There is a a strange thing which strong radio signals arrive frequently from spaceyet their source is still not completely known. Known as “long-period radio transmissions” (LPTs), these events are considered as the radio crackles which repeats at intervals ranging from several minutes to several hours. Only a dozen or so examples have been discovered within The Milky Wayand their physical nature has long remained a mystery.
Previous research has suggested that candidate sources of LPT include neutron stars known as magnets, which rotate very slowly, and the binary systems that comprise them white dwarfs and his co-stars. However, the magnetar hypothesis faces the problem of contradicting existing theoretical models.
On the other hand, while a few cases suggesting a white-dwarf binary merger have been reported, there have been no cases where the accretion process was directly confirmed to be occurring.
Due to this situation, an international research team led by the University of Sydney in Australia conducted a space survey using the Australian Square Kilometer Array Pathfinder (ASKAP) radio telescope and determined the true nature of a strange object called ASKAP J174508.9-505149. The results of this investigation it is 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 identified the nature of these signals,” he said Kovi Rose, a doctoral student at the University of Sydney’s School of Physics and the Commonwealth Scientific and Industrial Research Organisation, in a press release. “We have been able to show that the source of one of these short periods comes from a white dwarf pulling in material from its companion star.”
White Dwarf and Companion Star
Rose and his research team confirmed through spectroscopic observations that ASKAP J1745-5051 shows hydrogen emission lines (Balmer series) and helium emission lines (HeI and HeII). In particular, the strong HeII emission line is known as the optical characteristic of “catalyst magnetic variables.”
Catastrophes are a general term for closed binary systems in which a white dwarf has accreted matter from a companion star. Of these, those in which the dwarf star has a strong magnetic field and gas fields along the magnetic field lines are called “catastrophic magnetic variables.”
Furthermore, analysis of the radial velocities of the emission lines of the Balmer series revealed that the orbital period of this binary system is about 1.368 hours, which was confirmed to be consistent with the frequency of radio pulses, about 1.345 hours. Additionally, based on the orbital period, the mass of the senior star was estimated to be about 0.096 solar times, and its diameter about 0.13 solar times, indicating that it corresponds to an M6 class red dwarf.
In other words, ASKAP J1745-5051 is a binary system where a white dwarf and a red dwarf orbit each other at very close distances. A white dwarf is the remnant of a supermassive star that has reached the end of its life; although it is the size of the Earth, its mass is comparable to that of the sun. Its companion, a red dwarf, is larger but less dense, with a mass about one-tenth that of the Sun. The two stars orbit each other in less than an hour.
Two Secrets Revealed by Radio Waves and X-Rays
These studies have revealed that radio bursts and x-ray emissions are produced by different mechanisms. When a white dwarf accrets gas from its companion, the gas is heated 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 radio and x-ray emission peaks do not coincide, it is believed that they are produced at different locations within the system.
As for x-rays, data from the Chinese Academy of Sciences’ Einstein Probe observational satellite revealed radiation with a duration of about 1.32 hours. According to the researchers, the large amplitude of the x-ray fluctuations indicates that the accretion rate on the white dwarf can change over time.
