54 research outputs found
Tidal Disruption Flares: The Accretion Disk Phase
The evolution of an accretion disk, formed as a consequence of the disruption
of a star by a black hole, is followed by solving numerically the hydrodynamic
equations. The present investigation aims to study the dependence of resulting
light curves on dynamical and physical properties of such a transient disk
during its existence. One of main results derived from our simulations is that
black body fits of X-ray data tend to overestimate the true mean disk
temperature. The temperature derived from black body fits should be identified
with the color X-ray temperature rather than the average value derived from the
true temperature distribution along the disk. The time interval between the
beginning of the circularization of the bound debris and the beginning of the
accretion process by the black hole is determined by the viscous timescale,
which fixes also the raising part of the resulting light curve. The luminosity
peak coincides with the beginning of matter accretion by the black hole and the
late evolution of the light curve depends on the evolution of the debris
fallback rate. Peak bolometric luminosities are in the range 10^45-10^46 erg
s^-1 whereas peak luminosities in soft X-rays (0.2-2.0 keV) are typically one
order of magnitude lower. The timescale derived from our preferred models for
the flare luminosity to decay by two orders of magnitude is about 3-4 years.
Predicted soft X-ray light curves were fitted to data on galaxies in which a
variable X-ray emission, related to tidal events, was detected.Comment: 14 pages, 11 figures, Accepted for publication in Ap
Evolution of the phase-space density and the Jeans scale for dark matter derived from the Vlasov-Einstein equation
We discuss solutions of Vlasov-Einstein equation for collisionless dark
matter particles in the context of a flat Friedmann universe. We show that,
after decoupling from the primordial plasma, the dark matter phase-space
density indicator Q remains constant during the expansion of the universe,
prior to structure formation. This well known result is valid for
non-relativistic particles and is not "observer dependent" as in solutions
derived from the Vlasov-Poisson system. In the linear regime, the inclusion of
velocity dispersion effects permits to define a physical Jeans length for
collisionless matter as function of the primordial phase-space density
indicator: \lambda_J = (5\pi/G)^(1/2)Q^(-1/3)\rho_dm^(-1/6). The comoving Jeans
wavenumber at matter-radiation equality is smaller by a factor of 2-3 than the
comoving wavenumber due to free-streaming, contributing to the cut-off of the
density fluctuation power spectrum at the lowest scales. We discuss the
physical differences between these two scales. For dark matter particles of
mass equal to 200 GeV, the derived Jeans mass is 4.3 x 10^(-6) solar masses.Comment: 18 pages, 2 figures. Accepted for publication in JCA
Visser's Massive Gravity Bimetric Theory Revisited
A massive gravity theory was proposed by Visser in the late nineties. This
theory, based on a backgroung metric and on an usual
dynamical metric has the advantage of being free of ghosts
as well as discontinuities present in other massive theories proposed in the
past. In the present investigation, the equations of Visser's theory are
revisited with a particular care on the related conservation laws.\ It will be
shown that a multiplicative factor is missing in the graviton tensor originally
derived by Visser, which has no incidence on the weak field approach but
becomes important in the strong field regime when, for instance, cosmological
applications are considered. In this case, contrary to some previous claims
found in the literature, we conclude that a non-static background metric is
required in order to obtain a solution able to mimic the CDM
cosmology.Comment: 10 pages - Accepted for publication in Physical Review
Advanced VIRGO: detector optimization for gravitational waves by inspiralling binaries
For future configurations, we study the relation between the abatement of the
noise sources and the Signal to Noise Ratio (SNR) for coalescing binaries. Our
aim is not the proposition of a new design, but an indication of where in the
bandwidth or for which noise source, a noise reduction would be most efficient.
We take VIRGO as the reference for our considerations, solely applicable to the
inspiralling phase of a coalescing binary. Thus, only neutron stars and small
black holes of few solar masses are encompassed by our analysis. The
contributions to the SNR given by final merge and quasi-normal ringing are
neglected. It is identified that i) the reduction in the mirror thermal noise
band provides the highest gain for the SNR, when the VIRGO bandwidth is divided
according to the dominant noises; ii) it exists a specific frequency at which
lies the potential largest increment in the SNR, and that the enlargement of
the bandwidth, where the noise is reduced, produces a shift of such optimal
frequency to higher values; iii) the abatement of the pendulum thermal noise
provides the largest, but modest, gain, when noise sources are considered
separately. Our recent astrophysical analysis on event rates for neutron stars
leads to a detection rate of one every 148 or 125 years for VIRGO and LIGO,
respectively, while a recently proposed and improved, but still conservative,
VIRGO configuration would provide an increase to 1.5 events per year. Instead,
a bi-monthly event rate, similar to advanced LIGO, requires a 16 times gain. We
analyse the 3D (pendulum, mirror, shot noises) parameter space showing how such
gain could be achieved.Comment: Change of title (Virgo detector optimization for gravitational waves
by coalescing binaries) and partially of text. 6 figure
A Search for Very Active Stars in the Galaxy
We report the first results of a systematic search near the plane of the
Galaxy for the so called very active stars (VAS), which are characterized by a
hard X-ray spectrum and activity in the radio domain. Candidates with hard
X-ray binary-like spectra have been selected from the Bright ROSAT Source
Catalogue in the Zone of Avoidance () and were tentatively
identified in GB6/PMM/NVSS radio surveys. Most of them were observed with the
ATCA and VLA. Precise radio coordinates have led to unambiguous optical
identification for 60 candidates, and a sub-sample of five of themhas been
observed with the VLT. Also some discovery and confirmatory spectra were
obtained with the AAT (4-m) and BTA (6-m). Spectroscopy with moderate
dispersion, made with the FORS1 spectrograph of the VLT has revealed two
stellar objects (one of them, VASC J1628-41, is definitivelya binary VAS), one
new AGN and two featureless spectrum sources. One of these objects, VASC
J1353-66, shows a marginal evidence of proper motion, which, if confirmed,
would imply the discovery of a new type of galactic source.Comment: to appear in A&A, 7 figure
Indirect search for dark matter: prospects for GLAST
Possible indirect detection of neutralino, through its gamma-ray annihilation
product, by the forthcoming GLAST satellite from our galactic halo, M31, M87
and the dwarf galaxies Draco and Sagittarius is studied. Gamma-ray fluxes are
evaluated for the two representative energy thresholds, 0.1 GeV and 1.0 GeV, at
which the spatial resolution of GLAST varies considerably. Apart from dwarfs
which are described either by a modified Plummer profile or by a
tidally-truncated King profiles, fluxes are compared for halos with central
cusps and cores. It is demonstrated that substructures, irrespective of their
profiles, enhance the gamma-ray emission only marginally. The expected
gamma-ray intensity above 1 GeV at high galactic latitudes is consistent with
the residual emission derived from EGRET data if the density profile has a
central core and the neutralino mass is less than 50 GeV, whereas for a central
cusp only a substantial enhancement would explain the observations. From M31,
the flux can be detected above 0.1 GeV and 1.0 GeV by GLAST only if the
neutralino mass is below 300 GeV and if the density profile has a central cusp,
case in which a significant boost in the gamma-ray emission is produced by the
central black hole. For Sagittarius, the flux above 0.1 GeV is detectable by
GLAST provided the neutralino mass is below 50 GeV. From M87 and Draco the
fluxes are always below the sensitivity limit of GLAST.Comment: 14 Pages, 7 Figures, 3 Tables, version to appear on Physical Review
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