Pulsars with gigahertz-peaked spectra Kijak, J.; Lewandowski, W.; Maron, O. ...
Astronomy and astrophysics (Berlin),
07/2011, Letnik:
531
Journal Article
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Aims. We investigate a high frequency turn-over effect in radio spectra for pulsars with positive or flat spectral index. Methods. Using GMRT and Effelsberg observatory, we estimated the flux density ...to reconstruct pulsar spectra. Results. We find objects that have a maximum flux in their spectrum above 1 GHz, whose emitted energy decreases below 1 GHz and whose spectral indices are positive at lower frequencies. Some pulsars with a turn-over in their spectrum at high frequencies are found to exist in very interesting environments. We call these objects gigahertz-peaked spectra pulsars.
Transition metal oxides containing cubic B4O4 subcores are noted for their catalytic activity in water oxidation (OER). We synthesized a series of ternary spinel oxides, AB2O4, derived from LiMn2O4 ...by either replacement at the tetrahedral A site or Co substitution at the octahedral B site and measured their electrocatalytic OER activity. Atomic emission and powder X-ray diffraction confirmed spinel structure type and purity. Weak activation of the OER occurs upon A-site substitution: Zn2+ > Mg2+ > A-vacancy > Li+ = 0. Zn and Mg substitution is accompanied by (1) B-site conversion of Mn(IV) to Mn(III), resulting in expansion and higher symmetry of the Mn4O44+ core relative to LiMn2O4 (inducing greater flexibility of the core and lower reorganization barrier to distortions), and (2) the electrochemical oxidation potential for Mn(III)/IV) increases by 0.15–0.2 V, producing a stronger driving force for water oxidation. Progressive replacement of Mn(III/IV) by Co(III) at the B site (LiMn2–x Co x O4, 0 ≤ x ≤ 1.5) both symmetrizes the Mn4–x Co x O4 core and increases the oxidation potential for Co(III/IV), resulting in the highest OER activity within the spinel structure type. These observations point to two predictors of OER catalysis: (1) Among AMn2O4 spinels, those starting with Mn(III) in the resting lattice (prior to oxidation) result in longer, weaker Mn–O bonds for this eg 1 antibonding electronic configuration, yielding greater core flexibility and a higher oxidation potential to Mn(IV), and (2) a linear free energy relationship exists between the electrocatalytic rate and the binding affinity of the substrate oxygen (*OH and *OOH) to the B site.
The analysis presented in this paper verifies which of the mixing rules are best for real components of interstellar dust in possible wide range of wavelengths. The Discrete Dipole Approximation ...(DDA) method with elements of different components with various volume fractions has been used. We have considered six materials: ice, amorphous carbon, graphite, SiC, silicates and iron; and the following mixing rules: Maxwell-Garnett, Bruggeman, Looyenga, Hanay and Lichtenecker, which must satisfy rigorous bounds. The porous materials have also been considered. We have assumed simplified spatial distribution, shape and size of inclusions. The criteria given by Draine have been used to determine the range of wavelengths for the considered mixtures in order to calculate the Qext using the DDA. From all the chosen mixing rules for the examined materials in majority of cases (13 out of 20), the best results have been obtained using the Lichtenecker mixing rule. In five cases, this rule is better for some volume fraction of inclusions.
Aims. We aim to investigate the flux density modulation from pulsars and the specific behaviour of the modulation index versus frequency. Methods. Several pulsars were observed with the Effelsberg ...radio telescope at 8.35 GHz. Their flux density time series were corrected for interstellar scintillation effects. Results. We present measurements of modulation indices for eight pulsars. We confirm the presence of a critical frequency at ~1 GHz for these pulsars (including three new ones from this study). We derived intrinsic modulation indices for the resulting flux density time series. Our data analysis revealed strong single pulses detected from five pulsars.
Observations of pulsars at 9 millimetres Löhmer, O.; Jessner, A.; Kramer, M. ...
Astronomy and astrophysics (Berlin),
03/2008, Letnik:
480, Številka:
3
Journal Article
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Aims.The behaviour of the pulsar spectrum at high radio frequencies can provide decisive information about the nature of the radio emission mechanism. Methods.We report recent observations of a ...selected sample of pulsars at $\lambda=9$ mm (32 GHz) with the 100-m Effelsberg radio telescope. Results.Three pulsars, PSR B0144+59, PSR B0823+26, and PSR B2022+50, were detected for the first time at this frequency. We confirm the earlier flux density measurements for a sample of six pulsars, and we are able to place upper flux density limits for another 12 pulsars. We find that all pulsar spectra have a simple form that can be described using only three parameters, one of which is the lifetime of short nano-pulses in the emission region. The study of the transition region from coherent to incoherent emission needs further and more sensitive observations at even higher radio frequencies.
We present the computation of effective refractive coefficients for inhomogeneous two-component grains with three kinds of inclusions with mincl= 3.0 + 4.0i, 2.0 + 1.0i, 2.5 + 0.0001i and a matrix ...with mm= 1.33 + 0.01i for 11 volume fractions of inclusions from 0 to 50 per cent and wavelengths λ= 0.5, 1.0, 2.0 and 5.0 μm. The coefficients of extinction for these grains have been computed using a discrete dipole approximation. Computation of the extinction by the same method for grains composed of a matrix material with randomly embedded inclusions has been carried out for different volume fractions of inclusions. A comparison of extinction coefficients obtained for both models of grain material allows one to choose the best mixing rule for a mixture. In cases of inclusions with mincl= 2.0 + 1.0i and 2.5 + 0.0001i the best fit for the whole wavelength range and volume fractions of inclusions from 0 to 50 per cent has been obtained for the Lichtenecker mixing rule. In the case of mincl= 3.0 + 4.0i the fit for the whole wavelength range and volume fractions of inclusions from 0 to 50 per cent is not very significant but the best fit has been obtained for the Hanai rule. For volume fractions of inclusions from 0 to 15 per cent a very good fit has been obtained for the whole wavelength range for Rayleigh and Maxwell-Garnett mixing rules.
Pulsar spectra of radio emission Maron, O.; Kijak, J.; Kramer, M. ...
Astronomy & astrophysics. Supplement series,
12/2000, Letnik:
147, Številka:
2
Journal Article
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We have collected pulsar flux density observations and compiled spectra of 281 objects. The database of Lorimer et al. (1995) has been extended to frequencies higher than 1.4 GHz and lower than ...300 MHz. Our results show that above 100 MHz the spectra of the majority of pulsars can be described by a simple power law with average value of spectral index $<\alpha>$ = $-1.8~\pm~0.2$. A rigorous analysis of spectral fitting revealed only about 10% of spectra which can be modelled by the two power law. Thus, it seems that single power law is a rule and the two power law spectrum is a rather rare exception, of an unknown origin, to this rule. We have recognized a small number of pulsars with almost flat spectrum ($\alpha \geq -1.0$) in the wide frequency range (from 300 MHz to 20 GHz) as well as few pulsars with a turn-over at unusually high frequency (~ 1 GHz).
New pulsar observations at 8.35 GHz with the newly built highly sensitive receiver of the Effelsberg 100-m Radio Telescope are reported. They resulted in the first detection of 3 millisecond ...pulsars J1713+0747, B1855+09 and J2145-0750 at this high frequency. The measured flux densities indicate that the spectrum of all three pulsars follows a single power law. PSR B1855+09 shows a significant increase of the Interpulse-to-Mainpulse amplitude ratio at high frequencies.
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