共查询到20条相似文献，搜索用时 671 毫秒
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R. R. Shobbrook G. Handler D. Lorenz D. Mogorosi 《Monthly notices of the Royal Astronomical Society》2006,369(1):171-181
We have carried out single and multisite photometry of the three β Cephei stars β and 15 CMa as well as KZ Mus. For the two stars in CMa, we obtained 270 h of measurement in the Strömgren uvy and Johnson V filters, while 150 h of time-resolved Strömgren uvy photometry was acquired for KZ Mus. All three stars are multiperiodic variables, with three (β CMa) and four (15 CMa, KZ Mus) independent pulsation modes. Two of the mode frequencies of 15 CMa are new discoveries and one of the known modes showed amplitude variations over the last 33 yr. Taken together, this fully explains the diverse behaviour of the star reported in the literature.
Mode identification by means of the amplitude ratios in the different passbands suggests one radial mode for each star. In addition, β CMa has a dominant ℓ= 2 mode while its third mode is non-radial with unknown ℓ. The non-radial modes of 15 CMa, which are ℓ≤ 3 , form an almost equally split triplet that, if physical, would imply that we see the star under an inclination angle larger than 55°. The strongest non-radial mode of KZ Mus is ℓ= 2 , followed by the radial mode and a dipole mode. Its weakest known mode is non-radial with unknown ℓ, confirming previous mode identifications for the pulsations of the star.
The phased light curve for the strongest mode of 15 CMa has a descending branch steeper than the rising branch. A stillstand phenomenon during the rise to maximum light is indicated. Given the low photometric amplitude of this non-radial mode this is at first sight surprising, but it can be explained by the aspect angle of the mode. 相似文献
Mode identification by means of the amplitude ratios in the different passbands suggests one radial mode for each star. In addition, β CMa has a dominant ℓ= 2 mode while its third mode is non-radial with unknown ℓ. The non-radial modes of 15 CMa, which are ℓ≤ 3 , form an almost equally split triplet that, if physical, would imply that we see the star under an inclination angle larger than 55°. The strongest non-radial mode of KZ Mus is ℓ= 2 , followed by the radial mode and a dipole mode. Its weakest known mode is non-radial with unknown ℓ, confirming previous mode identifications for the pulsations of the star.
The phased light curve for the strongest mode of 15 CMa has a descending branch steeper than the rising branch. A stillstand phenomenon during the rise to maximum light is indicated. Given the low photometric amplitude of this non-radial mode this is at first sight surprising, but it can be explained by the aspect angle of the mode. 相似文献
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Stefano Ettori 《Monthly notices of the Royal Astronomical Society》2000,318(4):1041-1046
The spatial emission from the core of cooling-flow clusters of galaxies is inadequately described by a β -model. Spectrally, the central region of these clusters is well approximated with a two-temperature model, where the inner temperature represents the multiphase status of the core and the outer temperature is a measure of the ambient gas temperature. Following this observational evidence, I extend the use of the β -model to a two-phase gas emission, where the two components coexist within a boundary radius r cool and the ambient gas alone fills the volume shell at a radius above r cool . This simple model still provides an analytic expression for the total surface brightness profile (Note in the first term the different sign with respect to the standard β -model.) Based upon a physically meaningful model for the X-ray emission, this formula can be used (i) to improve significantly the modelling of the surface brightness profile of cooling flow clusters of galaxies when compared to the standard β -model results, (ii) to constrain properly the physical characteristics of the intracluster plasma in the outskirts, like, e.g., the ambient gas temperature. 相似文献
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A. M. Newsam I. M. McHardy L. R. Jones K. O. Mason 《Monthly notices of the Royal Astronomical Society》1999,310(1):255-261
At the faint end of the deepest X-ray surveys, a population of X-ray luminous galaxies is seen. In this paper, we present the results of a cross-correlation between the residual, unresolved X-ray photons in a very deep X-ray survey and the positions of faint galaxies, in order to examine the importance of these objects at even fainter flux levels. We measure a significant correlation on all angular scales up to ∼1 arcmin. This signal could account for a significant fraction of the unresolved X‐ray background, approximately 35 per cent if the clustering is similar to optically selected galaxies. However, the angular form of the correlation is seen to be qualitatively similar to that expected for clusters of galaxies and the X-ray emission could be associated with hot gas in clusters or with QSOs within galaxy clusters rather than emission from individual faint galaxies. The relative contribution from each of these possibilities cannot be determined with the current data. 相似文献
