Distribution of free electrons in the galactic plane

A method is proposed for constructing a map of the electron density distribution in the galactic plane. Data on the dispersion measures DM of more than 1500 pulsars and their distances, found by an independent method, are used. The independent distances of the pulsars are estimated using an empirical relation of the form L=γP ^α Ṗ ^β W ^δ between the radio luminosity L of the pulsars and their periods P, the rate of change Ṗ. of their periods, and the half width W of their pulses. A map of the electron density distribution in the galactic plane within a ±400 pc layer is provided. __________ Translated from Astrofizika, Vol. 49, No. 2, pp. 277–287 (May 2006).     

Age Dependence of Several Parameters of the Radiation from Radio Pulsars

It is shown that the magnitude of the changes in the periods (rates of change of the periods) of radiation from pulsars depends on their characteristic age. With age these changes proceed more slowly. It is also shown that there is a relationship between the changes in the period of the radiation and the period. This relationship becomes closer as pulsars evolve with age.__________Translated from Astrofizika, Vol. 48, No. 2, pp. 223–228 (May 2005).     

Statistics and Evolution of Pulsars’ Parameters

The rotation periods, surface magnetic field strengths, as well as the spatial distribution of the several kinds of pulsars discovered sofar are analyzed statistically. It is revealed that the spatial distribution of the millisecond pulsars is more dispersive than that of the normal radio pulsars. And that the spatial distribution of the pulsars in low-mass X-ray binaries (LMXBs) is also more dispersive than that of the pulsars in high-mass X-ray binaries (HMXBs). The distribution of rotation periods of the isolated millisecond pulsars has a peak at 4.7ms, and the corresponding peak values for the normal radio pulsars and the millisecond pulsars in binaries are 0.6 s and 3.5ms, respectively. The surface magnetic field strengths of the FERMI pulsars (the gamma-ray pulsars observed by the Large Area Telescope/Fermi Gamma-ray Space Telescope) and normal pulsars are all concentrated around 1012 Gs. It is found also that some young high-energy pulsars are associated with supernova remnants. In combination with the formation and evolution models of pulsars, we have made some remarks on the characteristics of these distributions.     

The radio luminosity of pulsars and the distribution of interstellar electron density

The radio luminosities of pulsars are given as functions of their period and the time variation of the period. The parameters of that dependence are calculated and independent distances are determined for pulsars. The average electron densities toward the pulsars are determined from the known dispersion measures. The results obtained are used to study the large-scale electron density distribution in the Galaxy. The distribution maximum lies in the vicinity of the Sagittarius spiral arm. The electron density falls off exponentially in the regions between spiral arms. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Effect of Lunisolar Precession on Pulsar Observations

In the measurements of pulsars’ periods and their rates of change there exists the effect of lunisolar precession. Via the analysis of this effect, we find that it can affect the accurate measurements of the periods of certain pulsars. An investigation of the periods of 1771 pulsars reveals that the measured periods of 81 pulsars suffer the influence of lunisolar precession in the range of the accuracy of measurements. With the improvement of the precision of measurements of pulsars’ periods and their change rates, this effect should get the corresponding corrections.     

The distribution of space velocities of pulsars

Using reasonable assumptions, we derive the distribution function of pulsar space velocities from the distribution of pulsar heights above the Galactic plane. We find that for 137 pulsars of the north Galactic hemisphere, the space velocity distribution is discrete, and forms two separate groups. We estimate the velocity ranges and the mean pulsar velocity of each group.Translated from Astrofizika, Vol. 37, No. 2, pp. 245–254, April–June, 1994.     

Pre-Breakdown Processes in the Hollow Cathode Region of a Transient Hollow Cathode Discharge

Discharge formation in the A–K space of transient hollow cathode discharge (THCD) is causally linked with the emission of high energy electron beams originating in the hollow cathode region (HCR). Ionization in the A–K gap proceeds through the formation of a moving virtual anode, whose time evolution is strongly correlated with different periods in the electron beam activity. Here, we report on time and space resolved observations of different ionization events inside the HCR, which are time correlated with ionization processes inside the A–K gap. The experiments have been performed in Hydrogen, at pressures between 50 and 400 mTorr. A statistical study of the characteristic times associated with the different ionization events, based on von Laue plots, shows that the time distribution of events is well described by a single Gaussian distribution.     

Velocities and birthplaces of pulsars

The values of the proper motions of 36 pulsars are analysed. It is shown that the birthplaces of pulsars are located within the OB-associations and/or in the galactic arms. Eleven cases of pulsar formation in associations, and fourteen cases of formation in the galactic arms, have been established. More precise parameters of pulsars — such as: distances, ages, spatial velocity components —, and the spatial velocity distribution of pulsars is constructed.     

