Distribution Of Neutron Stars Research Articles

Prospects for the Observation of Continuous Gravitational Waves from Spinning Neutron Stars Lensed by the Galactic Supermassive Black Hole

We study the prospects of detecting continuous gravitational waves (CGWs) from spinning neutron stars (NSs), gravitationally lensed by the galactic supermassive black hole. Assuming various astrophysically motivated spatial distributions of galactic NSs, we find that CGW signals from a few (∼0–6) neutron stars should be strongly lensed. Lensing will produce two copies of the signal (with time delays of seconds to minutes) that will interfere with each other. The relative motion of the NS with respect to the lensing optical axis will change the interference pattern, which will help us to identify a lensed signal. Accounting for the magnifications and time delays of the lensed signals, we investigate their detectability by ground-based detectors. Modeling the spin distribution of NSs based on that of known pulsars and assuming an ellipticity of ϵ = 10−7, lensed CGWs are unlikely to be detectable by LIGO and Virgo in realistic searches involving templates. However, third-generation detectors have a ∼2%–51% probability of detecting at least one lensed CGW signal. For an ellipticity of ϵ = 10−8, the detection probability reduces to ∼0%–18%. Though rare, such an observation will enable interesting probes of the supermassive black hole and its environment.

Detecting axion dark matter with radio lines from neutron star populations

It has been suggested that radio telescopes may be sensitive to axion dark matter that resonantly converts to radio photons in the magnetospheres surrounding neutron stars (NSs). In this work, we closely examine this possibility by calculating the radiated power from and projected sensitivity to axion dark matter conversion in ensembles of NSs within astrophysical systems like galaxies and globular clusters. We use population synthesis and evolution models to describe the spatial distributions of NSs within these systems and the distributions of NS properties. Focusing on three specific targets for illustration, the Galactic Center of the Milky Way, the globular cluster M54 in the Sagittarius dwarf galaxy, and the Andromeda galaxy, we show that narrow-band radio observations with telescopes such as the Green Bank Telescope and the future Square Kilometer Array may be able to probe the quantum chromodynamics axion over roughly two orders of magnitude in mass, starting at a fraction of a $\mu$eV.

Modeling the spatial distribution of neutron stars in the Galaxy

In this paper we investigate the space and velocity distributions of old neutron stars (aged 109 to 1010 yr) in our Galaxy. Galactic old Neutron Stars (NSs) population fills a torus-like area extending to a few tens kiloparsecs above the galactic plane. The initial velocity distribution of NSs is not well known, in this work we adopt a three component initial distribution, as given by the contribution of kick velocities, circular velocities and Maxwellian velocities. For the spatial initial distribution we use a Γ function. We then use Monte Carlo simulations to follow the evolution of the NSs under the influence of the Paczyński Galactic gravitational potential. Our calculations show that NS orbits have a very large Galactic radial expansion and that their radial distribution peak is quite close to their progenitors’ one. We also study the NS vertical distribution and find that it can well be described by a double exponential low. Finally, we investigate the correlation of the vertical and radial distribution and study the radial dependence of scale-heights.

Space and velocity distributions of neutron stars in the Milky Way

We study the long term dynamics of isolated neutron stars (NSs) for different initial conditions. From the resulting phase-space distributions, we estimate the fraction of NSs bound to the Milky Way. We also estimate the surface and spatial density of NSs in the solar neighborhood and the mean velocity of NSs in the Galactic disk. Estimates of the sky density of NSs towards specific lines of sight, i.e. the Galactic center and the Magellanic Clouds, are also given.

Probing isolated compact remnants with microlensing

We consider isolated compact remnants (ICoRs), i.e. neutrons stars and black holes that do not reside in binary systems and therefore cannot be detected as X-ray binaries. ICoRs may represent $\sim\,5$ percent of the stellar mass budget of the Galaxy, but they are very hard to detect. Here we explore the possibility of using microlensing to identify ICoRs. In a previous paper we described a simulation of neutron star evolution in phase space in the Galaxy, taking into account the distribution of the progenitors and the kick at formation. Here we first reconsider the evolution and distribution of neutron stars and black holes adding a bulge component. From the new distributions we calculate the microlensing optical depth, event rate and distribution of event time scales, comparing and contrasting the case of ICoRs and "normal stars". We find that the contribution of remnants to optical depth is slightly lower than without kinematics, owing to the evaporation from the Galaxy. On the other hand, the relative contribution to the rate of events is a factor $\sim\,5$ higher. In all, $\sim\,6-7$ percent of the events are likely related to ICoRs. In particular, $\sim\,30-40$ percent of the events with duration $>\,100$ days are possibly related to black holes. It seems therefore that microlensing observations are a suitable tool to probe the population of Galactic ICoRs.

Galactic neutron stars

Aims. Neutron stars (NSs) produced in the Milky Way are supposedly ten to the eighth - ten to the ninth, of which only $\sim 2 \times 10^{3}$ are observed. Constraining the phase space distribution of NSs may help to characterize the yet undetected population of stellar remnants. Methods. We perform Monte Carlo simulations of NS orbits, under different assumptions concerning the Galactic potential and the distribution of progenitors and birth velocities. We study the resulting phase space distributions, focusing on the statistical properties of the NS populations in the disk and in the solar neighbourhood. Results. It is shown that $\sim 80$ percent of NSs are in bound orbits. The fraction of NSs located in a disk of radius 20 kpc and width 0.4 kpc is $\lesssim 20$ percent. Therefore the majority of NSs populate the halo. Fits for the surface density of the disk, the distribution of heights on the Galactic plane and the velocity distribution of the disk, are given. We also provide sky maps of the projected number density in heliocentric Galactic coordinates (l, b). Our results are compared with previous ones reported in the literature. Conclusions. Obvious applications of our modelling are in the revisiting of accretion luminosities of old isolated NSs, the issue of the observability of the nearest NS and the NS optical depth for microlensing events. These will be the scope of further studies.