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Stefano Ettori 《Monthly notices of the Royal Astronomical Society》2000,311(2):313-316
I suggest that the β -model used to fit the X-ray surface brightness profiles of extended sources, like groups and clusters of galaxies, has to be corrected when the counts are collected in a wide energy band comparable to the mean temperature of the source, and a significant gradient in the gas temperature is observed. I present a revised version of the β -model for the X-ray brightness that applies to an intracluster gas with temperature and density related by a polytropic equation and extends the standard version that is strictly valid for an isothermal gas. Given a temperature gradient observed through an energy window of 1–10 keV typical for the new generation of X-ray observatories, the β parameter can change systematically by up to 20 per cent from the value obtained under isothermal assumption, i.e. by an amount larger than any statistical uncertainty obtained from the present data. Within the virial regions of typical clusters of galaxies, these systematic corrections affect the total gravitating mass estimate by 5–10 per cent, the gas mass by 10–30 per cent and the gas fraction value up to 50 per cent, when compared with the measurements obtained under the isothermal assumption. 相似文献
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A. Comastri F. Fiore C. Vignali G. Matt G.C. Perola F. La Franca 《Monthly notices of the Royal Astronomical Society》2001,327(3):781-787
The BeppoSAX High Energy Large Area Survey (HELLAS) has surveyed several tens of deg2 of the sky in the band down to a flux of about . The source surface density of at the survey limit corresponds to a resolved fraction of the X-ray background (XRB) of the order of per cent. The extrapolation of the HELLAS towards fainter fluxes with a Euclidean slope is consistent with the first XMM–Newton measurements, in the same energy band, which are a factor of 20 times more sensitive. The source counts in the hardest band so far surveyed by X-ray satellites are used to constrain XRB models. It is shown that in order to reproduce the counts over the range of fluxes covered by BeppoSAX and XMM–Newton a large fraction of highly absorbed , luminous active galactic nuclei is needed. A sizeable number of more heavily obscured, Compton-thick, objects cannot be ruled out but they are not required by the present data. The model predicts an absorption distribution consistent with that found from the hardness ratios analysis of the so far identified HELLAS sources. Interestingly enough, there is evidence of a decoupling between X-ray absorption and optical reddening indicators, especially at high redshifts/luminosities where several broad-line quasars show hardness ratios typical of absorbed power-law models with . 相似文献
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M. Bliton E. Rizza J. O. Burns F. N. Owen & M. J. Ledlow 《Monthly notices of the Royal Astronomical Society》1998,301(3):609-625
We examine the ROSAT PSPC X-ray properties of a sample of 15 Abell clusters containing 23 narrow-angle tailed (NAT) radio galaxies. We find that clusters with NATs show a significantly higher level of substructure than a similar sample of radio-quiet clusters, indicating that NAT radio sources are preferentially located in dynamically complex systems. Also, the velocity distribution of the NAT galaxies is similar to that of other cluster members; these velocities are inadequate for producing the ram pressure necessary to bend the radio jets. We therefore propose a new model for NAT formation, in which NATs are associated with dynamically complex clusters undergoing merger events. The U -shaped NAT morphology is produced in part by the merger-induced bulk motion of the ICM bending the jets. 相似文献
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H. Ebeling A. C. Edge S. W. Allen C. S. Crawford A. C. Fabian J. P. Huchra 《Monthly notices of the Royal Astronomical Society》2000,318(2):333-340
We present a low-flux extension of the X-ray-selected ROSAT Brightest Cluster Sample (BCS) published in Paper I of this series. Like the original BCS and employing an identical selection procedure, the BCS extension is compiled from ROSAT All-Sky Survey (RASS) data in the northern hemisphere ( δ ≥0°) and at high Galactic latitudes (| b |≥20°). It comprises 99 X-ray-selected clusters of galaxies with measured redshifts z ≤0.3 (as well as eight more at z >0.3) and total fluxes between 2.8×10−12 and 4.4×10−12 erg cm−2 s−1 in the 0.1–2.4 keV band (the latter value being the flux limit of the original BCS). The extension can be combined with the main sample published in 1998 to form the homogeneously selected extended BCS (eBCS), the largest and statistically best understood cluster sample to emerge from the RASS to date.
