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.     

Radio emission of RRAT pulsars at 111 MHz

Observations of the RRAT pulsars J0627+16, J0628+09, J1819?1458, J1826?1419, J1839?01, J1840?1419, J1846?0257, J1848?12, J1850+15, J1854+0306, J1919+06, J1913+1330, J1919+17, J1946+24, and J2033+00 observed earlier on the 64-m Parkes telescope (Australia) and the 300-m Arecibo radio telescope (Puerto Rico) at 1400 MHz were conducted at 111 MHz on the LSA radio telescope of the Pushchino Radio Astronomy observatory in 2010–2012. A characteristic feature of these pulsars is their sporadic radio emission during rare active epochs and the absence of radio emission during long time intervals. No appreciable flare activity of these pulsars was detected in the Pushchino observations. However, processing the observations using the Fast Folding Algorithm taking into account known information about the pulsar dispersion measures and periods shows that, even during quiescent intervals, the majority of the studied pulsars generate weak radio pulses with a period corresponding to that of the radio emission of the sporadic pulses observed at active epochs. The flux of this radio emission does not exceed 100 mJy at the pulse peak, even at the low frequency of 111 MHz. This considerably hinders detection of the radio emission of RRAT pulsars at high frequencies, since the radio fluxes of RRAT pulsars decreases with increasing frequency.     

The X-ray variability of Mrk 421

The results of monitoring the flaring activity of Mrk 421 with the XMM-Newton and RXTE orbiting observatories are presented. The ASM/RXTE data show that, during most of the observational period, 2002–2006, the source was in a very active state, with a mean 1.5–12 keV flux equals ∼17 mCrab. Analysis of daily and weekly averaged data shows strong X-ray variability of Mrk 421 on time scales from days to months. The long-term light curve demonstrates a rich variety of variability patterns and a complex hierarchical flare structure, in most cases superimposed on longer trends of rising and decreasing source activity. Quantitative estimates of the variability of Mrk 421 shows evidence of correlations between the source intensity in X-rays and the variability parameters, both for separate years and epochs and for most of the observational period. The soft and hard X-ray components (according to XMM-Newton data) are correlated with each other, and spectral changes (according to ASM/RXTE data) are characterized by increasing hardness with brightening of the object in X-rays. Moderate spectral variability at individual epochs and evidence for an energy dependence of the source variability are also observed. The 2–12 keV fluxes from Mrk 421 detected by XMM-Newton are consistent with the maximum 2–10 keV flux detected by ASCA ∼10 years ago. An analysis of simultaneous day-time-scale variability of Mrk 421 in X-rays (ASM/RXTE) and very-high-energy γ-rays (GT-48 Cherenkov telescope of the Crimean Astrophysical Observatory) in periods of increased activity of the source in December 2002 and April 2004 is also presented. Published in Russian in Astronomicheskiĭ Zhurnal, 2008, Vol. 85, No. 7, pp. 589–602. The article was translated by the authors.     

Polarization of individual pulses of radio pulsars at the low frequencies 40, 60, and 103 MHz

Measurements of the linear polarization of individual pulses at 40, 60, and 103 MHz are presented for ten pulsars. The degree and position angle of a linear polarization were measured with a temporal resolution of 1–7 ms, and the longitudinal distributions of these parameters were constructed for each pulsar at one or more of these frequencies. These are the first such measurements for pulsars B0031-07, B0320 + 39, B0628-28, and B2217 + 47. Apart from B0628-28, all the pulsars are characterized by the simultaneous presence of orthogonal polarization modes in at least one component of the integral profile. The secondary polarization mode increases at frequencies ≤100 MHz for pulsars whose integrated pulses contain pairs of conal components (B0031-07, B0329 + 54, B0834 + 06, B1133 + 16, B2020 + 28). This is manifested both as an expansion of the longitudinal range where the secondary polarization mode is observed and an increase in its contribution to the emission at a given longitude. New data confirming the dependence of the linear polarization of individual pulses on the intensity and mode of the pulsar emission have been obtained.     

