32 research outputs found
RX J0720.4--3125 as a Possible Example of the Magnetic Field Decay of Neutron Stars
We studied possible evolution of the rotational period and the magnetic field
of the X-ray source RX J0720.4-3125 assuming this source to be an isolated
neutron star accreting interstellar medium. Magnetic field of the source is
estimated to be G, and it is difficult to explain observed
rotational period 8.38 s without invoking hypothesis of the magnetic field
decay. We used the model of ohmic decay of the crustal magnetic field. The
estimates of accretion rate (), velocity of the
source relative to interstellar medium ( km/s), neutron star age
( yrs) are obtained.Comment: 12 pages (LATEX), 2 PostScript figures. Also available at
http://xray.sai.msu.su/~polar/ (with the Russian variant of the article
ROSAT X-ray sources and exponential field decay in isolated neutron stars
In this paper we semianalyticaly evaluate influence of the exponential decay
of magnetic field on the fate of isolated neutron stars. The fact of ROSAT
observations of several X-ray sources, which can be accreting old isolated
neutron stars gives us an opportunity to put some limits on the parameters of
the exponential decay.
We argue, that, if most part of neutron stars have approximately the same
decay and initial parameters, then the combinations of the bottom magnetic
momentum, , in the range and
characteristic time scale, , in the range for
standard initial magnetic momentum, , can be
excluded, because for that sets of parameters neutron stars never come to the
stage when accretion of the interstellar medium on their surfaces is possible
even for low velocity of neutron stars and relatively high density of the
interstellar medium. The region of excluded parameters increases with
decreasing.Comment: 5 pages, 4 PostScript figures (uses A&A style
Constrains on parameters of magnetic field decay for accreting isolated neutron stars
The influence of exponential magnetic field decay (MFD) on the spin evolution
of isolated neutron stars is studied. The ROSAT observations of several X-ray
sources, which can be accreting old isolated neutron stars, are used to
constrain the exponential and power-law decay parameters. We show that for the
exponential decay the ranges of minimum value of magnetic moment, , and
the characteristic decay time, , , are excluded assuming the
standard initial magnetic moment, . For these
parameters, neutron stars would never reach the stage of accretion from the
interstellar medium even for a low space velocity of the stars and a high
density of the ambient plasma. The range of excluded parameters increases for
lower values of .
We also show, that, contrary to exponential MFD, no significant restrictions
can be made for the parameters of power-law decay from the statistics of
isolated neutron star candidates in ROSAT observations.
Isolated neutron stars with constant magnetic fields and initial values of
them less than never come to the stage
of accretion.
We briefly discuss the fate of old magnetars with and without MFD, and
describe parameters of old accreting magnetars.Comment: 18 pages, 6 PostScript figures, to be published in the Proceedings of
the XXVIII ITEP Winter Schoo
Evolution of isolated neutron stars in globular clusters: number of Accretors
With a simple model from the point of view of population synthesis we try to
verify an interesting suggestion made by Pfahl & Rappaport (2001) that dim
sources in globular clusters (GCs) can be isolated accreting neutron stars
(NSs). Simple estimates show, that we can expect about 0.5-1 accreting isolated
NS per typical GC with in correspondence with observations.
Properties of old accreting isolated NSs in GCs are briefly discussed. We
suggest that accreting NSs in GCs experienced significant magnetic field decay.Comment: 6 pages, no figures. Submitted to Astronomical and Astrophysical
Transactions (style included
On the nature of the compact X-ray source inside RCW 103
I discuss the nature of the compact X-ray source inside the supernova remnant
RCW 103. Several models, based on the accretion onto a compact object such as a
neutron star or a black hole (isolated or binary), are analyzed. I show that it
is more likely that the X-ray source is an accreting neutron star than an
accreting black hole. I also argue that models of a binary system with an old
accreting neutron star are most favored.Comment: 6 pages, 1 PostScript figure, accepted to Astr. Astroph. Tran
Population synthesis of old neutron stars in the Galaxy
The paucity of old isolated accreting neutron stars in ROSAT observations is
used to derive a lower limit on the mean velocity of neutron stars at birth.
The secular evolution of the population is simulated following the paths of a
statistical sample of stars for different values of the initial kick velocity,
drawn from an isotropic Gaussian distribution with mean velocity . The spin-down, induced by dipole losses and the
interaction with the ambient medium, is tracked together with the dynamical
evolution in the Galactic potential, allowing for the determination of the
fraction of stars which are, at present, in each of the four possible stages:
Ejector, Propeller, Accretor, and Georotator. Taking from the ROSAT All Sky
Survey an upper limit of accreting neutron stars within pc
from the Sun, we infer a lower bound for the mean kick velocity, \ga
200-300 . The same conclusion is reached for both a constant
( G) and a magnetic field decaying exponentially with a
timescale yr. Present results, moreover, constrain the fraction of
low velocity stars, which could have escaped pulsar statistics, to \la 1%.Comment: 8 pages, 4 PostScript figures, to appear in the proceedings of IAU
Symposium 19
The elusiveness of old neutron stars
Old neutron stars (ONSs) which have radiated away their internal and
rotational energy may still shine if accreting the interstellar medium. Despite
their large number, only two promising candidates have been detected so far and
rather stringent limits on their observability follow from the analysis of
ROSAT surveys. This contrasts with optimistic theoretical estimates that
predicted a large number of sources in ROSAT fields. We have reconsidered the
issue of ONSs observability, accounting for the spin and magnetic field
evolution over the neutron star lifetime. In the framework of a spin-induced
field decay model, we show that the total number of ONSs which are, at present,
in the accretion stage is reduced by a factor ~5 over previous figures if the
characteristic timescale for crustal current dissipation is ~ 10^8 - 10^9 yr.
This brings theoretical predictions much closer to observational limits. Most
ONSs should be at present in the propeller phase and, if subject to episodic
flaring, they could be observable.Comment: 10 pages Latex, 5 ps figures. To be formatted with the AASTeX
package. Accepted for publication in Ap
Restrictions on parameters of power-law magnetic field decay for accreting isolated neutron stars
In this short note we discuss the influence of power-law magnetic field decay
on the evolution of old accreting isolated neutron stars. We show, that,
contrary to exponential field decay (Popov & Prokhorov 2000), no additional
restrictions can be made for the parameters of power-law decay from the
statistics of isolated neutron star candidates in ROSAT observations.
We also briefly discuss the fate of old magnetars with and without field
decay, and describe parameters of old accreting magnetars.Comment: 8 pages including 3 PostScript figure
Log N -- Log S distributions of accreting and cooling isolated neutron stars
We model populations of isolated neutron stars in the Galaxy following their
orbital and magneto-rotational evolution. It is shown that accretors become
more abundant than coolers at fluxes below erg cm
s and one can predict that about one accreting neutron star per square
degree should be observed at the {\it Chandra} and {\it Newton} flux limits of
erg cm s The soft ROSAT sources associated with
isolated neutron stars can be relatively young cooling objects only if the
neutron star birth rate in the Solar vicinity during the last yr is
higher than that inferred from radiopulsar observations.Comment: 5 pages, 3 PostScript figures, accepted to ApJ, new figures 2,