Black hole X-ray binaries (BHXBs) are known to display five distinct spectral states. In order of increasing luminosity these are the quiescent state, low state, intermediate state, high state and very high state. We present a self-consistent model of accretion flows around black holes which unifies all of these states except the very high state. The model is an extension of the following paradigm which has been applied successfully to the quiescent state. The accretion flow consists of two zones, an inner advection-dominated accretion flow (ADAF) which extends from the black hole horizon to a transition radius r tr , and an outer thin accretion disk that is present beyond r tr . Above the disk is a hot corona which is a continuation of the inner ADAF. The model consistently treats the dynamics of the accreting gas, the thermal balance of the ions and electrons in the two-temperature ADAF and corona, and the radiation processes that produce the observed spectrum.At low mass accretion rates,ṁ < ∼ 0.01 (in Eddington units), the inner ADAF zone in the model radiates extremely inefficiently, and the outer thin disk is restricted to large radii (r tr ∼ 10 2 − 10 4 , in Schwarzschild units). The luminosity therefore is low, and this configuration is identified with the quiescent state. Foṙ m > ∼ 0.01 and up to a critical valueṁ crit ∼ 0.08, the radiative efficiency of the ADAF increases rapidly and the system becomes fairly luminous. The spectrum is very hard and peaks around 100 keV. This is the low state. The exact value ofṁ crit depends on the viscosity parameter α (ṁ crit ∼ 1.3α 2 ; the paper assumes α = 0.25). For values ofṁ >ṁ crit and up to a second critical value about 10% higher, the ADAF progressively shrinks in size, the transition radius decreases, and the X-ray spectrum changes continuously from hard to soft. We identify this stage with the intermediate state. Finally, whenṁ is sufficiently large, the inner ADAF zone disappears altogether and the thin accretion disk extends down to the marginally stable orbit. The spectrum is dominated by an ultrasoft component with a weak hard tail. This is the high state. Model spectra calculated with this unified scenario agree well with observations of the quiescent, low, intermediate and high states. Moreover, the model provides a

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We present 1210 Johnson/Cousins B, V , R, and I photometric observations of 22 recent Type Ia supernovae (SNe Ia) : SNe 1993ac, 1993ae, 1994M, 1994S, 1994T, 1994Q, 1994ae, 1995D, 1995E, 1995al, 1995ac, 1995ak, 1995bd, 1996C, 1996X, 1996Z, 1996ab, 1996ai, 1996bk, 1996bl, 1996bo, and 1996bv. Most of the photometry was obtained at the Fred Lawrence Whipple Observatory of the HarvardSmithsonian Center for Astrophysics in a cooperative observing plan aimed at improving the database for SNe Ia. The redshifts of the sample range from cz \ 1200 to 37,000 km s~1 with a mean of cz \ 7000 km s~1.

We show that the model proposed by Esin, McClintock & Narayan (1997) for the low state, intermediate state and high state of the black hole soft X-ray transient, Nova Muscae 1991, is consistent with the spectral evolution of the black hole X-ray binary, Cyg X-1, during the hard-to-soft state transition observed in 1996. We also apply the model to the outbursts of two other black hole X-ray transients, GRO J0422+32 and GRO J1719−24.

We consider the X-ray luminosity difference between neutron star and black hole soft X-ray transients (NS and BH SXTs) in quiescence. The current observational data suggest that BH SXTs are significantly fainter than NS SXTs. The luminosities of quiescent BH SXTs are consistent with the predictions of binary evolution models for the mass transfer rate if (1) accretion occurs via an ADAF in these systems and (2) the accreting compact objects have event horizons. The luminosities of quiescent NS SXTs are not consistent with the predictions of ADAF models when combined with binary evolution models, unless most of the mass accreted in the ADAF is prevented from reaching the neutron star surface. We consider the possibility that mass accretion is reduced in quiescent NS SXTs because of an efficient propeller and develop a model of the propeller effect that accounts for the observed luminosities. We argue that modest winds from ADAFs are consistent with the observations while strong winds are probably not.Comment: LateX, 37 pages, 7 figures; Accepted for publication in The Astrophysical Journa

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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 ∼ 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 criticalṀ crit which depends on the radius R and viscosity parameter α. The solutions are advection-dominated for extremely low values ofṀ . However, for a range of intermediate accretion rates, the new solutions are both hot (T ∼ 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 coolingdominated and have a disk-like geometry, while the outer parts are fully advection-dominated and nearly quasi-spherical.

The deviation from the power-law decline of the optical flux observed in GRB 970228 and GRB 980326 has been used recently to argue in favor of the connection between gamma-ray bursts and supernovae. We consider an alternative explanation for this phenomenon, based on the scattering of a prompt optical burst by 0.1 M middle dot in circle dust located beyond its sublimation radius 0.1-1 pc from the burst. In both cases, the optical energy observed at the time of the first detection of the afterglow suffices to produce an echo after approximately 20-30 days, as observed. Prompt optical monitoring of future bursts and multiband photometry of the afterglows will enable us to test simple models of dust reprocessing quantitatively and to predict source redshift.