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1.
1 INTRODUCTION Many radio observations have been conducted on globular clusters in the past years, stimu-lated by the detection of pulsars and X-ray sources in them. Globular clusters are good placesfor hunting pulsars (Lyne et al. 2000). Until now, about 50 pulsars have been detected in 17globular clusters (Lyne et al. 1995; Biggs & Lyne 1996; D'Amico et al. 2001; Lyne et al. 2000;Camilo et al. 2000). Except for four long period pulsars, all of these pulsars are millisecondpulsars (M…  相似文献   

2.
Using a pulsar timing system developed at Urumqi Astronomical Observatory‘s 25-m telescope, we observed scintillation dynamic spectra for seven pulsars at the relatively high frequency of 1540 MHz over a wide frequency band of 320 MHz. Averaging observations at different epochs, we obtain time scales and decorrelation bandwidths for diffractive scintillation and show that these imply a power-law index for the electron density fluctuation close to 4.0. Assuming this value and that the scattering disk is approximately midway between the pulsar and the earth, we compute transverse velocities for the seven pulsars. These values are generally in good agreement with the proper motion velocities.  相似文献   

3.
We present an analysis of the variations seen in the dispersion measures (DMs) of 20-ms pulsars observed as part of the Parkes Pulsar Timing Array project. We carry out a statistically rigorous structure function analysis for each pulsar and show that the variations seen for most pulsars are consistent with those expected for an interstellar medium characterized by a Kolmogorov turbulence spectrum. The structure functions for PSRs J1045−4509 and J1909−3744 provide the first clear evidence for a large inner scale, possibly due to ion–neutral damping. We also show the effect of the solar wind on the DMs and show that the simple models presently implemented into pulsar timing packages cannot reliably correct for this effect. For the first time we clearly show how DM variations affect pulsar timing residuals and how they can be corrected in order to obtain the highest possible timing precision. Even with our presently limited data span, the residuals (and all parameters derived from the timing) for six of our pulsars have been significantly improved by correcting for the DM variations.  相似文献   

4.
In this paper, we study the variations of group galaxy properties according to the assembly history in Sloan Digital Sky Survey Data Release 6 (SDSS-DR6) selected groups. Using mock SDSS group catalogues, we find two suitable indicators of group formation time: (i) the isolation of the group, defined as the distance to the nearest neighbour in terms of its virial radius and (ii) the concentration, measured as the group inner density calculated using the fifth nearest bright galaxy to the group centre. Groups within narrow ranges of mass in the mock catalogue show increasing group age with isolation and concentration. However, in the observational data the stellar age, as indicated by the spectral type, only shows a correlation with concentration.
We study groups of similar mass and different assembly history, finding important differences in their galaxy population. Particularly, in high-mass SDSS groups, the number of members, mass-to-light ratios, red galaxy fractions and the magnitude difference between the brightest and second-brightest group galaxies, show different trends as a function of isolation and concentration, even when it is expected that the latter two quantities correlate with group age. Conversely, low-mass SDSS groups appear to be less sensitive to their assembly history.
The correlations detected in the SDSS are not consistent with the trends measured in the mock catalogues. However, discrepancies can be explained in terms of the disagreement found in the age-isolation trends, suggesting that the model might be overestimating the effects of environment. We discuss how the modelling of the cold gas in satellite galaxies could be responsible for this problem. These results can be used to improve our understanding of the evolution of galaxies in high-density environments.  相似文献   

5.
The explosion mechanism associated with thermonuclear supernovae (SNIa) is still a matter of debate. There is a wide agreement that high amounts of radioactive nuclei are produced during these events and they are expected to be strong γ-ray emitters. In the past, several authors have investigated the use of this γ-ray emission as a diagnostic tool. In this paper we have performed a complete study of the γ-ray spectra associated with all the different scenarios currently proposed. This includes detonation, delayed detonation, deflagration and the off-centre detonation. We have performed accurate simulations for this complete set of models in order to determine the most promising spectral features that could be used to discriminate among the different models. Our study is not limited to qualitative arguments. Instead, we have quantified the differences among the spectra and established distance limits for their detection. The calculations have been performed considering the best current response estimations of the SPI and IBIS instruments aboard INTEGRAL in such a way that our results can be used as a guideline to evaluate the capabilities of INTEGRAL in the study of Type Ia supernovae. For the purpose of completeness we have also investigated the nuclear excitation and spallation reactions as a possible secondary source of γ-rays present in some supernova scenarios. We conclude that this mechanism can be neglected because of its small contribution.  相似文献   

