Astronomers using the MAGIC (Major Atmospheric Gamma Imaging Cherenkov) telescopes have detected gamma rays from the 2021 outburst of RS Ophiuchi, a recurrent nova produced by a white dwarf and a red giant companion in a binary system.
“Birthplaces of a nova are systems in which two very different stars live in a parasitic relationship: a white dwarf (a small, burned-out and tremendously dense star) orbits a red giant (a star that will soon burn up),” said Max Planck Institute for Physics astrophysicist David Green and his colleagues.
“The giant star feeds the white dwarf with matter shedding its outer hydrogen layer as the gas flows onto the nearby white dwarf.”
“This flow of matter continues, until the white dwarf overheats itself.”
“The temperature and pressure in the newly gained stellar shells become too large and are flung away in a gigantic thermonuclear explosion,” they said.
“The dwarf star remains intact and the cycle begins again — until the spectacle repeats itself.”
RS Ophiuchi is a recurrent symbiotic nova with average time between major outbursts of 14.7 years.
First observed in 1898, the system is located some 5,000 light-years away in the constellation of Ophiuchus.
“The most recent outburst of RS Ophiuchi, in August 2021, was promptly reported in optical and high-energy gamma rays by the Large Area Telescope on board NASA’s Fermi Gamma-ray Space Telescope,” the astrophysicists said.
“The optical emission showed behavior similar to that of the 2006 outburst.”
Following these alerts, Dr. Green and co-authors used two MAGIC telescopes to observe RS Ophiuchi.
They recorded gamma rays with the value of 250 GeV (gigaelectronvolts), among the highest energies ever measured in a nova.
“The spectacular eruption of RS Ophiuchi shows that the MAGIC telescopes’ fast response really pays off: it takes them no more than 30 seconds to move to a new target,” Dr. Green said.
“After the explosion, several shock fronts propagated through the stellar wind from the red giant and the interstellar medium surrounding the binary system.”
“These shock waves work like a giant power plant in which particles are accelerated to near the speed of light.”
“The combined measurements suggest that the gamma rays emanate from energetic protons, nuclei of hydrogen atoms.”
“This also makes nova outbursts a source of cosmic rays,” he added.
“However, they tend to play the role of local heroes — meaning to only contribute to the cosmic rays in the close neighborhood.”
“The big players for cosmic rays are supernova remnants. The shock fronts created from stellar explosions are far more violent compared to novae.”
“To fully understand the complicated interplay of violent events with the interstellar medium in the Milky Way, more observations like those reported now will be necessary.”
“The MAGIC collaboration will therefore continue to look for ‘restless’ objects in our Galaxy and beyond.”
A paper on the findings appears in the journal Nature Astronomy.
V.A. Acciari et al. Proton acceleration in thermonuclear nova explosions revealed by gamma rays. Nat Astron, published online April 14, 2022; doi: 10.1038/s41550-022-01640-z
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