Two scales for the frequency dependence of the scattering of pulsar radio emission

We have measured the pulse broadening by scattering at 40, 60, and 111 MHz for the pulsars PSR B0809+74, B0950+08, B1919+21, and B2303+30. The frequency dependence of the scatter-broadening parameter is analyzed based on these measurements and data from the literature. The dependence obtained purely from the literature data is not consistent with the theory, and the scattering magnitudes differs considerably from the data of the catalog of 706 pulsars of Taylor et al. A two-component model for the frequency dependence of the scattering of the pulsar radio emission in the interstellar medium is proposed. Allowing for the presence of two scattering scales removes both inconsistencies between the observational data for these four pulsars and differences between the observed and theoretical frequency dependences for the scattering, as well as the need to invoke anomalous scattering magnitudes. The data of the catalog of Taylor et al. need to be corrected for the difference in the scattering magnitudes in the two branches of the frequency dependence.     

Distribution of interstellar plasma in the direction of PSR B0525+21 from data obtained on a ground–space interferometer

Observations on the RadioAstron ground–space interferometer with the participation of the Green Bank and Arecibo ground telescopes at 1668 MHz have enabled studies of the characteristics of the interstellar plasma in the direction of the pulsar PSR B0525+21. The maximum projected baseline for the ground–space interferometer was 233 600 km. The scintillations in these observations were strong, and the spectrum of inhomogeneties in the interstellar plasma was a power law with index n = 3.74, corresponding to a Kolmogorov spectrum. A new method for estimating the size of the scattering disk was applied to estimate the scattering angle (scattering disk radius) in the direction toward PSR B0525+21, θ _scat = 0.028 ± 0.002 milliarcsecond. The scattering in this direction occurs in a plasma layer located at a distance of 0.1Z from the pulsar, where Z is the distance from the pulsar to the observer. For the adopted distance Z = 1.6 kpc, the screen is located at a distance of 1.44 kpc from the observer.     

Distribution of inhomogeneities in the interstellar plasma in the directions of three distant pulsars from observations with the RadioAstron ground–space interferometer

The RadioAstron ground–space interferometer has been used to measure the angular sizes of the scattering disks of the three distant pulsars B1641–45, B1749–28, and B1933+16. The observations were carried out with the participation of the Westerbork Synthesis Radio Telescope; two 32-m telescopes at Torun, Poland and Svetloe, Russia (the latter being one antenna of the KVAZAR network); the Saint Croix VLBA antenna; the Arecibo radio telescope; the Parkes, Narrabri (ATCA), Mopra, Hobart, and Ceduna Australian radio telescopes; and the Hartebeesthoek radio telescope in South Africa. The full widths at half maximum of the scattering disks were 27 mas at 1668 MHz for B1641–45, 0.5 mas at 1668 MHz for B1749–28, and 12.3 at 316 MHz and 0.84 mas at 1668 MHz for B1933+16. The characteristic time scales for scatter-broadening of the pulses on inhomogeneities in the interstellar plasma τ_sc were also measured for these pulsars using various methods. Joint knowledge of the size of the scattering disk and the scatter-broadening time scale enables estimation of the distance to the effective scattering screen d. For B1641–45, d = 3.0 kpc for a distance to the pulsar D = 4.9 kpc, and for B1749–28, d = 0.95 kpc for D = 1.3 kpc. Observations of B1933+16 were carried out simultaneously at 316 and 1668 MHz. The positions of the screen derived using the measurements at the two frequencies agree: d ₁ = 2.6 and d ₂ = 2.7 kpc, for a distance to the pulsar of 3.7 kpc. Two screens were detected for this pulsar from an analysis of parabolic arcs in the secondary dynamic spectrum at 1668 MHz, at 1.3 and 3.1 kpc. The scattering screens for two of the pulsars are identified with real physical objects located along the lines of sight toward the pulsars: G339.1–04 (B1641–45) and G0.55–0.85 (B1749–28).     

