Peculiarities of giant pulses from the crab pulsar at frequencies of 594 and 2228 MHz

We present the results of our simultaneous observations of giant pulses from the Crab pulsar B0531+21 at frequencies of 594 and 2228 MHz with a high (62.5 ns) time resolution. The pulse broadening by scattering was found to be 25 and 0.4 µs at 594 and 2228 MHz, respectively. We obtained the original giant-pulse profiles compensated for interstellar scattering. The measured profile widths at the two frequencies are approximately equal, ≈0.5 µs; i.e., the giant pulses are narrower than the integrated profile by a factor of about 1000. We detected an extremely high brightness temperature of radio emission, T_b≥10³⁶ K radio emission, which is higher than the previous estimates of this parameter by five orders of magnitude. The decorrelation bandwidth of the radio-spectrum diffraction distortions has been determined for this pulsar for the first time: 10 kHz at 594 MHz and 300 kHz at 2228 MHz.     

Frequency dependence of the scattering pulse broadening for the Crab pulsar

We measured the frequency dependence of the pulsar pulse broadening by scattering over a wide frequency range, from 40 to 2228 MHz, based on direct measurements of this parameter using giant pulses from the pulsar PSR B0531+21 in the Crab Nebula. Our measurements were carried out at the following seven frequencies: 40, 60, and 111 MHz at the Pushchino Radio Astronomy Observatory (Astrospace Center, Lebedev Physical Institute, Russian Academy of Sciences), 406 MHz at the Medicina Observatory (Instituto di Radioastronomia, Italy), and 594, 1430, and 2228 MHz at the Kalyazin Radio Astronomy Observatory (Astrospace Center, Lebedev Physical Institute, Russian Academy of Sciences). The measured frequency dependence of the pulse broadening by scattering τ_SC (υ) ? υ^γ, where γ=?3.8±0.2, agrees with a model Gaussian distribution of interstellar inhomogeneities (γ=?4) but falls outside the error limits of correspondence to a Kolmogorov model spectrum of inhomogeneities (γ=?4.4).     

Scattering of the Low Frequency Pulsar Radiation

We report the results of the measurements and analysis of the pulse broadening due to interstellar scattering on 43 pulsars at 102 MHz. This is the largest uniform sample of direct measurements of pulsar scatteringτ_sc, which make it feasible to analyze the dependence of this value on other pulsar parameters. The measured dependence of τ_scon dispersion measure τ_sc (DM)=40(DM/100)^2.1 is close to theoretically expected relation τ_sc (DM)∝ DM². A frequency dependence of the scattering pulse broadening is weaker than commonly accepted τ_sc ∝ ν^-4.4. This revised version was published online in July 2006 with corrections to the Cover Date.     

Electric fields in coronal magnetic loops

We analyze spectra taken with the 40 cm coronograph at Sacramento Peak Observatory, for evidence of Stark effect on Balmer lines formed in coronal magnetic structures. Several spectra taken near the apex of a bright post-flare loop prominence show significant broadening from H₁₀ to the limit of Balmer line visibility in these spectra, at about H₂₀ The most likely interpretation of the increasing width is Stark broadening, although unresolved blends of Balmer emissions with metallic lines could also contribute to the trend. Less significant broadening is seen in 3 other post-flare loops, and the data from 5 other active coronal condensations observed in this study show no broadening tendency at all, over this range of Balmer number. The trend clearly observed in one post-flare loop requires an ion density of n _i ? 2 × 10¹² cm^?3, if it is to be explained entirely as Stark effect caused by pressure broadening. But mean electron densities measured directly from the Thomson scattering at λ3875 in the same SPO spectra, yield n _e ? 3?7 × 10¹⁰ cm^?3 for the same condensations observed within that loop. Comparison of this evidence from electron scattering, with densities derived from emission measures and line-intensity ratios, argues against a volume filling factor small enough to reconcile the values of n _i and n _e derived in this study. This discrepancy leads us to suggest that the Stark effect observed in these loops, and possibly also in flares, could be caused by macroscopic electric fields, rather than by pressure broadening. The electric field required to explain the Stark broadening in the brightest post-flare loop observed here is approximately 170 V cm^?1. We suggest an origin for such an electric field and discuss its implications for coronal plasma dynamics.     

