264 research outputs found
Rapid variability of accretion in AM Herculis
We present the last pointed observation of AM Her carried out during the life
of the BeppoSAX satellite. It was bright at the beginning of the observation,
but dropped to the lowest X-ray level ever observed so far. The X-ray emission
during the bright period is consistent with accretion occurring onto the main
pole of the magnetized white dwarf. The rapid change from the active state to
the low deep state indicates a drop by a factor of 17 in the accretion rate and
hence that accretion switched-off. The short timescale (less than one hour) of
this variation still remains a puzzle. Optical photometry acquired
simultaneousy during the low state shows that the white dwarf remains heated,
although a weak emission from the accretion stream could be still present.
Cyclotron radiation, usually dominating the V and R bands, is negligible thus
corroborating the possibility that AM Her was in an off-accretion state. The
X-ray emission during the inactive state is consistent with coronal emission
from the secondary late type star.Comment: 6 pages A&A-Latex, 6 Figures, accepted for publication in A&
What is a Political Value? Political Philosophy and Fidelity to Reality
This essay seeks to defend the claim that political philosophy ought to be appropriately guided by the phenomenon of politics that it seeks to both offer a theory of and, especially in its normative guise, offer a theory for. It does this primarily through the question of political values. It begins by arguing that for any value to qualify as a value for the political domain, it must be intelligible in relation to the constitutive features of politics as a human activity. It then examines the extent to which the preconditions for the realization of values in practice ought to figure in our considerations as to whether they are values that fit or belong to our social world. We can understand these parts of the essay as responding to two related questions, respectively: (i) Is this a political value at all? — which is to ask, is it a value that is appropriate for the political realm?; and then (ii) Is this a political value for us? The final section responds to the often-made complaint that political philosophy ought not to make any concessions to the actual world of politics as it really is, arguing that attending to the realities of politics, and in particular the constitutive conditions of political activity, gives meaning to the enterprise as the theorization of politics (and not something else). Furthermore those same conditions provide the limits of intelligibility beyond which ideals and values can no longer be, in any meaningful sense, ideals and values for the political sphere
On the Iwasawa-Taniguchi effect of radio-quiet AGN
The existence of an anti-correlation between the Equivalent Width (EW) of the
neutral narrow core of the iron Kalpha emission line and the 2-10 keV
luminosity (the so-called `X-ray Baldwin' or `Iwasawa-Taniguchi' effect) has
been debated in the last years. We aim at testing this claim on the largest
catalogue of radio quiet AGN high-quality X-ray spectra ever published. The
final sample comprises 157 objects. We search for a relation of the iron line
EW not only with the X-ray luminosity, but also with the Black Hole mass, the
Eddington ratio and the cosmological distance. The data presented here were
analyzed homogeneously, all spectra are from the same instrument and with high
Signal-to-Noise Ratio. A linear censored fit on the EW versus 2-10 keV
luminosity is highly significant and yields , where is the EW of the neutral iron
Kalpha line in eV and is the 2-10 keV X-ray luminosity in units of
erg s. The anti-correlation with the Eddington ratio is also
very significant, while no dependence of the iron EW on the BH mass is
apparent.Comment: 4 pages, 4 figures, accepted for publication in Astronomy and
Astrophysics Letter
High resolution X-ray spectroscopy and imaging of Mrk573
We present a detailed analysis of the XMM-Newton RGS high resolution X-ray
spectra of the Seyfert 2 galaxy, Mrk573. This analysis is complemented by the
study of the Chandra image, and its comparison to optical (HST) and radio (VLA)
data. The soft X-ray emission is mainly due to gas photoionised by the central
AGN, as indicated by the detection of radiative recombination continua from
OVII and OVIII, as well as by the prominence of the OVII forbidden line. This
result is confirmed by the best fit obtained with a self-consistent CLOUDY
photoionisation model. However, a collisionally excited component is also
required, in order to reproduce the FeXVII lines, accounting for about 1/3 of
the total luminosity in the 15-26 A band. Once adopted the same model in the
Chandra ACIS data, another photoionised component, with higher ionisation
parameter, is needed to take into account emission from higher Z metals. The
broadband ACIS spectrum also confirms the Compton-thick nature of the source.
