172 research outputs found
Quark model description of the tetraquark state X(3872) in a relativistic constituent quark model with infrared confinement
We explore the consequences of treating the X(3872) meson as a tetraquark
bound state. As dynamical framework we employ a relativistic constituent quark
model which includes infrared confinement in an effective way. We calculate the
decay widths of the observed channels X-> Jpsi+2\pi (3\pi) and X-> \bar
D0+D0+\pi0 via the intermediate off--shell states X-> Jpsi+\rho(\omega) and X->
\bar D + D*. For reasonable values of the size parameter of the X(3872) we find
consistency with the available experimental data. We also discuss the possible
impact of the X(3872) in a s-channel dominance description of the
Jpsi-dissociation cross section.Comment: 9 pages, 5 figures; discussion and references added, accepted in
Phys. Rev.
On the X-Ray Light Curve, Pulsed-Radio Emission, and Spin Frequency Evolution of the Transient Anomalous X-Ray Pulsar Xte J1810--197 During its X-Ray Outburst
We show that: (i) the long-term X-ray outburst light curve of the transient
AXP XTE J1810-197 can be accounted for by a fallback disk that is evolving
towards quiescence through a disk instability after having been heated by a
soft gamma-ray burst, (ii) the spin-frequency evolution of this source in the
same period can also be explained by the disk torque acting on the
magnetosphere of the neutron star, (iii) most significantly, recently observed
pulsed-radio emission from this source coincides with the epoch of minimum
X-ray luminosity. This is natural in terms of a fallback disk model, as the
accretion power becomes so low that it is not sufficient to suppress the beamed
radio emission from XTE J1810-197.Comment: 13 pages, 2 Figures, accepted for publication in Ap
Cessation of X-ray Pulsation of GX 1+4
We report results from our weekly monitoring campaign on the X-ray pulsar GX
1+4 with the {\em Rossi X-ray Timing Explorer} satellite. The spin-down trend
of GX 1+4 was continuing, with the pulsar being at its longest period ever
measured (about 138.7 s). At the late stage of the campaign, the source entered
an extended faint state, when its X-ray (2-60 keV) flux decreased significantly
to an average level of . It was
highly variable in the faint state; the flux dropped to as low as . In several observations during this
period, the X-ray pulsation became undetectable. We can, therefore, conclude
conservatively that the pulsed fraction, which is normally 70%
(peak-to-peak), must have decreased drastically in those cases. This is very
similar to what was observed of GX 1+4 in 1996 when it became similarly faint
in X-ray. In fact, the flux at which the cessation of X-ray pulsation first
occurred is nearly the same as it was in 1996. We suggest that we have, once
again, observed the propeller effect in GX 1+4, a phenomenon that is predicted
by theoretical models of accreting X-ray pulsars.Comment: 13 pages, 9 figures (available at
http://www.physics.purdue.edu/~cui/ftp/cuifigs.tar.gz). To appear in Ap
Can the anomalous X-ray pulsars be powered by accretion?
The nature of the 5-12 s "anomalous" X-ray pulsars remains a mystery. Among
the models that have been proposed to explain the properties of AXPs, the most
likely ones are: (1) isolated accreting neutron stars evolved from the
Thorne-\.{Z}ytkow objects due to complete spiral-in during the common envelope
evolution of high-mass X-ray binaries, and (2) magnetars, which are neutron
stars with ultra-high ( G) surface magnetic fields. We
have critically examined the predicted change of neutron star's spin in the
accretion model, and found that it is unable to account for the steady
spin-down observed in AXPs. A simple analysis also shows that any accretion
disk around an isolated neutron star has extremely limited lifetime. A more
promising explanation for such objects is the magnetar model.Comment: 9 pages, accepted for publication in Ap
Does Pulsar B1757--24 Have a Fallback Disk?
