211 research outputs found
The statistics of radio pulsars: A spark model
We use Monte Carlo techniques to relate a theoretical pulsar emission model to the observed distributions of pulse period, magnetic field strength, distance, and luminosity of radio pulsars. We assume that the radio luminosity of pulsars is proportional to the gap potential and current flow from the polar cap. The current is assumed to be nonuniform and clustered in sparks, but only those sparks swept by the line of sight contribute to the observed radio luminosity. We test our model by using the Ruderman-Sutherland vacuum gap potential and find that the simulated distributions are consistent with those observed, with the exception of the period distribution. The model predicts more long-period pulsars than are observed. This discrepancy may result from the model itself, a reduced sensitivity of surveys to long-period pulsars, or the nondipole spin-down of pulsars.published_or_final_versio
A Search for Pulsars in Quiescent Soft X-Ray Transients. I
We have carried out a deep search at 1.4 GHz for radio pulsed emission from
six soft X-ray transient sources observed during their X-ray quiescent phase.
The commonly accepted model for the formation of the millisecond radio pulsars
predicts the presence of a rapidly rotating, weakly magnetized neutron star in
the core of these systems. The sudden drop in accretion rate associated with
the end of an X-ray outburst causes the Alfv\`en surface to move outside the
light cylinder, allowing the pulsar emission process to operate. No pulsed
signal was detected from the sources in our sample. We discuss several
mechanisms that could hamper the detection and suggest that free-free
absorption from material ejected from the system by the pulsar radiation
pressure could explain our null result.Comment: accepted by Ap
The two-hour orbit of a binary millisecond X-ray pulsar
Typical radio pulsars are magnetized neutron stars that are born rapidly
rotating and slow down as they age on time scales of 10 to 100 million years.
However, millisecond radio pulsars spin very rapidly even though many are
billions of years old. The most compelling explanation is that they have been
"spun up" by the transfer of angular momentum during accretion of material from
a companion star in so-called low-mass X-ray binary systems, LMXBs. (LMXBs
consist of a neutron star or black hole accreting from a companion less than
one solar mass.) The recent detection of coherent X-ray pulsations with a
millisecond period from a suspected LMXB system appears to confirm this link.
Here we report observations showing that the orbital period of this binary
system is two hours, which establishes it as an LMXB. We also find an apparent
modulation of the X-ray flux at the orbital period (at the two per cent level),
with a broad minimum when the pulsar is behind this low-mass companion star.
This system seems closely related to the "black widow" millisecond radio
pulsars, which are evaporating their companions through irradiation. It may
appear as an eclipsing radio pulsar during periods of X-ray quiescence.Comment: 4 pages with 1 figure. Style files included. Fig. 2 deleted and text
revised. To appear in Nature. Press embargo until 18:00 GMT on 1998 July 2
Magnetar outbursts: an observational review
Transient outbursts from magnetars have shown to be a key property of their
emission, and one of the main way to discover new sources of this class. From
the discovery of the first transient event around 2003, we now count about a
dozen of outbursts, which increased the number of these strongly magnetic
neutron stars by a third in six years. Magnetar outbursts might involve their
multi-band emission resulting in an increased activity from radio to hard
X-ray, usually with a soft X-ray flux increasing by a factor of 10-1000 with
respect to the quiescent level. A connected X-ray spectral evolution is also
often observed, with a spectral softening during the outburst decay. The flux
decay times vary a lot from source to source, ranging from a few weeks to
several years, as also the decay law which can be exponential-like, a power-law
or even multiple power-laws can be required to model the flux decrease. We
review here on the latest observational results on the multi-band emission of
magnetars, and summarize one by one all the transient events which could be
studied to date from these sources.Comment: 34 pages, 6 figures. Chapter of the Springer Book ASSP 7395
"High-energy emission from pulsars and their systems", proceeding of the Sant
Cugat Forum on Astrophysics (12-16 April 2010). Review updated to January
201
A large age for the pulsar B1757-24 from an upper limit on its proper motion
The "characteristic age" of a pulsar usually is considered to approximate its true age, but this assumption has led to some puzzling results, including the fact that many pulsars with small characteristic ages have no associated supernova remnants. The pulsar B1757-24 is located just beyond the edge of a supernova remnant; the properties of the system indicate that the pulsar was born at the centre of the remnant, but that it has subsequently overtaken the expanding blast-wave. With a characteristic age of 16,000 yr, this implies an expected proper motion by the pulsar of 63-80 milliarcsec per year. Here we report observations of the nebula surrounding the pulsar which limit its proper motion to less than 25 mas/yr, implying a minimum age of 39,000 yr. A more detailed analysis argues for a true age as great as 170,000 yr, significantly larger than the characteristic age. From this result and other discrepancies associated with pulsars, we conclude that characteristic ages seriously underestimate the true ages of pulsars
Radio pulsar populations
The goal of this article is to summarize the current state of play in the
field of radio pulsar statistics. Simply put, from the observed sample of
objects from a variety of surveys with different telescopes, we wish to infer
the properties of the underlying sample and to connect these with other
astrophysical populations (for example supernova remnants or X-ray binaries).
