White dwarfs are known to steal material from a companion star and when this loot reaches the dwarf's surface… Bang! A thermonuclear reaction occurs and the dwarf suddenly becomes millions of times brighter. In some cases, some of these stars become visible to the naked eye and appear brand-new. For this reason, they are named after the Latin word for new: nova.
However, the way novas are powered exactly has been a matter of debate. Some consider the thermonuclear explosion enough to explain the brightness, others think that as the explosion happens, some of the material is thrown back into space at speeds so high that it heats up, generating light. Now, researchers reporting in Nature Astronomy have captured the first direct evidence that indeed the main source of the explosion's visible light comes from shock waves arising from the pressure and temperature changes formed in the material from the explosion.
This is the same principle behind a sonic boom – the sound (like an explosion) caused by shockwaves created when an object travels faster than the speed of sound. In the nova's case, the material is shocked to incredible energies and these shocks produce light at different wavelengths, including the most energetic type of all: gamma-rays.
The team used the serendipitous detection of nova V906 Carinae to show this. In March 2018, the All-Sky Automated Survey for Supernovae discovered the nova. At the same time, the Canadian satellite BRITE-Toronto was studying another object in the same patch of sky and caught the explosion, observing the nova’s changing brightness in incredible detail. NASA's Fermi Gamma-ray Space telescope almost missed it, being under maintenance, switching back on to catch the last three flares.
“Thanks to an especially bright nova and a lucky break, we were able to gather the best-ever visible and gamma-ray observations of a nova to date,” lead author Dr Elias Aydi, from Michigan State University said in a statement. “The exceptional quality of our data allowed us to distinguish simultaneous flares in both optical and gamma-ray light, which provides smoking-gun evidence that shock waves play a major role in powering some stellar explosions.”
They found fluctuations in both signals at the same time, indicating that the origin of one type of light was the same as the other, and that shocks must have been behind both.
"This is a new way of understanding the origin of the brightness of novae and other stellar explosions," Aydi explained. "Our findings present the first direct observational evidence, from unprecedented space observations, that shocks play a major role in powering these events."
Each nova explosion releases between 10,000 and 100,000 times the energy of the Sun in a year, and our galaxy experiences about 10 novae a year. More observations of nearby novae might expand on these findings and produce a clearer picture of what makes a nova shine. However, this research may explain more than just novae; shocks are common in many astrophysical phenomena.
