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Gaia22ayj is a magnetic accreting white dwarf, astronomers find

An international team of astronomers has conducted multiwavelength observations of a variable star designated Gaia22ayj. Results of the observation campaign indicate that this star is a magnetic accreting white dwarf. The findings are published Jan. 2 on the pre-print server arXiv.

Detecting and studying variable stars could offer important insights into aspects of stellar structure and evolution. Investigation of variables could also be helpful for a better understanding of the distance scale of the universe.

Cataclysmic variables (CVs) are binary star systems comprising a white dwarf (WD) and a normal star companion. They irregularly increase in brightness by a large factor, then drop back down to a quiescent state.

Polars are a subclass of cataclysmic variables distinguished from other CVs by the presence of a very strong magnetic field in their white dwarfs, while in intermediate polars (IPs) this magnetic field is weaker. Observations show that in polars, the WD spin is typically locked with the orbital period, while in IPs, the WD spins about 10–100 times faster than the orbital period.

Gaia22ayj was first identified in March 2022, at a distance of about 8,150 light years, when it underwent an optical outburst. Initial studies of this object suggested that it is a WD binary, based on the detected period of 9.36 minutes, which was proposed to be the orbital period of the system.

However, now, in a new study conducted by a group of astronomers, Antonio C. Rodriguez of the California Institute of Technology (Caltech) points to a rather different scenario.

"We present multiwavelength data to show that the 9.36-min period in Gaia22ayj is not the orbital period of a binary system, rather it is likely the WD spin (or spin-orbit beat) period of an accreting magnetic WD. In this sense, Gaia22ayj is like an IP, but its large photometric amplitude and spectroscopic modulation are reminiscent of polars, therefore suggesting this is, at the very minimum, a new empirically-defined subclass of magnetic CVs," the researchers explain.

The study found that Gaia22ayj pulsates at optical and near-infrared wavelengths on a 9.36-minute period. At optical wavelengths, the source pulsates at extreme levels, changing in brightness by a factor of 7.5–10 within the span of 2.5 minutes. The astronomers assume that broadband spectral modulation, reminiscent of cyclotron emission in polars, is responsible for such behavior.

The observations show that the 9.36-minute period accompanies a high amplitude (of about 2.0 mag) modulation. The researchers associate this period with a WD spin or spin-orbit beat period, not an orbital period as was previously thought. They add that the object is rapidly spinning down like WD pulsars.

Moreover, the study found that Gaia22ayj is a luminous X-ray source, with a luminosity at a level 270 nonillion erg/s in the 0.3–8 keV band, therefore comparable to most IPs and the most luminous polars. It also turned out that its linear polarization reaches 40%, which is a clear sign of magnetism.

The researchers add that the outburst of Gaia22ayj resembles those from IPs. This, together with broad double-peaked Balmer and helium emission lines, indicate ongoing accretion—like in polars.

All in all, the authors of the paper conclude that Gaia22ayj is magnetic accreting WD sharing properties of both WD pulsars and polars. Thus, the findings suggest that this system may be perceived as a missing link between the two classes of magnetic WD binaries.