Zooming towards the Event Horizon - mm-VLBI today and tomorrow. (arXiv:1305.2811v1 [astro-ph.HE]):
Global VLBI imaging at millimeter and sub-millimeter wavelength overcomes the
opacity barrier of synchrotron self-absorption in AGN and opens the direct view
into sub-pc scale regions not accessible before. Since AGN variability is more
pronounced at short millimeter wavelength, mm-VLBI can reveal structural
changes in very early stages after outbursts. When combined with observations
at longer wavelength, global 3mm and 1mm VLBI adds very detailed information.
This helps to determine fundamental physical properties at the jet base, and in
the vicinity of super-massive black holes at the center of AGN. Here we present
new results from multi-frequency mm-VLBI imaging of OJ287 during a major
outburst. We also report on a successful 1.3mm VLBI experiment with the APEX
telescope in Chile. This observation sets a new record in angular resolution.
It also opens the path towards future mm-VLBI with ALMA, which aims at the
mapping of the black hole event horizon in nearby galaxies, and the study of
the roots of jets in AGN.
Wednesday, May 15, 2013
Systematic Study of Event Horizons and Pathologies of Parametrically Deformed Kerr Spacetimes. (arXiv:1304.7786v2 [gr-qc] UPDATED)
Systematic Study of Event Horizons and Pathologies of Parametrically Deformed Kerr Spacetimes. (arXiv:1304.7786v2 [gr-qc] UPDATED):
In general relativity, all black holes in vacuum are described by the Kerr
metric, which has only two independent parameters: the mass and the spin. The
unique dependence on these two parameters is known as the no-hair theorem. This
theorem may be tested observationally by using electromagnetic or
gravitational-wave observations to map the spacetime around a candidate black
hole and measure potential deviations from the Kerr metric. Several parametric
frameworks have been constructed for tests of the no-hair theorem. Due to the
uniqueness of the Kerr metric, any such parametric framework must violate at
least one of the assumptions of the no-hair theorem. This can lead to
pathologies in the spacetime, such as closed timelike curves or singularities,
which may hamper using the metric in the strong-field regime. In this paper, I
analyze in detail several parametric frameworks and show explicitly the manner
in which they differ from the Kerr metric. I calculate the coordinate locations
of event horizons in these metrics, if any exist, using methods adapted from
the numerical relativity literature. I identify the regions where each
parametric deviation is unphysical as well as the range of coordinates and
parameters for which each spacetime remains a regular extension of the Kerr
metric and is, therefore, suitable for observational tests of the no-hair
theorem.
In general relativity, all black holes in vacuum are described by the Kerr
metric, which has only two independent parameters: the mass and the spin. The
unique dependence on these two parameters is known as the no-hair theorem. This
theorem may be tested observationally by using electromagnetic or
gravitational-wave observations to map the spacetime around a candidate black
hole and measure potential deviations from the Kerr metric. Several parametric
frameworks have been constructed for tests of the no-hair theorem. Due to the
uniqueness of the Kerr metric, any such parametric framework must violate at
least one of the assumptions of the no-hair theorem. This can lead to
pathologies in the spacetime, such as closed timelike curves or singularities,
which may hamper using the metric in the strong-field regime. In this paper, I
analyze in detail several parametric frameworks and show explicitly the manner
in which they differ from the Kerr metric. I calculate the coordinate locations
of event horizons in these metrics, if any exist, using methods adapted from
the numerical relativity literature. I identify the regions where each
parametric deviation is unphysical as well as the range of coordinates and
parameters for which each spacetime remains a regular extension of the Kerr
metric and is, therefore, suitable for observational tests of the no-hair
theorem.
Pygmies, giants, and skins as laboratory constraints on the equation of state of neutron-rich matter. (arXiv:1305.3202v1 [nucl-th])
Pygmies, giants, and skins as laboratory constraints on the equation of state of neutron-rich matter. (arXiv:1305.3202v1 [nucl-th]):
Laboratory experiments sensitive to the density dependence of the symmetry
energy may place stringent constraints on the equation of state of neutron-rich
matter and, thus, on the structure, dynamics, and composition of neutron stars.