Space and Velocity Distributions of Galactic Isolated Old Neutron Stars

ABSTRACTI present the results of Monte Carlo orbital simulations of Galactic Neutron Stars (NSs). The simulations take into account the up-to-date observed NS space and velocity distributions at birth, and account for their formation rate. I simulate two populations of NSs. NSs in the first population were born in the Galactic disk at a constant rate, in the past 12 Gyr. Those in the second population were formed simultaneously 12 Gyr ago in the Galactic bulge. I assume that the NSs born in the Galactic disk comprise 40% of the total NS population. Since the initial velocity distribution of NSs is not well known, I run two sets of simulations, each containing 3 × 106 simulated NSs. One set utilizes a bimodal initial velocity distribution and the other a unimodal initial velocity distribution, both are advocated based on pulsars observations. In light of recent observational results, I discuss the effect of dynamical heating by Galactic structure on NS space and velocity distributions and show it can be neglected. I present catalog of simulated NS space and velocity vectors in the current epoch, and catalog of positions, distances and proper motions of simulated NSs, relative to the Sun. Assuming there are 109 NSs in the Galaxy, I find that in the solar neighborhood the density of NSs is about 2 - 4 × 10-4 pc-3, and their scale height is about 0.3–0.6 kpc (depending on the adopted initial velocity distribution). Moreover, I estimate that about 60–70% of the NSs born in the Milky Way are gravitationally unbound to the Galaxy. These catalogs can be used to test the hypothesis that some radio transients are related to these objects.

Masses of neutron stars and blackholes born in close binary systems

Abstract The masses of some neutron stars (NSs), blackholes (BHs), radio and X-ray pulsars (PSRs) are known but with very different errors. We collected all of the considerably reliable mass values of these objects and we analysed these values. Using these data number-mass distributions of NSs and BHs are constructed. Among the 44 NSs none of them has a mass value > 2M ⊚. On the other hand, the average mass value for these NSs is very close to 1.4M ⊚. The distribution of the number of NSs with respect to their initial masses has a maximum at a value dose to 1.4M ⊚. We investigated the possibility of explaining very high mass values (∼ 6–10M ⊚) of BHs by accretion in dose binaries. Accretion in binary systems by itself cannot result in considerable changes in masses of compact objects, and one can hardly explain the gap between the masses of NSs and BHs and cannot explain the large masses of BHs. Therefore, the masses of BHs when they were born must be predominantly greater than 3–5M ⊚.

The Statistical Properties of X‐Ray Emission from Galactic Young Pulsars

We study statistical properties of the Galactic population of X-ray pulsars with ages of less than 106 yr using the Monte Carlo method. Combining the initial magnetic field and spatial and velocity distributions of neutron stars at birth (which are obtained from radio pulsar statistical studies) with either the X-ray pulsar model proposed by Cheng & Zhang or the expected X-ray flux obtained by the empirical formula proposed by Becker & Trumper, we obtain the distance, period, period derivative, and X-ray energy flux distributions of those X-ray pulsars whose X-ray and radio fluxes are above the detectable threshold fluxes. Our simulated results indicate that (1) the distance, period, period derivative, and X-ray energy flux distributions of simulated X-ray pulsars in both models are consistent with those of the known X-ray pulsars observed by ROSAT if the minimum detectable X-ray energy flux is 2 × 10-14 ergs cm-2 s-1, and (2) the expected X-ray pulsar numbers in both models are different; the ratio of expected X-ray pulsars in our model to those in Becker & Trumper's model is about 0.6. Moreover, as the minimum detectable X-ray energy flux decreases, the simulated distributions from the two models obviously differ except in the distance distributions, and especially so for the simulated distributions of the period and period derivative. As the minimum detectable X-ray energy flux decreases, those X-ray pulsars with long periods and small period derivatives become detectable. The next generation of X-ray satellites, e.g., XMM, may differentiate our model from that of Becker & Trumper.

Determination of ‘diffusion coefficients’ and stellar wind velocities for X-rays binaries

Abstract The distribution of neutron stars (NS's) is determined by stationary solution of the Fokker–Planck equation. In this work, using the observed period changes for four systems: Vela X-1, GX 301–2, Her X-1, and Cen X-3, we determined D, the ‘diffusion coefficient’ parameter, from the Fokker–Planck equation. Using strong dependence of D on the velocity, we determined the stellar wind velocity for Vela X-1 and GX 301–2, systems accreting from a stellar wind. For different assumptions for a turbulent velocity we obtained V = (660 – 1440) km s−1. It is in good agreement with the stellar wind velocity determined by other methods. We also determined the specific characteristic time scale for the ‘diffusion processes’ in X-ray pulsars. It is of the order of 200 sec for wind-fed pulsars and 1000–10000 sec for the disk accreting systems.