We present recent results of quiescent X-ray observations of recurrent novae (RNe) and related objects. Several RNe are luminous hard X-ray sources in quiescence, consistent with accretion onto a near Chandrasekhar mass white dwarf. Detection of similar hard X-ray emissions in old novae and other cataclysmic variables may lead to identification of additional RN candidates. On the other hand, other RNe are found to be comparatively hard X-ray faint. We present several scenarios that may explain this dichotomy, which should be explored further.
By definition, a recurrent nova (RN) has been seen to undergo multiple episodes of thermonuclear runaway within the last century or so. For the hydrogen-rich envelope to reach the high temperature and density required for a runaway in such a short period, the white dwarf must be massive and its accretion rate must be high. Although only ten Galactic RNe are currently known, the true number of RNe is likely to be much larger, considering the low discovery probability of nova outbursts (Schaefer 2009). Two important goals for RN observers in the context of Type Ia progenitors therefore are (i) observational determination of white dwarf mass and accretion rate; and (ii) search for hitherto undiscovered or unrecognized RNe.
In non magnetic cataclysmic variables (CVs) and symbiotic stars, X-rays are emitted in the boundary layer between the disk and the white dwarf. Optically thin boundary layers predominantly emit hard X-rays; even optically thick boundary layers are seen to retain some hard X-ray flux, presumably because the surface layer remains optically thin (Patterson & Raymond 1985). These hard X-rays are multi-temperature plasma emission whose maximum temperature is strongly constrained by the depth of the gravitational potential, i.e., the white dwarf mass. Thus, hard X-ray observations may be a viable alternative method to optical and UV spectroscopy in our study of white dwarf masses, particularly in cases of high interstellar extinction.
If the Keplerian flow just above the white dwarf surface is strongly shocked, then the shock temperature is half of the free-fall case, well known in the studies of magnetic CVs.
Figure 1. The expected maximum temperature of the optically thin X-rays for the magnetic (dashed line) and non-magnetic (solid) cases. The lower limit of 34 keV for V2487 Oph corresponds to 0.77 M⊙ (magnetic) or 1.08 M⊙ (non-magnetic) white dwarf. The best-fit temperature of 70 keV corresponds to 1.01 M⊙ and 1.33 M⊙, respectively.
Multiple groups have used X-ray spectroscopy to infer the white dwarf mass in magnetic CVs; the study of quiescent X-rays from dwarf novae (Byckling et al. 2010) suggests that this is also possible for non-magnetic CVs. One complication is that the hard X-ray emission of a dwarf nova usually becomes fainter and softer during outburst when a part of the boundary layer becomes optically thick (see below for a possible reason). Even so, the maximum temperature derived for the hard component sets a firm lower limit for the white dwarf mass.
In recent years, four symbiotic stars have been detected as luminous hard X-ray sources in the Swift BAT and INTEGRAL surveys Kennea et al. (2009). One of the four is T CrB, with a 15-150 keV luminosity of 7×10 33 × [d/1kpc] 2 erg s -1 . The current generation of hard X-ray all-sky surveys have a detection limit of order ∼ 10 -11 ergs cm -2 s -1 , or ∼ 10 33 erg s -1 at 1 kpc, below which many more hard X-ray bright symbiotics are likely to exist. In fact, Luna et al. (2010) have significantly increased the number of known hard X-ray sources among symbiotic stars through pointed observations with Swift XRT. Kennea et al. (2009) made the case that T CrB contains a near Chandrasekhar mass, non-magnetic white dwarf using a Bremsstrahlung fit to the BAT spectrum available at the time. We have updated their argument as follows. We fit the BAT spectrum below 100 keV from the Swift BAT 58-month survey with a cooling flow model, and obtain the maximum temperature of kT max = 46±6 keV. If the emission is from an optically thin boundary layer, it implies a white dwarf mass of M wd =1.2 M ⊙ . More likely, a significant portion of the boundary layer in T CrB is optically thick, given that the high UV luminosity (Selvelli et al. 1992). If this has resulted in the reduction of kT max by a factor of 1.7, as it does in SS Cyg in outburst, then M wd =1.35 M ⊙ in T CrB.
V2487 Oph is the first nova for with a pre-outburst X-ray detection (Hernanz & Sala 2002). It is also a hard X-ray source detected in the INTEGRAL and Swift BAT surveys. Although this is a CV and not a symbiotic system (the mass donor in V2487 Oph is not a red giant), its X-ray spectrum can be compared to that of T CrB. Using the BAT 58-month survey data and a cooling flow model, the best fit kT max is 70 keV; the 90% lower limit is 34 keV, while the upper limit cannot be constrained due primarily to the limited grid of plasma models available within the cooling flow model. For a distance of 12 kpc (Schaefer 2010), its 2-10 keV luminosity is close to 10 35 ergs s -1 . In contrast, intermediate polars (IPs), the most hard X-ray luminous subclass of magnetic CVs, usually do not exceed 10 34 ergs s -1 . V2487 Oph was established as an RN by Pagnotta et al. (2009), who discovered its 1900 outburst in photographic plates.
V2487 Oph is often considered a candidate IP, based on its X-ray appearances. However, the signature of a spin period, a key defining characteristic of IPs, has ne
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