Understanding the equation of state of neutron-rich matter is a central goal of
nuclear physics that cuts across a variety of disciplines. In this contribution
I focus on how laboratory experiments on neutron skins and on both Pygmy and
Giant resonances can help us elucidate the structure of neutron stars.
RKS Note: Clear explanation of nuclear physics situation vis-a-vis neutron star radii.
Laboratory experiments sensitive to the density dependence of the symmetry
energy may place stringent constraints on the equation of state of neutron-rich
matter and, thus, on the structure, dynamics, and composition of neutron stars.
Understanding the equation of state of neutron-rich matter is a central goal of
nuclear physics that cuts across a variety of disciplines. In this contribution
I focus on how laboratory experiments on neutron skins and on both Pygmy and
Giant resonances can help us elucidate the structure of neutron stars.
RKS Note: Clear explanation of nuclear physics situation vis-a-vis neutron star radii.
Sunday, May 12, 2013
Testing X-ray Measurements of Galaxy Cluster Gas Mass Fraction Using the Cosmic Distance-Duality Relation. (arXiv:1305.2077v1 [astro-ph.CO])
Testing X-ray Measurements of Galaxy Cluster Gas Mass Fraction Using the Cosmic Distance-Duality Relation. (arXiv:1305.2077v1 [astro-ph.CO]):
We propose a consistency test of some recent X-ray gas mass fraction
($f_{\rm{gas}}$) measurements in galaxy clusters, using the cosmic
distance-duality relation, $\eta_{\rm{theory}}=\dl(1+z)^{-2}/\da$, with
luminosity distance ($\dl$) data from the Union2 compilation of type Ia
supernovae. We set $\eta_{\rm{theory}}\equiv1$, instead of assigning any
redshift parameterizations to it, and constrain the cosmological information
preferred by $f_{\rm{gas}}$ data along with supernova observations. We adopt a
new binning method in the reduction of the Union2 data, in order to minimize
the statistical errors. Four data sets of X-ray gas mass fraction, which are
reported by Allen et al. (2 samples), LaRoque et al. and Ettori et al., are
detailedly analyzed against two theoretical modelings of $f_{\rm{gas}}$. The
results from the analysis of Allen et al.'s samples prove the feasibility of
our method. It is found that the preferred cosmology by LaRoque et al.'s sample
is consistent with its reference cosmology within 1-$\sigma$ confidence level.
However, for Ettori et al.'s $f_{\rm{gas}}$ sample, the inconsistency can reach
more than 3-$\sigma$ confidence level and this dataset shows special preference
to an $\Ol=0$ cosmology.
We propose a consistency test of some recent X-ray gas mass fraction
($f_{\rm{gas}}$) measurements in galaxy clusters, using the cosmic
distance-duality relation, $\eta_{\rm{theory}}=\dl(1+z)^{-2}/\da$, with
luminosity distance ($\dl$) data from the Union2 compilation of type Ia
supernovae. We set $\eta_{\rm{theory}}\equiv1$, instead of assigning any
redshift parameterizations to it, and constrain the cosmological information
preferred by $f_{\rm{gas}}$ data along with supernova observations. We adopt a
new binning method in the reduction of the Union2 data, in order to minimize
the statistical errors. Four data sets of X-ray gas mass fraction, which are
reported by Allen et al. (2 samples), LaRoque et al. and Ettori et al., are
detailedly analyzed against two theoretical modelings of $f_{\rm{gas}}$. The
results from the analysis of Allen et al.'s samples prove the feasibility of
our method. It is found that the preferred cosmology by LaRoque et al.'s sample
is consistent with its reference cosmology within 1-$\sigma$ confidence level.
However, for Ettori et al.'s $f_{\rm{gas}}$ sample, the inconsistency can reach
more than 3-$\sigma$ confidence level and this dataset shows special preference
to an $\Ol=0$ cosmology.
