4,665 research outputs found
Accretion onto Black Holes and Neutron Stars: Differences and Similarities
Accreting black holes and neutron stars at luminosities above 0.01 of the
critical Eddington luminosity have a lot of similarities, but also drastic
differences in their radiation and power density spectra. The efficiency of
energy release due to accretion onto a rotating neutron star usually is higher
than in the case of a black hole. The theory of the spreading layer on the
surface of an accreting neutron star is discussed. It predicts the appearance
of two bright belts equidistant from the equator. This layer is unstable and
its radiation flux must vary with high frequencies.Comment: 12 pages, 10 figures, invited talk, to appear in Proc. of ESO
Workshop Held in Garching 6-8 Sept. 1999 in Honour of Riaccardo Giacconi
(Springer, eds. L. Kaper et al.
Large scale motions in superclusters: their imprint in the CMB
We identify high density regions of supercluster size in high resolution
N-body simulations of a representative volume of three Cold Dark Matter
Universes. By assuming that (1) the density and peculiar velocities of baryons
trace those of the dark matter, and (2) the temperature of plasma is
proportional to the velocity dispersion of the dark matter particles in regions
where the crossing times is smaller than the supercluster free-fall time, we
investigate how thermal motions of electrons in the intra-cluster medium and
peculiar velocity of clusters can affect the secondary anisotropies in the
cosmic microwave background (CMB). We show that the thermal effect dominates
the kinematic effect and that the largest thermal decrements are associated
with the most massive clusters in superclusters. Thus, searching for the
presence of two or more close large CMB decrements represents a viable strategy
for identifying superclusters at cosmological distances. Moreover, maps of the
kinematic effect in superclusters are characterized by neighboring large peaks
of opposite signs. These peaks can be as high as ~ 10 microK at the arcminute
angular resolution. Simultaneous pointed observations of superclusters in the
millimeter and submillimeter bands with upcoming sensitive CMB experiments can
separate between the thermal and kinematic effect contributions and constrain
the evolution of the velocity field in large overdense regions.Comment: 4 pages, 5 figures, ApJ Letters, in press; revised version according
to referee's comment
Lensing reconstruction of cluster-mass cross-correlation with cosmic microwave background polarization
We extend our maximum likelihood method for reconstructing the cluster-mass
cross-correlation from cosmic microwave background (CMB) temperature
anisotropies and develop new estimators that utilize six different quadratic
combinations of CMB temperature and polarization fields. Our maximum likelihood
estimators are constructed with delensed CMB temperature and polarization
fields by using an assumed model of the convergence field and they can be
iteratively applied to a set of clusters, approaching to the optimal condition
for the lensing reconstruction as the assumed initial model is refined. Using
smoothed particle hydrodynamics simulations, we create a catalog of realistic
clusters obtainable from the current Sunyaev-Zel'dovich (SZ) surveys, and we
demonstrate the ability of the maximum likelihood estimators to reconstruct the
cluster-mass cross-correlation from the massive clusters. The iTT temperature
estimator provides a signal-to-noise ratio of a factor 3 larger than the iEB
polarization estimator, unless the detector noise for measuring polarization
anisotropies is controlled under 3 microK.Comment: 10 pages, 6 figures, accepted for publication in Physical Review
Maps of the Cosmos: The Cosmic Microwave Background
Since the IAU XXIV meeting in 2000, the CMB anisotropy has matured from being
one of a number of cosmological probes to forming the bedrock foundation for
what is now the standard model of cosmology. The large advances over the past
three years have come from making better and better maps of the cosmos. We
review the state of measurements of the anisotropy and outline some of what we
have learned since 2000. The recent advancements may be placed roughly into
three categories: 1) What we learn from the CMB with minimal input from other
cosmic measurements such as the Hubble constant; 2) What we learn from the CMB
in combination with other probes of large scale structure; and 3) What we learn
by using the CMB as a back light. Future directions are also discussed. It is
clear: we have much more to learn from the CMB anisotropy.Comment: 15 pgs, 6 figures, Proceedings from IAU 2003, M. Colless e
Close stars and an inactive accretion disk in Sgr A*: Eclipses and flares
A cold neutral and extremely dim accretion disk may be present as a remnant
of a past vigorous activity around the black hole in our Galactic Center (GC).
Here we discuss ways to detect such a disk through its interaction with
numerous stars present in the central ~0.1 parsec of the Galaxy. The first
major effect expected is X-ray and near infrared (NIR) flares arising when
stars pass through the disk. The second is eclipses of the stars by the disk.
