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Gavin Ramsay Peter J. Wheatley A. J. Norton Pasi Hakala Darren Baskill 《Monthly notices of the Royal Astronomical Society》2008,387(3):1157-1162
Intermediate polars (IPs) are a group of cataclysmic variables (CVs) which are thought to contain white dwarfs which have a magnetic field strength in the range ∼0.1–10 MG. A significant fraction of the X-ray sources detected in recent deep surveys has been postulated to consist of IPs. Until now two of the defining characteristics of IPs have been the presence of high (and complex) absorption in their X-ray spectra and the presence of a stable modulation in the X-ray light curve which is a signature of the spin period, or the beat period, of the accreting white dwarf. Three CVs, V426 Oph, EI UMa and LS Peg, have characteristics which are similar to IPs. However, there has been only tentative evidence for a coherent period in their X-ray light curve. We present the results of a search for coherent periods in XMM–Newton data of these sources using an autoregressive analysis which models the effects of red noise. We confirm the detection of a ∼760 s period in the soft X-ray light curve of EI UMa reported by Reimer et al. and agree that this represents the spin period. We also find evidence for peaks in the power spectrum of each source in the range 100–200 s which are just above the 3σ confidence level. We do not believe that they represent genuine coherent modulations. However, their X-ray spectra are very similar to those of known IPs. We believe that all three CVs are bona fide IPs. We speculate that V426 Oph and LS Peg do not show evidence for a spin period since they have closely aligned magnetic and spin axes. We discuss the implications that this has for the defining characteristics of IPs. 相似文献
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Analysis of the oscillations in HST observations of the quiescent SU UMa type dwarf nova WZ Sagittae
Warren Skidmore William F. Welsh Janet H. Wood M. S. Catalán Keith Horne 《Monthly notices of the Royal Astronomical Society》1999,310(3):750-758
An analysis of the UV oscillations in WZ Sge is presented, in which we obtain the oscillation amplitude spectra. We find a strong 27.9-s oscillation in our Hubble Space Telescope ( HST ) UV and zeroth-order light curves as well as weaker oscillations at 28.4 s in the UV and 29.1 s in the zeroth order. We find that the main oscillation amplitude spectrum can be fitted with static white dwarf spectra of about 17 000 K, an accretion hotspot of only a few 100 K hotter than the underlying white dwarf temperature or a variety of cool (<14 500 K) white dwarf pulsation amplitude spectra. A pulsating white dwarf can also explain the very blue colour of oscillations of different periods previously found in the optical. Comparing our results with those of Welsh et al., we see that the amplitude spectra of the main oscillations in WZ Sge measured with different periods in data sets from different epochs are similar to each other. Our results raise questions about using the magnetically accreting rotating white dwarf model to explain the oscillations. We suggest that the pulsating white dwarf model is still a viable explanation for the oscillations in WZ Sge. 相似文献
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G. Beekman M. Somers T. Naylor C. Hellier 《Monthly notices of the Royal Astronomical Society》2000,318(1):9-17
We present red spectra in the region ∼ λ 7000–8300 Å of the eclipsing dwarf nova IP Peg, with simultaneous narrow-band photometry centred at 7322 Å. We show that by placing a second star on the slit we can correct for the telluric absorption bands which have hitherto made the TiO features from the secondary star unusable. We use these TiO features to carry out a radial velocity study of the secondary star, and find this gives an improvement in the signal-to-noise ratio of a factor of 2 compared with using the Na i doublet. In contrast with previous results, we find no apparent ellipticity in the radial velocity curve. As a result we revise the semi-amplitude to K 2 =331.3±5.8 km s−1 , and thus the primary and secondary star masses to 1.05-0.07 +0.14 M⊙ and 0.33-0.05 +0.14 M⊙ respectively. Although this is the lowest mass yet derived for the secondary star, it is still overmassive for its observed spectral type. However, the revised mass and radius bring IP Peg into line with other cataclysmic variables in the mass–radius–period relationships.