The shocked outflow in NGC 4051 - momentum-driven feedback, UFO's and warm absorbers. (arXiv:1305.2046v1 [astro-ph.HE])
The shocked outflow in NGC 4051 - momentum-driven feedback, UFO's and warm absorbers. (arXiv:1305.2046v1 [astro-ph.HE]):
An extended XMM-Newton observation of the Seyfert 1 galaxy NGC 4051 in 2009
revealed an unusually rich absorption spectrum with outflow velocities, in both
RGS and EPIC spectra, up to ~ 9000 km/s (Pounds and Vaughan 2011). Evidence was
again seen for a fast ionised wind with velocity ~ 0.12c (Tombesi 2010, Pounds
and Vaughan 2012). Detailed modelling with the XSTAR photoionisation code now
confirms the general correlation of velocity and ionisation predicted by mass
conservation in a Compton-cooled shocked wind (King 2010). We attribute the
strong column density gradient in the model to the addition of strong two-body
cooling in the later stages of the flow, causing the ionisation (and velocity)
to fall more quickly, and confining the lower ionisation gas to a narrower
region. The column density and recombination timescale of the highly ionised
flow component, seen mainly in Fe K lines, determine the primary shell
thickness which, when compared with the theoretical Compton cooling length,
determines a shock radius of ~ 10^17 cm. Variable radiative recombination
continua (RRC) provide a key to scaling the lower ionisation gas, with the RRC
flux then allowing a consistency check on the overall flow geometry. We
conclude that the 2009 observation of NGC 4051 gives strong support to the idea
that a fast, highly ionised wind, launched from the vicinity of the
supermassive black hole, will lose much of its mechanical energy after shocking
against the ISM at a sufficiently small radius for strong Compton cooling.
However, the total flow momentum will be conserved, retaining the potential for
a powerful AGN wind to support momentum-driven feedback (King 2003; 2005). We
speculate that the `warm absorber' components often seen in AGN spectra result
from accumulation of shocked wind and ejected ISM.
An extended XMM-Newton observation of the Seyfert 1 galaxy NGC 4051 in 2009
revealed an unusually rich absorption spectrum with outflow velocities, in both
RGS and EPIC spectra, up to ~ 9000 km/s (Pounds and Vaughan 2011). Evidence was
again seen for a fast ionised wind with velocity ~ 0.12c (Tombesi 2010, Pounds
and Vaughan 2012). Detailed modelling with the XSTAR photoionisation code now
confirms the general correlation of velocity and ionisation predicted by mass
conservation in a Compton-cooled shocked wind (King 2010). We attribute the
strong column density gradient in the model to the addition of strong two-body
cooling in the later stages of the flow, causing the ionisation (and velocity)
to fall more quickly, and confining the lower ionisation gas to a narrower
region. The column density and recombination timescale of the highly ionised
flow component, seen mainly in Fe K lines, determine the primary shell
thickness which, when compared with the theoretical Compton cooling length,
determines a shock radius of ~ 10^17 cm. Variable radiative recombination
continua (RRC) provide a key to scaling the lower ionisation gas, with the RRC
flux then allowing a consistency check on the overall flow geometry. We
conclude that the 2009 observation of NGC 4051 gives strong support to the idea
that a fast, highly ionised wind, launched from the vicinity of the
supermassive black hole, will lose much of its mechanical energy after shocking
against the ISM at a sufficiently small radius for strong Compton cooling.
However, the total flow momentum will be conserved, retaining the potential for
a powerful AGN wind to support momentum-driven feedback (King 2003; 2005). We
speculate that the `warm absorber' components often seen in AGN spectra result
from accumulation of shocked wind and ejected ISM.
Discrete clouds of neutral gas between the galaxies M31 and M33. (arXiv:1305.1631v1 [astro-ph.CO])
Discrete clouds of neutral gas between the galaxies M31 and M33. (arXiv:1305.1631v1 [astro-ph.CO]):
Spiral galaxies must acquire gas to maintain their observed level of star
formation beyond the next few billion years (Leroy et al. 2008). A source of
this material may be the gas that resides between galaxies, but our
understanding of the state and distribution of this gas is incomplete (Shull et
al. 2012). Radio observations (Braun & Thilker 2004) of the Local Group of
galaxies have revealed hydrogen gas extending from the disk of the galaxy M31
at least halfway to M33. This feature has been interpreted to be the neutral
component of a condensing intergalactic filament (Dav\'e et al. 2001) which
would be able to fuel star formation in M31 and M33, but simulations suggest
that such a feature could also result from an interaction between both galaxies
within the past few billion years (Bekki 2008). Here we report radio
observations showing that about 50 per cent percent of this gas is composed of
clouds, while the rest is distributed in an extended, diffuse component. The
clouds have velocities comparable to those of M31 and M33, and have properties
suggesting they are unrelated to other Local Group objects. We conclude that
the clouds are likely to be transient condensations of gas embedded in an
intergalactic filament and are therefore a potential source of fuel for future
star formation in M31 and M33.