Radio sounding of the circumsolar plasma using polarized pulsar pulses

We present the results of radio sounding observations probing the inner solar wind near the minimum of the solar-activity cycle, using polarized pulses from PSR B0525+21 and PSR B0531+21 received when the lines of sight toward these pulsars were close to the Sun. The observations were obtained in June 2005 and June 2007 on the Large Phased Array of the Lebedev Physical Institute at 111 MHz. An upper limit for the scattering of giant pulses from PSR B0531+21 due to their passage through the turbulent solar-wind plasma is determined. The arrival-time delays for pulses from PSR B0531+21 are used to derive the radial dependence of the mean density of the circumsolar plasma. The resulting density distribution indicates that the acceleration of fast, high-latitude solar-wind outflows continues to heliocentric distances of 5–10R _⊙, where R _⊙ is the solar radius. The mean plasma density at heliocentric distances of about 5R _⊙ is 1.4 × 10⁴ cm^?3, substantially lower than at the solar-activity maximum. This is associated with the presence of polar coronal holes. The Faraday rotation measure at heliocentric distances of 6–7R _⊙ is estimated. Deviations of the spatial distribution of the magnetic field from spherical symmetry are comparatively modest in the studied range of heliocentric distances.     

The pulsar PSR B1931+24 as an orthogonal rotator

The angles of the magnetic moment μ and the line of sight L to the rotation axis Ω are estimated for the pulsar PSR B1921+24, which displays “on” and “off” periods in its radio emission. It is shown that this object is an orthogonal rotator, i.e., the angle β between μ and Ω is equal to 88°.2 and the angle between L and Ω is ζ = 98.7°, and that its rotation period should be twice the usually adopted value (P = 1.626 s). One possible reason for the peculiarities of this pulsar could be the precession of a relic disk in the equatorial region of the object. Further observations (in particular, in the infrared) are required to confirm the existence of such a disk. Polarization data for other pulsars whose radiation switches on and off (transients) are also required, to determine if they are likewise orthogonal rotators. Calculations for PSR B0656+14 show that β ∼ 20°, and the sharp increase of its pulse intensities is due to intrinsic reasons, and is not associated with a relic disk. Original Russian Text ? I.F. Malov, 2007, published in Astronomicheskiĭ Zhurnal, 2007, Vol. 84, No. 6, pp. 531–535.     

The asymmetry coefficient for interstellar scintillation of extragalactic radio sources

Comparing the asymmetry coefficients γ and scintillation indices m for observed time variations of the intensity of the radiation of extragalactic sources and the predictions of theoretical models is a good test of the nature of the observed variations. Such comparisons can be used to determine whether flux density variations are due to scintillation in the interstellar medium or are intrinsic to the source. In the former case, they can be used to estimate the fraction of the total flux contributed by the compact component (core) whose flux density variations are caused by inhomogeneities in the interstellar plasma. Results for the radio sources PKS 0405-385, B0917+624, PKS 1257-336, and J1819+3845 demonstrate that the scintillating component in these objects makes up from 50 to 100% of the total flux, and that the intrinsic angular sizes of the sources at 5 GHz are 10–40 microarcseconds. The characteristics of the medium giving rise to the scintillations are presented.     

On the existence of planets around the pulsar PSR B0329+54

Results of timing measurements of the pulsar PSR B0329+54 obtained in 1968–2012 using the Big Scanning Antenna of the Pushchino Radio Astronomy Observatory (at 102 and 111 MHz), the DSS 13 and DSS 14 telescopes of the Jet Propulsion Laboratory (2388 MHz), and the 64 m telescope of the Kalyazin Radio Astronomy Observatory (610 MHz) are presented. The astrometric and rotational parameters of the pulsar are derived at a new epoch. Periodic variations in the barycentric timing residuals have been found, which can be explained by the presence of a planet orbiting the pulsar, with an orbital period P₁ = 27.8 yr, mass m _c sin i = 2M_?, and orbital semi-major axis a = 10.26 AU. The results of this study do not confirm existence of a proposed second planet with orbital period P₂ = 3 yr.     

Refinement of the parameters of the binary pulsar B0655+64 based on 111 MHz observations

An analysis of monitoring observations for the pulsar PSR B0655+64, which is located in a binary system, at 111 MHz during 2002–2015 are presented. The Keplerian parameters of the pulsar have been refived: the longitude of periastron ω = 276_.^°5785 ± 0_.^°0005 and the orbital semi-major axis is a_p sin i = 4.124976± 0.000003 s. The parameters of the perturbed motion have been determined: the motion of periastron ω = 0_.^°315 ± 0_.^°005/ year, and the derivative of the period of the binary system ? = (-1.66 ± 0.11) × 10^-14 s/s = (-0.524 ± 0.038) µs/year. The estimated time scale for the decay of the PSR 0655+64 system is (1.7 ± 0.1) × 10¹¹ yrs.     

