Simultaneous dual-frequency observations of giant radio pulses from the millisecond pulsar B1937+21

Simultaneous dual-frequency observations of giant radio pulses from the millisecond pulsar B1937+21 were performed for the first time in January–February 2002 on the Westerbork Synthesis Radio Telescope (2210–2250 MHz) and the 64-m Kalyazin radio telescope (1414–1446 MHz). The total observing time was about three hours. Ten giant pulses with peak flux densities from 600 to 1800 Jy were detected at 2210–2250 MHz, and fifteen giant pulses with peak flux densities from 3000 to 10000 Jy were observed at 1414–1446 MHz. No events were found to occur simultaneously at both frequencies. Thus, the observed radio spectra of individual giant pulses of this pulsar are limited in frequency to scales of about \(\frac{{\Delta v}}{v} < 0.5\). The duration of the giant pulses is less than 100 ns and is consistent with the expected scattering timescale in these frequency ranges. Instantaneous radio spectra of the detected giant pulses were compared with the diffractive spectra obtained from ordinary pulses of the pulsar. In some cases, considerable deviations of the radio spectra of the giant pulses from the diffractive spectrum were revealed, which can be interpreted as indicating temporal structure of the giant pulses on timescales of 10–100 ns.     

Giant pulses of PSR B0531+21 at 112 MHz

Three series of 111.88 MHz observations of giant pulses of PSR B0531+21 have been carried out in 2005 and 2007. The scattering of pulses observed in various series varies by a factor of 1.7: 10.6±0.5 ms in November 2005, 18±1 ms in January 2007, and 16±0.8 ms in June 2007. The cumulative probability distribution for the peak intensities of the giant pulses for each of these series shows that the distribution is stable and is a power law with a single slope (n = 2.3). This testifies to stability of the mechanism generating the giant pulses. The distribution functions for the 2005 and 2007 data can be superposed after correcting the intensities with a coefficient equal to the ratio of the effective pulse widths. Consequently, in the range of 23MHz-9GHz the energy in the pulses is conserved; i.e., the increase in the pulse intensity is proportional to the decrease in the scattering. Refractive scintillations at low frequencies in measurements with large time separation lead to variations in the number of giant pulses exceeding a given amplitude, proportional to the ratio of the mean flux densities of the pulsar in the corresponding observational series. The maximum energy of the recorded giant pulses is 2.5 × 10⁷ Jy µs. A comparison with the statistical properties of the giant pulses observed at other frequencies shows that the frequency dependence of the maximum energy of the giant pulses in the range of 23 MHz-9GHz is a power-law with index 2.2±0.2. The degree of linear polarization of the giant pulses at 112 MHz does not exceed 12%.     

Instantaneous radio spectra of giant pulses from the crab pulsar from decimeter to decameter wavelengths

The results of simultaneous multifrequency observations of giant radio pulses from the Crab pulsar, PSR B0531+21, at 23, 111, and 600 MHz are presented and analyzed. Giant pulses were detected at a frequency as low as 23 MHz for the first time. Of the 45 giant pulses detected at 23 MHz, 12 were identified with counterparts observed simultaneously at 600 MHz. Of the 128 giant pulses detected at 111 MHz, 21 were identified with counterparts observed simultaneously at 600 MHz. The spectral indices for the power-law frequency dependence of the giant-pulse energies are from ?3.1 to ?1.6. The mean spectral index is ?2.7 ± 0.1 and is the same for both frequency combinations (600–111 MHz and 600–23 MHz). The large scatter in the spectral indices of the individual pulses and the large number of unidentified giant pulses suggest that the spectra of the individual giant pulses do not actually follow a simple power law. The observed shapes of the giant pulses at all three frequencies are determined by scattering on interstellar plasma inhomogeneities. The scatter-broadening of the pulses and its frequency dependence were determined as τ _sc = 20(ν/100)^?3.5±0.1 ms, where frequency ν is in MHz.     

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.     

