34,764 research outputs found
The superorbital variability and triple nature of the X-ray source 4U 1820-303
We perform a comprehensive analysis of the superorbital modulation in the
ultracompact X-ray source 4U 1820-303, consisting of a white dwarf accreting
onto a neutron star. Based on RXTE data, we measure the fractional amplitude of
the source superorbital variability (with a 170-d quasi-period) in the folded
and averaged light curves, and find it to be by a factor of about 2. As
proposed before, the superorbital variability can be explained by oscillations
of the binary eccentricity. We now present detailed calculations of the
eccentricity-dependent flow through the inner Lagrangian point, and find a
maximum of the eccentricity of about 0.004 is sufficient to explain the
observed fractional amplitude. We then study hierarchical triple models
yielding the required quasi-periodic eccentricity oscillations through the
Kozai process. We find the resulting theoretical light curves to match well the
observed ones. We constrain the ratio of the semimajor axes of the outer and
inner systems, the component masses, and the inclination angle between the
inner and outer orbits. Last but not least, we discover a remarkable and
puzzling synchronization between the observed period of the superorbital
variability (equal to the period of the eccentricity oscillations in our model)
and the period of the general-relativistic periastron precession of the binary.Comment: MNRAS, in pres
The superorbital variability and triple nature of the X-ray source 4U 1820-303
We perform a comprehensive analysis of the superorbital modulation in the ultracompact X-ray source 4U 1820-303, consisting of a white dwarf accreting onto a neutron star. Based on RXTE data, we measure the fractional amplitude of the source superorbital variability (with a 170-d quasi-period) in the folded and averaged light curves, and find it to be by a factor of about 2. As proposed before, the superorbital variability can be explained by oscillations of the binary eccentricity. We now present detailed calculations of the eccentricity-dependent flow through the inner Lagrangian point, and find a maximum of the eccentricity of about 0.004 is sufficient to explain the observed fractional amplitude. We then study hierarchical triple models yielding the required quasi-periodic eccentricity oscillations through the Kozai process. We find the resulting theoretical light curves to match well the observed ones. We constrain the ratio of the semimajor axes of the outer and inner systems, the component masses, and the inclination angle between the inner and outer orbits. Last but not least, we discover a remarkable and puzzling synchronization between the observed period of the superorbital variability (equal to the period of the eccentricity oscillations in our model) and the period of the general-relativistic periastron precession of the binary
Constraint on the early Universe by relic gravitational waves: From pulsar timing observations
Recent pulsar timing observations by the Parkers Pulsar Timing Array and
European Pulsar Timing Array teams obtained the constraint on the relic
gravitational waves at the frequency , which provides the
opportunity to constrain , the Hubble parameter when these waves crossed
the horizon during inflation. In this paper, we investigate this constraint by
considering the general scenario for the early Universe: we assume that the
effective (average) equation-of-state before the big bang nucleosynthesis
stage is a free parameter. In the standard hot big-bang scenario with ,
we find that the current PPTA result follows a bound H_*\leq
1.15\times10^{-1}\mpl, and the EPTA result follows H_*\leq
6.92\times10^{-2}\mpl. We also find that these bounds become much tighter in
the nonstandard scenarios with . When , the bounds become
H_*\leq5.89\times10^{-3}\mpl for the current PPTA and
H_*\leq3.39\times10^{-3}\mpl for the current EPTA. In contrast, in the
nonstandard scenario with , the bound becomes H_*\leq7.76\mpl for the
current PPTA.Comment: 8 pages, 3 figures, 1 table, PRD in pres
On Atkin and Swinnerton-Dyer Congruence Relations (2)
In this paper we give an example of a noncongruence subgroup whose
three-dimensional space of cusp forms of weight 3 has the following properties.
For each of the four residue classes of odd primes modulo 8 there is a basis
whose Fourier coefficients at infinity satisfy a three-term Atkin and
Swinnerton-Dyer congruence relation, which is the -adic analogue of the
three-term recursion satisfied by the coefficients of classical Hecke eigen
forms. We also show that there is an automorphic -function over
whose local factors agree with those of the -adic Scholl representations
attached to the space of noncongruence cusp forms.Comment: Last version, to appear on Math Annale
Topology induced anomalous defect production by crossing a quantum critical point
We study the influence of topology on the quench dynamics of a system driven
across a quantum critical point. We show how the appearance of certain edge
states, which fully characterise the topology of the system, dramatically
modifies the process of defect production during the crossing of the critical
point. Interestingly enough, the density of defects is no longer described by
the Kibble-Zurek scaling, but determined instead by the non-universal
topological features of the system. Edge states are shown to be robust against
defect production, which highlights their topological nature.Comment: Phys. Rev. Lett. (to be published
Irrational charge from topological order
Topological or deconfined phases of matter exhibit emergent gauge fields and
quasiparticles that carry a corresponding gauge charge. In systems with an
intrinsic conserved U(1) charge, such as all electronic systems where the
Coulombic charge plays this role, these quasiparticles are also characterized
by their intrinsic charge. We show that one can take advantage of the
topological order fairly generally to produce periodic Hamiltonians which endow
the quasiparticles with continuously variable, generically irrational,
intrinsic charges. Examples include various topologically ordered lattice
models, the three dimensional RVB liquid on bipartite lattices as well as water
and spin ice. By contrast, the gauge charges of the quasiparticles retain their
quantized values.Comment: 4 pages, 1 figure with two panel
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