An international team of astronomers has reported the observation of a new type of stellar outburst in binary systems: microns. They are millions and billions of times fainter than previously observed, last only a few hours but release as much energy in that time as the Sun emits in just one year.
Starbursts are massive phenomena accompanied by the release of enormous amounts of energy, which Earth-based telescopes can observe by dramatically increasing the brightness of a star by several orders of magnitude. Before the invention of modern telescopes, it was thought that new stars were formed this way. We now know that such events occur in binary systems, for example, where one star pulls matter from a companion star, accumulates it on its surface and then erupts as a so-called new star (or simply new), discarding the matter stored in its outer shell and starting the process all over again.
Astronomers have subsequently discovered stars that explode their outer shell into interstellar space, giving rise to even more massive events: supernova explosions. Among massive starbursts, there are kilonewaves (the merger of two neutron stars), hypernovae (the explosion of a star after the collapse of its core) and now micronovae.
Although these are still extremely powerful phenomena, they are 'micro' by astronomical standards: a micronova releases a million times less energy than a new outburst. An international team of astronomers first spotted these microbursts while analysing data from the TESS (Transiting Exoplanet Survey Satellite) space telescope, which is actually designed to search for exoplanets. An article with the findings of the discovery was published in the journal Nature.
"While reviewing astronomical data collected by NASA's TESS telescope, we detected something unusual: a bright flash of light in the optical range that lasted several hours. In a further search, we detected several similar signals," commented Nathalie Degenaar of the University of Amsterdam, who co-authored the study.
In total, the TESS telescope data has detected three similar events, which were then confirmed by observations on the Very Large Telescope (VLT). In all three cases, the microns have been observed in binary systems with a white dwarf star about the size of Earth, but similar in mass to the Sun. The observation of three similar systems at once has made it possible to determine the most likely cause of micronovae.
In the case of outbursts, a new white dwarf, pulling matter from the upper layers of its companion star and spreading it across its surface, will continue to fuel its thermonuclear reactions for several weeks, until the next outburst. Micronovae are thought to follow a similar scenario - except that the fusion and flare are localised near the magnetic poles of the white dwarf in the presence of a strong magnetic field.
"This phenomenon challenges our understanding of how fusion explosions occur in stars. We thought we had studied the process, but this discovery offers a whole new way of reaching them. It just goes to show how dynamic the universe is. Such events can actually be quite frequent, but because they happen so quickly, they are very difficult to observe," summed up Simone Scaringi, an astronomer at Durham University (UK) and lead author of the study.