NICER / ISS Science Nugget
for April 8, 2021

Dance of the White Dwarfs

HM Cancri (HM Cnc) is a binary star system comprising a pair of white dwarfs with the shortest orbital period of any known binary system, just 5.4 minutes. These stars are so close that the whole system is only about 100,000 km in size, and would easily fit between the Earth and Moon, with plenty of room to spare. The lower mass white dwarf, the donor star, probably weighing in at about 1/4 the mass of the Sun, is losing mass onto the heavier white dwarf, known as the accretor, which probably tips the scales at about half a solar mass. The accretion process is special in this system because the stars are so close. The stream of matter from the donor star impacts directly onto the accretor, forming a hot-spot that emits X-rays. As the two stars orbit each other, this spot comes in and out of view, producing an X-ray pulse once per period that can be measured with NICER. Because the stars are so close several interesting physical effects come into play. 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 this 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 component stars apart. NICER has been monitoring this object for a little more than three years, and has now found evidence that the accretion process is indeed also affecting the orbit. The new NICER data, combined with earlier Chandra measurements, show that the rate of decrease of the orbital period is slowing. This means that the rate of accretion is growing and that, eventually, it will start to push the stars further apart (increase the orbital period). The NICER data indicate that this will happen in about 1200 years. This is the expected behavior for the evolution of such a binary, but NICER's measurement represents the first time that such an effect has been observed directly.