Jupiter’s radio spectrum from 74 MHz up to 8 GHz

We carried out a brief campaign in September 1998 to determine Jupiter’s radio spectrum at frequencies spanning a range from 74 MHz up to 8 GHz. Eleven different telescopes were used in this effort, each uniquely suited to observe at a particular frequency. We find that Jupiter’s spectrum is basically flat shortwards of 1-2 GHz, and drops off steeply at frequencies greater than 2 GHz. We compared the 1998 spectrum with a spectrum (330 MHz-8 GHz) obtained in June 1994, and report a large difference in spectral shape, being most pronounced at the lowest frequencies. The difference seems to be linear with log(ν), with the largest deviations at the lowest frequencies (ν).We have compared our spectra with calculations of Jupiter’s synchrotron radiation using several published models. The spectral shape is determined by the energy-dependent spatial distribution of the electrons in Jupiter’s magnetic field, which in turn is determined by the detailed diffusion process across L-shells and in pitch angle, as well as energy-dependent particle losses. The spectral shape observed in September 1998 can be matched well if the electron energy spectrum at L = 6 is modeled by a double power law E^−a (1+(E/E₀))^−b, with a = 0.4, b = 3, E₀ = 100 MeV, and a lifetime against local losses τ₀ = 6 × 10⁷ s. In June 1994 the observations can be matched equally well with two different sets of parameters: (1) a = 0.6, b = 3, E₀ = 100 MeV, τ₀ = 6 × 10⁷ s, or (2) a = 0.4, b = 3, E₀ = 100 MeV, τ₀ = 8.6 × 10⁶ s. We attribute the large variation in spectral shape between 1994 and 1998 to pitch angle scattering, coulomb scattering and/or energy degradation by dust in Jupiter’s inner radiation belts.     

The two basic components of the interstellar neutral hydrogen and its relation with the galactic structure

The two basic components of the neutral hydrogen, cool dense clouds merged in a hotter tenuous medium, are studied using 21 cm absorption data of the Parkes Survey. The mean parameters obtained for the typical clouds next to the galactic plane are τ_p = 1.7, velocity half-width=3.3 km s^?1. Their temperatures areT _sc ≥40 K with a meanT _sc =63±12 K and the obtained hot gas density isn _HH=(0.15±0.05) atom cm^?3. Theoretical analysis following Giovanelli and Brown (1973) reveals that the pressure equilibrium condition (n _HH+2n _e )·T _SH≈n _HC·T _sc is compatible with the quoted values if it is assumed that the cosmic abundances in the interstellar medium are below the adopted normal solar abundance. This lack of heavy elements suggests accretion to grains which is consistent with the observed narrow concentration of the dark matter on the galactic layer (≤100 pc halfwidth). The same pressure condition leads to a mean cool cloud density ofn _HC～30 atom cm^?3 and a hot gas temperature ofT _SH～10 500 K. Comparison with data from Hii regions suggests that the cool clouds are somewhat denser and less extensive than such regions. An explanation for it is the expansion that the Hii regions went through in their origin. Comparison with 21 cm emission data shows that the cloud galactic layer is only about a quarter as thick as the hot gas layer. All the present results suggest that only such clouds can be spatially related with the typical I population associated with the spiral structure.     

Observation of the Comet Kohoutek (1973f) in the resonance light (A2Σ+−X2 ∏) of the OH radical

Two monochromatic pictures of the Comet Kohoutek (1973f) were taken on January 15, 1974 in the resonance light (A²Σ ? X² ∏) of the radical OH with a photographic telescope placed on board the NASA 990 Convair airplane. From an intensity profile we derive the production rate of OH radicals Q_OH = 4 xsx 10²⁸ moleculesec ^?1sr^?1 at 0.6 AU and the lifetime of the OH radical which is τ_OH = 4.5 × 10⁴ sec at 0.6 AU. This short lifetime (very similar to the lifetime of H₂O) combined with the high total production rate of gas in comets can explain the observed velocity of 8km sec^?1 for the H-atoms: The H-atoms produced by photodissociation of H₂O are thermalized at short distancesfrom the nucleus; the H-atoms produced by photodissociation of OH have a velocity of ?8km sec^?1 and can reach the outer part of the hydrogen envelope.     

