On the stability of self-gravitating accreting flows

Analytic methods show stability of the stationary accretion of test fluids but they are inconclusive in the case of self-gravitating stationary flows. We investigate numerically stability of those stationary flows onto compact objects that are transonic and rich in gas. In all studied examples solutions appear stable. Numerical investigation suggests also that the analogy between sonic and event horizons holds for small perturbations of compact support but fails in the case of finite perturbations.Comment: 10 pages, accepted for publication in PR

On the Magnitude of Dark Energy Voids and Overdensities

We investigate the clustering of dark energy within matter overdensities and voids. In particular, we derive an analytical expression for the dark energy density perturbations, which is valid both in the linear, quasi-linear and fully non-linear regime of structure formation. We also investigate the possibility of detecting such dark energy clustering through the ISW effect. In the case of uncoupled quintessence models, if the mass of the field is of order the Hubble scale today or smaller, dark energy fluctuations are always small compared to the matter density contrast. Even when the matter perturbations enter the non-linear regime, the dark energy perturbations remain linear. We find that virialised clusters and voids correspond to local overdensities in dark energy, with \delta_{\phi}/(1+w) \sim \Oo(10^{-5}) for voids, \delta_{\phi}/(1+w) \sim \Oo(10^{-4}) for super-voids and \delta_{\phi}/(1+w) \sim \Oo(10^{-5}) for a typical virialised cluster. If voids with radii of $100-300 {\rm Mpc}$ exist within the visible Universe then $\delta_{\phi}$ may be as large as $10^{-3}(1+w)$. Linear overdensities of matter and super-clusters generally correspond to local voids in dark energy; for a typical super-cluster: \delta_{\phi}/(1+w) \sim \Oo(-10^{-5}). The approach taken in this work could be straightforwardly extended to study the clustering of more general dark energy models.Comment: 20 pages, 14 figures. Accepted by the Astrophys.

A Classical Treatment of Island Cosmology

Computing the perturbation spectrum in the recently proposed Island Cosmology remains an open problem. In this paper we present a classical computation of the perturbations generated in this scenario by assuming that the NEC-violating field behaves as a classical phantom field. Using an exactly-solvable potential, we show that the model generates a scale-invariant spectrum of scalar perturbations, as well as a scale-invariant spectrum of gravitational waves. The scalar perturbations can have sufficient amplitude to seed cosmological structure, while the gravitational waves have a vastly diminished amplitude.Comment: 8 pages, 1 figur

On compressive radial tidal forces

Radial tidal forces can be compressive instead of disruptive, a possibility that is frequently overlooked in high level physics courses. For example, radial tidal compression can emerge in extended stellar systems containing a smaller stellar cluster. For particular conditions the tidal field produced by this extended mass distribution can exert on the cluster it contains compressive effects instead of the common disruptive forces. This interesting aspect of gravity can be derived from standard relations given in many textbooks and introductory courses in astronomy and can serve as an opportunity to look closer at some aspects of gravitational physics, stellar dynamics, and differential geometry. The existence of compressive tides at the center of huge stellar systems might suggest new evolutionary scenarios for the formation of stars and primordial galactic formation processes.Comment: 22 pages, 2 figure

Wiggles in the cosmic microwave background radiation: echoes from non-singular cyclic-inflation

In this paper we consider a unique model of inflation where the universe undergoes rapid asymmetric oscillations, each cycle lasting millions of Planck time. Over many-many cycles the space-time expands to mimic the standard inflationary scenario. Moreover, these rapid oscillations leave a distinctive periodic signature in ln(k) in the primordial power spectrum, where k denotes the comoving scale. The best fit parameters of the cyclic-inflation model provides a very good fit to the 7-year WMAP data.Comment: Computational details and a figure adde

Flux density measurements of a complete sample of faint blazars

We performed observations with the Effelsberg 100-m radio telescope to measure flux densities and polarised emission of sources selected from the "Deep X-ray Radio Blazar Survey" (DXRBS) to better define their spectral index behaviour in the radio band, with the aim to construct a homogeneous sample of blazars. Sources were observed at four different frequencies with the Effelsberg 100-m telescope. We complemented these measurements with flux density data at 1.4GHz derived from the NRAO VLA Sky Survey.The spectral indices of a sample of faint blazars were computed making use of almost simultaneous measurements. Sixty-six percent of the sources can be classified as "bona fide" blazars. Seven objects show a clearly inverted spectral index. Seventeen sources previously classified as flat spectrum radio quasars (FSRQs) are actually steep spectrum radio quasars (SSRQs). The flux densities obtained with the Effelsberg 100-m telescope at 5GHz are compared with the flux densities listed in the Green Bank GB6 survey and in the Parkes-MIT-NRAO PMN catalogue. About 43% of the sources in our sample exhibit flux density variations on temporal scales of 19 or 22 years. We confirm that 75 out of 103 sources of the DXRBS are indeed FSRQs. Twenty-seven sources show a spectral index steeper than -0.5 and should be classified as SSRQs. Polarised emission was detected for 36 sources at 4.85GHz. The median value of the percentage of polarised emission is (5.8+-0.9$)%. Five sources show rotation measure (RM) values >200 rad m^-2.Comment: Accepted for publication in Astronomy and Astrophysic

