The result of the analysis identifies novae as a new type of very high-energy gamma-ray source. This event is the first one detected at these energies and allows a better understanding of these class of eruptions and their potential role in the production of the mysterious cosmic rays that inhabit the Milky Way.

A new type of very high energy gamma ray source

The end of a star after its death depends on its mass. In about five billion years, when the Sun dies down, it will expand into a Red Giant star, then collapse into a stellar corpse known as a white dwarf. These stellar remnants are very dense and under certain circumstances can produce large explosions. In binary systems in which the white dwarf has a red giant star as a companion, hydrogen from the outer layers of the Red Giant can succumb to the gravitational pull of the white dwarf and accumulate on its surface. This “vampirism” of a white dwarf on a star in active phase results in a nuclear explosion on its surface, which causes it to expel most of the hydrogen and the products of fusion into interstellar space, at speeds of about two at four thousand kilometers per second. This type of explosion is very bright, and can be up to 100,000 times brighter than our Sun and are known as “nova”. If the cycle of material transfer between the two stars begins again, it can restart the process and will eventually lead again to an explosion in the systems known as recurrent.

When the RS Ophiuchi nova explosion alert was received on August 8, 2021, an extensive tracking device was activated. “The RS Oph eruption is a very rare event in the gamma-rays: it is the most luminous nova and with the highest flux detected in gamma rays to date, and we observed it just in time”, says Rubén López-Coto, researcher at the INFN Padova and the IAA-CSIC of Granada, one of the main authors of the result. A series of observations followed, making this the first nova detected over such a wide energy range both from Earth and from space. On August 9, the MAGIC collaboration used its twin Cherenkov telescope system, located at the Roque de los Muchachos Observatory on the island of La Palma, to look in the direction of RS Oph, detecting the source.

Thanks to the excellent observing conditions on La Palma, the fast reaction of the collaboration and the high sensitivity of MAGIC, the nova could be detected at energies one hundred billion times higher than visible light. “This work has identified novae as a new type of very high-energy gamma-ray source. Therefore, a new line of research has been opened in very high energy gamma ray astronomy”, adds Alicia López-Oramas, researcher at the IAC, and also one of the main authors of this work.

Novas, cosmic proton accelerators

Together with data from other wavelengths, the research team was able to uncover a new fact: the nova explosion was energetic enough to produce strong shock waves in the medium surrounding the star system. These shock waves are responsible for accelerating the small subatomic particles present in the interstellar medium to speeds close to those of light. In the case of nova RS Ophiuchi 2021, the model that best describes the observations from MAGIC and other telescopes says that very high-energy gamma rays are produced by protons, positively charged particles that make up the nuclei of hydrogen atoms. Although novae eruptions are less energetic than their cousins ​​supernovae, in which a star dies in a catastrophic explosion, they are also much more frequent. The results obtained by the MAGIC collaboration group and their colleagues indicate that, although most of the cosmic rays that permeate the Milky Way are generated from other sources, novae can be surprisingly efficient at accelerating protons in their vicinity.

“MAGIC has been unsuccessfully tracking nova explosions for some time. It is gratifying when you see that the effort is worth it and we manage to open new windows that bring a deeper knowledge of our Universe”, points out Oscar Blanch, spokesperson for the MAGIC collaboration. “It is the fruit of the work of many people” he concludes.

To fully understand the complicated relationship between violent events in the interstellar medium in the Milky Way, more observations like those presented in this article will be necessary. The MAGIC collaboration will continue the celestial surveillance of stellar remnants in the Milky Way and other galaxies.

The Spanish community has participated in MAGIC since its inception. Currently members of MAGIC are: the Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), the Instituto de Astrofísica de Canarias (IAC), the Institut de Física d’Altes Energies (IFAE), the Universidad Autónoma de Barcelona (UAB), the Instituto de Ciencias del Cosmos de la Universidad de Barcelona (ICCUB), the Universidad Complutense de Madrid (UCM) and thel Instituto de Astrofísica de Andalucía (IAA). The Institut d’Estudis Espacials de Catalunya (IEEC) participates in this project through researchers from the ICCUB units and the Centre d’Estudis i Recerca Espacials (CERES-UAB). In addition, the MAGIC data center is the Port d’Informació Científica (PIC), a collaboration between IFAE and CIEMAT.