Spiral galaxies must acquire gas to maintain their observed level of star
formation beyond the next few billion years (Leroy et al. 2008). A source of
this material may be the gas that resides between galaxies, but our
understanding of the state and distribution of this gas is incomplete (Shull et
al. 2012). Radio observations (Braun & Thilker 2004) of the Local Group of
galaxies have revealed hydrogen gas extending from the disk of the galaxy M31
at least halfway to M33. This feature has been interpreted to be the neutral
component of a condensing intergalactic filament (Dav\'e et al. 2001) which
would be able to fuel star formation in M31 and M33, but simulations suggest
that such a feature could also result from an interaction between both galaxies
within the past few billion years (Bekki 2008). Here we report radio
observations showing that about 50 per cent percent of this gas is composed of
clouds, while the rest is distributed in an extended, diffuse component. The
clouds have velocities comparable to those of M31 and M33, and have properties
suggesting they are unrelated to other Local Group objects. We conclude that
the clouds are likely to be transient condensations of gas embedded in an
intergalactic filament and are therefore a potential source of fuel for future
star formation in M31 and M33.
Wednesday, May 1, 2013
Inner Accretion Disk Edges in a Kerr-Like Spacetime. (arXiv:1304.8106v1 [gr-qc])
Inner Accretion Disk Edges in a Kerr-Like Spacetime. (arXiv:1304.8106v1 [gr-qc]):
According to the no-hair theorem, astrophysical black holes are uniquely
described by the Kerr metric. In order to test this theorem with observations
in either the electromagnetic or gravitational-wave spectra, several Kerr-like
spacetimes have been constructed which describe potential deviations from the
Kerr spacetime in parametric form. For electromagnetic tests of the no-hair
theorem, such metrics allow for the proper modeling of the accretions flows
around candidate black holes and the radiation emitted from them. In many of
these models, the location of the inner edge of the accretion disk is of
special importance. This inner edge is often taken to coincide with the
innermost stable circular orbit (ISCO), which can serve as a direct probe of
the spin and the deviation from the Kerr metric. In certain cases, however, an
ISCO does not exist, and the inner edge of an accretion disk is instead
determined by an instability against small perturbations in the direction
vertical to the disk. In this paper, I analyze the properties of accretion
disks in the Kerr-like metric proposed by Johannsen and Psaltis (2011), whose
inner edges are located at the radii where this vertical instability occurs. I
derive expressions of the energy and axial angular momentum of disk particles
that move on circular equatorial orbits and calculate the locations of the
inner disk edges. As a possible observable of such accretion disks, I simulate
profiles of relativistically broadened iron lines and show that they depend
significantly on the values of the spin and the deviation parameter.
According to the no-hair theorem, astrophysical black holes are uniquely
described by the Kerr metric. In order to test this theorem with observations
in either the electromagnetic or gravitational-wave spectra, several Kerr-like
spacetimes have been constructed which describe potential deviations from the
Kerr spacetime in parametric form. For electromagnetic tests of the no-hair
theorem, such metrics allow for the proper modeling of the accretions flows
around candidate black holes and the radiation emitted from them. In many of
these models, the location of the inner edge of the accretion disk is of
special importance. This inner edge is often taken to coincide with the
innermost stable circular orbit (ISCO), which can serve as a direct probe of
the spin and the deviation from the Kerr metric. In certain cases, however, an
ISCO does not exist, and the inner edge of an accretion disk is instead
determined by an instability against small perturbations in the direction
vertical to the disk. In this paper, I analyze the properties of accretion
disks in the Kerr-like metric proposed by Johannsen and Psaltis (2011), whose
inner edges are located at the radii where this vertical instability occurs. I
derive expressions of the energy and axial angular momentum of disk particles
that move on circular equatorial orbits and calculate the locations of the
inner disk edges. As a possible observable of such accretion disks, I simulate
profiles of relativistically broadened iron lines and show that they depend
significantly on the values of the spin and the deviation parameter.
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