Sandra Baumgartner
Compact Galactic binary systems with orbital periods of a few hours are expected to be detected in gravitational waves (GW) by LISA or a similar mission. At present, these so-called verification binaries provide predictions for GW frequency and amplitude. A full polarisation prediction would provide a new method to calibrate LISA and other GW observatories, but requires resolving the orientation of the binary on the sky, which is not currently possible. We suggest a method to determine the elusive binary orientation and hence predict the GW polarisation, using km-scale optical intensity interferometry. The most promising candidate is CD–30◦ 11223, consisting of a hot helium subdwarf with 𝑚𝐵 = 12 and a much fainter white dwarf companion, in a nearly edge-on orbit with period 70.5 min. We estimate that the brighter star is tidally stretched by 6%. Resolving the tidal stretching would provide the binary orientation. The resolution needed is far beyond any current instrument, but not beyond current technology. We consider scenarios where an array of telescopes with km- scale baselines and/or the Very Large Telescope (VLT) and Extremely Large Telescope (ELT) are equipped with recently-developed kilo-pixel sub-ns single-photon counters and used for intensity interferometry. We estimate that a team-up of the VLT and ELT could measure the orientation to ±1◦ at 2𝜎 confidence in 24 hours of observation.
But even the largest and brightest LISA verification binary, namely CD -30° 11223, which consists of a hot helium subdwarf and a faint white dwarf, is too small on the sky to resolve the position angle with a direct observation.
Here we propose a novel approach to the problem: intensity interferometry. This technique is based on the Hanbury Brown and Twiss effect, which is that the arrival times of photons from a single source at two distant telescopes in a narrow wavelength band are correlated. The correlation is basically the spatial Fourier transform of the source distribution. In CD -30° 11223 only one star in the binary is bright, but the fainter star has the effect of tidally elongating the brighter star. We suggest that the Very Large Telescope and the Extremely Large Telescope operating together as an intensity interferometer with a 23 km baseline could resolve the direction of the elongation, and hence the position angle of the orbit. A new generation of multi-channel single-photon counters now being developed would make it feasible to obtain sufficient signal-to-noise within a reasonable observation time.