Previous research has suggested that candidates for the source of the LPT include neutron stars known as magnetars, which rotate extremely slowly, and binary systems consisting of white dwarfs with companion stars. However, the magnetar hypothesis faces the problem of contradicting existing theoretical models.
On the other hand, while there have been some cases suggesting an association with white dwarf binaries, there have been no cases in which the accretion process has actually been confirmed to actually occur.
In this backdrop, an international research team led by the University of Sydney in Australia conducted a sky-survey using the Australian Square Kilometer Array Pathfinder (ASKAP) radio telescope and identified the true nature of a mysterious object named ASKAP J174508.9-505149. These observational results are said to be the strongest evidence to date pointing to LPT as one of the sources of this phenomenon.
“For the first time we have discovered the origin of these signals,” Covey Rose, a doctoral student at the University of Sydney’s School of Physics and the Commonwealth Scientific and Industrial Research Organisation, said in a press release. “We have been able to show that the source of one of these transients comes from a white dwarf that is actively drawing material from a companion star.”
A white dwarf and a companion star
Rose and his research team confirmed through spectroscopic observations that ASKAP J1745-5051 exhibits hydrogen emission lines (Balmer series) and helium emission lines (HeI and HeII). In particular, the strong HeII emission line is known to be an optical characteristic feature of “magnetic cataclysmic variables”.
Cataclysmic variable is a general term for close binary systems in which a white dwarf collects material from a companion star. Of these, those in which the white dwarf has a strong magnetic field and gas is clustered along magnetic field lines are called “magnetic cataclysmic variables”.
Furthermore, analysis of the radial velocities of the Balmer series emission lines revealed that this binary system has an orbital period of approximately 1.368 hours, which was confirmed to match the recurrence period of the radio pulses, approximately 1.345 hours. Furthermore, based on the orbital period, the mass of the companion star was estimated to be about 0.096 times that of the Sun, and its radius was about 0.13 times that of the Sun, indicating that it corresponds to an M6-class red dwarf.
In other words, ASKAP J1745-5051 is a binary system consisting of a white dwarf and a red dwarf orbiting each other at extremely close distances. A white dwarf is the high-density remnant of a star that has reached the end of its life; Although it is the size of Earth, its mass is comparable to that of the Sun. Its companion, the red dwarf, is larger but less dense, with a mass only one-tenth that of the Sun. Both stars revolve around each other in a short period of just over an hour.
Double mystery revealed by radio waves and X-rays
These observations have shown that radio bursts and X-ray emissions are produced by different mechanisms. When the white dwarf collects gas from its companion, that gas heats up and emits X-rays. At the same time, powerful radio bursts occur in the region where the magnetic fields of the two stars interact. However, since the peaks of the radio and X-ray emissions do not match, they are thought to originate at different locations within the system.
Regarding X-rays, data from the Chinese Academy of Sciences’ Einstein Probe observation satellite revealed radiation with a period of about 1.32 hours. According to the researchers, the large amplitude of the X-ray fluctuations suggests that the accretion rate on the white dwarf is changing over time.
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