Pulsars suggest that ultra–low-frequency gravitational waves are rippling through the cosmos. The signal seen by international pulsar timing array collaborations in 2023 could come from a stochastic gravitational-wave background—the sum of many distant sources—or from a single nearby binary of supermassive black holes. To tell these apart, Hideki Asada, theoretical physicist and Professor at Hirosaki University, and Shun Yamamoto, researcher at the Graduate School of Science and Technology, Hirosaki University, propose a method that exploits beat phenomena between gravitational waves at nearly the same frequency, searching for their imprint in the tiny shifts of pulsars’ radio-pulse arrival times. 

Kyoto, Japan --  White dwarfs are the compact remnants of stars that have stopped nuclear burning, a fate that will eventually befall our sun. These extremely dense objects are degenerate stars because their structure is counterintuitive: the heavier they are, the smaller they are.

White dwarfs often form binary systems, in which two stars orbit one another. The majority of these are ancient even by galactic standards, and have cooled to surface temperatures of about 4,000 degrees Kelvin. However, recent studies have revealed a class of short period binary systems in which the stars orbit each other faster than once per hour. Contrary to theoretical models, these stars are inflated to twice the size as expected due to surface temperatures of 10 to 30 thousand degrees Kelvin.

This inspired a team of researchers, led by Lucy Olivia McNeill of Kyoto University, to investigate the theory of tides and use it to predict the temperature increase of white dwarfs in short period binary orbits. Tidal forces often deform celestial bodies in binary orbits, determining their orbital evolution.