275 research outputs found
Quasiperiodic oscillations in bright galactic-bulge X-ray sources
Quasiperiodic oscillations with frequencies in the range 5-50 Hz have recently been discovered in X-rays from two bright galactic-bulge sources and Sco X-1. These sources are weakly magnetic neutron stars accreting from disks which the plasma is clumped. The interaction of the magnetosphere with clumps in the inner disk causes oscillations in the X-ray flux with many of the properties observed
Stability of radiation-pressure dominated disks. I. The dispersion relation for a delayed heating alpha-viscosity prescription
We derive and investigate the dispersion relation for accretion disks with
retarded or advanced heating. We follow the alpha-prescription but allow for a
time offset (\tau) between heating and pressure perturbations, as well as for a
diminished response of heating to pressure variations. We study in detail
solutions of the dispersion relation for disks with radiation-pressure fraction
1 - \beta . For \tau <0 (delayed heating) the number and sign of real solutions
for the growth rate depend on the values of the time lag and the ratio of
heating response to pressure perturbations, \xi . If the delay is larger than a
critical value (e.g., if \Omega \tau <-125 for \alpha =0.1, \beta =0 and \xi
=1) two real solutions exist, which are both negative. These results imply that
retarded heating may stabilize radiation-pressure dominated accretion disks.Comment: 11 pages, 10 figures, to be submitted to A&
Superfluid Friction and Late-time Thermal Evolution of Neutron Stars
The recent temperature measurements of the two older isolated neutron stars
PSR 1929+10 and PSR 0950+08 (ages of and yr,
respectively) indicate that these objects are heated. A promising candidate
heat source is friction between the neutron star crust and the superfluid it is
thought to contain. We study the effects of superfluid friction on the
long-term thermal and rotational evolution of a neutron star. Differential
rotation velocities between the superfluid and the crust (averaged over the
inner crust moment of inertia) of rad s for PSR
1929+10 and rad s for PSR 0950+08 would account for their
observed temperatures. These differential velocities could be sustained by
pinning of superfluid vortices to the inner crust lattice with strengths of
1 MeV per nucleus. Pinned vortices can creep outward through thermal
fluctuations or quantum tunneling. For thermally-activated creep, the coupling
between the superfluid and crust is highly sensitive to temperature. If pinning
maintains large differential rotation ( rad s), a feedback
instability could occur in stars younger than yr causing
oscillations of the temperature and spin-down rate over a period of . For stars older than yr, however, vortex creep occurs
through quantum tunneling, and the creep velocity is too insensitive to
temperature for a thermal-rotational instability to occur. These older stars
could be heated through a steady process of superfluid friction.Comment: 26 pages, 1 figure, submitted to Ap
The formation of the double neutron star pulsar J0737--3039
We find that the orbital period (2.4 hours), eccentricity (0.09), dipole
magnetic field strength (6.9 x 10^9 Gauss) and spin period (22 ms) of the new
highly relativistic double neutron star system PSR J0737-3039 can all be
consistently explained if this system originated from a close helium star plus
neutron star binary (HeS-NS) in which at the onset of the evolution the helium
star had a mass in the range 4.0 to 6.5 M_sun and an orbital period in the
range 0.1 to 0.2 days. Such systems are the post-Common-Envelope remnants of
wide Be/X-ray binaries (orbital period ~ 100 to 1000 days) which consist of a
normal hydrogen-rich star with a mass in the range 10 - 20 M_sun and a neutron
star. The close HeS-NS progenitor system went through a phase of mass transfer
by Roche-lobe overflow at a high rate lasting a few times 10^4 years; assuming
Eddington-limited disk accretion onto the neutron star this star was spun up to
its present rapid spin rate. At the moment of the second supernova explosion
the He star had a mass in the range 2.3 to 3.3 M_sun and in order to obtain the
present orbital parameters of PSR J0737-3039 a kick velocity in the range 70 -
230 km/s must have been imparted to the second neutron star at its birth.Comment: accepted by MNRA
Quasi-periodic X-ray brightness fluctuations in an accreting millisecond pulsar
The relativistic plasma flows onto neutron stars that are accreting material
from stellar companions can be used to probe strong-field gravity as well as
the physical conditions in the supranuclear-density interiors of neutron stars.