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8.
Electron temperatures derived from the He  i recombination line ratios, designated T e(He  i ), are presented for 48 planetary nebulae (PNe). We study the effect that temperature fluctuations inside nebulae have on the T e(He  i ) value. We show that a comparison between T e(He  i ) and the electron temperature derived from the Balmer jump of the H  i recombination spectrum, designated T e(H  i ), provides an opportunity to discriminate between the paradigms of a chemically homogeneous plasma with temperature and density variations, and a two-abundance nebular model with hydrogen-deficient material embedded in diffuse gas of a 'normal' chemical composition (i.e. ∼solar), as the possible causes of the dichotomy between the abundances that are deduced from collisionally excited lines and those deduced from recombination lines. We find that T e(He  i ) values are significantly lower than T e(H  i ) values, with an average difference of  〈 T e(H  i ) − T e(He  i )〉= 4000 K  . The result is consistent with the expectation of the two-abundance nebular model but is opposite to the prediction of the scenarios of temperature fluctuations and/or density inhomogeneities. From the observed difference between T e(He  i ) and T e(H  i ), we estimate that the filling factor of hydrogen-deficient components has a typical value of 10−4. In spite of its small mass, the existence of hydrogen-deficient inclusions may potentially have a profound effect in enhancing the intensities of He  i recombination lines and thereby lead to apparently overestimated helium abundances for PNe.  相似文献   

9.
We investigate the first stage of the dynamical evolution of Oort cloud comets entering the planetary region for the first time. To this purpose, we integrate numerically the motions of a large number of fictitious comets pertaining to two samples, both with perihelion distances up to 5.7 au and random inclinations; the first sample is composed of comets whose orbits have at least one node close to 5.2 au, while the second is not subject to this constraint. We examine the orbits when the comets come to aphelion after their first perihelion passage within the planetary region, and find that there is a clear statistical dependence of the energy perturbations on the Tisserand parameter. There appear to be two main processes, of comparable importance, governing the shortening of semimajor axes to values of less than 1000 au, i.e. planetary close encounters, especially with Jupiter, and indirect perturbations due to the shifting of the motion from barycentric to heliocentric and back; the former process mostly affects comets crossing the ecliptic at about 5.2 au, or on low-inclination orbits, while the latter mostly affects comets of small perihelion distance. This last result may help to understand the relative paucity of Halley-type comets with perihelion distances larger than about 1.5 au.  相似文献   

10.
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11.
The isothermal Lane–Emden equation arises in many astrophysical problems, in particular in modelling of a self-gravitating, polytropic gas in a spherically symmetric configuration. In this work, the isothermal Lane–Emden equation is investigated using the fractional approximation technique. The method provides an efficient and accurate way of obtaining approximate analytic solution to the Lane–Emden equation thus is useful in the modelling of self-gravitating gaseous spheres in astrophysics.  相似文献   