Upgraded technique to analyze the vertical structure of the aerosol component of the atmospheres of giant planets

To analyze the behavior of the optical thickness of aerosols or the ratio of the optical thicknesses of the aerosol and gas components in the spectral absorption bands of atmospheric gases with depth, we developed a software package. The package structure includes the units for the following operations: (1) to calculate the Legendre expansion coefficients x _i of the phase function and the volume scattering coefficient σ₀ of the polydisperse medium with the specified refractive index and the size distribution function N(r) with the use of the code developed by M.I. Mishchenko; (2) to generate the array containing the pairs of the single scattering albedo ω and the geometric albedo A _g for the models of a semi-infinite homogeneous medium with the parameters determined in the previous step; (3) to determine the single scattering albedo values from the comparison of the calculated and measured values of the geometric albedo for each of the measured points in the examined absorption band of the atmospheric gas (accounting for the change of the phase function due to Rayleigh scattering); (4) to calculate the spectral values of the effective optical depths τ_eff of the levels, where the intensity field of light diffusely reflected by the investigated atmosphere is formed; (5) to derive the scattering and absorption components of the effective optical depth (τ _eff ^s and τ _eff ^v ) from the values of ω and τ_eff; (6) from the values of τ _eff ^v to determine the amount of the absorbing gas NL (in km-amg) along the line of sight and, from these values, the atmospheric pressure p(NL) and the gas component of the scattering portion of the optical depth τ _g (λ₀) at the wavelength λ₀ = 887.2 nm; (7) from the values of τ _eff ^s (λ) and τ _g (λ, NL), to find the aerosol component τ _a (λ, NL); (8) to build the plots of τ _a (λ) or the ratio τ _a (λ)/τ _g (λ) reduced to λ₀ = 887.2 nm versus the pressure. The software package was validated in the analysis of the spectrophotometric data obtained in the measurements of the whole disk of Jupiter in the profiles of the strong absorption bands of methane centered at λλ 841.6, 864, and 887.2 nm under the assumption on two versions of the size distribution of particles (the modified gamma distribution and the log-normal one). It was found that the model with the gamma distribution is analyzed several times more quickly than the log-normal one that yields the close results in computations performed for the same medium.     

Effective temperature vs. line‐depth ratio for ELODIE spectra: Gravity and rotational velocity effects

The dependence on the temperature of photospheric line‐depth ratios (LDRs) in the spectral range 6190–6280 Å is investigated by using a sample of 174 ELODIE Archive stellar spectra of luminosity class from V to III. The rotational broadening effect on LDRs is also studied. We provide useful calibrations of effective temperature versus LDRs for giant and main sequence stars with 3800 ≃ T_eff ≃6000 K and v sin i in the range 0–30 km s^–1. We found that, with the exception of very few line pairs, LDRs, measured at a spectral resolution as high as 42 000, depend on v sin i and that, by neglecting the rotational broadening effect, the T_eff determination can be wrong by ∼100 K in the worst cases. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Axel dust on Saturn and Titan

The scattering and absorption properties of Axel dust were investigated by means of Mie theory. We find that a flat distribution of particle radii between 0 and 0.1 μm, and an imaginary part of the index of refraction which varies as λ^?2.5 produce a good fit to the variation of Titan's geometric albedo with wavelength (λ) provided that τ_ext, the extinction optical depth of Titan's atmosphere at 5000 Å, is about 10. The real part of the complex index is taken to be 2.0. The model assumes that the mixing ratio of Axel dust to gas is uniform above the surface of Titan. The same set of physical properties for Axel dust also produces a good fit to Saturn's albedo if τ_ext = 0.7 at 5000 Å. To match the increase in albedo shortward of 3500 Å, a clear layer (containing about 7 km-am H₂) is required above the Axel dust. Such a layer is also required to explain the limb brightening in the ultraviolet. These models can be used to analyze the observed equivalent widths of the visible methane bands. The analysis yields an abundance of the order of 1000 m-am CH₄ in Titan's atmosphere. The derived CH₄/H₂ mixing ratio for Saturn is about 3.5 × 10^?3 or an enhancement of about 5 over the solar ratio.     