The dynamics and structure of plasma inhomogeneities in the Crab Nebula

The results of studies of the dynamics and structure of plasma inhomogeneities in the Crab Nebula carried out during 2002–2015 at 111 MHz on the Large Phased Array of the Pushchino Radio Astronomy Observatory are presented. Giant pulses of the pulsar PSR B0531+21 were observed and analyzed using specialized software designed to enable characterization of the scattering of a pulse via modeling of its passage through the scattering medium. The results of this analysis for the scattering of giant pulses are compared to variations in the dispersion measure, derived using data from Jodrell Bank Observatory (United Kingdom). Numerous non-stationary events associated with enhanced scattering are identified during the indicated period. The strongest scattering was observed during 2012–2014. The corresponding data are interpreted as eclipses of the pulsar by filaments in the Crab Nebula. A correlation between the variations in the scattering and dispersion measure is observed.     

Giant pulses—The main component of the radio emission of the crab pulsar

The paper presents an analysis of dual-polarization observations of the Crab pulsar obtained on the 64-m Kalyazin radio telescope at 600 MHz with a time resolution of 250 ns. A lower limit for the intensities of giant pulses is estimated by assuming that the pulsar radio emission in the main pulse and interpulse consists entirely of giant radio pulses; this yields estimates of 100 and 35 Jy for the peak flux densities of giant pulses arising in the main pulse and interpulse, respectively. This assumes that the normal radio emission of the pulse occurs in the precursor pulse. In this case, the longitudes of the giant radio pulses relative to the profile of the normal radio emission turn out to be the same for the Crab pulsar and the millisecond pulsar B1937+21, namely, the giant pulses arise at the trailing edge of the profile of the normal radio emission. Analysis of the distribution of the degree of circular polarization for the giant pulses suggests that they can consist of a random mixture of nanopulses with 100% circular polarization of either sign, with, on average, hundreds of such nanopulses within a single giant pulse.     

Probing cosmic plasma with giant pulses from the crab nebula pulsar

A review and comparative analysis of results from studies of the effects of scattering on the interstellar medium using giant pulses of the Crab Nebula pulsar (B0531+21) are presented. This analysis was based on eight epochs of Very Long Baseline Interferometry (VLBI) radio observations carried out as part of the scientific program of the Radio Astron mission during 2011–2015. The scintillation timescale t _scint and spectral index γ for the power-law energy distribution of the pulses were obtained for each observing epoch. The measured scintillation timescales are t _scint = 7.5?123 s at 1668 MHz and t _scint = 2.9 s at 327 MHz. The spectral indices are ?1.6...?2.5. The frequency and time characteristics of the scattering were measured using two independent methods: based on the decorrelation bandwidth Δν _d and the scattering timescale τ _SC. The angular size of the scattering disk θ _H of the pulsar was obtained, the phase structure functions constructed, and the distance to the effective scattering screen estimated. The derived diameter of the scattering disk θ _H at 1668 MHz ranges from 0.4 to 1.3 mas, while the scatteringdisk diameter at 327 MHz is 14.0 mas. The measured distance to the effective scattering screen ranges from 0.7 to 1.9 kpc, and varies from observation to observation in the same way as the scattering timescale and decorrelation bandwidth: τ _SC ≈ 0.9?5.8 μs and Δν _d ≈ 40.7?161 kHz at 1668 MHz. The scattering timescale and decorrelation bandwidth at 327 MHz are 2340 μs and 68 Hz.     

Monitoring of the radio emission of the Crab Nebula pulsar at low frequencies

Results of long-term (2002–2010) monitoring of giant radio pulses of the pulsar PSR B0531+21 in the Crab Nebula at ν = 44, 63, and 111 MHz are reported. The observations were conducted on the LPA and DKR-1000 radio telescopes of the Lebedev Physical Institute. The giant pulses were analyzed using specialized software for calculating the magnitude of the scattering τ _sc , signal-to-noise ratio, and other required parameters by modeling the propagation of a pulse in the scattering interstellar medium. Three pronounced sharp increases in the scattering were recorded in 2002–2010. Analysis of the dependence between the variations of the scattering and dispersion measure (data of Jodrell Bank Observatory) shows a strong correlation at all frequencies, ≈0.9. During periods of anomalous increase in scattering and the dispersion measure, the index γ in the frequency dependence of the scattering in the Crab Nebula, τ _sc (ν) ∝ ν ^−γ , was smaller than the generally accepted values γ = 4.0 for a Gaussian and γ = 4.4 for a Kolmogorov distribution. This difference in combination with the piece-wise power-law spectrum may be due to the presence of a dense plasma structure with developed Langmuir turbulence in the nebula, along the pulsar’s line of sight. The magnetic field in the Crab Nebula estimated from measurements of the rotation measure toward the pulsar is 100 μG.     

