12 AM CVn-Type Stars Detectable in GWR and Electromagnetic Spectrum

Figure 17View Image [287Jump To The Next Citation Point] shows the distributions vs. orbital periods for the total number of AM CVn systems with P ≤ 1500 s and for AM CVn LISA sources that have optical and/or X-ray counterparts. The interrelations between numbers of sources emitting in different wavebands are shown in the legend to the right of the figure. Out of 11,000 systems detectable in GWR, 2,060 are expected to be in the direct-impact (DI) stage and only 325 in the mass-transfer via disk stage. Thus, the majority of the DI systems are expected to be detectable in GWR; some 5% of DI systems are expected to emit X-rays. There are 1,336 systems detectable in the optical waveband and 326 in X-rays; 106 members of the latter samples may be detected in both spectral ranges.

View Image

Figure 17: Short-period AM CVn systems, subdivided in different types. Each panel shows the total population as the white histogram. The top left panel shows 11,000 systems that can be resolved by LISA in gray, and they are subdivided into the ones that have optical counterparts (GWR + Opt), X-ray counterparts (GWR + X), and both (GWR + Opt + X). The top right panel shows the systems that are in the direct impact phase of accretion in gray, and they are subdivided in GWR and X-ray sources. The bottom two panels show (again in gray) the populations that are detectable in the optical band (left panel) and the X-ray band (right panel). The distribution of sources detectable both in optical and X-ray bands is shown as hatched bins in both lower panels (Opt + X). (Figure from [287Jump To The Next Citation Point].)

An additional piece of information may be obtained from eclipsing AM CVn-stars: They would provide radii of the components and orbital inclinations of the systems. A systematical study of the possibility of eclipses was never carried out, but an estimate for a “typical” system with initial masses of components (0.25 + 0.60)M ⊙ shows that the probability for eclipsing of the accretor is about 30% at P = 1000 s, and even higher for eclipsing (a part of) the accretion disc. The first detection of an eclipsing AM CVn-type star – SDSS J0926+3624 (Porb = 28.3 min) – was recently reported by Anderson et al. [8Jump To The Next Citation Point].

For WD + WD pairs detectable by LISA the prospects of optical identification are negligible, since for them cooling luminosity is the only source of emission. Most of the potentially detectable dwarfs are located close to the Galactic center and will be very faint. Estimates based on the model [287Jump To The Next Citation Point] predict for the bulk of them V ≈ 35 mag, with only 75 objects detectable with V < 25 mag. Even inclusion of brightening of the dwarfs close to contact under the assumption of efficient tidal heating [166] increases this number to ≈ 130 only (G. Nelemans, private communication).

In the discussion above, we considered the X-ray flux of AM CVn-type systems in the ROSAT waveband: 0.1 – 2.4 keV. It may be compared with the expected flux in the Chandra and XMM bands: 0.1 – 15 keV. Since most of the spectra of model AM CVn-stars are rather soft, the flux ℱ in the latter band is generally not much larger than in the ROSAT band: 80% of systems have ℱXMM ∕ℱROSAT < 1.5; 96% have ℱXMM ∕ℱROSAT < 3. However, Chandra and XMM have much higher sensitivity. For instance, the Chandra observations of the Galactic centre have a completeness limit of 3 × 10–15 erg cm–2 s–1, almost two orders of magnitude deeper than our assumed ROSAT limit [264]. The expected number of X-ray sources in the 0.1 – 15 keV band detectable down to 10–14 erg cm–2 s–1 is 644 and it is 1085 down to 10–15 erg cm–2 s–1. In the Chandra mosaic image of the Galactic centre [441], roughly down to 10–14 erg cm–2 s–1 there are ∼ 1,000 point sources, presumably associated with accreting white dwarfs, neutron stars, and black holes. Model [287] predicts 16 X-ray systems in this region.

 12.1 Effects of finite entropy
 12.2 Going further

  Go to previous page Go up Go to next page