7,338 research outputs found
SO(3) family symmetry and axions
Motivated by the idea of Comprehensive Unification, we consider a gauged
flavor extension of the Standard Model, including right-handed
neutrinos and a Peccei-Quinn symmetry. The model accommodates the observed
fermion masses and mixings and yields a characteristic, successful relation
among them. The Peccei-Quinn symmetry is an essential ingredient.Comment: Contribution to the 2019 EW session of the 54th Rencontres de Morion
Spectral states in Be/X-ray pulsars
In the last quarter of a century, a unified characterization of the spectral
evolution of low-mass X-ray binaries, both containing a neutron star and a
black hole, was possible. In this context, the notion of source states
characterizing the X-ray emission from black-hole binaries and neutron-star
low-mass X-ray binaries revealed to be a very useful tool to disentangle the
complex spectral and aperiodic phenomenology displayed by those classes of
accreting objects. Be/X-ray binaries constitute another major class of
transient accreting binaries, for which very little work has been done on the
correlated timing and spectral variability. Especially, no definition of source
states exists for this class, in spite of their highly variable X-ray emission.
When active, Be/X-ray binaries are among the brightest objects in the X-ray sky
and are characterized by dramatic variability in brightness on timescales
ranging from seconds to years. It is then worth it to ask whether a definition
of spectral states is possible for these systems. In this work, we try to
address such a question, investigating whether accreting X-ray pulsars display
source states and characterizing those states through their spectral
properties. Our results show that Be/X-ray pulsars trace two different branches
in their hardness-intensity diagram: the horizontal branch, a low-intensity
state, and the diagonal branch, a high-intensity state that only appears when
the X-ray luminosity exceeds a critical limit. We propose that the two branches
are the phenomenological signature of two different accretion modes -- in
agreement with recently proposed models -- depending on whether the luminosity
of the source is above or below a critical value.Comment: Proceedings of "An INTEGRAL view of the high-energy sky (the first 10
years)" the 9th INTEGRAL Workshop, October 15-19, 2012, Paris, France, in
Proceedings of Science (INTEGRAL 2012), Eds. A. Goldwurm, F. Lebrun and C.
Winkler, (http://pos.sissa.it/cgi-bin/reader/conf.cgi?confid=176), id 02
Disc-loss episode in the Be shell optical counterpart to the high-mass X-ray binary IGR J21343+4738
The main goal of this work is to determine the properties of the optical
counterpart to the INTEGRAL source IGR J21343+4738, and study its long-term
optical variability. We present optical photometric BVRI and spectroscopic
observations covering the wavelength band 4000-7500 A. We find that the optical
counterpart to IGR J21343+4738 is a V=14.1 B1IVe shell star located at a
distance of ~8.5 kpc. The Halpha line changed from an absorption dominated
profile to an emission dominated profile, and then back again into absorption.
In addition, fast V/R asymmetries were observed once the disc developed.
Although the Balmer lines are the most strongly affected by shell absorption,
we find that shell characteristics are also observed in He I lines. The optical
spectral variability of IGR J21343+4738 is attributed to the formation of an
equatorial disc around the Be star and the development of an enhanced density
perturbation that revolves inside the disc. We have witnessed the formation and
dissipation of the circumstellar disc. The strong shell profile of the Halpha
and He I lines and the fact that no transition from shell phase to a pure
emission phase is seen imply that we are seeing the system near edge-on.Comment: accepted for publication in A&
Warped disks during type II outbursts in Be/X-ray binaries: evidence from optical polarimetry
Current models that explain giant (type II) X-ray outbursts in Be/X-ray
binaries (BeXB), are based on the idea of highly distorted disks. They are
believed to occur when a misaligned and warped disk becomes eccentric, allowing
the neutron star to capture a large amount of material. The BeXB 4U 0115+63
underwent two major outbursts in 2015 and 2017. Our aim is to investigate
whether the structural changes in the disk expected during type II outbursts
can be detected through optical polarimetry. We present the first optical
polarimetric observations and new optical spectra of the BeXB 4U 0115+63
covering the period 2013-2017. We study in detail the shape of the H
line profile and the polarization parameters before, during, and after the
occurrence of a type II X-ray outburst. We find significant changes in
polarization degree and polarization angle and highly distorted line profiles
during the 2017 X-ray outburst. The degree of polarization decreased by
1%, while the polarization angle, which is supposed to be related with the disk
orientation, first increased by in about two months and then
decreased by a similar amount and on a similar timescale once the X-ray
