Kavli Affiliate: Scott A. Hughes
| First 5 Authors: Joheen Chakraborty, Kevin B. Burdge, Saul A. Rappaport, James Munday, Hai-Liang Chen
| Summary:
We report the discovery of three ultracompact binary white dwarf systems
hosting accretion disks, with orbital periods of 7.95, 8.68, and 13.15 minutes.
This significantly augments the population of mass-transferring binaries at the
shortest periods, and provides the first evidence that accretors in
ultracompacts can be dense enough to host accretion disks even below 10 minutes
(where previously only direct-impact accretors were known). In the two
shortest-period systems, we measured changes in the orbital periods driven by
the combined effect of gravitational wave emission and mass transfer; we find
$dot{P}$ is negative in one case, and positive in the other. This is only the
second system measured with a positive $dot{P}$, and it the most compact
binary known that has survived a period minimum. Using these systems as
examples, we show how the measurement of $dot{P}$ is a powerful tool in
constraining the physical properties of binaries, e.g. the mass and mass-radius
relation of the donor stars. We find that the chirp masses of ultracompact
binaries at these periods seem to cluster around $mathcal{M}_c sim 0.3
M_odot$, perhaps suggesting a common origin for these systems or a selection
bias in electromagnetic discoveries. Our new systems are among the
highest-amplitude known gravitational wave sources in the millihertz regime,
providing exquisite opportunity for multi-messenger study with future
space-based observatories such as textit{LISA} and TianQin; we discuss how
such systems provide fascinating laboratories to study the unique regime where
the accretion process is mediated by gravitational waves.
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