H.E.S.S. watches a nova outburst for the first time

Last August marked the first opportunity to observe a nova outburst in very-high-energy gamma rays and to follow both its growth and subsequent fading over a month, i.e. to witness the acceleration process itself. The members of the H.E.S.S. collaboration now report on this unique discovery in Science. Together with the data from the Fermi satellite, these observations provide revealing insights into the processes underlying gamma-ray emission. Surprisingly, the nova "RS Ophiuchi" seems to accomplish particle acceleration reaching the theoretical limit.

In the constellation of Ophiuchus (the Serpent Bearer) there is a double star system called "RS Ophiuchi", about 7500 light years away, consisting of a white dwarf and a red giant. Their separation is only about one and a half times the distance between the Earth and the Sun, close enough for the white dwarf to continuously pull matter from the atmosphere of its companion. Episodically, enough material is accumulated on the white dwarf's surface for the strong gravity to ignite a thermonuclear explosion. Between 1898 and 2006, astronomers have previously observed eight such nova outbursts of RS Ophiuchi. On the 8th of August 2021, another outburst occurred that was visible even to the naked eye.

Starting the following night, H.E.S.S. set its sights on the star and was indeed able to detect it. “This is the very first observation of a nova in very-high-energy gamma radiation,” says Alison Mitchell of the Friedrich-Alexander-Universität Erlangen-Nürnberg and principal investigator of the H.E.S.S Nova programme. For an unprecedented period of one month – with a brief period of interruption around full moon – H.E.S.S. was able to track the evolving luminosity of the nova.

Complementing the H.E.S.S. observations were lower-energy gamma-ray measurements by the Fermi satellite. Combining data from the two instruments revealed unique insights into the processes underlying the gamma-ray emission and their evolution. In both energy ranges, the rate of gamma-rays received by the instruments displays a period of steady increase before decaying at the same rate. However, the peak in the emission as measured by H.E.S.S. was delayed with respect to that in the Fermi energy range by two days. Moreover, the energy spectra measured by the two instruments are consistent with a continuous curve that gradually evolves from night to night: it becomes flatter, but broader with time (see figure). This suggests that the gamma rays observed by Fermi and H.E.S.S. have a common origin: accelerated particles whose maximum energy increases in the first days after the explosion.

“The most likely scenario is that protons and other nuclei are accelerated efficiently at the expanding explosion shock front and collide with compressed material from the stellar wind of the red giant. This releases gamma rays” explains Brian Reville of the MPI for Nuclear Physics, who played a major role in the model calculations needed to interpret the data. This implies that such nova outbursts are efficient cosmic accelerators, accelerating particles to energies that reach their theoretical maximum and supplying their surroundings with abundant high-energy particles – a significant contribution to cosmic rays in the nova’s neighbourhood. This finding supports the prevailing theory that the fast shocks of young supernova remnants expanding into the dense winds of massive stars – extreme analogues of the nova RS Ophiuchi – drive production of the highest-energy Galactic cosmic rays.

The High Energy Stereoscopic System, or H.E.S.S. for short, located in Namibia, consists of five Cherenkov telescopes that are used to study very-high-energy cosmic gamma rays. The large telescope is equipped with a state-of-the-art camera (FlashCam) that has been developed under the leadership of the Max Planck Institute for Nuclear Physics in Heidelberg. “The new camera has been in operation since the end of 2019, and with this measurement it has proven the potential of this technology,” says Simon Steinmaßl, a doctoral student at the MPI for Nuclear Physics, who was involved in the analysis of the camera data. The FlashCam design is currently being further optimised for the next-generation gamma-ray observatory, the Cherenkov Telescope Array (CTA).

Original publication:

Time-resolved hadronic particle acceleration in the recurrent Nova RS Ophiuchi, H.E.S.S. Collaboration, Science 10.03.2022, DOI: 10.1126/science.abn0567

Division Hinton at MPIK
Group Reville at MPIK


Dr. Brian Reville
Phone: +49 6221 516-589
Email: brian.reville@mpi-hd.mpg.de

Prof. Dr. Jim Hinton
Phone: +49 6221 516-140
Email: jim.hinton@mpi-hd.mpg.de

Simon Steinmaßl
Phone: +49 6221 516-144
Email: simon.steinmassl@mpi-hd.mpg.de

The large H.E.S.S. telescope. (©MPIK, Christian Föhr)

Artist's impression of the white dwarf and red giant binary system following the nova outburst. Material ejected from the surface of the white dwarf generates shockwaves that rapidly expand, forming an hour-glass shape. Particles are accelerated at these shock fronts, which collide with the dense wind of the red giant star to produce very-high-energy gamma-ray photons. (© DESY/H.E.S.S., Science Communication Lab)

The energy spectrum of gamma rays from RS Ophiuchi measured by Fermi and H.E.S.S. for the 9th and 13th August 2021. (© H.E.S.S. Collaboration)