The main problem we need to tackle is the fact that, like many areas of
science, the observed populations are often heavily biased by a variety of
selection effects. After a review of the main effects relevant to radio
pulsars, I discuss techniques to correct for them and summarize some of the
most recent results. Perhaps the main point I would like to make in this
article is that current models to describe the population are far from complete
and often suffer from strong covariances between input parameters. That said,
there are a number of very interesting conclusions that can be made concerning
the evolution of neutron stars based on current data. While the focus of this
review will be on the population of isolated Galactic pulsars, I will also
briefly comment on millisecond and binary pulsars as well as the pulsar content
of globular clusters and the Magellanic Clouds.Comment: 16 pages, 6 figures, to appear in Proceedings of ICREA Workshop on
The High-Energy Emission from Pulsars and their Systems, Sant Cugat, Spain,
2010 April 12-16 (Springer
An increased estimate of the merger rate of double neutron stars from observations of a highly relativistic system
The merger of close binary systems containing two neutron stars should
produce a burst of gravitational waves, as predicted by the theory of general
relativity. A reliable estimate of the double-neutron-star merger rate in the
Galaxy is crucial in order to predict whether current gravity wave detectors
will be successful in detecting such bursts. Present estimates of this rate are
rather low, because we know of only a few double-neutron-star binaries with
merger times less than the age of the Universe. Here we report the discovery of
a 22-ms pulsar, PSR J0737-3039, which is a member of a highly relativistic
double-neutron-star binary with an orbital period of 2.4 hours. This system
will merge in about 85 Myr, a time much shorter than for any other known
neutron-star binary. Together with the relatively low radio luminosity of PSR
J0737-3039, this timescale implies an order-of-magnitude increase in the
predicted merger rate for double-neutron-star systems in our Galaxy (and in the
rest of the Universe).Comment: 6 pages, 2 figure
Binary and Millisecond Pulsars at the New Millennium
We review the properties and applications of binary and millisecond pulsars.
Our knowledge of these exciting objects has greatly increased in recent years,
mainly due to successful surveys which have brought the known pulsar population
to over 1300. There are now 56 binary and millisecond pulsars in the Galactic
disk and a further 47 in globular clusters. This review is concerned primarily
with the results and spin-offs from these surveys which are of particular
interest to the relativity community.Comment: 59 pages, 26 figures, 5 tables. Accepted for publication in Living
Reviews in Relativity (http://www.livingreviews.org
Entangled-State Cycles of Atomic Collective-Spin States
We study quantum trajectories of collective atomic spin states of
effective two-level atoms driven with laser and cavity fields. We show that
interesting ``entangled-state cycles'' arise probabilistically when the (Raman)
transition rates between the two atomic levels are set equal. For odd (even)
, there are () possible cycles. During each cycle the
-qubit state switches, with each cavity photon emission, between the states
, where is a Dicke state in a rotated
collective basis. The quantum number (), which distinguishes the
particular cycle, is determined by the photon counting record and varies
randomly from one trajectory to the next. For even it is also possible,
under the same conditions, to prepare probabilistically (but in steady state)
the Dicke state , i.e., an -qubit state with excitations,
which is of particular interest in the context of multipartite entanglement.Comment: 10 pages, 9 figure
Radio emission from Supernova Remnants
The explosion of a supernova releases almost instantaneously about 10^51 ergs
of mechanic energy, changing irreversibly the physical and chemical properties
of large regions in the galaxies. The stellar ejecta, the nebula resulting from
the powerful shock waves, and sometimes a compact stellar remnant, constitute a
supernova remnant (SNR). They can radiate their energy across the whole
electromagnetic spectrum, but the great majority are radio sources. Almost 70
years after the first detection of radio emission coming from a SNR, great
progress has been achieved in the comprehension of their physical
characteristics and evolution. We review the present knowledge of different
aspects of radio remnants, focusing on sources of the Milky Way and the
Magellanic Clouds, where the SNRs can be spatially resolved. We present a brief
overview of theoretical background, analyze morphology and polarization
properties, and review and critical discuss different methods applied to
determine the radio spectrum and distances. The consequences of the interaction
between the SNR shocks and the surrounding medium are examined, including the
question of whether SNRs can trigger the formation of new stars. Cases of
multispectral comparison are presented. A section is devoted to reviewing
recent results of radio SNRs in the Magellanic Clouds, with particular emphasis
on the radio properties of SN 1987A, an ideal laboratory to investigate
dynamical evolution of an SNR in near real time. The review concludes with a
summary of issues on radio SNRs that deserve further study, and analyzing the
prospects for future research with the latest generation radio telescopes.Comment: Revised version. 48 pages, 15 figure
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