930 research outputs found
Evidence of Fast Magnetic Field Evolution in an Accreting Millisecond Pulsar
The large majority of neutron stars (NSs) in low mass X-ray binaries (LMXBs)
have never shown detectable pulsations despite several decades of intense
monitoring. The reason for this remains an unsolved problem that hampers our
ability to measure the spin frequency of most accreting NSs. The accreting
millisecond X-ray pulsar (AMXP) HETE J1900.1--2455 is an intermittent pulsar
that exhibited pulsations at about 377 Hz for the first 2 months and then
turned in a non-pulsating source. Understanding why this happened might help to
understand why most LMXBs do not pulsate. We present a 7 year long coherent
timing analysis of data taken with the Rossi X-ray Timing Explorer. We discover
new sporadic pulsations that are detected on a baseline of about 2.5 years. We
find that the pulse phases anti-correlate with the X-ray flux as previously
discovered in other AMXPs. We place stringent upper limits of 0.05% rms on the
pulsed fraction when pulsations are not detected and identify an enigmatic
pulse phase drift of ~180 degrees in coincidence with the first disappearance
of pulsations. Thanks to the new pulsations we measure a long term spin
frequency derivative whose strength decays exponentially with time. We
interpret this phenomenon as evidence of magnetic field burial.Comment: Accepted by ApJ Letter
Neutron Star Planets: Atmospheric processes and habitability
Of the roughly 3000 neutron stars known, only a handful have sub-stellar
companions. The most famous of these are the low-mass planets around the
millisecond pulsar B1257+12. New evidence indicates that observational biases
could still hide a wide variety of planetary systems around most neutron stars.
We consider the environment and physical processes relevant to neutron star
planets, in particular the effect of X-ray irradiation and the relativistic
pulsar wind on the planetary atmosphere. We discuss the survival time of planet
atmospheres and the planetary surface conditions around different classes of
neutron stars, and define a neutron star habitable zone. Depending on as-yet
poorly constrained aspects of the pulsar wind, both Super-Earths around
B1257+12 could lie within its habitable zone.Comment: Submitted to A&
A Deep Pulse Search in Eleven Low Mass X-Ray Binaries
We present a systematic coherent X-ray pulsation search in eleven low mass
X-ray binaries (LMXBs). We select a relatively broad variety of LMXBs,
including persistent and transient sources and spanning orbital periods between
0.3 and 17 hours. We use about 3.6 Ms of data collected by the Rossi X-Ray
Timing Explorer (RXTE) and XMM-Newton and apply a semi-coherent search strategy
to look for weak and persistent pulses in a wide spin frequency range. We find
no evidence for X-ray pulsations in these systems and consequently set upper
limits on the pulsed sinusoidal semi-amplitude between 0.14% and 0.78% for ten
outbursting/persistent LMXBs and 2.9% for a quiescent system. These results
suggest that weak pulsations might not form in (most) non-pulsating LMXBs.Comment: submitted to ApJ, 8 page
The Spin Distribution of Fast Spinning Neutron Stars in Low Mass X-Ray Binaries: Evidence for Two Sub-Populations
We study the current sample of rapidly rotating neutron stars in both
accreting and non-accreting binaries in order to determine whether the spin
distribution of accreting neutron stars in low-mass X-ray binaries can be
reconciled with current accretion torque models. We perform a statistical
analysis of the spin distributions and show that there is evidence for two
sub-populations among low-mass X-ray binaries, one at relatively low spin
frequency, with an average of ~300 Hz and a broad spread, and a peaked
population at higher frequency with average spin frequency of ~575 Hz. We show
that the two sub-populations are separated by a cut-point at a frequency of
~540 Hz. We also show that the spin frequency of radio millisecond pulsars does
not follow a log-normal distribution and shows no evidence for the existence of
distinct sub-populations. We discuss the uncertainties of different accretion
models and speculate that either the accreting neutron star cut-point marks the
onset of gravitational waves as an efficient mechanism to remove angular
momentum or some of the neutron stars in the fast sub-population do not evolve
into radio millisecond pulsars.Comment: Submitted to Ap
Radio pulsars around intermediate mass black holes in super stellar clusters
We study accretion in binaries hosting an intermediate mass black hole (IMBH)
of 1000 solar masses, and a donor star more massive than 15 solar masses. These
systems experience an active X-ray phase characterized by luminosities varying
over a wide interval, from <10^36 erg/s up to a few 10^40 erg/s typical of the
ultra luminous X-ray sources (ULXs). Roche lobe overflow on the zero-age main
sequence and donor masses above 20 solar masses can maintain a long-lived
accretion phase at the level required to feed a ULX source. In wide systems,
wind transfer rates are magnified by the focusing action of the IMBH yielding
wind luminosities around 10^38 erg/s. These high mass-IMBH binaries can be
identified as progenitors of IMBH-radio pulsar (PSR) binaries. We find that the
formation of an IMBH-PSR binary does not necessarely require the transit
through a ULX phase, but that a ULX can highlight a system that will evolve
into an IMBH-PSR, if the mass of the donor star is constrained to lie within 15
to 30 solar masses. We show that binary evolution delivers the pre-exploding
helium core in an orbit such that after explosion, the neutron star has a very
high probability to remain bound to the IMBH, at distances of 1-10 AU. The
detection of an IMBH-PSR binary in the Milky Way has suffered, so far, from the
same small number of statistics limit affecting the population of ULXs in our
Galaxy. Ongoing deeper surveys or next generation radio telescopes like SKA
will have an improved chance to unveil such intriguing systems. Timing analysis
of a pulsar orbiting around an IMBH would weigh the black hole in the still
uncharted interval of mass around 1000 solar massesComment: 10 pages, 8 figures, accepted for publication in MNRAS. Few comments
added (minor revision
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