Energy of anomalously intense pulsar pulses at decameter wavelengths

Cumulative and differential energy distributions are derived for the subpulse radio emission from the pulsars B0809+74, B0943+10, B0950+08, and B1133+16 at decameter wavelengths. The obtained cumulative distributions are compared with the analogous characteristic distributions for giant pulsar pulses. The analysis suggests that the amplification mechanisms in pulsar magnetospheres producing giant pulses and anomalously intense pulses may be similar.     

Measurements of the scattering of pulsar radio emission

Measurements of the broadening of pulsar pulses by scattering in the interstellar medium are presented for a complete sample of 100 pulsars with Galactic longitudes from 6° to 311° and distances to three kiloparsec. The dependences of the scattering on the dispersion measure (τ _sc(DM) ∝ DM^α), frequency (τ _sc(v) ∝ v ^?γ ), Galactic longitude, and distance to the pulsar are analyzed. The dependence of the scattering on the dispersion measure in the near-solar neighbourhood can be represented by the power law τ _sc(DM) ∝ DM^2.2±0.1). Measurements at the low frequencies 111, 60, and 40 MHz and literature data are used to derive the frequency dependence of the scattering (τ _sc(v) ∝ V ^?γ ) over a wide frequency interval (covering a range of less than 10: 1) with no fewer than five frequencies. The index for the frequency dependence, γ = 4.1 ± 0.3, corresponds to a normal distribution for inhomogeneities in the turbulence in the scattering medium. Based on an analysis of the dependence of the scattering on the distance to the pulsar and on Galactic longitude, on average, the turbulence level C _n ² is the same in all directions and at all distances out to about three kpc, testifying to the statistical homogeneity of the turbulence of the scattering medium in the near-solar region of the Galaxy.     

Phase and intensity distributions of individual pulses of PSR B0950+08

The distribution of the intensities of individual pulses of PSR B0950+08 as a function of the longitudes at which they appear is analyzed. The flux density of the pulsar at 111 MHz varies strongly from day to day (by up to a factor of 13) due to the passage of the radiation through the interstellar plasma (interstellar scintillation). The intensities of individual pulses can exceed the amplitude of the mean pulse profile, obtained by accumulating 770 pulses, by more than an order of magnitude. The intensity distribution along the mean profile is very different for weak and strong pulses. The differential distribution function for the intensities is a power law with index n = ?1.1 ± 0.06 up to peak flux densities for individual pulses of the order of 160 Jy.     

A spindown mechanism for short-period radio pulsars

It is shown that a model with accretion in a “quasi-propeller” mode can explain the observed spindown of pulsars with periods P<0.1 s. The mean accretion rate for 39 selected objects is \(\dot M = 5.6 \times 10^{ - 11} M_ \odot /year\). If \(\dot M\) is constant during the pulsar’s lifetime, the neutron star will stop rotating after 10⁷ years. The mean magnetic field at the neutron-star surface calculated in this model, \(\bar H_0 = 6.8 \times 10^8 G\), is consistent to an order of magnitude with the values of H₀ for millisecond pulsars from known catalogs. However, the actual value of H₀ for particular objects can differ from the catalog values by appreciable factors, and these quantities must be recalculated using more adequate models. The accretion disk around the neutron star should not impede the escape of the pulsar’s radiation, since this radiation is generated near the light cylinder in pulsars with P<0.1 s. Pulsars such as PSR 0531+21 and PSR 0833-45 have probably spun down due to the effect of magnetic-dipole radiation. If the difference in the braking indices for these objects from n=3 is due to the effect of accretion, the accretion rate must be of the order of 10¹⁸ g/s.     

The parameters of pulsar subpulse emission at decameter wavelengths

An original method for determining the main parameters of the radio emission of pulsar subpulses at decameter wavelengths is proposed. The method involves the combined use of spectral and correlation analyses for the recorded signals. The novelty of the method is connected with two conditions that must be fulfilled to determine all the characteristics of the subpulse decameter emission. First, the signal-to-noise ratio in the output data must be increased, which can be done only by accumulating more data. Second, the phase characteristics of the subpulse component in the main pulse window must be preserved during the accumulation process. The method proposed makes it possible to fulfill these conditions simultaneously. A reference transfer function obtained from a spectral analysis of data with a relatively high number of individual detected pulses is used in the correlation analysis. The method is used to determine the drift rate, subpulse component width, individual subpulse width, secondary periods P ₂ and P ₃, and the subpulse structure coherence timescale recorded for the pulsar PSR B0809+74 at the central frequency 23.7 MHz. Perspectives for future application of the method are discussed.     

