3,500 research outputs found
The mass and radius of the M-dwarf in the short period eclipsing binary RR Caeli
We present new photometry and spectroscopy of the eclipsing white dwarf -
M-dwarf binary star RR Cae. We use timings of the primary eclipse from
white-light photo-electric photometry to derive a new ephemeris for the
eclipses. We find no evidence for any period change greater than Pdot/P ~ 5E-12
over a timescale of 10 years. We have measured the effective temperature of the
white dwarf, T_WD, from an analysis of two high resolution spectra of RR Cae
and find T_WD = (7540 +- 175)K. We estimate a spectral type of M4 for the
companion from the same spectra. We have combined new spectroscopic orbits for
the white dwarf and M-dwarf with an analysis of the primary eclipse and cooling
models for helium white dwarfs to measure the mass and radius of the M-dwarf.
The mass of the M-dwarf is (0.182 - 0.183) +- 0.013 Msun and the radius is
(0.203 - 0.215) +- 0.013 Rsun, where the ranges quoted for these values reflect
the range of white dwarf models used. In contrast to previous studies, which
lacked a spectroscopic orbit for the white dwarf, we find that the mass and
radius of the M-dwarf are normal for an M4 dwarf. The mass of the white dwarf
is (0.440 +-0.022) Msun. With these revised masses and radii we find that RR
Cae will become a cataclysmic variable star when the orbital period is reduced
from its current value of 7.3 hours to 121 minutes by magnetic braking in 9-20
Gyr. We note that there is night-to-night variability of a few seconds in the
timing of primary eclipse caused by changes to the shape of the primary
eclipse. We speculate as to the possible causes of this phenomenon. (Abridged)Comment: Accepted for publication in MNRAS. The paper contains 10 figures and
3 table
A common behavior in the late X-ray afterglow of energetic GRB-SN systems
The possibility to divide GRBs in different subclasses allow to understand
better the physics underlying their emission mechanisms and progenitors. The
induced gravitational collapse scenario proposes a binary progenitor to explain
the time-sequence in GRBs-SNe. We show the existence of a common behavior of
the late decay of the X-ray afterglow emission of this subclass of GRBs,
pointing to a common physical mechanism of their late emission, consistent with
the IGC picture.Comment: 3 pages, to appear in the proceedings of the Gamma-Ray Burst
Symposium 2012 - IAA-CSIC - Marbella, editors: Castro-Tirado, A. J.,
Gorosabel, J. and Park, I.
Negative Komar Mass of Single Objects in Regular, Asymptotically Flat Spacetimes
We study two types of axially symmetric, stationary and asymptotically flat
spacetimes using highly accurate numerical methods. The one type contains a
black hole surrounded by a perfect fluid ring and the other a rigidly rotating
disc of dust surrounded by such a ring. Both types of spacetime are regular
everywhere (outside of the horizon in the case of the black hole) and fulfil
the requirements of the positive energy theorem. However, it is shown that both
the black hole and the disc can have negative Komar mass. Furthermore, there
exists a continuous transition from discs to black holes even when their Komar
masses are negative.Comment: 7 pages, 2 figures, document class iopart. v2: changes made
(including title) to coincide with published versio
Induced gravitational collapse at extreme cosmological distances: the case of GRB 090423
CONTEXT: The induced gravitational collapse (IGC) scenario has been
introduced in order to explain the most energetic gamma ray bursts (GRBs),
Eiso=10^{52}-10^{54}erg, associated with type Ib/c supernovae (SNe). It has led
to the concept of binary-driven hypernovae (BdHNe) originating in a tight
binary system composed by a FeCO core on the verge of a SN explosion and a
companion neutron star (NS). Their evolution is characterized by a rapid
sequence of events: [...]. AIMS: We investigate whether GRB 090423, one of the
farthest observed GRB at z=8.2, is a member of the BdHN family. METHODS: We
compare and contrast the spectra, the luminosity evolution and the
detectability in the observations by Swift of GRB 090423 with the corresponding
ones of the best known BdHN case, GRB 090618. RESULTS: Identification of
constant slope power-law behavior in the late X-ray emission of GRB 090423 and
its overlapping with the corresponding one in GRB 090618, measured in a common
rest frame, represents the main result of this article. This result represents
a very significant step on the way to using the scaling law properties, proven
in Episode 3 of this BdHN family, as a cosmological standard candle.
CONCLUSIONS: Having identified GRB 090423 as a member of the BdHN family, we
can conclude that SN events, leading to NS formation, can already occur already
at z=8.2, namely at 650 Myr after the Big Bang. It is then possible that these
BdHNe originate stem from 40-60 M_{\odot} binaries. They are probing the
Population II stars after the completion and possible disappearance of
Population III stars.Comment: 9 pages, 9 figures, to appear on A&
A white dwarf merger as progenitor of the anomalous X-ray pulsar 4U 0142+61?