Results of three-frequency monitoring of giant pulses from the crab pulsar

We present the results of long-term, three-frequency monitoring of giant pulses from the Crab pulsar on the 64-m radio telescope in Kalyazin. The total monitoring time was 160 hours. The signal power was recorded simultaneously at 600, 1650, and 4850 MHz via direct sampling of the received signals in the total receiver bandwidth without any compensation for interstellar dispersion. In total, 1117 and 352 giant pulses were detected at 600 and 4850 MHz, respectively. The frequency band centered at 1650 MHz was contaminated by interference, and was used only to identify events found in other frequency bands. The cumulative energy distribution of the giant pulses follows a power law at 600 and 4850 MHz up to the highest energies. A deep modulation in the radio spectra of individual giant pulses was observed on both large (Δv/v ≈ 0.5) and small (Δv/v ≈ (2?4) × 10^?3) frequency scales. The simultaneous appearance of giant pulses at the interpulse longitudes at high (4850 MHz) and low (1650 and/or 600 MHz) frequencies testifies to their common origin, in spite of the observed differences in other parameters.     

Giant pulses from the pulsar PSR B0950+08

Analysis of individual pulses of the pulsar B0950+08 at 112 MHz has shown that giant pulses with intensities exceeding the peak amplitude of the mean profile at these longitudes by two orders of magnitude are observed at the longitudes of all three components of the mean pulsar profile (the precursor and two-component main pulse). The maximum peak flux density of a recorded pulse is 15 240 Jy, and the energy of this pulse exceeds the mean pulse energy by a factor of 153. Strong but infrequent pulses at the longitude of the first component (precursor) can reach peak flux densities of 5750 Jy, exceeding the amplitude of the mean profile at this longitude by a factor of 490. It is shown that the emission at the precursor longitudes is virtually absent when giant pulses appear at the main-pulse longitudes, and vice versa: the presence of giant pulses at the precursor longitude results in the absence or considerable attenuation of the emission at other longitudes. The analysis shows that the cumulative probability function of the pulse peak flux densities has a piecewise power-law form. The power-law index for pulses with intensities exceeding 600 Jy appearing at the longitudes of the main pulse in the mean profile varies from n ₁ = ?1.25 ± 0.04 to n ₂ = ?1.84 ± 0.07. The obtained pulse energy distribution also has an inflection at E > 3000 Jy ms and a power-law form with the same index. The distribution of the pulse intensities at the precursor longitude was obtained, and forms a power law with index n = ?1.5 ± 0.1. The studied properties of the pulses at various longitudes of the mean profile are interpreted in the framework of induced scattering of the main-pulse emission by particles of ultrarelativistic, strongly magnetized plasma in the pulsar magnetosphere.     

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).     

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.     

Parameters of giant pulses from the Crab pulsar measured with the Mark5A VLBI system

Radio observations of the Crab pulsar were performed on the 100-m radio telescope of the Green Bank Observatory at a frequency of 2100 MHz in a 64-MHz band in two channels with right-and left-circular polarization. The Mark5A recording system was used. During 15 min of observing time, 609 giant pulses were recorded; the brightest had a peak flux density of 670 kJy. The energy distribution has been constructed, polarization properties have been analyzed, and the characteristic temporal and frequency scales in the radio emission of the detected giant pulses have been found. Comparison of these parameters indicates that the properties of giant pulses detected at the main-pulse and interpulse longitudes do not differ, as is clearly observed at frequencies above 4 GHz. Probable origins of the frequency evolution of the properties of giant pulses are discussed.     