Microstructure of pulsar radio pulses measured with a time resolution of 62.5 ns at 1650 MHz

We present an analysis of pulsar observations carried out on two frequency channels at 1634 MHz and 1650 MHz with a time resolution of 62.5 ns on the 70-m radio telescope of the NASA Deep Space Network in Tidbinbilla. The data were recorded using the S2 system, intended primarily for VLBI observations. Microstructure with characteristic timescales of 270, 80, and 150 µs was detected in pulsars B0833-45, B1749-28, and B1933 + 16, respectively. The distribution of microstructure timescales for the Vela pulsar (B0833-45) is characterized by a gradual growth with decreasing timescale to 200 µs; the distribution has a maximum at 20–200 µs and falls off sharply for timescales below 20 µs. The statistical relation between the microstructure modulation index m and the corresponding timescale τ_µ can be approximated by the power law dependence R∝τ _⊙ ^0.5 ; i.e., the intensity is higher for micropulses with longer durations. This contradicts the predictions of nonlinear models for the formation of micropulses by supercompact soliton wave packets. In all the pulsars studied, the time delays of the micropulses between the two frequency channels deviate from the expected dispersion laws for the interstellar plasma. In particular, the micropulses in the low-frequency channel arrive earlier than predicted by the dispersion measures derived previously from the mean pulse profiles. The deviation from the dispersion delay is determined most accurately for B0833-45, and is 4.9±0.2 µs. Such anomalous delays are probably associated with the effects of propagation of the radio emission within the pulsar magnetosphere.     

Studies of cosmic plasma using radioastron VLBI observations of giant pulses of the pulsar B0531+21

The structure of the interstellar plasma in the direction of the pulsar in the Crab Nebula is studied using several sets of space-VLBI observations obtained with networks of ground telescopes and the RadioAstron space antenna at 18 and 92 cm. Six observing sessions spanning two years are analyzed. Giant pulses are used to probe the cosmic plasma, making it possible to measure the scattering parameters without averaging. More than 4000 giant pulses were detected. The interferometer responses (visibility functions) on ground and ground–space baselines are analyzed. On the ground baselines, the visibility function as a function of delay is dominated by a narrow feature at zero delay with a width of δ_τ ~ 1/B, where B is the receiver bandwidth. This is typical for compact continuum sources. On the ground–space baselines, the visibility function contains a set of features superposed on each other and distributed within a certain interval of delays, which we identify with the scattering time for the interfering rays τ. The amplitude of the visibility function on ground baselines falls with increasing baseline; the scattering disk is partially resolved at 18 cmand fully resolved at 92 cm. Estimates of the scattering angle ? give 0.5–1.3mas at 18 cm and 14.0 mas at 92 cm. The measured values of ? and τ are compared to estimate the distance from the source to the effective scattering screen, which is found at various epochs to be located at distances from 0.33 to 0.96 of the distance from the observer to the pulsar, about 2 kpc. The screen is close to the Crab Nebula at epochs of strong scattering, confirming that scattering on inhomogeneities in the plasma in the vicinity of the nebula itself dominates at these epochs.     

Search for and detection of pulsars inmonitoring observations at 111 MHz

In the course of monitoring interplanetary scintillations of a large number of sources using the Big Scanning Antenna of the Lebedev Physical Institute, a search for pulsars with periods ≥0.4 s at declinations ?9? < δ < 42? and right ascensions 0h < α < 24h was simultaneously carried out. The search was conducted using four years of observations carried out at 110.25MHz in six frequency channels making up a 2.5 MHz band and having a time resolution of 100 ms. The initial identification of pulsar candidates was done using Fourier power spectra averaged over the entire observational period; the pulsar candidates were then verified using observations with higher frequency and time resolution: 32 frequency channels and a time resolution of 12.5 ms. Eighteen new pulsars were discovered in the studied area, whose main characteristics are presented.     

Observations of giant pulses from B1237+25 (J1239+2453) at 111 MHz. Detection and classification

An analysis of data from monitoring of individual pulses of the second-period pulsar PSR B1237+25 (J1239+2453) carried out on the Large Phased Array (LPA) of the Pushchino Radio Astronomy Observatory at 111 MHz during 2012–2015 is presented. The aim of this observing program is a search for anomalously strong and giant pulses. The regular generation of powerful individual pulses at the longitudes of three of five components in the main profile of PSR B1237+25 has been detected. The distribution of these strong pulses in flux density is bimodal, and has the power-law form characteristic for giant pulses, with power-law indices n = ?1.26 ± 0.05 and ?3.36 ± 0.34, which differentiates them from the regular pulses of pulsars, having a log–normal distribution. The characteristic pulse widths at the half-intensity level are 3–5 ms, which comprises 50–100% of the width of the corresponding component in the mean profile. The most powerful of the detected pulses had a peak flux density of 900 ± 160 Jy, and the strongest pulse exceeded the session-mean profile by a factor of 65.     