Calculating Method and Characteristics of the Distribution of Neutron Exposures in the Radiative S-process Nucleosynthesis Model for AGB Stars

A study on the distribution of neutron exposures in a low-mass asymptotic giant branch (AGB) star is presented, according to the s-process nucleosynthesis model with the ¹²C(α, n)¹⁶O reaction occurred under radiative conditions in the interpulse phases. The model parameters, such as the over- lap factor r of two successive convective thermal pulses, the mass ratio q of the ¹³C shell with respect to the He intershell, and the effective mass of ¹³C in the ¹³C shell, vary with the pulse number. Considering these factors, a calculating method for the distribution of neutron exposures in the He intershell has been presented. This method has the features of simplicity and universality. Using this method, the exposure distribution for the stellar model of a star with the mass of 3 M_? and the solar metallicity has been calculated. The results suggest that under the reasonable assumption that the number density of neutrons is uniform in the ¹³C shell, the ?nal exposure distribution approaches to an exponential distribution. For a stellar model with the de?nite initial mass and metallicity, there is a de?nite relation between the mean neutron exposure τ₀ and the neutron exposure Δτ of each pulse, namely τ₀ = 0.434λ(q1, q2, …, qm_max +1, …, r₁, r₂, …, rm_max +1)Δτ, where m_max is the total number of thermal pulses with the third dredge-up episode, and the proportional coeffcient λ(q₁, q₂, …, qm_max +1, …, r₁, r₂, …, rm_max +1) can be determined by an exponential curve ?tting to the ?nal exposure distribution. This new formula quantitatively uni?es the classical model with the s-process nu- cleosynthesis model by means of neutron exposure distribution, and makes the classical model continue to offer guidance and constraints to the s-process nu- merical calculations of stellar models.     

Drifting ELF/VLF Emissions Observed at Low Latitude Ground Station During Geomagnetic Storm

We have reported for the first time total seven strong events of drifting ELF/VLF discrete emissions observed on 28th–29th April, 1990 in the pre-midnight sector at Varanasi (Geomag. lat. 14°55′N, long. 154°E, L = 1.07). The events exhibit a regular increasing as well as decreasing frequency drifts and are mainly discrete periodic emissions of riser, faller and hook types observed during a geomagnetic storm period, with minimum Dst-index ?98 nT and K _p -index ≥ 5. The frequency drift in ELF/VLF emissions at low latitudes seems to be a rare phenomenon. The repetition period and the frequency drift rate have been evaluated for all the recorded events. The frequency drifts have been interpreted in terms of a combined effect of L-shell drift of interacting energetic electrons and the change in convection electric fields during the storm developments. The computed maximum spectral power density $ \left\langle {B_{f}^{2} } \right\rangle_{\max } $ of the wave varies between 1.8 × 10^?21 and 4.08 × 10^?22 Gauss²/Hz, whereas frequency drift rates are in agreement with the observed values.     

Structure of the object W3 OH in hydroxyl maser lines

The structure of the gas-dust complex W3 OH has been investigated in the main hydroxyl radio line at 1665 MHz with the QUASAR system in left- and right-hand circular polarizations. The emission dominates in right-hand circular polarization. Spectra have been taken on single dishes and interferometers. The spectral features become narrower with increasing baseline length, which is determined by compact knots in sources. The correlated flux densities on maximum-length baselines do not exceed 20 and 10 Jy in right- and left-hand circular polarizations, respectively. Radio maps have been obtained in both polarizations with various angular resolutions from 5 to 200 mas. The object W3 OH located in a Galactic arm is observed in the transverse direction, which reduces considerably the scattering φ _sc ? 2 mas. A number of fragments are observed only in one of the polarizations; the emission from three sources is observed in both polarizations with a frequency separation reaching Δf = ±10.6 kHz, corresponding to the Zeeman splitting in a magnetic field H = 7.6 mG.     

