Signatures of Energetic Protons in Hot Accretion Flows: Synchrotron Cooling of Protons in Strongly Magnetized Pulsars

The existence of hot, two-temperature accretion flows is essential to the recent discussions of the low luminosity, hard X-ray emission from accreting neutron stars and black holes in Galactic binaries and massive black holes in low luminosity galactic nuclei. In these flows, protons are essentially virialized and relativistic energies for non-thermal protons are likely. Observational confirmation of the energetic protons' presence could further support the two-temperature accretion flow models. We point out that synchrotron emission from nonthermal relativistic protons could provide an observational signature in strongly magnetized neutron star systems. The self-absorbed synchrotron emission from an accreting neutron star with the magnetic moment $\sim 10^{30} G cm^3$ is expected to exhibit a spectrum \nu I_{\nu}~ \nu**2 with the luminosity ~ a few x 10^**33 (L_x/10**36 erg/s)**0.4 erg/s at \nu~10**15 Hz where L_x is the X-ray luminosity from the neutron star surface. The detection of the expected synchrotron signature in optical and UV bands during the low luminosity state of the pulsar systems such as 4U 1626-67 and GX 1+4 could prove the existence of the hot, two-temperature accretion flows during their spin-down episodes. The detected optical emission in 4U 1626-67 has a spectral shape and luminosity level very close to our predictions.Comment: 10 pages, ApJ

Advection-Dominated Accretion: Self-Similarity and Bipolar Outflows

We consider axisymmetric viscous accretion flows where a fraction f of the viscously dissipated energy is advected with the accreting gas as stored entropy and a fraction 1-f is radiated. When f is small (i.e. very little advection), our solutions resemble standard thin disks in many respects except that they have a hot tenuous corona above. In the opposite {\it advection-dominated} limit ($f\rightarrow1$), the solutions approach nearly spherical accretion. The gas is almost at virial temperature, rotates at much below the Keplerian rate, and the flow is much more akin to Bondi accretion than to disk accretion. We compare our exact self-similar solutions with approximate solutions previously obtained using a height-integrated system of equations. We conclude that the height- integration approximation is excellent for a wide range of conditions. We find that the Bernoulli parameter is positive in all our solutions, especially close to the rotation axis. This effect is produced by viscous transport of energy from small to large radii and from the equator to the poles. In addition, all the solutions are convectively unstable and the convection is especially important near the rotation axis. For both reasons we suggest that a bipolar outflow will develop along the axis of the flows, fed by material from the the surface layers of the equatorial inflow.Comment: 22 Pages, 5 Figures are available by request to [email protected], Plain Tex, CfA Preprint No. 3931, To Appear in Astrophysical Journal 5/1/9

Relativistic Conic Beams and Spatial Distribution of Gamma-Ray Bursts

We study the statistics of gamma-ray bursts, assuming that gamma-ray bursts are cosmological and they are beamed in the form of a conical jet with a large bulk Lorentz factor $\sim 100$. In such a conic beam, the relativistic ejecta may have a spatial variation in the bulk Lorentz factor and the density distribution of gamma-ray emitting jet material. An apparent luminosity function arises because the axis of the cone is randomly oriented with respect to the observer's line of sight. The width and the shape of the luminosity function are determined by the ratio of the beam opening angle of the conical jet to the inverse of the bulk Lorentz factor, when the bulk Lorentz factor and the jet material density is uniform on the photon emitting jet surface. We calculate effects of spatial variation of the Lorentz factor and the spatial density fluctuations within the cone on the luminosity function and the statistics of gamma-ray bursts. In particular, we focus on the redshift distribution of the observed gamma-ray bursts. The maximum distance to and the average redshift of the gamma-ray bursts are strongly affected by the beaming-induced luminosity function. The bursts with the angle-dependent Lorentz factor which peaks at the center of the cone have substantially higher average gamma-ray burst redshifts. When both the jet material density and the Lorentz factor are inhomogeneous in the conical beam, the average redshift of the bursts could be 5 times higher than that of the case in which relativistic jet is completely homogeneous and structureless. Even the simplest models for the gamma-ray burst jets and their apparent luminosity distributions have a significant effect on the redshift distribution of the gamma-ray bursts.Comment: 15 pages, 4 figures, submitted to ApJ

A New Model for Black Hole Soft X-ray Transients in Quiescence

We present models of the soft X-ray transients, A0620-00, V404 Cyg, and X-ray Nova Mus 1991, in quiescence. In each source, we postulate that there is an outer region, extending outward from about 3000 Schwarzschild radii, where the accretion flow is in the form of a standard thin disk. The outer disk produces most of the radiation we observe in the infrared, optical and UV bands. We propose that the disk undergoes an instability at its inner edge, perhaps by the mechanism described recently by Meyer \& Meyer-Hofmeister for cataclysmic variables. The accreting gas is thereby converted into a hot corona which flows into the black hole as a nearly virial two-temperature flow. We describe the hot inner flow by means of a recently discovered stable solution of optically thin advection-dominated accretion. In this flow, most of the thermal energy released by viscous dissipation is advected into the black hole and only a small fraction, $\sim10^{-4}-10^{-3}$, of the energy is radiated. The radiation is in the form of Comptonized synchrotron and bremsstrahlung emission, and has a broad spectrum extending from optical to soft gamma-rays. The models we present are consistent with all the available data in the three sources. In particular, the X-ray emission from the hot inner flow fits the observed flux and spectral index of A0620-00. We derive a mass accretion rate of \sim10^{-11}\msyr in A0620-00 and Nova Mus, and \sim{\rm few}\times10^{-10}\msyr in V404 Cyg. The best fit to the data is obtained for a viscosity parameter $\alpha\sim0.1-0.3$ in the hot flow. The models predict that all three sources must have substantial flux in hard X-rays and soft $\gamma$-rays. This prediction is testable in the case of V404 Cyg with current instruments. A necessary feature of our proposal is that most of the viscousComment: 32 Pages, 6 Figures included, Compressed Postscript, To Appear in Astrophysical Journa

On Companion-Induced Off-Center Supernova-Like Explosions

We suggest that a neutron star with a strong magnetic field, spiraling into the envelope of a companion star, can generate a ``companion induced SN-like off-center explosion". The strongly magnetized neutron star ("magnetar") is born in a supernova explosion before entering into an expanding envelope of a supergiant companion. If the neutron star collapses into a black hole via the hypercritical accretion during the spiral-in phase, a rapidly rotating black hole with a strong magnetic field at the horizon results. The Blandford-Znajek power is sufficient to power a supernova-like event with the center of explosion displaced from the companion core. The companion core, after explosion, evolves into a C/O-white dwarf or a neutron star with a second explosion. The detection of highly eccentric black-hole, C/O-white dwarf binaries or the double explosion structures in the supernova remnants could be an evidence of the proposed scenario.Comment: 5 page