A new look at pulsar statistics — Birthrate and evidence for injection     

M. Vivekanand  R. Narayan 《Journal of Astrophysics and Astronomy》1981,2(3):315-337

We make a statistical analysis of the periodsP and period-derivativesP of pulsars using a model independent theory of pulsar flow in theP-P diagram. Using the available sample ofP andP values, we estimate the current of pulsars flowing unidirectionally along theP-axis, which is related to the pulsar birthrate. Because of radio luminosity selection effects, the observed pulsar sample is biased towards lowP and highP. We allow for this by weighting each pulsar by a suitable scale factor. We obtain the number of pulsars in our galaxy to be 6.05_−2.80 ^+3.32 × 10⁵ and the birthrate to be 0.048_−0.011 ^+0.014 pulsars yr⁻¹ galaxy⁻¹. The quoted errors refer to 95 per cent confidence limits corresponding to fluctuations arising from sampling, but make no allowance for other systematic and random errors which could be substantial. The birthrate estimated here is consistent with the supernova rate. We further conclude that a large majority of pulsars make their first appearance at periods greater than 0.5 s. This ‘injection’, which runs counter to present thinking, is probably connected with the physics of pulsar radio emission. Using a variant of our theory, where we compute the current as a function of pulsar ‘age’ (1/2P/P), we find support for the dipole braking model of pulsar evolution upto 6 × 10⁶ yr of age. We estimate the mean pulsar braking index to be 3.7_−0.8 ^+0.8.  相似文献   

Turbulence‐driven angular momentum transport in modulated Kepler flows     

G. Rüdiger  A. Drecker 《Astronomische Nachrichten》2001,322(3):179-187

The velocity fluctuations in a spherical shell arising from sinusoidal perturbations of a Keplerian shear flow with a free amplitude parameter ε are studied numerically by means of fully 3D nonlinear simulations. The investigations are performed at high Reynolds numbers, i.e. 3000 < Re < 5000. We find Taylor‐Proudman columns of large eddies parallel to the rotation axis for sufficiently strong perturbations. An instability sets in at critical amplitudes with ε_crit ∝ Re^—1. The whole flow turns out to be almost axisymmetric and nonturbulent exhibiting, however, a very rich radial and latitudinal structure. The Reynolds stress 〈u′_ru′_ϕ〉 is positive in the entire computational domain, from its Gaussian radial profile a positive viscosity‐alpha of about 10^—4 is derived. The kinetic energy of the turbulent state is dominated by the azimuthal component 〈u^′2_ϕ whereas the other components are smaller by two orders of magnitude. Our simulations reveal, however, that these structures disappear as soon as the perturbations are switched off. We did not find an “effective” perturbation whose amplitude is such that the disturbance is sustained for large times (cf. Dauchot & Daviaud 1995) which is due to the effective violation of the Rayleigh stability criterion. The fluctuations rapidly smooth the original profile towards to pure Kepler flow which, therefore, proves to be stable in that sense.  相似文献   

Photometry of the contact binary system V471 Cas     

Liu Xuefu  Tan Huisong 《Astrophysics and Space Science》1991,183(2):237-244

UBV photoelectric observations of the W UMa-type binary V471 Cas were made from September to November 1984. Its colour indices were found to be(B-V)=0 _. ^m 771±0 _. ^m 02 and(U-B)=0 _. ^m 196±0 _. ^m 02. TheU, B, andV light curves of V471 Cas show some photometric fluctuations. We found that its orbital period is not 0.335998 days which was given by GCVS (1986), but 0.405356 days.Photometric orbital elements of V471 Cas were found using the Wilson-Devinney method. V471 Cas is a contact system, in which the overcontact factor is 0.19, its mass ratio of two components is 0.5947, and orbital minclination is in 83.29 degrees.  相似文献   

Snow cover monitoring in the Northern Patagonia Icefield using MODIS satellite images (2000–2006)     

