3,511 research outputs found
A method to determine the parameters of black holes in AGNs and galactic X-ray sources with periodic modulation of variability
We propose a simple and unambiguous way to deduce the parameters of black
holes which may reside in AGNs and some types of X-ray binaries. The black-hole
mass and angular momentum are determined in physical units. The method is
applicable to the sources with periodic components of variability, provided one
can assume the following: (i) Variability is due to a star or a stellar-mass
compact object orbiting the central black hole and passing periodically through
an equatorial accretion disk (variability time-scale is given by the orbital
period). (ii) The star orbits almost freely, deviation of its trajectory due to
passages through the disk being very weak (secular); the effect of the star on
the disk, on the other hand, is strong enough to yield observable photometric
and spectroscopic features. (iii) The gravitational field within the nucleus is
that of the (Kerr) black hole, the star and the disk contribute negligibly.Comment: 17 pages, 4 figures, 2 tables; LaTeX2.09 (aas2pp4.sty); submitted to
the Astrophysical Journal; also available at
http://astro.mff.cuni.cz/karas/papers.htm with additional illustration
Vainshtein mechanism in Gauss-Bonnet gravity and Galileon aether
We derive field equations of Gauss-Bonnet gravity in 4 dimensions after
dimensional reduction of the action and demonstrate that in this scenario
Vainshtein mechanism operates in the flat spherically symmetric background. We
show that inside this Vainshtein sphere the fifth force is negligibly small
compared to the gravitational force. We also investigate stability of the
spherically symmetric solution, clarify the vocabulary used in the literature
about the hyperbolicity of the equation and the ghost-Laplacian stability
conditions. We find superluminal behavior of the perturbation of the field in
the radial direction. However, because of the presence of the non linear terms,
the structure of the space-time is modified and as a result the field does not
propagate in the Minkowski metric but rather in an "aether" composed by the
scalar field . We thereby demonstrate that the superluminal behavior
does not create time paradoxes thank to the absence of Causal Closed Curves. We
also derive the stability conditions for Friedmann Universe in context with
scalar and tensor perturbations.Comment: 9 pages, 5 figures, references added, more details on the
cosmological analysis included, results and conclusions unchanged, final
version to appear in PR
Advances in the management of HPV-related oropharyngeal cancer
Patients with human papillomavirus- (HPV-) related oropharyngeal squamous cell carcinoma (OPSCC) have a better prognosis
than HPV-negative OPSCC when treated with standard high-dose cisplatin-based chemoradiotherapy. Consistent with this
assertion and due to younger age at diagnosis, novel approaches tominimize treatment sequelaewhile preserving survival outcomes
become of paramount importance. Here, we critically reviewed the evidence-based literature supporting the deintensification
strategies in HPV-related OPSCC management, including radiotherapy dose and/or volume reduction, replacement of cisplatin
radiosensitising chemotherapy, and the use of transoral surgery. Undoubtedly, further researches are needed before changing the
standard of care in this setting of patients
Excess of weight: is it a modifiable predictive and prognostic factor in locally advanced rectal cancer?
To evaluate the relationship between body mass index (BMI) and rates of treatment tolerance and clinical outcomes in patients with locally advanced rectal cancer treated with a multimodality approach.
PATIENTS AND METHODS:
This study was conducted on 56 patients with histologically proven rectal adenocarcinoma, staged T3-4, and/or node-positive tumor, which underwent intensified radiochemotherapy (RT-CHT) treatment before surgery. We calculated adiposity indices and analyzed their influence on treatment tolerance and clinical outcomes.
RESULTS:
Distribution of the 56 patients according to BMI was BMI < 25 kg/m2 (n = 19; 33.9%), BMI 25-29 kg/m2 (n = 29; 51.8%) and BMI ≥ 30 kg/m2 (n = 8; 14.3%). BMI had no significant influence on neo-adjuvant treatment-related toxicity. With a median follow-up of 23 months (range 11-47), the 2-year survival was 85.7%. We did not observe any significant difference among the three BMI categories for any of the outcomes.
CONCLUSIONS:
This study suggested no evident links between overweight and survival in patients with locally advanced rectal carcinoma treated with neo-adjuvant RT-CHT. Overweight patients tolerate treatment as normal-weight patients
Cosmological perturbations of a perfect fluid and noncommutative variables
We describe the linear cosmological perturbations of a perfect fluid at the
level of an action, providing thus an alternative to the standard approach
based only on the equations of motion. This action is suited not only to
perfect fluids with a barotropic equation of state, but also to those for which
the pressure depends on two thermodynamical variables. By quantizing the system
we find that (1) some perturbation fields exhibit a noncommutativity quite
analogous to the one observed for a charged particle moving in a strong
magnetic field, (2) local curvature and pressure perturbations cannot be
measured simultaneously, (3) ghosts appear if the null energy condition is
violated.Comment: 4 pages, uses RevTeX. Title modified, references and comments added
Modulation of galactic protons in the heliosphere during the unusual solar minimum of 2006 to 2009
The last solar minimum activity period, and the consequent minimum modulation
conditions for cosmic rays, was unusual. The highest levels of galactic protons
were recorded at Earth in late 2009 in contrast to expectations. Proton spectra
observed for 2006 to 2009 from the PAMELA cosmic ray detector on-board the
Resurs-DK1 satellite are presented together with the solutions of a
comprehensive numerical model for the solar modulation of cosmic rays. The
model is used to determine what mechanisms were mainly responsible for the
modulation of protons during this period, and why the observed spectrum for
2009 was the highest ever recorded. From mid-2006 until December 2009 we find
that the spectra became significantly softer because increasingly more low
energy protons had reached Earth. To simulate this effect, the rigidity
dependence of the diffusion coefficients had to decrease significantly below ~3
GeV. The modulation minimum period of 2009 can thus be described as relatively
more "diffusion dominated" than previous solar minima. However, we illustrate
that drifts still had played a significant role but that the observable
modulation effects were not as well correlated with the waviness of the
heliospheric current sheet as before. Protons still experienced global gradient
and curvature drifts as the heliospheric magnetic field had decreased
significantly until the end of 2009, in contrast to the moderate decreases
observed during previous minimum periods. We conclude that all modulation
processes contributed to the observed increases in the proton spectra for this
period, exhibiting an intriguing interplay of these major mechanisms
Galileon gravity and its relevance to late time cosmic acceleration
We consider the covariant galileon gravity taking into account the third
order and fourth order scalar field Lagrangians L_3(\pi) and L_4(\pi)
consisting of three and four 's with four and five derivatives acting on
them respectively. The background dynamical equations are set up for the system
under consideration and the stability of the self accelerating solution is
demonstrated in general setting. We extended this study to the general case of
the fifth order theory. For spherically symmetric static background, we spell
out conditions for suppression of fifth force effects mediated by the galileon
field . We study the field perturbations in the fixed background and
investigate conditions for their causal propagation. We also briefly discuss
metric fluctuations and derive evolution equation for matter perturbations in
galileon gravity.Comment: 11 pages, no figure, minor clarifications and few refs added, to
appear in pr
- …