We point out conditions under which the properties of the expected X-ray flares
are similar to those recently discovered by Chandra. Since orbits of bright
stars are now being precisely measured, the combination of the expected flares
and eclipses offers an invaluable tool for constraining the disk density, size,
plane and even direction of rotation. The winds of the O-type stars are
optically thick to free-free absorption in radio frequencies. If present near
Sgr A* core, such powerful stellar winds can modulate and even occult the radio
source.Comment: typo in eq. 3 correcte
Preheating of the Universe by cosmic rays from primordial supernovae at the beginning of cosmic reionization
The 21-cm signal from the cosmic reionization epoch can shed light on the
history of heating of the primordial intergalactic medium (IGM) at z~30-10. It
has been suggested that X-rays from the first accreting black holes could
significantly heat the Universe at these early epochs. Here we propose another
IGM heating mechanism associated with the first stars. As known from previous
work, the remnants of powerful supernovae (SNe) ending the lives of massive
Population III stars could readily expand out of their host dark matter
minihalos into the surrounding IGM, aided by the preceeding photoevaporation of
the halo's gas by the UV radiation from the progenitor star. We argue that
during the evolution of such a remnant a significant fraction of the SN kinetic
energy can be put into low-energy (E<30 MeV) cosmic rays that will eventually
escape into the IGM. These subrelativistic cosmic rays could propagate through
the Universe and heat the IGM by ~10-100 K by z~15, before more powerful
reionization/heating mechanisms associated with the first galaxies and quasars
came into play. Future 21-cm observations could thus constrain the energetics
of the first supernovae and provide information on the magnetic fields in the
primordial IGM.Comment: 10 pages, 1 figure, accepted for publication in MNRA
Relative velocity of dark matter and barions in clusters of galaxies and measurements of their peculiar velocities
The increasing sensitivity of current experiments, which nowadays routinely
measure the thermal SZ effect within galaxy clusters, provide the hope that
peculiar velocities of individual clusters of galaxies will be measured rather
soon using the kinematic SZ effect. Also next generation of X-ray telescopes
with microcalorimeters, promise first detections of the motion of the intra
cluster medium (ICM) within clusters. We used a large set of cosmological,
hydrodynamical simulations, which cover very large cosmological volume, hosting
a large number of rich clusters of galaxies, as well as moderate volumes where
the internal structures of individual galaxy clusters can be resolved with very
high resolution to investigate, how the presence of baryons and their
associated physical processes like cooling and star-formation are affecting the
systematic difference between mass averaged velocities of dark matter and the
ICM inside a cluster. We, for the first time, quantify the peculiar motion of
galaxy clusters as function of the large scale environment. We also demonstrate
that especially in very massive systems, the relative velocity of the ICM
compared to the cluster peculiar velocity add significant scatter onto the
inferred peculiar velocity, especially when measurements are limited to the
central regions of the cluster. Depending on the aperture used, this scatter
varies between 50% and 20%, when going from the core (e.g. ten percent of the
virial radius) to the full cluster (e.g. the virial radius).Comment: 17 pages, 18 figures, submitted to MNRA
Equivalent width, shape and proper motion of the iron fluorescent line emission from the molecular clouds as an indicator of the illuminating source X-ray flux history
Observations of the diffuse emission in the 8--22 keV energy range, elongated
parallel to the Galactic plane (Sunyaev et al. 1993) and detection of the
strong 6.4 keV fluorescent line with 1 keV equivalent width from some
giant molecular clouds (e.g. Sgr B2) in the Galactic Centre region (Koyama
1994) suggest that the neutral matter of these clouds is (or was) illuminated
by powerful X-ray radiation, which gave rise to the reprocessed radiation. The
source of this radiation remains unknown. Transient source close to the Sgr B2
cloud or short outburst of the X-ray emission from supermassive black hole at
the Galactic Centre are the two prime candidates under consideration. We argue
that new generation of X-ray telescopes combining very high sensitivity and
excellent energy and angular resolutions would be able to discriminate between
these two possibilities studying time dependent changes of the morphology of
the surface brightness distribution, the equivalent width and the shape of the
fluorescent line in the Sgr B2 and other molecular clouds in the region. We
note also that detection of broad and complex structures near the 6.4 keV line
in the spectra of distant AGNs, which are X-ray weak now, may prove the
presence of violent activity of the central engines of these objects in the
past. Accurate measurements of the line shape may provide an information on the
time elapsed since the outburst. Proper motion (super or subluminal) of the
fluorescent radiation wave front can give additional information on the
location of the source. Observations of the described effects can provide
unique information on the matter distribution inside Sgr B2 and other giant
molecular clouds.Comment: 14 pages, 10 figures, accepted for publication in MNRA
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