Paulina Lopez  Pascal Sirguey  Yves Arnaud  Bernard Pouyaud  Pierre Chevallier 《Global and Planetary Change》2008,61(3-4):103-116

The snow cover of the Northern Patagonia Icefield (NPI) was monitored after applying the Normalized Difference Snow Index (NDSI) and the Red/NIR band ratio to 134 Moderate Resolution Imaging Spectroradiometer (MODIS) images captured between 2000 and 2006. The final results show that the snow cover extent of the NPI fluctuates a lot in winter, in addition to its seasonal behaviour. The minimum snow cover extent of the period (3600 km²) was observed in March 2000 and the maximum (11,623 km²) in August 2001. We found that temperature accounts for approximately 76% of the variation of the snow cover extent over the entire icefield. We also show two different regimes of winter snow cover fluctuations corresponding to the eastern and the western sides of the icefield. The seasonality of the snow cover on the western side was determined by temperature rather than precipitation, while on the east side the seasonality of the snow cover was influenced by the seasonal behaviour of both temperature and precipitation. This difference can be explained by the two distinct climates: coastal and continental. The fluctuations in the winter snow cover extent were more pronounced and less controlled by temperature on the western side than on the eastern side of the icefield. Snow cover extent was correlated with temperature R² = 0.75 and R² = 0.74 for the western and eastern sides, respectively. Since limited meteorological data are available in this region, our investigation confirmed that the change in snow cover is an interesting climatic indicator over the NPI providing important insights in mass balance comprehension. Since snow and ice were distinguished snow cover fluctuations can be associated to fluctuations in the snow accumulation area of the NPI. In addition, days with minimum snow covers of summer season can be associated to the period in which Equilibrium Line Altitude (ELA) is the highest.  相似文献   

A new period and period changes of GG Cassiopeiae     

R. K. Srivastava 《Astrophysics and Space Science》1987,132(2):331-335

The O–C diagram of the eclipsing binary GG Cassiopeiae has been presented for the first time, and the period changes present in the system have been analysed. In all three period changes are noted. The strongest period change has been found to occur in the time-interval 1942 to 1966. The total period change in different portions of the O–C diagram ranges from 7.1×10^–7 d to 2.0×10^–5 d. The stronger period changes appear to have occurred after 1942; prior to it, the system has shown a negligible period change. The overall picture of the O–C diagram suggests that the O–C values of the system GG Cas are negative after 1942. The presence of a third body does not appear probable. The period fluctuations are also appreciable. A new period (P=3 _. ^d 758733) has been presented.  相似文献   

Search for outbursts in the narrow 511-keV line from compact sources based on INTEGRAL data     

S. S. Tsygankov  E. M. Churazov 《Astronomy Letters》2010,36(4):237-247

We present the results of a systematic search for outbursts in the narrow positron annihilation line on various time scales (5 × 10⁴–10⁶ s) based on the SPI/INTEGRAL data obtained from 2003 to 2008. We show that no outbursts were detected with a statistical significance higher than ∼6σ for any of the time scales considered over the entire period of observations. We also show that, given the large number of independent trials, all of the observed spikes could be associated with purely statistical flux fluctuations and, in part, with a small systematic prediction error of the telescope’s instrumental background. Based on the exposure achieved in ∼6 yr of INTEGRAL operation, we provide conservative upper limits on the rate of outbursts with a given duration and flux in different parts of the sky.  相似文献   

Power Spectrum of Cosmic-ray Fluctuations During Consecutive Solar Minimum and Maximum Periods     