The imaging analysis shows the close morphological correspondence between the
soft X-ray and the [OIII] emission. The radio emission appears much more
compact, although clearly aligned with the narrow line region. The collisional
phase of the soft X-ray emission may be due to starburst, requiring a star
formation rate of M yr, but there is no clear
evidence of this kind of activity from other wavelengths. On the other hand, it
may be related to the radio ejecta, responsible for the heating of the plasma
interacting with the outflow, but the estimated pressure of the hot gas is much
larger than the pressure of the radio jets, assuming equipartition and under
reasonable physical parameters.Comment: 12 pages, 6 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society Main Journa
The recent X-ray history of NGC 5506
We present a detailed discussion of the spectral and spatial components of
NGC 5506, based on XMM-Newton, Chandra and BeppoSAX observations. The overall
picture consists of a nucleus absorbed by cold gas with column density of
cm and surrounded by a Compton-thick torus, whose
existence is inferred by a cold reflection component and an iron K
line. On a much larger scale, a photoionized gas extended on pc
reprocesses the nuclear radiation, producing a soft excess and ionized iron
lines. Noteworthy, we find no evidence for the presence of the accretion disc.Comment: 9 pages, 9 figures, accepted for publication in A&
Strong field effects on emission line profiles: Kerr black holes and warped accretion disks
If an accretion disk around a black hole is illuminated by hard X-rays from
non-thermal coronae, fluorescent iron lines will be emitted from the inner
region of the accretion disk. The emission line profiles will show a variety of
strong field effects, which may be used as a probe of the spin parameter of the
black hole and the structure of the accretion disk. In this paper we generalize
the previous relativistic line profile models by including both the black hole
spinning effects and the non-axisymmetries of warped accretion disks. Our
results show different features from the conventional calculations for either a
flat disk around a Kerr black hole or a warped disk around a Schwarzschild
black hole by presenting, at the same time, multiple peaks, rather long red
tails and time variations of line profiles with the precession of the disk. We
show disk images as seen by a distant observer, which are distorted by the
strong gravity. Although we are primarily concerned with the iron K-shell lines
in this paper, the calculation is general and is valid for any emission lines
produced from a warped accretion disk around a black hole.Comment: 22 pages, 8 figures. Accepted for publication in Ap
X-ray spectral properties of Seyfert galaxies and the unification scheme
Aims: The unification scheme of Seyfert galaxies predicts that the observed
differences between type 1 and type 2 Seyfert galaxies are solely due to the
differing orientations of the toroidal-shaped obscuring material around AGN.
The observed X-ray spectra of Seyfert type 2s compared to type 1s are expected
to be affected by higher absorbing column density due to the edge-on view of
the obscuring torus. We study the 0.5 - 10 keV X-ray spectral properties of
Seyfert type 1s and type 2s with the aim to test the predictions of Seyfert
unification scheme in the X-ray regime. Methods: We use an optically selected
Seyfert sample in which type 1s and type 2s have matched distributions in the
orientation independent parameters of AGN and host galaxy. Results: The 0.5 -
10 keV XMM-Newton pn X-ray spectra of Seyfert galaxies are in general best
fitted with a model consists of an absorbed power-law, a narrow Gaussian fitted
to the Fe K{\alpha} emission line and an often seen soft excess component
characterized by either a thermal plasma model with temperature kT \sim 0.1 -
1.0 keV and/or a steep power-law. The 2.0 - 10 keV hard X-ray continuum
emission in several Seyfert type 2s is reflection dominated and suggests the
Compton-thick obscuration. Results on the statistical comparison of the
distributions of the observed X-ray luminosities in the soft (0.5 - 2.0 keV)
and hard (2.0 - 10.0 keV) bands, the X-ray absorbing column densities, the
equivalent widths of Fe K{\alpha} line and the flux ratios of hard X-ray to
[OIII] {\lambda}5007{\AA} for the two Seyfert subtypes are consistent with the
obscuration and orientation based unification scheme.Comment: 15 pages, 3 figures, 6 tables, accepted for publication in A&
Global 3D Simulations of Disc Accretion onto the classical T Tauri Star BP Tauri
The magnetic field of the classical T Tauri star BP Tau can be approximated
as a superposition of dipole and octupole moments with respective strengths of
the polar magnetic fields of 1.2 kG and 1.6 kG (Donati et al. 2008). We adopt
the measured properties of BP Tau and model the disc accretion onto the star.