Plasma inhomogeneities orbiting a few kilometres above the stars are observable
as X-ray brightness fluctuations on the millisecond dynamical timescale of the
flows. Two frequencies in the kilohertz range dominate these fluctuations: the
twin kilohertz quasi-periodic oscillations (kHz QPOs). Competing models for the
origins of these oscillations (based on orbital motions) all predict that they
should be related to the stellar spin frequency, but tests have been difficult
because the spins were not unambiguously known. Here we report the detection of
kHz QPOs from a pulsar whose spin frequency is known. Our measurements
establish a clear link between kHz QPOs and stellar spin, but one not predicted
by any current model. A new approach to understanding kHz QPOs is now required.
We suggest that a resonance between the spin and general relativistic orbital
and epicyclic frequencies could provide the observed relation between QPOs and
spin.Comment: Published in the 2003 July 3 issue of Natur
Suzaku Observation of AXP 1E 1841-045 in SNR Kes 73
Anomalous X-ray pulsars (AXPs) are thought to be magnetars, which are neutron
stars with ultra strong magnetic field of -- G. Their energy
spectra below 10 keV are modeled well by two components consisting of a
blackbody (BB) (0.4 keV) and rather steep power-law (POW) function
(photon index 2-4). Kuiper et al.(2004) discovered hard X-ray component
above 10 keV from some AXPs. Here, we present the Suzaku observation of
the AXP 1E 1841-045 at the center of supernova remnant Kes 73. By this
observation, we could analyze the spectrum from 0.4 to 50 keV at the same time.
Then, we could test whether the spectral model above was valid or not in this
wide energy range. We found that there were residual in the spectral fits when
fit by the model of BB + POW. Fits were improved by adding another BB or POW
component. But the meaning of each component became ambiguous in the
phase-resolved spectroscopy. Alternatively we found that NPEX model fit well
for both phase-averaged spectrum and phase-resolved spectra. In this case, the
photon indices were constant during all phase, and spectral variation seemed to
be very clear. This fact suggests somewhat fundamental meaning for the emission
from magnetars.Comment: To appear in the proceedings of the "40 Years of Pulsars: Millisecond
Pulsars, Magnetars and More" conference, held 12-17 August 2007, in Montreal
QC (AIP, in press, eds: C. Bassa, Z. Wang, A. Cumming, V. Kaspi
THERMAL RADIATION FROM MAGNETIZED NEUTRON STARS: A look at the Surface of a Neutron Star.
Surface thermal emission has been detected by ROSAT from four nearby young
neutron stars. Assuming black body emission, the significant pulsations of the
observed light curves can be interpreted as due to large surface temperature
differences produced by the effect of the crustal magnetic field on the flow of
heat from the hot interior toward the cooler surface. However, the energy
dependence of the modulation observed in Geminga is incompatible with blackbody
emission: this effect will give us a strong constraint on models of the neutron
star surface.Comment: 10 pages. tar-compressed and uuencoded postcript file. talk given at
the `Jubilee Gamow Seminar', St. Petersburg, Sept. 1994
Implications of the measured parameters of PSR J1903+0327 for its progenitor neutron star
Using the intrinsic PSR J1903+0327 parameters evaluated from radio
observations (mass, rotation period and dipole magnetic field deduced from the
timing properties) we calculate the mass of its neutron star progenitor, M_i,
at the onset of accretion. Simultaneously, we derive constraints on average
accretion rate Mdot and the pre-accretion magnetic field B_i. Spin-up is
modelled by accretion from a thin disk, using the magnetic-torque disk-pulsar
coupling model proposed by Kluzniak and Rappaport (2007), improved for the
existence of relativistic marginally-stable circular orbit. Orbital parameters
in the disk are obtained using the space-time generated by a rotating neutron
star in the framework of General Relativity. We employ an observationally
motivated model of the surface magnetic field decay. We also seek for the
imprint of the poorly known equation of state of dense matter on the spin-up
tracks - three equations of state of dense matter, consistent with the