M.A. El-Borie  S.S. Al-Thoyaib 《Solar physics》2002,209(2):397-407

Power spectral analysis of cosmic-ray intensity recorded by eight stations was carried out over a wide range of frequencies from 2.3 × 10^–8 Hz to 5.8 × 10^–6 Hz (2–500 days) during the period 1964–1995. Spectrum results of large-scale fluctuations have revealed the existence of a broad peak near 250–285 days and a narrower peak at 45–50 days during the studied epochs as a stable feature in all neutron monitors covering a wide rigidity range. The cosmic-ray power spectrum displayed significant peaks of varying amplitude with the solar rotation period (changed inversely with the particle rigidities) and its harmonics. The amplitudes of 27-day and 13.5-day fluctuations are greater during the positive-polarity epochs of the interplanetary magnetic field (qA>0) than during the qA<0 epochs. The comparison of cosmic-ray power spectra during the four successive solar activity minima have indicated that at the low-rigidity particles the spectrum differences between the qA>0 and qA<0 epochs are significantly large. Furthermore, the spectrum for even solar maximum years are higher and much harder than the odd years. There are significant differences in the individual spectra of solar maxima for different cycles.  相似文献   

Spectrum of the galactic magnetic fields     

A. A. Ruzmaikin  A. M. Shukurov 《Astrophysics and Space Science》1982,82(2):397-407

The magnetic fields observed in the galactic disc are generated by the differential rotation and the helical turbulent motions of interstellar gas. On the scalesl=2k ^–1 which lie in the intervall ₀<l<l _e (l ₀100 pc is the energy-range scale of the galactic turbulence), the spectral density of the kinetic energy of turbulence (k ^–5/3) exceeds the magnetic energy spectral density (k ^–1). The equipartition between magnetic and kinetic energies is reached atl=l _e 6 pc in the intercloud medium and is maintained down to the scalel=l _d 0.03 pc. In dense interstellar cloudsl _e is determined by the individual cloud size andl _d 0.1 pc.The internal turbulent velocities in Hi clouds (cloud size is assumed to be 10 pc) lie in the range from 1.8 to 5.6km s^–1, fitting well within the observed range of internal rms velocities. Dissipation of the interstellar MHD turbulence leads to creation of temperature fluctuations with amplitudes of 150 K and 65 K in dense clouds and intercloud medium, respectively. The small-scale fluctuations observed in the interstellar medium may arise from such perturbations due to the thermal instability, for instance. Dissipation of the MHD turbulence energy provides nearly half of the energy supply needed to maintain the thermal balance of the interstellar medium.  相似文献   

Simultaneous observations of second and sub-second time structures in H α , radio and hard X-ray data due to the periodical particle acceleration and MHD waves in the November 1, 2004 flare     

Huang Guangli  Ji Haisheng 《Astrophysics and Space Science》2007,312(1-2):127-138

Second and sub-second structures were simultaneously detected in optical, radio and hard X-ray (HXR) band, respectively by the GanYu Station of Purple Mountain Observatory, Nobeyama Radio Observatory, and RHESSI satellite in the November 1, 2004 flare (Ji et al., in Astrophys. J. 636:L173, 2006), which may be contributed to the energy transport of the continuous heat flux from the hot corona or chromosphere evaporation and of the accelerated electrons. The linear correlations between the amplitudes of these fluctuations and their flare emissions, and those between the cross-correlation coefficients of the fluctuations at two H _α kernels, or two radio frequencies, or two X-ray energies and their flare emissions may support the causal relationship of the flare and these time structures. While, the cross-correlations of the fluctuations at three different bands suggest that the fluctuations are caused by the common thermal or nonthermal processes in the flare. Moreover, some new features of the fluctuations are reported in the flare: (1) The sub-second fluctuations in radio and HXR bands have a same timescale, which is evidently larger than that in H-alpha band. The difference may be explained by the downward movements of nonthermal electrons or the upward motion of chromosphere evaporation. (2) The power-law distributions of the amplitudes of the second and the sub-second structures are obtained at optical, radio and HXR bands with different indices. (3) The peak-to-peak correspondence of Stokes I and V components in the sub-second structures at radio band suggests that they may be resulted from a periodical particle acceleration and particle injection in this event. However, the second structures may be caused by the modulations of Alfvén waves with an upward speed of 10³ km/s.  相似文献