2022-05-30 10:43:34
In ancient times, the firmament surprised human beings from time to time by showing a new star, a “nova”, which challenged the belief in the immutability of the heavens. They were sporadic, unpredictable events that could arise at any time and place in the sky. Now, the study of those phenomena has made it possible to discover that some novae have recurring behaviors and increase their brightness dramatically from time to time. Most of these “symbiotic novae”, as they are known in astronomy, undergo repeated explosions in periods of hundreds or thousands of years, but a few follow shorter cycles, compatible with human life. Only a dozen of these “recurring novae” are known and one of them, called RS Ophiuchi, is the protagonist in this Talking with Scientists program.
Capturing the moment in which RS Ophiuchi exploded, releasing an enormous amount of energy, was an event that scientists had been waiting for more than 15 years, including our guests, Ruben López Coto, Fellini Researcher at the Institute of Nuclear Physics in Padova, Italy, and Alicia López Oramas, a researcher at the Instituto de Astrofísica de Canarias in the Astroparticles Group.
Alicia and Rubén belong to the corporation MAGIC and, together with a large international group of researchers, they have published the results of the RS Ophiuchi observations in the journal Nature Astronomy.
The firmament is very extensive and the number of objects to be observed is so great that many events would be missed if it were not for thousands of amateur astronomers who spend their nights scrutinizing the skies. On August 8, 2021, one of those fans, the Brazilian Alexandre Amorim, observed a region of the Ofiuco constellation with his telescope. To his surprise, a star that is normally magnitude 12, that is, impossible to see with the naked eye, began to brighten rapidly. Before long, it became a bright star, visible to the naked eye. He was not the only one to observe the phenomenon, other members of the American Variable Star Association (AAVSO) also saw it and raised the alarm to the scientific community.
For variable star hunters like Amorim and researchers like Rubén López-Coto and Alicia López Oramas, RS Ophouchi was an old acquaintance. Their bursts of energy have been observed for over a century. The first outburst for which data are available was observed in 1898 and since then it has experienced sudden increases in brightness in 1933, 1958, 1967, 1985 and 2006. An easy calculation reveals an average periodicity of 14.7 years, for that reason it was expected a new outbreak at any moment in 2021, although, of course, no one could know the exact moment when it would happen.
A recurring nova like RS Ophiuchi is actually made up of two stars. One, the one that undergoes these sudden eruptions, is a white dwarf, that is, the stellar remnant left behind by a solar-type star when it has consumed all its nuclear fuel and suffers a contraction such that it can accommodate a mass similar to that of the Sun in an object the size of the Earth. The other component is a red giant, that is, a star that after consuming its fuel expels its outer layers, greatly increasing its size. As both stars are very close, the material ejected by the red giant is being picked up by the companion white dwarf. This material accumulates in the outer layer of the star, which gradually increases its mass, until, as if it were an astronomical geyser, the star explodes, releasing an enormous amount of energy. This is how, every 15 years or so, enough material from the red giant accumulates on the surface of the white dwarf to cause a thermonuclear explosion.
The outbursts of a nova usually last weeks or months and are repeated over and over again with a frequency that depends on the masses of the stars that make up the system and the ability to exchange matter between them.
The detection of the burst in August 2021 put professional astronomers on alert and they quickly pointed their instruments towards RS Ophiuchi. In orbit, the Fermi space telescope detected the arrival of optical and high-energy gamma rays. On land, the members of the corporation MAGICincluding Alicia López Oramas and Rubén López-Coto, , also joined the observation.
MAGIC began RS Ophiuchi observations on August 9, 2021 at 22:27 UT, that is, approximately 1 day after the first detections. MAGIC It is made up of two reflecting telescopes each 17 meters in diameter, located 2,200 meters above sea level on the Canary Island of La Palma. MAGIC it detects very high energy gamma rays by studying the cascade of particles that these generate upon entering the Earth’s atmosphere. Thus, that day in August 2021, MAGIC captured the very high energy gamma ray emission from RS Ophiuchi.
Data obtained by the Fermi space telescope and MAGIC They have been studied by scientists and they interpret that during the explosion accelerated electrons and protons were generated which, when colliding with the matter ejected by the red giant, created a shock wave that was the origin of the detected gamma rays.
I invite you to listen to Ruben López Coto, Fellini Researcher at the Institute of Nuclear Physics in Padova, Italy, and Alicia López Oramas, researcher at the Instituto de Astrofísica de Canarias in the Astroparticles Group.
References:
Acciari, V.A., Ansoldi, S., Antonelli, L.A. et al. Proton acceleration in thermonuclear nova explosions revealed by gamma rays. Nat Astron (2022). https://doi.org/10.1038/s41550-022-01640-z
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