共查询到20条相似文献，搜索用时 15 毫秒
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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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Hong-Xing Yin Cheng-Min Zhang Yong-Heng Zhao Li-Ming Song Fan Zhang National Astronomical Observatories Chinese Academy of Sciences Beijing Institute of High Energy Physics Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2005,5(6):595-600
For the bright neutron star low-mass X-ray binary Sco X-1, we analyzed all updated frequencies of the twin kilohertz quasi-periodic oscillations (kHz QPOs), their correlations and distributions. We found that the frequency separation of the kHz QPO peaks appears not to be a constant, rather, it decreases with increasing inferred mass accretion rate. We show that the currently available data of Sco X-1 by Rossi X-ray Timing Explorer are inconsistent with the proposals of the beat model that the frequency separation is a constant. Our conclusions are consistent with those of some previous researchers and we discuss further implications for the kilohertz QPO models. 相似文献
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L. M. Heil S. Vaughan T. P. Roberts 《Monthly notices of the Royal Astronomical Society》2009,397(2):1061-1072
We present results from a study of short-term variability in 19 archival observations by XMM–Newton of 16 ultraluminous X-ray sources (ULXs). Eight observations (six sources) showed intrinsic variability with power spectra in the form of either a power-law or broken power-law-like continuum and in some cases quasi-periodic oscillations (QPOs). The remaining observations were used to place upper limits on the strength of possible variability hidden within. Seven observations (seven sources) yielded upper limits comparable to, or higher than, the values measured from those observations with detectable variations. These represented the seven faintest sources, all with fx < 3 × 10−12 erg cm−2 s−1 . In contrast, there are four observations (three sources) that gave upper limits significantly lower than both the values measured from the ULX observations with detectable variations, and the values expected by comparison with luminous Galactic black hole X-ray binaries (BHBs) and active galactic nuclei (AGN) in the observed frequency bandpass (10−3 –1 Hz). This is the case irrespective of whether one assumes characteristic frequencies appropriate for a stellar mass (10 M⊙ ) or an intermediate mass (1000 M⊙ ) black hole, and means that in some ULXs the variability is significantly suppressed compared to bright BHBs and AGN. We discuss ways to account for this unusual suppression in terms of both observational and intrinsic effects and whether these solutions are supported by our results. 相似文献
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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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Elena Rossi Davide Lazzati Martin J. Rees 《Monthly notices of the Royal Astronomical Society》2002,332(4):945-950
Gamma-ray bursts are often modelled as jet-like outflows directed towards the observer; the cone angle of the jet is then commonly inferred from the time at which there is a steepening in the power-law decay of the afterglow. We consider an alternative model in which the jet has a beam pattern where the luminosity per unit solid angle (and perhaps also the initial Lorentz factor) decreases smoothly away from the axis, rather than having a well-defined cone angle within which the flow is uniform. We show that the break in the afterglow light curve then occurs at a time that depends on the viewing angle. Instead of implying a range of intrinsically different jets – some very narrow, and others with a similar power spread over a wider cone – the data on afterglow breaks could be consistent with a standardized jet, viewed from different angles. We discuss the implication of this model for the luminosity function. 相似文献
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A. De Luca P. A. Caraveo S. Mereghetti A. Tiengo G. F. Bignami 《Astrophysics and Space Science》2007,308(1-4):231-238
1E 161348-5055 (1E) is a compact object lying at the center of the 2000 year old Supernova Remnant (SNR) RCW103. Its original
identification as an isolated, radio-quiet neutron star has been questioned in recent years by the observation of a significant
long-term variability, as well as by reports of a possible periodicity at ∼6 hours. Here we report conclusive evidence for
a strong (nearly 50%) periodic modulation of 1E at 6.67±0.03 hours, discovered during a long (90 ks) XMM-Newton observation
performed in August 2005, when the source was in a “low state”. The source spectrum varies along the 6.67 hr cycle. No fast
pulsations are seen. 1E could be a very young binary system, possibly composed of a compact object and a low-mass star in
an eccentric orbit. This would be the first example of a low-mass X-ray binary (LMXB) associated with a SNR, and thus the
first LMXB for which we know the precise birth date, just 2000 years ago. Alternatively, if it is an isolated neutron star,
the unprecedented combination of age, period and variability may only fit in a very unusual scenario, featuring a peculiar
magnetar, dramatically slowed-down over 2000 years, possibly by a supernova debris disc.