Differences in the parameters of radio pulsars with short and long periods

A comparative analysis of various parameters of pulsars with short (P < 0.1 s) and long (P > 0.1 s) periods is carried out. There is no correlation between the radio and gamma-ray luminosities of the pulsars and their surfacemagnetic fields, but there is a correlation between the X-ray luminosity and the surfacemagnetic field. A dependence of the X-ray and gamma-ray luminosities on the magnetic field at the light cylinder is also found. This result provides evidence for the formation of hard, non-thermal emission at the periphery of the magnetosphere. An appreciable positive correlation between the luminosity and the rate of rotational energy loss by the neutron star is observed, supporting the idea that all radio pulsars have the same basic source of energy. The efficiency of the transformation of rotational energy into radiation is significantly higher in long-period pulsars. The dependence of the pulse width on the pulsar period is steeper for pulsars with short periods than for those with long periods. The results obtained support earlier assertions that there are differences in the processes generating the emission in pulsars with P < 0.1 s and those with P > 0.1 s.     

Synchrotron spectra of short-period pulsars

A model with synchrotron radiation near the light cylinder is proposed to explain the observed spectra of short-period pulsars (P≤0.1 s). These spectra can be described if a power-law energy distribution of the emitting electrons with exponent γ=2–8 is assumed. For most pulsars, the peak frequency ν_m is below 10 MHz. The ν_m(γ) dependence is derived, and shows that the peak frequencies for pulsars with spectral indices α<1.5 may fall in the observable range. In particular, ν_m may be ν_m ～ 100 MHz for PSR J0751 + 1807 and PSR J1640 + 2224. The observed radio spectrum of Geminga (PSR J0633 + 1746) can be described by a synchrotron model with a monoenergetic or Maxwellian distribution of relativistic electrons and a small angle β between the spin axis and magnetic moment (β ～ 10°).     

Masses of X-ray pulsars in binary systems with OB supergiants

We describe the results of a statistical approach to analyzing the combined radial-velocity curves of X-ray binaries with OB supergiants in a Roche model, both with and without allowance for the anisotropy of the stellar wind. We present new mass estimates for the X-ray pulsars in the close binary systems Cen X-3, LMC X-4, SMC X-1, 4U 1538-52, and Vela X-1.     

Secondary dynamical spectra of pulsars as indicators of inhomogeneities in the interstellar plasma

Observations of ten bright pulsars were obtained on the Giant Meter-wavelength Radio Telescope (GMRT, India) in order to study the effects of scattering of their radio waves by contructing and analyzing secondary dynamical spectra. The observations were conducted at 610 and 1420 MHz using a digital spectral analyzer operating in a real-time regime. The frequency resolution was 32.5 or 65.1 kHz, and the readout time was from 61.44 to 512 μs. Archival data for five pulsars at 327 MHz were also used. Procedures for normalizing the spectra and for constructing the secondary dynamical spectra were developed. Parabolic arcs were found in the secondary spectra of four pulsars (B1642-03, B1556-44, B2154+40, and B2021+51). The curvature of these arcs can be used to determine the distance to the effective scattering screen. In all cases, these screens are located relatively near the pulsars themselves.     

Rotational periods and other parameters of magnetars

The rotational periods P, period derivatives dP/dt, and magnetic fields B in the region where the emission of anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) is generated are calculated using a model that associates the emission of these objects with the existence of drift waves at the periphery of the magnetosphere of a neutron star. The values obtained for these parameters are P = 11?737 ms, dP/dt = 3.7 × 10^?16?5.5 × 10^?12, and log B (G) = 2.63?6.25. We find a dependence between the X-ray luminosity of AXPs and SGRs, L _x, and the rate at which they lose rotational energy, dE/dt, which is similar to the L _x(dE/dt) dependence for radio pulsars with detected X-ray emission. Within the errors, AXPs/SGRs and radio pulsars with short periods (P < 0.1 s) display the same slopes for their log(dP/dt)-log P relations and for the dependence of the efficiency of their transformation of rotational energy into radiation on their periods. A dipole model is used to calculate the surface magnetic fields of the neutron stars in AXPs and SGRs, which turn out to be, on average, comparable to the surface fields of normal radio pulsars (〈log B _s (G)〉 = 11.90).     