Probable causes of long-period variations in the Uranian geometrical albedo

We use the technique developed in our previous studies to determine the ratios of optical depth components τ _a /τ _R and τ_κ/τ _S on the basis of the observational data on the Uranian geometrical albedo for the years 1981, 1993, and 1995. Here τ _a and τ _R are the aerosol and gas scattering components, τ _S = τ _a + τ _R , and τ_κ is the absorption component of the effective optical depth at which the intensity of the diffusely reflected radiation is formed. The ratios turn out to be different for different years. This phenomenon is caused by the horizontal inhomogeneity of the aerosol component distribution over the Uranian disk.     

An Observational Study of the Strong Single Pulses of PSR J0034-0721

Using the 25m radio telescope of the Urumqi Observatory, the strong single pulses of the pulsar PSR J0034-0721 were observed at 1.54 GHz on 6th Aug. 2007. With the technique of single-pulse detection, 116 single pulses with the signal-to-noise ratios of R_SN≥5 were detected from the observed data of 1 h. At 1.54 GHz, the signal-to-noise ratios of the detected single pulses are in the range from 5 to 10.5, and the peak flux densities of these pulses are approximately 14∼29 times that of the average pulse (AP), much less than the ratios between the intensities of typical giant pulses and the intensity of AP. The cumulative distribution of the intensities of these pulses is basically a powerlaw distribution with the spectral index α = −4.3 ± 0.4. The detection rates for the pulses of R_SN≥5 and R_SN≥10 are 3% and 0.08%, respectively. For these pulses, the half-peak width W₅₀ ranges from 1.6 ms to 8 ms, 3.9 ms in average. The phases of the vast majority of the strong single pulses are concentrated around the peak position of AP, but 2 strong pulses of R_SN≥8.5 are detected at the phases about 33 ms earlier than the phase of the AP peak. This implies that there probably exist two emission regions of strong pulses, and this is consistent with the previously observed results at 40 MHz and 111 MHz, except that at 1.54 GHz the profile of AP exhibits only one component.     

Saturn's rings: II. Condensations of light and optical thickness of Cassini's division

“Condensations” of light have been observed when Saturn's rings are seen almost edge on, and the Sun and the Earth are on opposite sides of the ring plane. These condensations are associated with ring C and Cassini's division. If the relative brightness between the two condensations and the optical thickness of ring C are known, we can calculate the optical thickness of Cassini's division, τ_CASS. Using Barnard's and Sekiguchi's measurements, we have obtained 0.01 ? τ_CASS ? 0.05. A brightness profile of the condensations which agrees well with visual observations is also presented.We are able to set an upper limit of about 0.01 for the optical thickness of any hypothetical outer ring. This rules out a ring observed by C. Cragg in 1954, but does not eliminate the D′ ring observed by Feibelman in 1967.It is known that the outer edge of ring B is almost at the position of the 1/2 resonance with Mimas. Franklin, Colombo, and Cook explained this fact in 1971, postulating a total mass of ring B of 10^?6M_SATURN. We have derived a formula for the mass of the rings, which is a linear function of the mean particle size. We find that 10^?6M_SATURN implies large particles (～70m). If the particles are small (～10cm), as currently believed, the total mass of ring B is not enough to shift the outer edge. We conclude that the above explanation and current size estimates are inconsistent.     