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L. Hjalmarsdotter A. A. Zdziarski S. Larsson V. Beckmann M. McCollough D. C. Hannikainen O. Vilhu 《Monthly notices of the Royal Astronomical Society》2008,384(1):278-290
The X-ray binary Cygnus X-3 (Cyg X-3) is a highly variable X-ray source that displays a wide range of observed spectral states. One of the main states is significantly harder than the others, peaking at ∼20 keV, with only a weak low-energy component. Due to the enigmatic nature of this object, hidden inside the strong stellar wind of its Wolf–Rayet companion, it has remained unclear whether this state represents an intrinsic hard state, with truncation of the inner disc, or whether it is just a result of increased local absorption. We study the X-ray light curves from RXTE /ASM and CGRO /BATSE in terms of distributions and correlations of flux and hardness and find several signs of a bimodal behaviour of the accretion flow that are not likely to be the result of increased absorption in a surrounding medium. Using INTEGRAL observations, we model the broad-band spectrum of Cyg X-3 in its apparent hard state. We find that it can be well described by a model of a hard state with a truncated disc, despite the low cut-off energy, provided the accreted power is supplied to the electrons in the inner flow in the form of acceleration rather than thermal heating, resulting in a hybrid electron distribution and a spectrum with a significant contribution from non-thermal Comptonization, usually observed only in soft states. The high luminosity of this non-thermal hard state implies that either the transition takes place at significantly higher L / L E than in the usual advection models, or the mass of the compact object is ≳20 M⊙ , possibly making it the most-massive black hole observed in an X-ray binary in our Galaxy so far. We find that an absorption model as well as a model of almost pure Compton reflection also fit the data well, but both have difficulties explaining other results, in particular the radio/X-ray correlation. 相似文献
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N. S. Loaring M. J. Page G. Ramsay 《Monthly notices of the Royal Astronomical Society》2003,345(3):865-870
We present results from XMM–Newton observations of the obscured quasi-stellar object 1SAX J1218.9+2958. We find that the previously reported optical and soft X-ray counterpart positions are incorrect. However, we confirm the spectroscopic redshift of 0.176. The optical counterpart has a K magnitude of 13.5 and an R – K colour of 5.0 and is therefore a bright extremely red object. The X-ray spectrum is well described by a power law (Γ= 2.0 ± 0.2) absorbed by an intrinsic neutral column density of 8.2+1.1 −0.7 × 1022 cm−2 . We find that any scattered emission contributes at most 0.5 per cent to the total X-ray flux. From the optical/near-infrared colour we estimate that the active nucleus must contribute at least 50 per cent of the total flux in the K band and that the ratio of extinction to X-ray absorption is 0.1–0.7 times that expected from a Galactic dust–gas ratio and extinction curve. If 1SAX J1218.9+2958 were 100 times less luminous it would be indistinguishable from the population responsible for most of the 2–10 keV X-ray background. This has important implications for the optical/infrared properties of faint absorbed X-ray sources. 相似文献
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This paper presents the results of optical spectroscopic observations of two ROSAT bright sources, 1RXS J020928.9 283243 and 1RXS J042332.8 745300. The low-dispersion spectra suggest the cataclysmic variable classification for the two objects.Further photometric observations are expected to reveal the variable features and to confirm the classifications. 相似文献
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Doron Lemze Sharon Sadeh Yoel Rephaeli 《Monthly notices of the Royal Astronomical Society》2009,397(4):1876-1884
Extensive measurements of the X-ray background (XRB) yield a reasonably reliable characterization of its basic properties. Having resolved most of the cosmic XRB into discrete sources, the levels and spectral shapes of its main components can be used to probe both the source populations and also alternative cosmological and large-scale structure models. Recent observations of clusters seem to provide evidence that clusters formed earlier and are more abundant than predicted in the standard Λ cold dark matter (ΛCDM) model. This motivates interest in alternative models that predict enhanced power on cluster scales. We calculate predicted levels and spectra of the superposed emission from groups and clusters of galaxies in ΛCDM and in two viable alternative non-Gaussian (χ2 ) and early dark energy models. The predicted levels of the contribution of clusters to the XRB in the non-Gaussian models exceed the measured level at low energies and levels of the residual XRB in the 2–8 keV band; these particular models are essentially ruled out. Our work demonstrates the diagnostic value of the integrated X-ray emission from clusters, by considering also its dependences on different metallicities, gas and temperature profiles, Galactic absorption, merger scenarios and on a non-thermal pressure component. We also show that the XRB can be used for an upper limit for the concentration parameter value. 相似文献
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