Period clustering of anomalous X-ray pulsars

The question of why the observed periods of anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) cluster in the range 2–12 s is discussed. The possibility that AXPs and SGRs are the descendants of high-mass X-ray binaries that have disintegrated in core-collapse supernova explosions is investigated. The spin periods of neutron stars in high-mass X-ray binaries evolve towards the equilibrium period, which is a few seconds, on average. After the explosion of its massive companion, the neutron star becomes embedded in a dense gaseous envelope, and accretion from this envelope leads to the formation of a residual magnetically levitating disk. It is shown that the expected mass of the disk in this case is 10^?7–10^?8 M_⊙, which is sufficient to support accretion at the rate 10¹⁴–10¹⁵ g/s over a few thousand years. During this period, the star manifests itself as an isolated X-ray pulsar with a number of parameters similar to those of AXPs and SGRs. The periods of such pulsars can cluster if the lifetime of the residual disk does not exceed the spin-down timescale of the neutron star.     

Spin evolution of long-period X-ray pulsars

The spin evolution of X-ray pulsars in high-mass X-ray binaries is discussed under various assumptions about the geometry and physical parameters of the accretion flow. The torque applied to the neutron star by the accretion flow and the equilibrium periods of the pulsars are estimated. It is shown that the observed spin evolution of the pulsars can be explained in a scenario in which the neutron star accretes material from a magnetized stellar wind.     

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.     

Long-term,multi-frequency monitoring of the blazar S0528+134 (Nimfa)

Flux-density variations of the quasar S0528+134 (Nimfa) are analyzed based on long-term monitoring at five radio frequencies between 4.8 and 37 GHz, performed at the Crimean Astrophysical Observatory, the Metsähovi Radio Observatory of Aalto University, and the University of Michigan Radio Astronomy Observatory. The dynamics of a powerful flare in 1996 are analyzed using gamma-ray (0.1–300 GeV), X-ray (2–10 keV, 15–50 keV), and radio observations. The delays of the flare between different spectral ranges and between different radio wavelengths have been measured. The dependence for the delays at different radio wavelengths relative to the X-ray and optical flares is established based on long-term observations in the X-ray, optical, and radio obtained from 2004 to 2013. Multi-frequency monitoring in the radio is used to estimate the orbital and precession periods in the binary supermassive black hole system S0528+134 and the physical characteristics of this system.     

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.     

Interpretation of the light curves of “quiescent” X-ray novae in a model with a noncollisional interaction between the flow and disk. The XTE J1118+480 system

We have analyzed the orbital light curve of the X-ray nova XTE J1118+480 in a “disk + hot line” model based on three-dimensional gas-dynamical computations of gas flows in interacting binary systems. As a result, we have been able to derive reliable parameters for the system: i = 80 _?4 ⁺⁴ degrees, _MBH = 7.1 _?0.1 ^+0.5 M_⊙, M _opt = 0.39 _?0.07 ^+0.15 M_⊙.     

Tree-ring-based mass-balance estimates for the past 300 years at Peyto Glacier, Alberta, Canada

Tree rings were used to reconstruct mass balance for Peyto Glacier in the Canadian Rocky Mountains from A.D. 1673 to 1994. Summer balance was reconstructed from tree-ring estimates of summer temperature and precipitation in the Canadian Rockies. Winter balance was derived from tree-ring data from sites bordering the Gulf of Alaska and in western British Columbia. The models for winter and summer balance each explain over 40% of the variance in the appropriate mass-balance series. Over the period 1966-1994 the correlation between the reconstructed and measured net balances is 0.71. Strong positive mass balances are reconstructed for 1695-1720 and 1810-1825, when higher winter precipitation coincided with reduced ablation. Periods of reconstructed positive mass balance precede construction of terminal moraines throughout the Canadian Rockies ca. 1700-1725 and 1825-1850. Positive mass balances in the period 1845-1880 also correspond to intervals of glacier readvance. Mass balances were generally negative between 1760 and 1805. From 1673 to 1883 the mean annual net balance was +70 mm water equivalent per year (w.e./yr.), but it averaged −317 mm w.e./yr from 1884 to 1994. This reconstructed mass balance history provides a continuous record of glacier change that appears regionally representative and consistent with moraine and other proxy climate records.     

