Artist concept of a micronova. Image: ESO/M. Kornmesser, L. Calçada
Scientists have discovered an entirely new type of star explosion, called a “micronova,” which is like a miniature version of the energetic nova eruptions that illuminate certain dead stars. The breakthrough not only unveils an entirely novel stellar phenomenon, it also solves a decades-long puzzle involving “unexplained rapid bursts” spotted in a system about 1,680 light years from Earth, reports a new study.
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Novae are among the most dazzling sights in the night sky, and have been witnessed by astronomers for many centuries. Unlike supernovae, which are the pyrotechnic swan songs of giant stars, novae occur when white dwarfs, the compact corpses of stars like our Sun, end up in binary systems with another star. As the two objects orbit each other, the gravitational pull of the white dwarf tugs stellar material off of its companion as part of a process called accretion, which fuels radiant thermonuclear bursts across the entire surface of the dead star.For about 40 years, scientists have been perplexed by flashes emitted by one of these white dwarfs, known as TV Columbae, which is in a binary system with a low-mass star. The bursts are relatively dim and they only last for a few hours, whereas typical novae are bright and can shine for several weeks. Now, researchers led by Simone Scaringi, an astronomer at Durham University, believe they have solved this long-standing enigma using NASA’s Transiting Exoplanet Survey Satellite (TESS). The team suggests the bursts at TV Columbae, along with two similar white dwarfs called EI Ursae Majoris and ASASSN-19bh, are powered by micronovae that are localized to the poles of these dead stars, a process that may be “more common than previously thought,” according to a study published on Wednesday in Nature.
“The discovery was an unexpected surprise,” Scaringi said in an email. “One of my interests is studying accretion physics. I mostly do this through the use of TESS. We've been running a program with TESS to monitor the brightness variations of a few hundred accreting white dwarfs (as well as candidate accreting [white dwarfs]),” including the three dead stars featured in the new study. “These systems are monitored continuously for at least a month and up to a year,” he added. “It was the long observations which allowed us to observe the bright and fast micronovae in action.”As its name suggests, TESS is primarily tasked with spotting exoplanets, which are worlds that orbit other stars. For this reason, the observatory is designed to flag extremely small variations in the brightness of stars that might hint at the presence of planets crossing in front of them, from our perspective on Earth. Scaringi notes that this extreme sensitivity to variable light also distinguishes TESS as an excellent platform for studying accretion bursts around white dwarfs, especially these extremely subtle micronovae, which are so much shorter and dimmer than their novae counterparts. (Though the micronovae are many orders of magnitude smaller than novae, they are still massively energetic eruptions that burn up an amount of material equivalent to 3.5 billion Great Pyramids of Giza.)
By examining the trio of white dwarfs featured in the study with TESS, as well as the European Southern Observatory’s Very Large Telescope, the team was able to rule out a host of mechanisms that have been previously presented to explain the strange bursts. Ultimately, the observations revealed a completely novel scenario in which the magnetic fields of white dwarfs channel accreted material to the poles, where they explode in a localized pattern that is distinct from the global eruptions of regular novae. Scientists already knew about these “intermediate polars”—the term for white dwarfs that magnetically direct material to their poles—but the discovery that these systems can generate thermonuclear blasts was completely unanticipated.“We were surprised,” Scaringi said. “After having discovered the first one in the data, we spent over a year trying to explain the observation with the models we had at hand. It was only when we discovered two other systems in the data displaying micronovae, and noticed that they contained a magnetic accreting white dwarf as well, that we started to join the dots.” “Finally, when we made the comparison to the thermonuclear bursts observed in accreting neutron stars it appeared clear to us that what we were seeing were bursts of radiation from localized thermonuclear explosions on white dwarfs,” he added.Now that this new type of star explosion has been introduced to the scientific community, the team hopes that follow-up observations will capture more transient micronovae out there in our galaxy. These discoveries will help to unravel some of the open questions about the mechanisms behind these bizarre events. “To find more, we will need long monitoring observations similar to what TESS is already providing,” Scaringi said. “We are also hoping to promptly follow up these events and catch them in action with X-ray space-based observatories and optical ground-based ones. Given how quickly micronovae happen and fade, this will be challenging!” “I think this goes to show just how dynamic the Universe is,” he concluded “These micronova are fast flashes of light that may be quite common out there. It also goes to show that thermonuclear explosions can occur on localized areas (as opposed to the entire surface) of white dwarfs, something that was unexpected and surprising.”