666 research outputs found
Modelling the closest double degenerate system RXJ0806.3+1527 and its decreasing period
In the hypothesis that the 5.4m binary RXJ0806.3+1527 consists of a low mass
helium white dwarf (donor) transferring mass towards its more massive white
dwarf companion (primary), we consider as possible donors white dwarfs which
are the result of common envelope evolution occurring when the helium core mass
of the progenitor giant was still very small (~ 0.2Msun), so that they are
surrounded by a quite massive hydrogen envelope (~1/100Msun or larger), and
live for a very long time supported by proton--proton burning. Mass transfer
from such low mass white dwarfs very probably starts during the hydrogen
burning stage, and the donor structure will remain dominated by the burning
shell until it loses all the hydrogen envelope and begins transferring helium.
We model mass transfer from these low mass white dwarfs, and show that the
radius of the donor decreases while they shed the hydrogen envelope. This
radius behavior, which is due to the fact that the white dwarf is not fully
degenerate, has two important consequences on the evolution of the binary: 1)
the orbital period decreases, with a timescale consistent with the period
decrease of the binary RXJ0806.3+1527; 2) the mass transfer rate is a factor of
about 10 smaller than from a fully degenerate white dwarf, easing the problem
connected with the small X-ray luminosity of this object. The possibility that
such evolution describes the system RXJ0806.3+1527 is also consistent with the
possible presence of hydrogen in the optical spectrum of the star, whose
confirmation would become a test of the model.Comment: 13 pages, 4 figures, accepted for publication on ApJ, main journa
Search for pulsations at high radio frequencies from accreting millisecond X-ray pulsars in quiescence
It is commonly believed that millisecond radio pulsars have been spun up by
transfer of matter and angular momentum from a low-mass companion during an
X-ray active mass transfer phase. A subclass of low-mass X-ray binaries is that
of the accreting millisecond X-ray pulsars, transient systems that show periods
of X-ray quiescence during which radio emission could switch on. The aim of
this work is to search for millisecond pulsations from three accreting
millisecond X-ray pulsars, XTE J1751-305, XTE J1814-338, and SAX J1808.4-3658,
observed during their quiescent X-ray phases at high radio frequencies (5 - 8
GHz) in order to overcome the problem of the free-free absorption due to the
matter engulfing the system. A positive result would provide definite proof of
the recycling model, providing the direct link between the progenitors and
their evolutionary products. The data analysis methodology has been chosen on
the basis of the precise knowledge of orbital and spin parameters from X-ray
observations. It is subdivided in three steps: we corrected the time series for
the effects of (I) the dispersion due to interstellar medium and (II) of the
orbital motions, and finally (III) folded modulo the spin period to increase
the signal-to-noise ratio. No radio signal with spin and orbital
characteristics matching those of the X-ray sources has been found in our
search, down to very low flux density upper limits. We analysed several
mechanisms that could have prevented the detection of the signal, concluding
that the low luminosity of the sources and the geometric factor are the most
likely reasons for this negative result.Comment: 5 pages, 3 figures. Accepted for publication by A&
A Firm Upper Limit to the Radius of the Neutron Star in SAX J1808.4-3658
We show that observations of X-ray pulsing from SAX J1808.4-3658 place a firm
upper limit of 13.8 m^{1/3} km on the radius of the neutron star, where m is
its mass in solar units. The limit is independent of distance or assumptions
about the magnetospheric geometry, and could be significantly tightened by
observations of the pulsations in the near future. We discuss the implications
for the equation of state and the possible neutron star mass.Comment: (7 pages, 1 figure, accepted for publication in ApJ Letters
The magnetospheric radius of an inclined rotator in the magnetically threaded disk model
The estimate of the magnetospheric radius in a disk-fed neutron star X-ray
binary is a long standing problem in high energy Astrophysics. We review the
magnetospheric radius calculations in the so-called magnetically threaded disk
model, comparing the simplified approach originally proposed by Ghosh & Lamb
(1979) with the revised version proposed by Wang (1987), Wang (1995), and Wang
(1997). We show that for a given set of fixed parameters (assuming also a
comparable screening factor of the neutron star magnetic field by the currents
induced on the disk surface) the revised magnetically threaded disk model
predicts a magnetospheric radius that is significantly smaller than that
derived from the Ghosh & Lamb (1979) treatment. For a fixed value of the
neutron star magnetic field and a wide range of mass accretion rates, the
inclusion of a large inclination angle between the neutron star rotation and
magnetic field axes (60 deg) leads to a further decrease of the
magnetospheric radius. To illustrate the relevance of these calculations, we
consider, as an example, the case of the transitional pulsars. During the
so-called "high mode" of their sub-luminous accretion disk state, these sources
have shown X-ray pulsations interpreted as due to accretion at an unprecedented
low luminosity level compared to other neutron stars in X-ray binaries. In the
context of the magnetic threaded disk model, we show that accretion at
luminosities of 10 erg s (and thus accretion-driven X-ray
pulsations) can be more easily explained when the prescription of the
magnetospheric radius provided by Wang (1997) is used. This avoids the need of
invoking very strong propeller outflows in the transitional pulsars, as
proposed in other literature works.Comment: Accepted for publication in A&
The Broad Band Spectrum of MXB 1728-34 Observed by BeppoSAX
We report on the results of a broad band (0.1-100 keV) spectral analysis of
the bursting atoll source MXB 1728-34 observed by the BeppoSAX satellite. Three
bursts were present during this observation. The spectrum during the bursts can
be fitted by a blackbody with a temperature of 2 keV. From the bursts we also
estimate a distance to the source of 5.1 kpc. MXB 1728-34 was in a rather soft
state during the BeppoSAX observation. The persistent spectrum is well fitted
by a continuum consisting of a soft blackbody emission and a comptonized
spectrum. We interpreted the soft component as the emission from the accretion
disk. Taking into account a spectral hardening factor of 1.7, we estimated that
the inner disk radius is km, where i is the
inclination angle. The comptonized component could originate in a spherical
corona, with temperature of 10 keV and optical depth of 5, surrounding the
neutron star. A broad gaussian emission line at 6.7 keV is observed in the
spectrum, probably emitted in the ionized corona or in the inner part of the
disk. Another emission line is present at 1.66 keV.Comment: 12 pages, accepted by Ap
A Radio Pulsar in SN1987A?
