of radiation from quasar and its stochastic variabilities is not well understood. In this paper, we present following new methods to reveal the structure of quasar via gravitational lens ``telescope''.
1. Microlensing of accretion disk in quasar Light curves of microlensing events are affected by the size and the structure of microlensed sources. Therefore, we show that wide-waveband observations of microlensing at the same time reveal the detailed structure of an accretion disk in quasar on scales down to a few AU.
2. Source size limitation from time delay in a lensed quasar In a gravitationally lensed quasar, it is well known that there is a time delay of variabilities between each lensed image. However, if the source size is large, nicely correlated variabilites, e.g.,as known in the case of Q0957+567 (so-called ``double quasar''), can not be observed because of time delay difference in only one lensed image is not negligibly small. Therefore, we can limit the size of variabilities and moreover, the origin of the variabilities. Thus, gravitational lensing is one of the powerful tool to diagnose the surroundings of supermassive black hole in quasar.

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