We observed in simulations that the disc is disrupted by the dipole component
and matter flows towards the star in two funnel streams which form two
accretion spots below the dipole magnetic poles. The octupolar component
becomes dynamically important very close to the star and it redirects the
matter flow to higher latitudes. The spots are meridionally elongated and are
located at higher latitudes, compared with the pure dipole case, where
crescent-shaped, latitudinally elongated spots form at lower latitudes. The
position and shape of the spots are in good agreement with observations. The
disk-magnetosphere interaction leads to the inflation of the field lines and to
the formation of magnetic towers above and below the disk. The magnetic field
of BP Tau is close to the potential only near the star, inside the
magnetospheric surface, where magnetic stress dominates over the matter stress.
A series of simulation runs were performed for different accretion rates. They
show that an accretion rate is lower than obtained in many observations, unless
the disc is truncated close to the star. The torque acting on the star is about
an order of magnitude lower than that which is required for the rotational
equilibrium. We suggest that a star could lose most of its angular momentum at
earlier stages of its evolution.Comment: 11 pages, 13 figures, submitted to MNRA
FERO: Finding Extreme Relativistic Objects. I. Statistics of Relativistic Fe Kalpha lines in Radio-Quiet Type 1 AGN
Accretion models predict that fluorescence lines broadened by relativistic
effects should arise from reflection of X-ray emission onto the inner region of
the accretion disc surrounding the central black hole of active galactic nuclei
(AGN). The theory behind the origin of relativistic lines is well established,
and observational evidence from a moderate number of sources seems to support
the existence of these lines. The aim of this work is to establish the fraction
of AGN with relativistic Fe Kalpha lines, and study possible correlations with
source physical properties. An XMM-Newton collection of 149 radio-quiet Type 1
AGN has been systematically and uniformly analyzed in order to search for
significant evidence of a relativistically broadened Fe Kalpha line. To enable
statistical studies, an almost complete, flux-limited subsample of 31 sources
has been defined. The 2-10 keV spectra of the FERO sources have been compared
with a complex model including most of the physical components observed in the
X-ray spectra of Seyfert galaxies: a power law primary continuum modified by
non-relativistic Compton reflection and warm absorption, plus a series of
narrow Fe line reflection features. The observed fraction of sources in the
flux-limited sample that show significant evidence of a relativistic Fe Kalpha
line is 36%. The average line Equivalent Width (EW) is of the order of 100 eV,
while the average disc inclination angle is 28+/-5 deg and the average
power-law index of the radial disc emissivity law is 2.4+/-0.4. The spin value
is well constrained only in 2 cases (MCG-6-30-15 and MRK509), and in the rest
of the cases, whenever a constraint can be placed, it always implies the
rejection of the static black hole solution. The Fe Kalpha line EW does not
correlate with disc parameters or with system physical properties, such as
black hole mass, accretion rate and hard X-ray luminosity.Comment: 22 pages, 7 figures. Accepted for publication in A&
Critical temperature for kaon condensation in color-flavor locked quark matter
We study the behavior of Goldstone bosons in color-flavor-locked (CFL) quark
matter at nonzero temperature. Chiral symmetry breaking in this phase of cold
and dense matter gives rise to pseudo-Goldstone bosons, the lightest of these
being the charged and neutral kaons K^+ and K^0. At zero temperature,
Bose-Einstein condensation of the kaons occurs. Since all fermions are gapped,
this kaon condensed CFL phase can, for energies below the fermionic energy gap,
be described by an effective theory for the bosonic modes. We use this
effective theory to investigate the melting of the condensate: we determine the
temperature-dependent kaon masses self-consistently using the two-particle
irreducible effective action, and we compute the transition temperature for
Bose-Einstein condensation. Our results are important for studies of transport
properties of the kaon condensed CFL phase, such as bulk viscosity.Comment: 24 pages, 8 figures, v2: new section about effect of electric
neutrality on critical temperature added; references added; version to appear
in J.Phys.
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