A complex three-ring shaped structure has been reported extending some arcseconds around the SN1987A central spot by several authors. This structure is believed to arise when a thin shell of matter surrounding the supernova was illuminated by the initial extreme-ultra-violet flash from the explosion of the progenitor. In this view the two
ring-shaped outer loops, 300 in size, are interpreted as limb brightening of an hourglass-shaped nebula surrounding the supernova, while the smaller central ring is located at the waist of the hourglass. Our explanation of the two external loops is different: keeping the hourglass-shaped nebula scenario, we believe that these loops result from the interaction of this nebula with a double beam of relativistic particles emitted by a young pulsar formed by the supernova
A Radio Pulsar in SN1987A?
A complex three-ring shaped structure has been reported extending some arcseconds around the SN1987A central spot by several authors. This structure is believed to arise when a thin shell of matter surrounding the supernova was illuminated by the initial extreme-ultra-violet flash from the explosion of the progenitor. In this view the two
ring-shaped outer loops, 300 in size, are interpreted as limb brightening of an hourglass-shaped nebula surrounding the supernova, while the smaller central ring is located at the waist of the hourglass. Our explanation of the two external loops is different: keeping the hourglass-shaped nebula scenario, we believe that these loops result from the interaction of this nebula with a double beam of relativistic particles emitted by a young pulsar formed by the supernova
Measuring the spin up of the Accreting Millisecond Pulsar XTE J1751-305
We perform a timing analysis on RXTE data of the accreting millisecond pulsar
XTE J1751-305 observed during the April 2002 outburst. After having corrected
for Doppler effects on the pulse phases due to the orbital motion of the
source, we performed a timing analysis on the phase delays, which gives, for
the first time for this source, an estimate of the average spin frequency
derivative = (3.7 +/- 1.0)E-13 Hz/s. We discuss the torque resulting
from the spin-up of the neutron star deriving a dynamical estimate of the mass
accretion rate and comparing it with the one obtained from X-ray flux.
Constraints on the distance to the source are discussed, leading to a lower
limit of \sim 6.7 kpc.Comment: 7 pages, 3 figures, Accepted for publication by MNRA
The spin and orbit of the newly discovered pulsar IGR J17480-2446
We present an analysis of the spin and orbital properties of the newly
discovered accreting pulsar IGR J17480-2446, located in the globular cluster
Terzan 5. Considering the pulses detected by the Rossi X-ray Timing Explorer at
a period of 90.539645(2) ms, we derive a solution for the 21.27454(8) hr binary
system. The binary mass function is estimated to be 0.021275(5) Msun,
indicating a companion star with a mass larger than 0.4 Msun. The X-ray pulsar
spins up while accreting at a rate of between 1.2 and 1.7E-12 Hz/s, in
agreement with the accretion of disc matter angular momentum given the observed
luminosity.
We also report the detection of pulsations at the spin period of the source
during a Swift observation performed ~2 d before the beginning of the RXTE
coverage. Assuming that the inner disc radius lies in between the neutron star
radius and the corotation radius while the source shows pulsations, we estimate
the magnetic field of the neutron star to be within ~2E8 G and ~2.4E10 G. From
this estimate, the value of the spin period and of the observed spin-up rate,
we associate this source with the still poorly sampled population of slow,
mildly recycled, accreting pulsars.Comment: 5 pages, accepted by A&A Letters on 2010 Nov 30. Timing solution
derived on a longer time interval with respect to the previous versio
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