The nominal completeness of the combined sample (defined with respect to a power-law fit to the bright end of the BCS log N –log S distribution) is relatively low at 75 per cent (compared with 90 per cent for the high-flux sample of Paper I). However, just as for the original BCS, this incompleteness can be accurately quantified, and thus statistically corrected for, as a function of X-ray luminosity and redshift.
In addition to its importance for improved statistical studies of the properties of clusters in the local Universe, the low-flux extension of the BCS is also intended to serve as a finding list for X-ray-bright clusters in the northern hemisphere which we hope will prove useful in the preparation of cluster observations with the next generation of X-ray telescopes such as Chandra and XMM-Newton .
An electronic version of the eBCS can be obtained from the following URL: http://www.ifa.hawaii.edu/~ebeling/clusters/BCS.html. 相似文献
The nominal completeness of the combined sample (defined with respect to a power-law fit to the bright end of the BCS log N –log S distribution) is relatively low at 75 per cent (compared with 90 per cent for the high-flux sample of Paper I). However, just as for the original BCS, this incompleteness can be accurately quantified, and thus statistically corrected for, as a function of X-ray luminosity and redshift.
In addition to its importance for improved statistical studies of the properties of clusters in the local Universe, the low-flux extension of the BCS is also intended to serve as a finding list for X-ray-bright clusters in the northern hemisphere which we hope will prove useful in the preparation of cluster observations with the next generation of X-ray telescopes such as Chandra and XMM-Newton .
An electronic version of the eBCS can be obtained from the following URL: http://www.ifa.hawaii.edu/~ebeling/clusters/BCS.html. 相似文献
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C. Brocksopp R. L. C. Starling P. Schady K. O. Mason E. Romero-Colmenero E. M. Puchnarewicz 《Monthly notices of the Royal Astronomical Society》2006,366(3):953-968
We present XMM–Newton /EPIC spectra for the Laor et al. sample of Palomar Green (PG) quasars. We find that a power law provides a reasonable fit to the 2–5 keV region of the spectra. Excess soft X-ray emission below 2 keV is present for all objects, with the exception of those known to contain a warm absorber. However, a single power law is a poor fit to the 0.3–10.0 keV spectrum and instead we find that a simple model, consisting of a broken power law (plus an iron line), provides a reasonable fit in most cases. The equivalent width of the emission line is constrained in just 12 objects but with low (<2σ) significance in most cases. For the sources whose spectra are well fitted by the broken-power-law model, we find that various optical and X-ray line and continuum parameters are well correlated; in particular, the power-law photon index is well correlated with the FWHM of the Hβ line and the photon indices of the low- and high-energy components of the broken power law are well correlated with each other. These results suggest that the 0.3–10 keV X-ray emission shares a common (presumably non-thermal) origin, as opposed to suggestions that the soft excess is directly produced by thermal disc emission or via an additional spectral component. We present XMM–Newton Optical Monitor (OM) data, which we combine with the X-ray spectra so as to produce broad-band spectral energy distributions (SEDs), free from uncertainties due to long-term variability in non-simultaneous data. Fitting these optical–UV spectra with a Comptonized disc model indicates that the soft X-ray excess is independent of the accretion disc, confirming our interpretation of the tight correlation between the hard and soft X-ray spectra. 相似文献
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I. Georgantopoulos K. Nandra M. Brotherton A. Georgakakis I. E. Papadakis P. O'Neill 《Monthly notices of the Royal Astronomical Society》2006,367(4):1727-1734
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