72 research outputs found
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 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 (), 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 . 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, , 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 in the hot flow. The models predict
that all three sources must have substantial flux in hard X-rays and soft
-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
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