The cyclotron spectrum of anisotropic ultrarelativistic electrons: Interpretation of X-ray pulsar spectra

The spectrum of cyclotron radiation produced by electrons with a strongly anisotropic velocity distribution is calculated taking into account higher harmonics. The motion of the electrons is assumed to be ultrarelativistic along the magnetic field and nonrelativistic across the field. One characteristic feature of the resulting spectrum is that harmonics of various orders are not equally spaced. The physical properties and observed spectra of four X-ray pulsars displaying higher cyclotron harmonics are analyzed. It was shown that at least in one of them, the cyclotron feature can apparently be only an emission line. Moreover, the observed harmonics are not equidistant, and display certain other properties characteristic of emission by strongly anisotropic ultrarelativistic electrons. In addition, there are indirect theoretical arguments that the electrons giving rise to cyclotron features in the spectra of X-ray pulsars are ultrarelativistic and characterized by strongly anisotropic distributions. As a result, estimates of the magnetic fields of X-ray pulsars (which are usually derived from the energies of cyclotron lines) and certain other physical parameters require substantial revision.     

Gamma-ray bursts from a close pulsar binary system?

The close neutron-star binary system comprised of the radio pulsars PSR J0737-3039 A,B is discussed. An analysis of the observational data indicates that the wind from pulsar A, which is more powerful than the wind from pulsar B, strongly distorts the magnetosphere of pulsar B. A shock separating the relativistic wind from pulsar A and the corotating magnetosphere of pulsar B should form inside the light cylinder of pulsar B. A weakly diverging “tail” of magnetic field is also formed, which stores a magnetic energy on the order of 10³⁰ erg. This energy could be liberated over a short time on the order of 0.1 s as a result of reconnection of the magnetic-force lines in this “tail,” leading to an outburst of electromagnetic radiation with energies near 100 keV, with an observed flux at the Earth of 4 × 10^?11 erg cm^?2 s^?2. Such outbursts would occur sporadically, as in the case of magnetic substorms in the Earth’s magnetosphere.     

Harmonic oscillations of the X-ray emission of a solar flare

An analysis is presented for the class-M9.3 solar flare of November 6, 2004, whose decay phase displayed weakly damped harmonic oscillations of the predominantly thermal X-ray flux detected by the RHESSI spacecraft (at energies ≲25 keV). The period of these oscillations was ≈78 s, and their characteristic decay time ≈100 min. Similar quasi-periodic pulsations were observed in the decimeter-centimeter radio flux (pulsations of a type-IV radio outburst), but were less pronounced in the non-thermal hard X-ray flux (≳25 keV). The area of the quasi-stationary X-ray source, which was located primarily at the apex of a set of flare loops (≲15 keV) that were cooled primarily via thermal conduction, was found to be in anti-phase with the oscillating X-ray flux it emitted. The observed oscillations are interpreted as harmonic modulations of the radiation flux emitted by the heated thermal flare-loop plasma, due to the global, standing, sausage mode of fast magnetoacoustic waves excited in the loop.     

A comparative analysis of the integrated radio luminosities of normal and millisecond pulsars

We present the results of a comparative statistical analysis of the integrated radio luminosities of millisecond and normal pulsars and their dependences on other parameters of the pulsars. The analysis is based on our own measurements of the flux densities, spectra, and integrated radio luminosities of the millisecond pulsars, as well as data from the literature used to determine the integrated radio luminosities for 545 pulsars, 50 of them millisecond pulsars. Despite large differences in their periods P, period derivatives $\dot P$ , magnetic fields B, and ages τ, the integrated radio luminosities of the millisecond and normal pulsars and their dependences on other parameters are approximately the same. The integrated radio luminosity depends on the parameter B/P ², which is proportional to the potential difference in the polar-cap gap; this may indicate that the radio energy of pulsars is determined by the energy of primary particles accelerated in the polar-cap gap. Secular decreases in the radio luminosities of both normal and millisecond pulsars were also detected.