NICER / ISS Science Nugget for October 11, 2018
Probing the Evolution of the Most Compact Binary Known
HM Cnc is a binary system consisting of two white-dwarf stars (a so-called "double degenerate" binary). It holds the record for the shortest orbital period known, at just 321.5 seconds. We see this 5.36-minute period as a large-amplitude brightness modulation in soft X-rays (see figure). The X-rays are likely from a hot-spot produced by direct-impact accretion: one of the stars is transferring mass to the other, but instead of forming a disk, the inflowing matter impacts the accreting star's surface directly
With such a short orbital period, HM Cnc is expected to be emitting gravitational radiation with a luminosity comparable to, or larger than, that in X-rays, and its orbital evolution should be strongly influenced by the angular momentum lost to gravitational radiation. Indeed, HM Cnc and similar binaries are likely to be among the first gravitational-wave sources detected by the Laser Interferometer Space Array (LISA), a future space-based gravitational-wave telescope. Past X-ray observations have shown that the orbital period is in fact decreasing – the stars are spiraling in toward each other – at a rate consistent with that expected for a detached system radiating gravitational waves; long-term accretion, however, should eventually drive the components apart. Evolutionary calculations suggest the inspiral phase can persist past initial contact, and we may be seeing this in the X-ray timing, but a longer temporal baseline is needed to test this hypothesis.
NICER observed HM Cnc in 2017 October and November, extending the long-term X-ray timing baseline to more than 16 years. Combined with prior measurements, this enables a new, sensitive probe of the system's orbital evolution. NICER clearly detects the 5.36 minute orbital period; the X-ray modulation is consistent with the trend derived from past observations, setting new constraints on the long-term evolution of the orbit and our theoretical understanding of evolutionary scenarios in this and similar systems.