By fitting the phase-resolved spectra around the TiO bands to a mean spectrum, we attempt to isolate the light curve of the secondary star. The resulting light curve has marked deviations from the expected ellipsoidal shape. The largest difference is at phase 0.5, and can be explained as an eclipse of the secondary star by the disc, indicating that the disc is optically thick when viewed at high inclination angles. 相似文献
By fitting the phase-resolved spectra around the TiO bands to a mean spectrum, we attempt to isolate the light curve of the secondary star. The resulting light curve has marked deviations from the expected ellipsoidal shape. The largest difference is at phase 0.5, and can be explained as an eclipse of the secondary star by the disc, indicating that the disc is optically thick when viewed at high inclination angles. 相似文献
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G. H. A. Roelofs G. Nelemans P. J. Groot 《Monthly notices of the Royal Astronomical Society》2007,382(2):685-692
The AM Canum Venaticorum (AM CVn) stars are rare interacting white dwarf binaries, whose formation and evolution are still poorly known. The Sloan Digital Sky Survey provides, for the first time, a sample of six AM CVn stars (out of a total population of 18) that are sufficiently homogeneous that we can start to study the population in some detail.
We use the Sloan sample to 'calibrate' theoretical population synthesis models for the space density of AM CVn stars. We consider optimistic and pessimistic models for different theoretical formation channels, which yield predictions for the local space density that are more than two orders of magnitude apart. When calibrated with the observations, all models give a local space density ρ0 = 1–3 × 10−6 pc−3 , which is lower than expected.
We discuss the implications for the formation of AM CVn stars, and conclude that at least one of the dominant formation channels (the double-degenerate channel) has to be suppressed relative to the optimistic models. In the framework of the current models this suggests that the mass transfer between white dwarfs usually cannot be stabilized. We furthermore discuss evolutionary effects that have so far not been considered in population synthesis models, but which could be of influence for the observed population. We finish by remarking that, with our lower space density, the expected number of Galactic AM CVn stars resolvable by gravitational-wave detectors like the Laser Interferometer Space Antenna ( LISA ) should be lowered from current estimates, to about 1000 for a mission duration of 1 yr. 相似文献
We use the Sloan sample to 'calibrate' theoretical population synthesis models for the space density of AM CVn stars. We consider optimistic and pessimistic models for different theoretical formation channels, which yield predictions for the local space density that are more than two orders of magnitude apart. When calibrated with the observations, all models give a local space density ρ
We discuss the implications for the formation of AM CVn stars, and conclude that at least one of the dominant formation channels (the double-degenerate channel) has to be suppressed relative to the optimistic models. In the framework of the current models this suggests that the mass transfer between white dwarfs usually cannot be stabilized. We furthermore discuss evolutionary effects that have so far not been considered in population synthesis models, but which could be of influence for the observed population. We finish by remarking that, with our lower space density, the expected number of Galactic AM CVn stars resolvable by gravitational-wave detectors like the Laser Interferometer Space Antenna ( LISA ) should be lowered from current estimates, to about 1000 for a mission duration of 1 yr. 相似文献
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L. Morales-Rueda M.D. Still P. Roche J.H. Wood J.J. Lockley 《Monthly notices of the Royal Astronomical Society》2002,329(3):597-604
We study the absorption lines present in the spectra of the long-period cataclysmic variable GK Per during its quiescent state, which are associated with the secondary star. By comparing quiescent data with outburst spectra we infer that the donor star appears identical during the two states and the inner face of the secondary star is not noticeably irradiated by flux from the accreting regions. We obtain new values for the radial velocity semi-amplitude of the secondary star, , a projected rotational velocity, and consequently a measurement of the stellar mass ratio of GK Per, . The inferred white dwarf radial velocities are greater than those measured traditionally using the wings of Doppler-broadened emission lines suspected to originate in an accretion disc, highlighting the unsuitability of emission lines for mass determinations in cataclysmic variables. We determine mass limits for both components in the binary, and . 相似文献