1,393 research outputs found

### Finite-time singularities in f(R, T) gravity and the effect of conformal anomaly

We investigate $f(R,T)$ gravity models ($R$ is the curvature scalar and $T$
is the trace of the stress-energy tensor of ordinary matter) that are able to
reproduce the four known types of future finite-time singularities. We choose a
suitable expression for the Hubble parameter in order to realise the cosmic
acceleration and we introduce two parameters, $\alpha$ and $H_s$, which
characterise each type of singularity. We address conformal anomaly and we
observe that it cannot remove the sudden singularity or the type IV one, but,
for some values of $\alpha$, the big rip and the type III singularity may be
avoided. We also find that, even without taking into account conformal anomaly,
the big rip and the type III singularity may be removed thanks to the presence
of the $T$ contribution of the $f(R,T)$ theory.Comment: 18 pages; Accepted for publication in Canadian Journal of Physics
(CJP

### Diffuse Hard X-ray Sources Discovered with the ASCA Galactic Plane Survey

We found diffuse hard X-ray sources, G11.0+0.0, G25.5+0.0, and G26.6-0.1 in
the ASCA Galactic plane survey data. The X-ray spectra are featureless with no
emission line, and are fitted with both models of a thin thermal plasma in
non-equilibrium ionization and a power-law function. The source distances are
estimated to be 1-8 kpc, using the best-fit NH values on the assumption that
the mean density in the line of sight is 1 H cm^-3. The source sizes and
luminosities are then 4.5-27 pc and (0.8-23)x10^33 ergs/s. Although the source
sizes are typical to supernova remnants (SNR) with young to intermediate ages,
the X-ray luminosity, plasma temperature, and weak emission lines in the
spectra are all unusual. This suggests that these objects are either shell-like
SNRs dominated by X-ray synchrotron emission, like SN 1006, or, alternatively,
plerionic SNRs. The total number of these classes of SNRs in our Galaxy is also
estimated.Comment: 17 pages, 9 figures; to appear in Ap

### Phantom crossing in viable $f(R)$ theories

We review the equation of state for dark energy in modified gravity theories.
In particular, we summarize the generic feature of the phantom divide crossing
in the past and future in viable $f(R)$ gravity models.Comment: 7 pages, 3 figures, talk presented at the 2nd International Workshop
on Dark Matter, Dark Energy and Matter-Antimatter Asymmetry, Hsinchu, Taiwan,
5-6 Nov 201

### Helical Magnetic Fields from Inflation

We analyze the generation of seed magnetic fields during de Sitter inflation
considering a non-invariant conformal term in the electromagnetic Lagrangian of
the form $-\frac14 I(\phi) F_{\mu \nu} \widetilde{F}^{\mu \nu}$, where
$I(\phi)$ is a pseudoscalar function of a non-trivial background field $\phi$.
In particular, we consider a toy model, that could be realized owing to the
coupling between the photon and either a (tachyonic) massive pseudoscalar field
and a massless pseudoscalar field non-minimally coupled to gravity, where $I$
follows a simple power-law behavior $I(k,\eta) = g/(-k\eta)^{\beta}$ during
inflation, while it is negligibly small subsequently. Here, $g$ is a positive
dimensionless constant, $k$ the wavenumber, $\eta$ the conformal time, and
$\beta$ a real positive number. We find that only when $\beta = 1$ and $0.1
\lesssim g \lesssim 2$ astrophysically interesting fields can be produced as
excitation of the vacuum, and that they are maximally helical.Comment: 17 pages, 1 figure, subsection IIc and references added; accepted for
publication in IJMP

### Chandra Observations of A Galactic Supernova Remnant Vela Jr.: A New Sample of Thin Filaments Emitting Synchrotron X-Rays

A galactic supernova remnant (SNR) Vela Jr. (RX J0852.0$-$4622, G266.6$-$1.2)
shows sharp filamentary structure on the north-western edge of the remnant in
the hard X-ray band. The filaments are so smooth and located on the most outer
side of the remnant. We measured the averaged scale width of the filaments
($w_u$ and $w_d$) with excellent spatial resolution of {\it Chandra}, which are
in the order of the size of the point spread function of {\it Chandra} on the
upstream side and 49.5 (36.0--88.8) arcsec on the downstream side,
respectively. The spectra of the filaments are very hard and have no line-like
structure, and were well reproduced with an absorbed power-law model with
$\Gamma =$2.67 (2.55--2.77), or a {\tt SRCUT} model with $\nu_{rolloff}$ = 4.3
(3.4--5.3)$\times 10^{16}$ Hz under the assumption of $p=0.3$. These results
imply that the hard X-rays are synchrotron radiation emitted by accelerated
electrons, as mentioned previously. Using a correlation between a function
${\cal B} \equiv \nu_{rolloff}/w_d^2$ and the SNR age, we estimated the
distance and the age of Vela Jr.: the estimated distance and age are 0.33
(0.26--0.50) kpc and 660 (420--1400) years, respectively. These results are
consistent with previous reports, implying that ${\cal B}$--age relation may be
a useful tool to estimate the distance and the age of synchrotron X-ray
emitting SNRs.Comment: 19 pages, 8 figures, ApJ, in pres

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