The defining characteristics of intermediate polars – the case of three-candidate systems |
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Authors: | Gavin Ramsay Peter J Wheatley A J Norton Pasi Hakala Darren Baskill |
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Institution: | Armagh Observatory, College Hill, Armagh BT61 9DG;Department of Physics, University of Warwick, Coventry CV4 7AL;Department of Physics &Astronomy, The Open University, Milton Keynes MK7 6AA;Tuorla Observatory, University of Turku, Väisäläntie 20, FIN-21500 Piikkiö, Finland;Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH;Royal Observatory Greenwich, National Maritime Museum, Park Row, Greenwich, London SE10 9NF |
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Abstract: | 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. |
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Keywords: | binaries: close stars: individual: EI UMa stars: individual: LS Peg stars: individual: V426 Oph novae cataclysmic variables X-rays: binaries |
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