12.
From 2001 January to 2002 June, we monitored PSRs B0329+54, B0823+26, B1929+10, B2020+28 and B2021+51 using the Nanshan 25-m radio telescope of the Urumqi Observatory to study their diffractive interstellar scintillation (DISS). The average interval between observations was about 9 d and the observation duration ranged between 2 and 6 h depending on the pulsar. Wide variations in the DISS parameters were observed over the 18-month data span. Despite this, the average scintillation velocities are in excellent agreement with the proper motion velocities. The average two-dimensional autocorrelation function for PSR B0329+54 is well described by a thin-screen Kolmogorov model, at least along the time and frequency axes. Observed modulation indices for the DISS time-scale and bandwidth and the pulsar flux density are greater than values predicted for a Kolmogorov spectrum of electron density fluctuations. Correlated variations over times that are long compared to the nominal refractive scintillation time are observed, suggesting that larger scale density fluctuations are important. For these pulsars, the scintillation bandwidth as a function of frequency has a power-law index  (∼3.6)  much less than that expected for Kolmogorov turbulence (∼4.4). Sloping fringes are commonly observed in the dynamic spectra, especially for PSR B0329+54. The detected range of fringe slopes are limited by our observing resolution. Our observations are sensitive to larger-scale fringes and hence smaller refractive angles, corresponding to the central part of the scattering disc.  相似文献   

13.
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14.
We present stellar evolution calculations for Population III stars for both single- and binary-star evolutions. Our models include 10- and  16.5-M⊙  single stars and a  10-M⊙  model star that undergoes an episode of accretion resulting in a final mass of  16.1 M⊙  . For comparison, we present the evolution of a solar heavy element abundance model. We use the structure from late-stage evolution models to calculate simulated supernova light curves. Light curve comparisons are made between accretion and non-accretion progenitor models, and models for single-star evolution of comparable masses. Where possible, we make comparisons to previous works. Similar investigations have been carried out, but primarily for solar or near-solar heavy metal abundance stars and not including both the evolution and the supernova explosions in one work.  相似文献   

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17.
Within the framework of the internal–external shocks model for γ -ray bursts, we study the various mechanisms that can give rise to quiescent times in the observed γ -ray light curves. In particular, we look for the signatures that can provide us with evidence as to whether or not the central engine goes dormant for a period of time comparable to the duration of the gaps. We show that the properties of the prompt γ -ray and X-ray emission can, in principle, determine whether the quiescent episodes are caused by a modulated relativistic wind or a switching off of the central engine. We suggest that detailed observations of the prompt afterglow emission from the reverse shock will strongly constrain the possible mechanisms for the production of quiescent times in γ -ray bursts.  相似文献   

18.
Recent observations suggest that long-duration γ -ray bursts and their afterglows are produced by highly relativistic jets emitted in core-collapse explosions. As the jet makes its way out of the stellar mantle, a bow shock runs ahead and a strong thermal precursor is produced as the shock breaks out. Such erupting fireballs produce a very bright γ -ray precursor as they interact with the thermal break-out emission. The prompt γ -ray emission propagates ahead of the fireball before it becomes optically thin, leading to e± pair loading and radiative acceleration of the external medium. The detection of such precursors would offer the possibility of diagnosing not only the radius of the stellar progenitor and the initial Lorentz factor of the collimated fireball, but also the density of the external environment.  相似文献   

19.
We present Chandra and XMM–Newton observations of 12 bright  [ f (2–10 keV) > 10−13 erg cm−2 s−1]  sources from the ASCA search for the High Energy Extragalactic Population (SHEEP) survey. Most of these have been either not observed or not detected previously with the ROSAT mission, and therefore they constitute a sample biased towards hard sources. The Chandra observations are important in locating the optical counterpart of the X-ray sources with accuracy. Optical spectroscopic observations show that our sample is associated with both narrow-line (NL) (six objects) and broad-line (BL) active galactic nuclei (AGN) (five objects), with one source remaining unidentified. Our sources cover the redshift range 0.04–1.29, spanning luminosities from 1042 to  1045 erg s−1  (2–10 keV). The NL sources have preferentially lower redshift (and luminosity) compared to the BL ones. This can be most easily explained in a model where the NL AGN are intrinsically less luminous than the BL ones in line with the results of Steffen et al. The X-ray spectral fittings show a roughly equal number of obscured  ( N H > 1022 cm−2)  and unobscured  ( N H < 1022 cm−2)  sources. There is a clear tendency for obscured sources to be associated with NL AGN and unobscured sources with BL ones. However, there is a marked exception with the highest obscuring column observed at a BL AGN at a redshift of z = 0.5.  相似文献   

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