Gamma-ray emission from a nova confirms a 2006 prediction

The burst of a recurring nova that was detected in gamma rays confirms predictions made in 2006.

The predictions were made by Margarita Hernanz and Vincent Tatischeff. Hernanz is a researcher at the Institute for Space Studies of Catalonia (IEEC) at the Institute of Space Sciences (ICE) attached to the Higher Council for Scientific Research (CSIC) in Spain. Tatischeff is from the IJCLab, attached to the University of Paris-Saclay in France.

RS Ophiuchi (RS Oph), located 5,000 light-years from Earth, is an unstable binary system consisting of a red giant star and a white dwarf star. Every 15 to 20 years, enough material from the red giant accumulates on the surface of the white dwarf to trigger a thermonuclear explosion. In astronomy, these stars in the exploding phase are known as novae, since the increase in light generated by the explosion made in the past, and sometimes also today, a star too dim to be seen from Earth with available technology, to appear. suddenly in the firmament, as if it were a “new” star (“nova” in Latin).

RS Oph outbursts have been recorded in 1898, 1933, 1958, 1967, 1985, 2006, and 2021. In fact, all novae are recurrent, but only a few have recurrence periods of less than 100 years.

Its last eruption, in 2021, was observed by the LAT (Large Area Telescope) telescopes of the Fermi mission and by the Cherenkov HESS telescope array in Namibia and MAGIC in the Canary Islands (Spain). This was the first time that, in the Milky Way, such an explosion had been observed in very high-energy gamma rays using ground-based telescopes.

Artist’s impression of a white dwarf star sucking matter from its companion star, a red giant. (Illustration: NASA/CXC/M.Weiss)

The outburst of RS Oph in 2021 confirms the predictions that Margarita Hernanz made in 2006 about the emission of very high energy gamma rays related to the acceleration of particles in this recurring nova. That study was published in the academic journal The Astrophysical Journal.

Particle acceleration was not considered feasible in novae until the RS Oph observations, so it has become a key object for studying the microphysics of particle acceleration in astrophysical shock waves.

In the 2006 study, co-authored by Hernanz with V. Tatischeff, analyzes were made of observations of the blast with radio telescopes and infrared and X-ray telescopes, and the shock wave generated by the explosion that struck thousands kilometers per second to the wind of the red giant star.

Hernanz and Tatischeff then predicted an acceleration of protons in this shock wave to energies of several teraelectronvolts (TeV) and a potentially observable gamma emission for several days after the explosion. This was the first prediction of such a process in a nova explosion.

The researchers deduced from observations at all wavelengths in 2006 that particle acceleration had occurred in RS Oph as a consequence of strong collisions between the nova’s ejecta and the red giant’s wind. At that time, gamma rays could not be detected due to the absence of sufficiently sensitive gamma telescopes. Nonetheless, the observation of the RS Oph explosion in 2021 confirms the 2006 predictions very accurately.

The existing observations at different wavelengths of this nova explosion in 2006 were crucial to discover the acceleration of the particles (protons and electrons) and contributed to the study of the theory of diffusive acceleration in novae, previously applied only to explosions. of supernova. “The fact that the process is much faster in novae than in supernovae – because less mass is involved in novae explosions – will allow us to study these processes in more detail,” says Hernanz. (Source: IEEC / ICE / CSIC)