existence of 2 Msun neutron star, are considered. We find that the minimum
average accretion rate should be larger than 2-8 10^(-10) Msun/yr, the highest
lower bound corresponding to the stiffest equation of state. We conclude that
the influence of magnetic field in the "recycling" process is crucial - it
leads to a significant decrease of spin-up rate and larger accreted masses, in
comparison to the B=0 model. Allowed B_i-dependent values of M_i are within
1.0-1.4 Msun, i.e., much lower than an oversimplified but widely used B=0
result, where one gets M_i>1.55 Msun. Estimated initial neutron-star mass
depends on the assumed dense-matter equation of state.Comment: 11 pages, 10 figures; A&A accepte
Progenitor neutron stars of the lightest and heaviest millisecond pulsars
The recent mass measurements of two binary millisecond pulsars, PSR
J1614-2230 and PSR J0751+1807 with a mass M=1.97+/-0.04 Msun and M= 1.26 +/-
0.14 Msun, respectively, indicate a wide range of masses for such objects and
possibly also a broad spectrum of masses of neutron stars born in core-collapse
supernovae.
Starting from the zero-age main sequence binary stage, we aim at inferring
the birth masses of PSR J1614-2230 and PSR J0751+1807 by taking the differences
in the evolutionary stages preceding their formation into account.
Using simulations for the evolution of binary stars, we reconstruct the
evolutionary tracks leading to the formation of PSR J1614-2230 and PSR
J0751+1807. We analyze in detail the spin evolution due to the accretion of
matter from a disk in the intermediate-mass/low-mass X-ray binary. We consider
two equations of state of dense matter, one for purely nucleonic matter and the
other one including a high-density softening due to the appearance of hyperons.
Stationary and axisymmetric stellar configurations in general relativity are
used, together with a recent magnetic torque model and
observationally-motivated laws for the decay of magnetic field.
The estimated birth mass of the neutron stars PSR J0751+1807 and PSR
J1614-2230 could be as low as 1.0 Msun and as high as 1.9 Msun, respectively.
These values depend weakly on the equation of state and the assumed model for
the magnetic field and its accretion-induced decay.
The masses of progenitor neutron stars of recycled pulsars span a broad
interval from 1.0 Msun to 1.9 Msun. Including the effect of a slow Roche-lobe
detachment phase, which could be relevant for PSR J0751+1807, would make the
lower mass limit even lower. A realistic theory for core-collapse supernovae
should account for this wide range of mass.Comment: 13 pages, 10 figures, accepted in A&
Improved estimate of the detectability of gravitational radiation from a magnetically confined mountain on an accreting neutron star
We give an improved estimate of the detectability of gravitational waves from
magnetically confined mountains on accreting neutron stars. The improved
estimate includes the following effects for the first time: three-dimensional
hydromagnetic ("fast") relaxation, three-dimensional resistive ("slow")
relaxation, realistic accreted masses M_a \la 2 \times 10^{-3} M_\odot,
(where the mountain is grown ab initio by injection), and verification of the
curvature rescaling transformation employed in previous work. Typically, a
mountain does not relax appreciably over the lifetime of a low-mass X-ray
binary. The ellipticity reaches for
. The gravitational wave spectrum for triaxial
equilibria contains an additional line, which, although weak, provides valuable
information about the mountain shape. We evaluate the detectability of magnetic
mountains with Initial and Advanced LIGO. For a standard, coherent matched
filter search, we find a signal-to-noise ratio of for Initial LIGO, where is the distance and is
the observation time. From the nondetection of gravitational waves from
low-mass X-ray binaries to date, and the wave strain limits implied by the spin
frequency distribution of these objects (due to gravitational wave braking), we
conclude that there are other, as yet unmodelled, physical effects that further
reduce he quadrupole moment of a magnetic mountain, most notably sinking into
the crust.Comment: accepted by MNRA
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