Spatial Distribution of Pulsars

The spatial distributions of pulsars with ages of less than 10⁶ years and more than 10⁶ years are shown to be quite different. The spatial distributions of pulsars with ages of less than 106 years and of the remnants of supernova outbursts are essentially the same. They lie near the galactic plane in a thin zone of width 400 pc. The overwhelming majority of pulsars with ages exceeding 106 years lie outside this zone. These facts suggest a genetic relationship between pulsars and supernova remnants. The spatial distribution of pulsars with different emission periods also supports this point.     

Searching for Runaway OB Stars in Supernova Remnants

Searching for runaway stars in Supernova remnants gives us the possibility to estimate the mass ratio in binary systems in which there occur Supernova explosion. Such a method also gives information on portions of spectroscopic and close binaries among the stars on the main sequence with mass > 7–8 M⊙. More importantly, we can learn more about space velocities and spin periods of pulsars by this way. We have searched for runaway OB stars in central parts of 48 Supernova remnants with distances less than about 3 kpc. In 16 of the remnants in our sample, there is no candidate O or B type star and we have shown that pulsars (point sources) might be born not only in spectroscopic and close binaries. We have represented a list of stars which are candidates for runaway B type stars located in Supernova remnants. Spectroscopic investigations on these candidates could provide solutions for the problems mentioned above.__________Published in Astrofizika, Vol. 48, No. 3, pp. 393–408 (August 2005).     

Population statistics study of radio and gamma-ray pulsars in the Galactic plane

We present results of our pulsar population synthesis of normal and millisecond pulsars in the Galactic plane. Over the past several years, a program has been developed to simulate pulsar birth, evolution and emission using Monte Carlo techniques. We have added to the program the capability to simulate millisecond pulsars, which are old, recycled pulsars with extremely short periods. We model the spatial distribution of the simulated pulsars by assuming that they start with a random kick velocity and then evolve through the Galactic potential. We use a polar cap/slot gap model for γ-ray emission from both millisecond and normal pulsars. From our studies of radio pulsars that have clearly identifiable core and cone components, in which we fit the polarization sweep as well as the pulse profiles in order to constrain the viewing geometry, we develop a model describing the ratio of radio core-to-cone peak fluxes. In this model, short period pulsars are more cone-dominated than in our previous studies. We present the preliminary results of our recent study and the implications for observing these pulsars with GLAST and AGILE.      

Doppler period variations of high-velocity pulsars

The effect of the source motion on the period variation of pulsars is investigated. For some pulsars, a velocity 100–300 km/s can give a Doppler contribution to the second period derivative P which is comparable or larger than the intrinsic variation predicted by theoretical models, whereas larger velocities are required in order to give an appreciable kinematic effect on the first derivative P. The possibility of experimental detection of P and proper motion for known pulsars is discussed.     

Massive X-ray binaries: Their physics and evolution

We review various aspects of the evolutionary history of massive X-ray binaries. It is expected that moderately massive close binaries evolve to Be X-ray binaries, while very massive systems evolve to standard X-ray binaries.The compact objects are formed through supernova explosions. The fairly low galactic latitudes of those systems indicate that the explosion should, in general, not have accelerated the system to a velocity larger than 50kms^–1. This implies that the mass of the exploding stars is in general less than 5 to 6M .After the explosion, tidal forces will circularize the orbit of short period systems. Even if the tidal evolution has been completed, the expansion of the optical star during the course of its evolution will continously disturb the stability of the orbit. Short period systems with large mass ratio may eventually become tidally unstable. Cen X-3 may be an example of such a system. The predicted rate of the orbital period decrease of Cen X-3 is in agreement with the observed rate.A way to represent the rotational and magnetic evolution of neutron stars in close binary systems is presented. The observed distribution of the pulsation periods of X-ray pulsars with Be companions is consistent with initial magnetic fields of 10¹²–10¹³ G of the neutron stars. We suggest that the fast X-ray pulsars 4U 0115+63 and A 0538-66 are young neutron stars, while Cen X-3 and SMC X-1 are recycled pulsars.The evolutionary relationship between massive X-ray binaries, binary pulsars, and millisecond pulsars is also discussed.Invited paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

Strong-field tests of gravity using pulsars and black holes

The sensitivity of the SKA enables a number of tests of theories of gravity. A Galactic Census of pulsars will discover most of the active pulsars in the Galaxy beamed toward us. In this census will almost certainly be pulsar–black hole binaries as well as pulsars orbiting the super-massive black hole in the Galactic centre. These systems are unique in their capability to probe the ultra-strong field limit of relativistic gravity. These measurements can be used to test the Cosmic Censorship Conjecture and the No-Hair theorem.The large number of millisecond pulsars discovered with the SKA will also provide a dense array of precision clocks on the sky. These clocks will act as the multiple arms of a huge gravitational wave detector, which can be used to detect and measure the stochastic cosmological gravitational wave background that is expected from a number of sources.     

