An article published in the journal “Nature Astronomy” reports a study of the recurrent nova known as RS Ophiuchi based on the detection of the gamma rays emitted by the last outburst that occurred on the white dwarf part of the pair. MAGIC Collaboration researchers examined data collected by the MAGIC telescopes and concluded that outbursts of that type generate some of the gamma rays present in the Milky Way.

About 8,000 light-years from Earth, RS Ophiuchi is a binary system consisting of a white dwarf and a companion that is likely a red giant. Despite its small size, the white dwarf regularly steals gas from its companion and, at intervals between 15 and 20 years, the amount of gas accumulated on its surface reaches a critical mass that triggers a colossal thermonuclear explosion. That’s what is called a recurrent nova precisely because this is one of the systems in which that process is repeating.

The first outburst in the RS Ophiuchi system was observed in photographic plates taken in 1898 and the subsequent ones were observed with increasingly powerful and sensitive instruments. The outbursts in the RS Ophiuchi system aren’t regular but it’s kept under observation in order to capture each new event. On August 8, 2021, NASA’s Fermi Space Telescope and the High Energy Stereoscopic System (HESS) telescope system in Namibia detected a new outburst. In March 2022, the HESS Collaboration published an article describing the results of examining those findings in the journal “Science”.

The various studies conducted on this new outburst offered interesting results. RS Ophiuchi seems to be a particular case of recurrent nova due to the power of the outburst, which caused out-of-the-ordinary consequences compared to other novae observed in the past.

The HESS Collaboration observed a particle acceleration following the nova at energies hundreds of times higher than those observed in other novae. The shockwave generated by the colossal thermonuclear explosion on the white dwarf’s surface is at the origin of that process, compared to a cosmic particle accelerator. In RS Ophiuchi, the speed of the particles reached the theoretical limits calculated in the models. This suggests that similar processes could also take place in supernovae.

The MAGIC Collaboration also used data collected from other instruments of various types in other bands of the electromagnetic spectrum. However, gamma rays are at the center of their study and the analysis of those emissions led to the conclusion that novae are among the sources of gamma rays present in the Milky Way and that they are emitted by very energetic protons.

Gamma-ray emissions are due to the shockwave that accelerates particles into interstellar space. In this case, the conclusion of the MAGIC Collaboration is that novae are also sources of cosmic rays. However, novae are local sources only, in relatively limited areas of space compared to supernovae, which are far bigger sources of cosmic rays.

These observations of the outburst that occurred in the RS Ophiuchi system in 2021 have a much higher quality than those of previous outbursts. This made it possible to conduct much more in-depth and precise studies that showed for the first time the processes taking place during that type of event.

Not everything was clarified and studies are still ongoing to examine data collected by a number of instruments in various bands of the electromagnetic spectrum. For example, it remains to be ascertained that the companion “robbed” of its hydrogen is indeed a red giant and if its nature has any consequences on the power of the outbursts.

Another piece of data to ascertain concerns the white dwarf’s mass, which is important because the estimates place it close to the limit for this type of star remnant. That means that if it steals enough gas from its companion, it will collapse further and turn into a neutron star.

The instruments useful to study novae continue to be improved, so the next outburst in the RS Ophiuchi system will be studied in higher quality detail. Meanwhile, there are other recurrent novae to study in various types of astronomical research.