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Astronomers detect new ultracompact binary system with unusually bright, infrequent outbursts

An international team of astronomers reports the discovery of a new ultracompact binary of the AM CVn type exhibiting infrequent outbursts. The detection of the newfound system, designated TCP J07222683+6220548, was detailed in a paper May 27 on the arXiv preprint server.

Cataclysmic variables (CVs) are binary star systems that include a white dwarf (WD) and a normal star companion from which the WD accretes matter. Observations show that the brightness of CVs increases irregularly by a large factor, then returns to a quiescent state.

The so-called AM CVn stars (named after the star AM Canum Venaticorum) are a rare type of CV in which a WD accretes hydrogen-poor matter from a compact companion. These systems are helium-rich binaries, not showing traces of hydrogen in their spectra, with orbital periods ranging from five minutes to about an hour.

Now, a group of astronomers, led by Alexander Tarasenkov of the Russian Academy of Sciences, reports that on January 20, 2025, a new AM CVn was identified as part of the New Milky Way survey (NMW). The system was detected during an outburst lasting seven days, which was followed by multiple rebrightenings in January–March 2025. Its AM CVn nature was confirmed by follow-up observations.

According to the paper, TCP J07222683+6220548, or J0722, is located some 1,874 light years away. Its spectrum shows a blue continuum with prominent broad absorption lines of helium and lacks the Balmer hydrogen lines. Such spectrum is typical for an AM CVn system in an outburst.

The outburst of J0722 peaked at 12.45 mag, which makes it one of the brightest AM CVn outbursts ever observed. Moreover, the peak absolute magnitude of the outburst, at a level of 3.4 mag, suggests that the accretion disk in J0722 is viewed almost face-on, which maximizes its apparent brightness.

The study detected periodic modulation with a period of approximately 46.87 minutes in the light curve of J0722. The astronomers assume that this modulation represents positive superhumps—a type of photometric variation observed in some dwarf novae and related systems.

The researchers noted that the superhumps became clearly visible during the first re-brightening of J0722, which started on day 18 and ended on day 24 after the initial outburst. They added that no convincing change in the superhump period was detected.

Summing up the results, the authors of the study underlined that the overall shape and duration of the outburst of J0722 are similar to those found in long-period AM CVn systems. Their findings highlight the need for follow-up observations to identify new AM CVn binaries.

"Some outbursting AM CVn systems lacking such detailed follow-up may remain unrecognized among the newly discovered cataclysmic variable candidates," the scientists concluded.