The distance to the pulsar B1818-04 and the distribution of turbulent interstellar plasma toward B0833-45, B1818-04, and B1933+16

The dependence of the scatter broadening of extragalactic sources on the dispersion measures of distant pulsars observed along nearby lines of sight and the dependence of broadening of pulsar pulses on the scatter broadening observed for the pulsars themselves and for extragalactic sources observed along nearby lines of sight are constructed and analyzed. These dependences can be used to study turbulent plasma in the Galaxy. The effective scattering layer in the direction toward the pulsar B1933+16 is located in the Sagittarius arm at a distance of ≈3.4 kpc from the observer, and has an extent of ≈0.55 kpc. The scatter broadening and pulse broadening of B0833-45 are due to the turbulent medium in the shell of the Gum Nebula. The distance from the pulsar to the center of the scattering layer is≈43 pc. Data on scattering of the radiation of the pulsar B1818-04 and of the extragalactic source J1821-0502, together with data on the distribution of OB stars in the direction toward this pulsar, are used to show that the distance to the pulsar is ≈0.6 kpc; an H II region around the O7V star HD 171198, located 0.42 kpc from the Sun, is responsible for the scattering of this pulsar’s radiation.     

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.     

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.     

Characteristics of the pulsed emission of the peculiar pulsar B1822-09 at low radio frequencies

The pulse structure of the pulsar B1822-09 has been studied at 112, 62, and 42 MHz. The observations were conducted in 2010 on the Large Scanning Antenna and the DKR-1000 radio telescope of the Pushchino Radio Astronomy Observatory. The shape of the main pulse and interpulse undergo considerable changes at low radio frequencies. In the main pulse, the precursor disappears and is replaced by a new component that trails 50 ms behind the main component. At 62 MHz, the interpulse acquires a pronounced two-peaked shape. At 62 and 112 MHz, as well as at higher frequencies, the brighter second component of the interpulse follows the main pulse at 185° and has a relative amplitude of about 5%. The main pulse width changes with frequency according to the power law W _0.5 ∼ ν ^−0.15 in the frequency range 42–4750-MHz. The interpulse width follows this law only in the range 325–4750 MHz; at 112, 102, and 62 MHz, the interpulse is almost a factor of three broader than themain pulse. The parameters of the pulse’s scattering on interstellar plasma inhomogeneities and the initial pulse width before it enters the scattering medium have been measured at 62 and 42 MHz. The frequency dependence of the characteristic scale for scattering of the pulses of B1822-09 corresponds to a Kolmogorov spectrum for the electron-density fluctuations in the interstellar medium in the direction toward this pulsar.     

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.     

Anisotropic Scattering of the Radio Emission of the Pulsar B0833–45 in the Vela Supernova Remnant

Very Long Baseline Interferometry (VLBI) observations of the pulsar B0833–45 have been carried out as part of the scientific program of the RadioAstron mission. Ground support was provided by the Long Baseline Array, which includes radio telescopes in Australia and other countries in the southern hemisphere. The VLBI observations of the pulsar are analyzed in order to derive the parameters characterizing the scattering of the pulsar radio emission: the angular size of the scattering disk, the spatial scale of the diffraction pattern, the drift velocity of this pattern relative to the observer, the pulse scattering time scale, and the characteristic scintillation time and frequency scales, as well as the index of the electrondensity fluctuation spectrum. Comparison of these values with the predictions of the theory of scattering on a thin screen enables the determination of the position of the effective screen along the line of sight. Estimates made using various methods give distances to the screen from the observer of 0.79 to 0.87 times the total distance to the pulsar. Although the position of the screen is beyond the boundary of the Vela supernova remnant, this object may play the dominant role in the scattering. The scattering disk is an ellipse with a 2:1 axis ratio and with the inferred position angle of the major axis being ≈ 50°, based on the changes in the visibility-function amplitude for various orientations of the projected baseline. This conclusion is supported by the shape of the visibility-function amplitude as a function of the delay.

     

New observations of anomalous X-ray pulsars at low frequencies

We report the results of new observations of three anomalous X-ray pulsars: 1E 2259+586, 4U 0142+61, and XTE J1810-197. The observations were carried out on high-sensitivity radio telescopes of the Pushchino Radio Astronomy Observatory: the Large Phased Array at 111MHz and the DKR-1000 at 62 MHz. New, digital, multi-channel receivers designed for pulsar observations were used. Pulse profiles and dynamical spectra for the three pulsars are presented. The mean flux density for XTE J1810-197 is estimated to be ∼160 mJy at 62 MHz. An estimated spectral index for this pulsar is also presented.