It has been recently proposed that massive fast-rotating highly-magnetized
white dwarfs could describe the observational properties of some of Soft
Gamma-Ray Repeaters (SGRs) and Anomalous X-Ray Pulsars (AXPs). Moreover, it has
also been shown that high-field magnetic (HFMWDs) can be the outcome of white
dwarf binary mergers. The products of these mergers consist of a hot central
white dwarf surrounded by a rapidly rotating disk. Here we show that the merger
of a double degenerate system can explain the characteristics of the peculiar
AXP 4U 0142+61. This scenario accounts for the observed infrared excess. We
also show that the observed properties of 4U 0142+6 are consistent with an
approximately 1.2 M_{\sun} white dwarf, remnant of the coalescence of an
original system made of two white dwarfs of masses 0.6\, M_{\sun} and 1.0\,
M_{\sun}. Finally, we infer a post-merging age kyr,
and a magnetic field G. Evidence for such a magnetic
field may come from the possible detection of the electron cyclotron absorption
feature observed between the and bands at Hz in the
spectrum of 4U 0142+61.Comment: to appear in ApJ Letter
General Relativistic Radiant Shock Waves in the Post-Quasistatic Approximation
An evolution of radiant shock wave front is considered in the framework of a
recently presented method to study self-gravitating relativistic spheres, whose
rationale becomes intelligible and finds full justification within the context
of a suitable definition of the post-quasistatic approximation. The spherical
matter configuration is divided into two regions by the shock and each side of
the interface having a different equation of state and anisotropic phase. In
order to simulate dissipation effects due to the transfer of photons and/or
neutrinos within the matter configuration, we introduce the flux factor, the
variable Eddington factor and a closure relation between them. As we expected
the strength of the shock increases the speed of the fluid to relativistic
values and for some critical ones is larger than light speed. In addition, we
find that energy conditions are very sensible to the anisotropy, specially the
strong one. As a special feature of the model, we find that the contribution of
the matter and radiation to the radial pressure are the same order of magnitude
as in the mant as in the core, moreover, in the core radiation pressure is
larger than matter pressure.Comment: To appear in Journal of Physics:Conference Series:"XXIX Spanish
Relativity Meeting (ERE 2006): Einstein's Legacy: From the Theoretical
Paradise to Astrophysical Observations
GRB 081024B and GRB 140402A: two additional short GRBs from binary neutron star mergers
Theoretical and observational evidences have been recently gained for a
two-fold classification of short bursts: 1) short gamma-ray flashes (S-GRFs),
with isotropic energy ~erg and no BH formation, and 2) the
authentic short gamma-ray bursts (S-GRBs), with isotropic energy
~erg evidencing a BH formation in the binary neutron star
merging process. The signature for the BH formation consists in the on-set of
the high energy (--~GeV) emission, coeval to the prompt emission, in
all S-GRBs. No GeV emission is expected nor observed in the S-GRFs. In this
paper we present two additional S-GRBs, GRB 081024B and GRB 140402A, following
the already identified S-GRBs, i.e., GRB 090227B, GRB 090510 and GRB 140619B.
We also return on the absence of the GeV emission of the S-GRB 090227B, at an
angle of from the \textit{Fermi}-LAT boresight. All the correctly
identified S-GRBs correlate to the high energy emission, implying no
significant presence of beaming in the GeV emission. The existence of a common
power-law behavior in the GeV luminosities, following the BH formation, when
measured in the source rest-frame, points to a commonality in the mass and spin
of the newly-formed BH in all S-GRBs.Comment: 16 pages, submitted to ApJ, second version addressing the comments by
the refere
Correlating densities of centrality and activities in cities : the cases of Bologna (IT) and Barcelona (ES)
This paper examines the relationship between street centrality and densities of commercial and service activities in cities. The aim is to verify whether a correlation exists and whether some 'secondary' activities, i.e. those scarcely specialized oriented to the general public and ordinary daily life, are more linked to street centrality than others. The metropolitan area of Barcelona (Spain) is investigated, and results are compared with those found in a previous work on the city of Bologna (Italy). Street centrality is calibrated in a multiple centrality assessment (MCA) model composed of multiple measures such as closeness, betweenness and straightness. Kernel density estimation (KDE) is used to transform data sets of centrality and activities to one scale unit for correlation analysis between them. Results indicate that retail and service activities in both Bologna and Barcelona tend to concentrate in areas with better centralities, and that secondary activities exhibit a higher correlation
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