Omnidirectional induced compton scattering by relativistic electrons

Quasars, pulsars and other cosmic sources of intense radiation are known to have large brightness temperature (kT _b?mc ²) and relativistic electron density values. In this case the induced Compton scattering by relativistic electrons should be considered. The probability of scattering with decreasing radiation frequency is derived for isotropic radiation scattering. When induced scattering takes place, the relativistic electron obtains its energy by transforming high-frequency quanta into the low-frequency ones. In the most intensive sources electrons would receive energiesE?mc ² ××(kT _b/mc ²)^1/7 due to the heating rate proportional toE ^?5 with the cooling rate proportional toE ². Considerable distortion of the quasar spectrum is possible for reasonably large values of relativistic electron density (N?10⁶cm^?3) notwithstanding that the heating is negligible. In pulsars relativistic electron heating and spectrum distortion appear to depend more on the induced Compton scattering.     

Modeling the linear polarization of optical emission from red giants

We present the results of solving the radiative transfer equation for the Stokes vector in the case of light scattering by spherical forsterite dust particles in an axisymmetric circumstellar envelope of a red giant. We have assumed that the surfaces of constant scattering-particle density are prolate or oblate spheroids, the particle density decreases with radius as N _d ∝ r ⁻², and the dust particles at the inner boundary of the envelope are in thermal equilibrium with the stellar emission at solid-phase evaporation temperature T _ev = 800 K. In the wavelength range 0.27 μm ≤ λ ≤ 1 μm, particles with radii 0.03 μm ≲ a ≲ 0.2 μm make a major contribution to the linear polarization of the stellar emission. The increase in scattering efficiency factor with decreasing wavelength λ is mainly responsible for the growth of polarization toward the short wavelengths known from observations. However, at a mean number of scatterings 1.2 ≤ N _sca ≤ 1.6, the polarization ceases to grow due to depolarization effects and decreases rapidly as the wavelength decreases further. The wavelength of the polarization maximum is determined mainly by two quantities: the particle radius and the mass loss rate. The upper limits for the degree of linear polarization in the case of light scattering in circumstellar dust envelopes with the geometries of prolate and oblate spheroids are p ≈ 3 and 5%, respectively. The polarization for light scattering by enstatite particles is higher than that for light scattering by forsterite particles approximately by 0.3%. Original Russian Text ? Yu.A. Fadeyev, 2007, published in Pis’ma v Astronomicheskiĭ Zhurnal, 2007, Vol. 33, No. 2, pp. 123–133.     

Can the known millisecond pulsars help in the detection of intermediate-mass black holes at the centers of globular clusters?

We consider the possibility of detecting intermediate-mass (10³–10⁴ M _⊙) black holes, whose existence at the centers of globular clusters is expected from optical and infrared observations, using precise pulse arrival timing for the millisecond pulsars in globular clusters known to date. For some of these pulsars closest to the cluster centers, we have calculated the expected delay times of pulses as they pass in the gravitational field of the central black hole. The detection of such a time delay by currently available instruments for the known pulsars is shown to be impossible at a black hole mass of 10³ M _⊙ and very problematic at a black hole mass of 10⁴ M _⊙. In addition, the signal delay will have a negligible effect on the pulsar periods and their first derivatives compared to the current accuracy of their measurements.     

Radial pulsations of red giant branch stars

We performed hydrodynamic computations of nonlinear stellar pulsations of population I stars at the evolutionary stages of the ascending red giant branch and the following luminosity drop due to the core helium flash. Red giants populating this region of the Hertzsprung–Russel diagram were found to be the fundamental mode pulsators. The pulsation period is the largest at the tip of the red giant branch and for stars with initial masses from 1.1 M _⊙ to 1.9 M _⊙ ranges from ∏ ≈ 254 day to ∏ ≈ 33 day , respectively. The rate of period change during the core helium flash is comparable with rates of secular period change in Mira type variables during the thermal pulse in the helium shell source. The period change rate is largest (∏?/∏ ≈ ?10^?2 yr^?1) in stars with initial mass M ^ZAMS = 1.1 M ^⊙ and decreases to ∏?/∏ ～ ?10^?3 yr^?1 for stars of the evolutionary sequence M ^ZAMS = 1.9 M ^⊙. Theoretical light curves of red giants pulsating with periods ∏ > 200 day show the presence of the secondary maximum similar to that observed in many Miras.