X-ray pulsars: their masses,radii and equations of state

The semi-empirical mass — radius relations of X-ray pulsars, which depend on the exterior behavior of X-ray pulsars, are calculated for four X-ray pulsars, Cen X-3, 4U 1626–67, Her X-1 and EXO 2030+375, based on the observed data and our modified accretion torque model. Comparison between the calculated and the theoretical mass — radius relations shows that three of them, Cen X-3, 4U 1626–67 and EXO 2030+375, are consistent with the stiff or intermediate stiff equations of state, yielding the masses and radii around the typical values of neutron stars. Her X-1 seems not to be well described by the standard equations of state, though it may be reconciled with the soft equation of state.     

Rate of stellar collapses in the Galaxy

From an analysis of pulsar spatial and luminosity distributions, the number density of observed pulsars in the local region is determined to be 1.1±0.4×10^–7 pulsar pc^–3. Multiplication by the detection factor and by the ratio of Galaxy mass to local matter density and division by a mean lifetime of pulsars of 3×10⁶ yr suggests a pulsar birth every 4 yr. A stellar collapse might occur even more often.Supported in part by the U.S. Energy Research and Development Administration and by the National Science Foundation.     

An Estimation of the Initial Period of Pulsars

The initial period of a pulsar is an important factor in our understanding of the formation of neutron stars and of the nature of the equation of state of neutron star matter.Up to now this quantity can only be obtained for a few pulsars for which accurate age and braking index are known.Based on the theory of the offcenter dipole emission,in which pulsars obtain theiry high velocities depending on the initial periods,we calculate the initial period using the proper motion data,Because the orbital velocity of the progenitor and asymmetric kick in the supernova explosion may also contribute to the observed velocity of the pusar,the derived values of initial periods are lower limits.For normal pulsars,the initial periods are in the range of 0.6～2.6ms.For the millisecond pulsars,the initial periods are comparable to their current periods,and the ratio between the initial period and the current period increases with the decrease of the current period.For PSR B1937 21 with the shortest period of 1.56ms,the ratio is 0.77.     

A search for soft X-ray radiation from pulsars with the Astronomical Netherlands Satellite

We present the results of a search for X-ray emission in the energy range 0.2–0.28 keV and 1–7 keV from a number of radio pulsars, including Crab, Vela and the binary pulsar PSR 1913+16, using the soft X-ray experiment aboard ANS. Except for the Crab no pulsed flux has been found. From the Vela pulsar we have detected continuous flux in agreement with earlier observations. Upper limits are given.Paper presented at the COSPAR Symposium on Fast Transients in X-and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975.     

How rapidly do neutron stars spin at birth? Constraints from archival X-ray observations of extragalactic supernovae

Traditionally, studies aimed at inferring the distribution of birth periods of neutron stars are based on radio surveys. Here we propose an independent method to constrain the pulsar spin periods at birth based on their X-ray luminosities. In particular, the observed luminosity distribution of supernovae (SNe) poses a constraint on the initial rotational energy of the embedded pulsars, via the     correlation found for radio pulsars, and under the assumption that this relation continues to hold beyond the observed range. We have extracted X-ray luminosities (or limits) for a large sample of historical SNe observed with Chandra , XMM and Swift , which have been firmly classified as core-collapse SNe. We have then compared these observational limits with the results of Monte Carlo simulations of the pulsar X-ray luminosity distribution for a range of values of the birth parameters. We find that a pulsar population dominated by millisecond periods at birth is ruled out by the data.     

The Parkes Multibeam Pulsar Survey – VI. Discovery and timing of 142 pulsars and a Galactic population analysis

We present the discovery and follow-up observations of 142 pulsars found in the Parkes 20-cm multibeam pulsar survey of the Galactic plane. These new discoveries bring the total number of pulsars found by the survey to 742. In addition to tabulating spin and astrometric parameters, along with pulse width and flux density information, we present orbital characteristics for 13 binary pulsars which form part of the new sample. Combining these results from another recent Parkes multibeam survey at high Galactic latitudes, we have a sample of 1008 normal pulsars which we use to carry out a determination of their Galactic distribution and birth rate. We infer a total Galactic population of  30 000 ± 1100  potentially detectable pulsars (i.e. those beaming towards us) having 1.4-GHz luminosities above 0.1 mJy kpc². Adopting the Tauris & Manchester beaming model, this translates to a total of  155 000 ± 6000  active radio pulsars in the Galaxy above this luminosity limit. Using a pulsar current analysis, we derive the birth rate of this population to be  1.4 ± 0.2  pulsars per century. An important conclusion from our work is that the inferred radial density function of pulsars depends strongly on the assumed distribution of free electrons in the Galaxy. As a result, any analyses using the most recent electron model of Cordes & Lazio predict a dearth of pulsars in the inner Galaxy. We show that this model can also bias the inferred pulsar scaleheight with respect to the Galactic plane. Combining our results with other Parkes multibeam surveys we find that the population is best described by an exponential distribution with a scaleheight of 330 pc. Surveys underway at Parkes and Arecibo are expected to improve the knowledge of the radial distribution outside the solar circle, and to discover several hundred new pulsars in the inner Galaxy.