Magnetically Accreting Isolated Old Neutron Stars

Previous work on the emission from isolated old neutron stars (IONS) accreting the inter-stellar medium (ISM) focussed on gravitational capture - Bondi accretion. We propose a new class of sources which accrete via magnetic interaction with the ISM. While for the Bondi mechanism, the accretion rate decreases with increasing NS velocity, in magnetic accretors (MAGACs="magics") the accretion rate increases with increasing NS velocity. MAGACs will be produced among high velocity (~> 100 km s-1) high magnetic field (B> 1e14 G) radio pulsars - the ``magnetars'' - after they have evolved first through magnetic dipole spin-down, followed by a ``propeller'' phase (when the object sheds angular momentum on a timescale ~< 1e10 yr). The properties of MAGACS may be summarized thus: dipole magnetic fields of B~>1e14 G; minimum velocities relative to the ISM of >25-100 km s-1, depending on B, well below the median in the observed radio-pulsar population; spin-periods of >days to years; accretion luminosities of 1e28- 1e31 ergs s-1 ; and effective temperatures kT=0.3 - 2.5 keV if they accrete onto the magnetic polar cap. We find no examples of MAGACs among previously observed source classes (anomalous X-ray pulsars, soft-gamma-ray repeaters or known IONS). However, MAGACs may be more prevelant in flux-limited X-ray catalogs than their gravitationally accreting counterparts.Comment: ApJ, accepte

Accretion of Ghost Condensate by Black Holes

The intent of this letter is to point out that the accretion of a ghost condensate by black holes could be extremely efficient. We analyze steady-state spherically symmetric flows of the ghost fluid in the gravitational field of a Schwarzschild black hole and calculate the accretion rate. Unlike minimally coupled scalar field or quintessence, the accretion rate is set not by the cosmological energy density of the field, but by the energy scale of the ghost condensate theory. If hydrodynamical flow is established, it could be as high as tenth of a solar mass per second for 10MeV-scale ghost condensate accreting onto a stellar-sized black hole, which puts serious constraints on the parameters of the ghost condensate model.Comment: 5 pages, 3 figures, REVTeX 4.0; discussion expande

The cosmological BCS mechanism and the Big Bang Singularity

We provide a novel mechanism that resolves the Big Bang Singularity present in FRW space-times without the need for ghost fields. Building on the fact that a four-fermion interaction arises in General Relativity when fermions are covariantly coupled, we show that at early times the decrease in scale factor enhances the correlation between pairs of fermions. This enhancement leads to a BCS-like condensation of the fermions and opens a gap dynamically driving the Hubble parameter $H$ to zero and results in a non-singular bounce, at least in some special cases.Comment: replaced to match the journal versio

The Evolution of the M-sigma Relation

(Abridged) We examine the evolution of the black hole mass - stellar velocity dispersion (M-sigma) relation over cosmic time using simulations of galaxy mergers that include feedback from supermassive black hole growth. We consider mergers of galaxies varying the properties of the progenitors to match those expected at redshifts z=0-6. We find that the slope of the resulting M-sigma relation is the same at all redshifts considered. For the same feedback efficiency that reproduces the observed amplitude of the M-sigma relation at z=0, there is a weak redshift-dependence to the normalization that results from an increasing velocity dispersion for a given galactic stellar mass. We develop a formalism to connect redshift evolution in the M-sigma relation to the scatter in the local relation at z=0. We show that the scatter in the local relation places severe constraints on the redshift evolution of both the normalization and slope of the M-sigma relation. Furthermore, we demonstrate that cosmic downsizing introduces a black hole mass-dependent dispersion in the M-sigma relation and that the skewness of the distribution about the locally observed M-sigma relation is sensitive to redshift evolution in the normalization and slope. In principle, these various diagnostics provide a method for differentiating between theories for producing the M-sigma relation. In agreement with existing constraints, our simulations imply that hierarchical structure formation should produce the relation with small intrinsic scatter.Comment: 12 pages, 6 figures, version accepted by Ap