Radio pulsars are thought to spin-down primarily due to torque from magnetic
dipole radiation (MDR) emitted by the time-varying stellar magnetic field as
the star rotates. This assumption yields a `characteristic age' for a pulsar
which has generally been assumed to be comparable to the actual age. Recent
observational limits on the proper motion of pulsar B1757-24, however, revealed
that the actual age (>39 kyr) of this pulsar is much greater than its MDR
characteristic age (16 kyr) - calling into question the assumption of pure MDR
spin-down for this and other pulsars. To explore the possible cause of this
discrepancy, we consider a scenario in which the pulsar acquired an accretion
disk from supernova ejecta, and the subsequent spin-down occurred under the
combined action of MDR and accretion torques. A simplified model of the
accretion torque involving a constant mass inflow rate at the pulsar
magnetosphere can explain the age and period derivative of the pulsar for
reasonable values of the pulsar magnetic field and inflow rate. We discuss
testable predictions of this model.Comment: Accepted by ApJ Letters. 15 pages with 1 figur
On the Nature of Part Time Radio Pulsars
The recent discovery of rotating radio transients and the quasi-periodicity
of pulsar activity in the radio pulsar PSR B193124 has challenged the
conventional theory of radio pulsar emission. Here we suggest that these
phenomena could be due to the interaction between the neutron star
magnetosphere and the surrounding debris disk. The pattern of pulsar emission
depends on whether the disk can penetrate the light cylinder and efficiently
quench the processes of particle production and acceleration inside the
magnetospheric gap. A precessing disk may naturally account for the
switch-on/off behavior in PSR B193124.Comment: 9 pages, accepted to ApJ
The Compact Central Object in the Supernova Remnant G266.2-1.2
We observed the compact central object CXOU J085201.4--461753 in the
supernova remnant G266.2--1.2 (RX J0852.0--4622) with the Chandra ACIS detector
in timing mode. The spectrum of this object can be described by a blackbody
model with the temperature kT=404 eV and radius of the emitting region R=0.28
km, at a distance of 1 kpc. Power-law and thermal plasma models do not fit the
source spectrum. The spectrum shows a marginally significant feature at 1.68
keV. Search for periodicity yields two candidate periods, about 301 ms and 33
ms, both significant at a 2.1 sigma level; the corresponding pulsed fractions
are 13% and 9%, respectively. We find no evidence for long-term variability of
the source flux, nor do we find extended emission around the central object. We
suggest that CXOU J085201.4--461753 is similar to CXOU J232327.9+584842, the
central source of the supernova remnant Cas A. It could be either a neutron
star with a low or regular magnetic field, slowly accreting from a fossil disk,
or, more likely, an isolated neutron star with a superstrong magnetic field. In
either case, a conservative upper limit on surface temperature of a 10 km
radius neutron star is about 90 eV, which suggests accelerated cooling for a
reasonable age of a few thousand years.Comment: Accepted to ApJ, 13 pages, 1 figur
The quiescent X-ray emission of three transient X-ray pulsars
We report on BeppoSAX and Chandra observations of three Hard X-Ray Transients
in quiescence containing fast spinning (P<5 s) neutron stars: A 0538-66, 4U
0115+63 and V 0332+53. These observations allowed us to study these transients
at the faintest flux levels thus far. Spectra are remarkably different from the
ones obtained at luminosities a factor >10 higher, testifying that the
quiescent emission mechanism is different. Pulsations were not detected in any
of the sources, indicating that accretion of matter down to the neutron star
surface has ceased. We conclude that the quiescent emission of the three X-ray
transients likely originates from accretion onto the magnetospheric boundary in
the propeller regime and/or from deep crustal heating resulting from
pycnonuclear reactions during the outbursts.Comment: Accepted for publication on ApJ (5 pages and 2 figures
Magnetically Torqued Thin Accretion Disks
We compute the properties of a geometrically thin, steady accretion disk
surrounding a central rotating, magnetized star. The magnetosphere is assumed
to entrain the disk over a wide range of radii. The model is simplified in that
we adopt two (alternate) ad hoc, but plausible, expressions for the azimuthal
component of the magnetic field as a function of radial distance. We find a
solution for the angular velocity profile tending to corotation close to the
central star, and smoothly matching a Keplerian curve at a radius where the
viscous stress vanishes. The value of this ''transition'' radius is nearly the
same for both of our adopted B-field models. We then solve analytically for the
torques on the central star and for the disk luminosity due to gravity and
magnetic torques. When expressed in a dimensionless form, the resulting
quantities depend on one parameter alone, the ratio of the transition radius to
the corotation radius. For rapid rotators, the accretion disk may be powered
mostly by spin-down of the central star. These results are independent of the
viscosity prescription in the disk. We also solve for the disk structure for
the special case of an optically thick alpha disk. Our results are applicable
to a range of astrophysical systems including accreting neutron stars,
intermediate polar cataclysmic variables, and T Tauri systems.Comment: 9 sharper figs, updated reference
The Propeller Regime of Disk Accretion to a Rapidly Rotating Magnetized Star
The propeller regime of disk accretion to a rapidly rotating magnetized star
is investigated here for the first time by axisymmetric 2.5D
magnetohydrodynamic simulations. An expanded, closed magnetosphere forms in
which the magnetic field is predominantly toroidal. A smaller fraction of the
star's poloidal magnetic flux inflates vertically, forming a magnetically
dominated tower. Matter accumulates in the equatorial region outside
magnetosphere and accretes to the star quasi-periodically through elongated
funnel streams which cause the magnetic field to reconnect. The star spins-down
owing to the interaction of the closed magnetosphere with the disk. For the
considered conditions, the spin-down torque varies with the angular velocity of
the star omega* as omega*^1.3 for fixed mass accretion rate. The propeller
stage may be important in the evolution of X-ray pulsars, cataclysmic variables
and young stars. In particular, it may explain the present slow rotation of the
classical T Tauri stars.Comment: 5 pages with 4 figures, LaTeX, macros: emulapj.sty, avi movies are
available at http://www.astro.cornell.edu/us-russia/disk_prop.ht
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