43,160 research outputs found
Signature of the Van der Waals like small-large charged AdS black hole phase transition in quasinormal modes
We calculate the quasinormal modes of massless scalar perturbations around
small and large four-dimensional Reissner-Nordstrom-Anti de Sitter (RN-AdS)
black holes. We find a dramatic change in the slopes of quasinormal frequencies
in small and large black holes near the critical point where the Van der Waals
like thermodynamic phase transition happens. This further supports that the
quasinormal mode can be a dynamic probe of the thermodynamic phase transition.Comment: 20 pages,11 figures.The new version is accepted for publication in
JHE
Observational Bounds on Modified Gravity Models
Modified gravity provides a possible explanation for the currently observed
cosmic accelaration. In this paper, we study general classes of modified
gravity models. The Einstein-Hilbert action is modified by using general
functions of the Ricci and the Gauss-Bonnet scalars, both in the metric and in
the Palatini formalisms. We do not use an explicit form for the functions, but
a general form with a valid Taylor expansion up to second order about redshift
zero in the Riemann-scalars. The coefficients of this expansion are then
reconstructed via the cosmic expansion history measured using current
cosmological observations. These are the quantities of interest for theoretical
considerations relating to ghosts and instabilities. We find that current data
provide interesting constraints on the coefficients. The next-generation dark
energy surveys should shrink the allowed parameter space for modifed gravity
models quite dramatically.Comment: 23 pages, 5 figures, uses RevTe
Modelling and interpreting the dependence of clustering on the spectral energy distributions of galaxies
We extend our previous physically-based halo occupation distribution models
to include the dependence of clustering on the spectral energy distributions of
galaxies. The high resolution Millennium Simulation is used to specify the
positions and the velocities of the model galaxies. The stellar mass of a
galaxy is assumed to depend only on M_{infall}, the halo mass when the galaxy
was last the central dominant object of its halo. Star formation histories are
parametrized using two additional quantities that are measured from the
simulation for each galaxy: its formation time (t_{form}), and the time when it
first becomes a satellite (t_{infall}). Central galaxies begin forming stars at
time t_{form} with an exponential time scale tau_c. If the galaxy becomes a
satellite, its star formation declines thereafter with a new time scale tau_s.
We compute 4000 \AA break strengths for our model galaxies using stellar
population synthesis models. By fitting these models to the observed abundances
and projected correlations of galaxies as a function of break strength in the
Sloan Digital Sky Survey, we constrain tau_c and tau_s as functions of galaxy
stellar mass. We find that central galaxies with large stellar masses have
ceased forming stars. At low stellar masses, central galaxies display a wide
range of different star formation histories, with a significant fraction
experiencing recent starbursts. Satellite galaxies of all masses have declining
star formation rates, with similar e-folding times, tau_s ~ 2.5Gyr. One
consequence of this long e-folding time is that the colour-density relation is
predicted to flatten at redshifts > 1.5, because star formation in the majority
of satellites has not yet declined by a significant factor. This is consistent
with recent observational results from the DEEP and VVDS surveys.Comment: 14 pages, 11 figures, submitted to MNRA
The kHz QPOs as a probe of the X-ray color-color diagram and accretion-disk structure for the atoll source 4U 1728-34
We have taken the kHz QPOs as a tool to probe the correlation between the
tracks of X-ray color-color diagram (CCD) and magnetosphere-disk positions for
the atoll source 4U 1728-34, based on the assumptions that the upper kHz QPO is
ascribed to the Keplerian orbital motion and the neutron star (NS)
magnetosphere is defined by the dipole magnetic field. We find that from the
island to the banana state, the inner accretion disk gradually approaches the
NS surface with the radius decreasing from r ~33.0km to ~15.9 km, corresponding
to the magnetic field from B(r) ~4.8*10^6 G to ~4.3*10^7 G. In addition, we
note the characteristics of some particular radii of magnetosphere-disk -r are:
firstly, the whole atoll shape of the CCD links the disk radius range of ~15.9
- 33.0 km, which is just located inside the corotation radius of 4U 1728-34
-r_co ( ~34.4 km), implying that the CCD shape is involved in the NS spin-up
state. Secondly, the island and banana states of CCD correspond to the two
particular boundaries: (I)-near the corotation radius at r ~27.2 - 33.0 km,
where the source lies in the island state; (II)-near the NS surface at r ~15.9
- 22.3 km, where the source lies in both the island and banana states. Thirdly,
the vertex of the atoll shape in CCD, where the radiation transition from the
hard to soft photons occurs, is found to be near the NS surface at r ~16.4 km.
The above results suggest that both the magnetic field and accretion
environment are related to the CCD structure of atoll track, where the
corotation radius and NS hard surface play the significant roles in the
radiation distribution of atoll source.Comment: 6 pages, 3 figures, 1 table, accepted by Astronomy & Astrophysic
- …