225 research outputs found
Jet-dominated advective systems of all mass scales
We show that the radio emission of black hole (BH) and neutron star (NS)
X-ray binaries (XRBs) follows the analytical prediction of a jet model where
the jet carries a constant fraction of the accretion power. The radio emission
can therefore be used as a tracer of the accretion rate. This measure is
normalised with efficiently radiating objects. As it is independent of the
X-ray fluxes, the measure allows us to compare the accretion rate dependency of
the bolometric X-ray luminosity of BHs and NSs. For NSs, it scales linearly
with accretion rate while the scaling for BHs is quadratic - as expected for
inefficient accretion flows. We find the same behaviour in AGN. This new
approach uses the jet power to obtain the accretion rate. Thus, we know both
the jet power and the radiated power of an accreting BH. This allows us to show
that some accretion power is likely to be advected into the black hole, while
the jet power dominates over the bolometric luminosity of a hard state BH.Comment: conference proceedings of the Sixth Microquasar workshop:
Microquasars and Beyond, 18-22 September 2006 in Como, Italy (eds: T. Belloni
et al. 2006
Neutral hydrogen absorption towards Fast Radio Bursts
If Fast Radio Bursts (FRBs) are truly at astronomical, in particular
cosmological, distances, they represent one of the most exciting discoveries in
astrophysics of the past decade. However, the distance to FRBs has, to date,
been estimated purely from their excess dispersion, and has not been
corroborated by any independent means. In this paper we discuss the possibility
of detecting neutral hydrogen absorption against FRBs both from spiral arms
within our own galaxy, or from intervening extragalactic HI clouds. In either
case a firm lower limit on the distance to the FRB would be established.
Absorption against galactic spiral arms may already be detectable for bright
low-latitude bursts with existing facilities, and should certainly be so by the
Square Kilometre Array (SKA). Absorption against extragalactic HI clouds, which
would confirm the cosmological distances of FRBs, should also be detectable
with the SKA, and maybe also Arecibo. Quantitatively, we estimate that SKA1-MID
should be able to detect extragalactic HI absorption against a few percent of
FRBs at a redshift z~1.Comment: Accepted for publications as a Letter in MNRA
The closest black holes
Starting from the assumption that there is a large population (> 10^8) of
isolated, stellar-mass black holes (IBH) distributed throughout our galaxy, we
consider the detectable signatures of accretion from the interstellar medium
(ISM) that may be associated with such a population. We simulate the nearby
(radius 250 pc) part of this population, corresponding to the closest ~35 000
black holes, using current best estimates of the mass distribution of stellar
mass black holes combined with two models for the velocity distribution of
stellar-mass IBH which bracket likely possibilities. We distribute this
population of objects appropriately within the different phases of the ISM and
calculate the Bondi-Hoyle accretion rate, modified by a further dimensionless
efficiency parameter \lambda. Assuming a simple prescription for radiatively
inefficient accretion at low Eddington ratios, we calculate the X-ray
luminosity of these objects, and similarly estimate the radio luminosity from
relations found empirically for black holes accreting at low rates. The latter
assumption depends crucially on whether or not the IBH accrete from the ISM in
a manner which is axisymmetric enough to produce jets. Comparing the predicted
X-ray fluxes with limits from hard X-ray surveys, we conclude that either the
Bondi-Hoyle efficiency parameter \lambda, is rather small (< 0.01), the
velocities of the IBH are rather high, or some combination of both. The
predicted radio flux densities correspond to a population of objects which,
while below current survey limits, should be detectable with the Square
Kilometre Array (SKA). Converting the simulated space velocities into proper
motions, we further demonstrate that such IBH could be identified as faint high
proper motion radio sources in SKA surveys.Comment: Accepted for publication in MNRA
Iron Emission Lines from Extended X-ray Jets in SS 433: Reheating of Atomic Nuclei
Powerful relativistic jets are among the most ubiquitous and energetic
observational consequences of accretion around supermassive black holes in
active galactic nuclei and neutron stars and stellar-mass black holes in x-ray
binary (XRB) systems. But despite more than three decades of study, the
structure and composition of these jets remain unknown. Here we present
spatially resolved x-ray spectroscopy of arc second-scale x-ray jets from the
XRB SS 433 analized with the Chandra advanced charge-coupled device imaging
spectrometer. These observations reveal evidence for a hot continuum and
Doppler-shifted iron emission lines from spatially resolved regions.
Apparently, in situ reheating of the baryonic component of the jets takes place
in a flow that moves with relativistic bulk velocity even more than 100 days
after launch from the binary core
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