8 research outputs found
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
1RXS J214303.7+065419/RBS 1774: A New Isolated Neutron Star Candidate
We report on the identification of a new possible Isolated Neutron Star
candidate in archival ROSAT observations. The source 1RXS J214303.7+065419,
listed in the ROSAT Bright Survey as RBS 1774, is very soft, exhibits a thermal
spectrum well fitted by a blackbody at eV and has a low column
density, . Catalogue searches revealed
no known sources in other energy bands close to the X-ray position of RBS 1774.
Follow-up optical observations with NTT showed no peculiar object within the
X-ray error circle. The absence of any plausible optical counterpart down to
results in an X-ray to optical flux ratio in excess of 1000.Comment: LaTeX (A&A style files), 5 pages, 3 figures. Accepted for publication
in Astronomy and Astrophysics Letters. Minor correction
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
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
Evolution of Isolated Neutron Stars
In this paper we briefly review our recent results on evolution and properties of isolated neutron stars (INSs) in the Galaxy. As the first step we discuss stochastic period evolution of INSs. We briefly discuss how an INS’s spin period evolves under influence of interaction with turbulized interstellar medium. To investigate statistical properties of the INS population we calculate a census of INSs in our Galaxy. We infer a lower bound for the mean kick velocity of NSs, < V>∼(200-300) kms −1. The same conclusion is reached for both a constant magnetic field (B ∼ 10 12 G) and for a magnetic field decaying exponentially with a timescale ∼ 10 9 yr. These results, moreover, constrain the fraction of low velocity NSs, which could have escaped pulsar statistics, to ∼few percents. Then we show that for exponential field decay the range of minimum value of magnetic moment, µb: ∼ 10 29.5 ≥ µb ≥ 10 28 Gcm 3, and the characteristic decay time, td: ∼ 10 8 ≥ td ≥ 10 7 yrs, can be excluded assuming the standard initial magnetic momentum, µ0 = 10 30 Gcm 3, if accreting INSs are observed. For these parameters an INS would never reach the stage of accretion from the interstellar medium even for a low space velocity of the star and high density of the ambient plasma. The range of excluded parameters increases for lower values of µ0. It is shown that old accreting INSs become more abundant than young cooling INSs at X-ray fluxes below ∼ 10 −13 erg cm −2 s −1. We can predict that about one accreting INS per square degree should be observed at the Chandra and Newton flux limits of ∼ 10 −16 erg cm −2 s −1. The weak ROSAT sources, associated with INSs, can be young cooling objects, if the NSs birth rate in the solar vicinity during the last ∼ 10 6 yr was much higher than inferred from radiopulsar observations