The origin of the rotational and spatial velocities of radio pulsars

We analyze possible origins of the observed high rotational and spatial velocities of radio pulsars. In particular, these can be understood if all radio pulsars originate in close binary systems with orbital periods of 0.1–100 days, with the neutron star being formed by a type Ib,c supernova. The high spatial velocities of pulsars (v _p up to 1000 km/s) reflect the high Keplerian velocities of the components of these binaries, while their short periods of rotation (P _p < 4 s) are due to the rapid rotation of the presupernova helium-star components with masses of 2.5–10 M_⊙, which is synchronous with their orbital rotation. Single massive stars or components in wide binaries are likely to produce only slowly rotating (P _p > 4 s) neutron stars or black holes, which cannot be radio pulsars. As a result, the rate of formation of radio pulsars should be a factor of a few lower than the rate of type II and type Ib,c supernovae estimated from observations. This scenario for the formation of radio pulsars is supported by (i) the bimodal spatial velocity distribution of radio pulsars; (ii) the coincidence of the observed spatial velocities of radio pulsars with the orbital velocities of the components of close binaries with nondegenerate helium presupernovae; (iii) the correlation between the orbital and rotational periods for 22 observed radio pulsars in binaries with elliptical orbits; and (iv) the similarity of the observed rate of formation of radio pulsars and the rate of type Ib,c supernovae.     

Measurement of the inner turbulence scale of the interstellar plasma toward the pulsar PSR B2111+46

The pulsar PSR B2111+46 has been observed at 112 MHz, and a new approach to analyzing pulsar pulses scattered in turbulent interstellar plasma applied. This method is based on the dependence of the normalized energy in the trailing part of a pulse on the intrapulse time. Since the trailing edge of a pulse follow exponential law to high accuracy, the inner turbulence scale of the interstellar plasma exceeds the field coherence scale. The measured scattering parameter is τ _sc = 147 ± 1 ms. Analysis of the parameters of diffractive and refractive scintillations of the pulsar at 610 MHz together with the 112 MHz data shows that the spectrum of the interstellar plasma toward PSR B2111+46 is a piecewise power law: on scales of 10¹³–10¹⁴ cm, the exponent of the turbulence spectrum is n ≃ 4, whereas n = 3.5 on scales of 2 × 10⁸−10¹³ cm. The spectrum flattens with approach to the inner turbulence scale l: n = 3–3.2. The obtained inner turbulence scale is l = (3.5 ± 1.5) × 10⁷ cm. The distribution of the interstellar plasma toward the pulsar is close to statistically homogeneous. The local density (N _e = 0.4 cm⁻³) and filling factor (F = 0.04) of the interstellar plasma have been estimated. The similarity of N _e estimates obtained from the inner scale of the inhomogeneities and the ratio of the emission measure to the dispersion measure provides evidence that the inner turbulence scale corresponds to the ion inertial length.     

Magnetic fields and the $$\dot P - P$$ diagram for radio pulsars

Various mechanisms for the loss of angular momentum of neutron stars are analyzed. Theoretical predictions about the evolution of the period are compared with the observed distribution of pulsars on the log\(\dot P\)log(P) diagram. Pulsars with short periods (P≤0.1 s) cannot be fit well by any of the models considered. Their braking index is n=?1, which requires the development of a new braking mechanism. The evolution of pulsars with P>1.25 s is described by the law \(\dot P \propto P^2\), probably due to processes internal to the neutron stars. The observational data for pulsars with 0.1<P≤1.25 s can be fit with a hybrid model incorporating internal processes and magnetic-dipole losses. The magnetic fields in pulsar catalogs should be recomputed in accordance with the results obtained. For example, the magnetic fields obtained for two magnetars with P=5.16 s and P=7.47 s are B _s =1.7×10¹³ and 2.9×10¹³ G, which are lower than the critical field B_cr=4.4×10¹³ G. For a substantial fraction of pulsars, their characteristic ages \(\tau = P/2\dot P\) cannot serve as measures of their real ages.