activity ceased.We interpret the polarimetric and spectroscopic variability as
evidence for the presence of a warped disk
Accretion regimes in the X-ray pulsar 4U 1901+03
The source 4U 1901+03 is a high-mass X-ray pulsar than went into outburst in
2003. Observation performed with the Rossi X-ray Timing Explorer showed
spectral and timing variability, including the detection of flares,
quasi-periodic oscillations, complex changes in the pulse profiles, and pulse
phase dependent spectral variability. We re-analysed the data covering the 2003
X-ray outburst and focused on several aspects of the variability that have not
been discussed so far. These are the 10 keV feature and the X-ray spectral
states and their association with accretion regimes, including the transit to
the propeller state at the end of the outburst. We find that 4U 1901+03 went
through three accretion regimes over the course of the X-ray outburst. At the
peak of the outburst and for a very short time, the X-ray flux may have
overcome the critical limit that marks the formation of a radiative shock at a
certain distance above the neutron star surface. Most of the time, however, the
source is in the subcritical regime. Only at the end of the outburst, when the
luminosity decreased below ~10^{36} (d/10 kpc)^2 erg/s, did the source enter
the propeller regime. Evidence for the existence of these regimes comes from
the pulse profiles, the shape of the hardness-intensity diagram, and the
correlation of various spectral parameters with the flux. The 10 keV feature
appears to strongly depend on the X-ray flux and on the pulse phase, which
opens the possibility to interpret this feature as a cyclotron line.Comment: accepted for publication in A&A. arXiv admin note: text overlap with
arXiv:astro-ph/9704084 by other author
The time-lag -- photon-index correlation in GX 339--4
Black-hole transients exhibit a correlation between the time lag of hard
photons with respect to softer ones and the photon index of the hard X-ray
power law. The correlation is not very tight and therefore it is necessary to
examine it source by source. The objective of the present work is to
investigate in detail the time-lag -- photon-index correlation in GX 339-4. We
have obtained RXTE energy spectra and light curves and have computed the photon
index and the time lag of the keV photons with respect to the
keV ones. The observations cover the first stages of the hard state, the pure
hard state, and the hard-intermediate state. At low , the correlation
is positive and it becomes negative at large . By assuming that the
hard X-ray power law index is produced by inverse Compton scattering
of soft disk photons in the jet, we have reproduced the entire correlation by
varying two parameters in the jet: the radius of the jet at its base and
the Thomson optical depth along the jet . We have found that,
as the luminosity of the source increases, initially increases and then
decreases. This behavior is expected in the context of the Cosmic Battery. As a
further test of our model, we predict the break frequency in the radio spectrum
as a function of the photon index during the rising part of an outburst
Spontaneous proton decay and the origin of Peccei-Quinn symmetry
We propose a new interpretation of Peccei-Quinn symmetry within the Standard
Model, identifying it with the axial symmetry i.e. . This new interpretation retains all the attractive
features of Peccei-Quinn solution to strong CP problem but in addition also
leads to several other new and interesting consequences. Owing to the
identification the axion also behaves
like Majoron inducing small seesaw masses for neutrinos after spontaneous
symmetry breaking. Another novel feature of this identification is the
phenomenon of spontaneous (and also chiral) proton decay with its decay rate
associated with the axion decay constant. Low energy processes which can be
used to test this interpretation are pointed out.Comment: 8 pages, 1 figure, a brief section on axial Baryon number as PQ
symmetry added, additional refs added, conclusions unchanged, published
versio
Cosmic implications of a low-scale solution to the axion domain wall problem
The post-inflationary breaking of Peccei-Quinn (PQ) symmetry can lead to the
cosmic domain wall catastrophe. In this Letter we show how to avoid domain
walls implementing the Instanton Interference Effect (IIE) with a new
interaction which itself breaks PQ symmetry and confines at an energy scale
smaller than . We give a general description of the mechanism
and consider its cosmological implications and constraints within a minimal
model. Contrary to other mechanisms we do not require an inverse phase
transition neither fine-tuned bias terms. Incidentally, the mechanism leads to
the introduction of new self-interacting dark matter candidates and the
possibility of producing gravitational waves in the frequency range of SKA.
Unless a fine-tuned hidden sector is introduced, the mechanism predicts a QCD
axion in the mass range .Comment: 12 pages, 4 figures; matches published versio
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