Figure 17 [287
] shows the distributions vs. orbital periods for the total number of AM CVn
systems with
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.
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
shows that the probability for eclipsing of the accretor is about 30% at
,
and even higher for eclipsing (a part of) the accretion disc. The first detection of an eclipsing AM CVn-type
star – SDSS J0926+3624 (
) – was recently reported by
Anderson et al. [8
].
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 [287] predict for the bulk of them
, with only 75 objects detectable with
. 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 in the latter
band is generally not much larger than in the ROSAT band: 80% of systems have
;
96% have
. 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.
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