Capture the moment a white dwarf explodes

Center for The High Angular Resolution Astronomy (CHARA Array) at Georgia State University has produced detailed images of the early stages of two supernova explosions discovered in 2021. By measuring near-infrared interferometry, a process that combines light from multiple telescopes, the CHARA array was able to capture rapidly changing conditions in the post-explosion phase with high resolution.

A nova is an astronomical phenomenon that occurs in a binary system when a white dwarf strips its companion star of hydrogen-rich gas, causing a runaway thermonuclear reaction on the surface of the white dwarf. The name is derived from the sudden brightness that makes it appear as if a new star has appeared in the night sky. However, the ejecta immediately following the explosion is small and challenging to observe, and until now astronomers have only been able to infer the early stages through indirect methods.

“The images give us a close-up look at how material is being flung away from the star during the explosion,” explains Gale Schiffer, director of the CHARA Array. “Catching these fleeting events requires flexibility to adapt our nightly schedule while discovering new targets of opportunity.”

Explosive results

Schäfer and her team observed V1674 Herculis, a nova in the constellation Hercules, and V1405 Cassiopeia, a nova in the constellation Cassiopeia. V1674 was one of the fastest supernova ever recorded, reaching peak brightness in less than 16 hours after its discovery and then quickly fading within a few days. In contrast, V1405 took 53 days to reach peak brightness and remained bright for about 200 days.

Images were taken 2.2 days (left) and 3.2 days (middle) after the explosion caused by nova V1674 Herculis. As indicated by the arrows, two ejection flows were formed. On the right is an illustration of an image of the explosion.

The image of V1674, taken a few days after its discovery, shows an explosion that is clearly not spherical; There are two jet flows, one to the northwest and one to the southeast with an elliptical structure radiating almost perpendicular to them. This is direct evidence that the explosion involved many projectiles interacting with each other.

Spectroscopic observations also revealed different velocity components in the Balmer series of hydrogen atoms. While the velocity of the absorption line before the peak was about 3800 km/s, the component that appeared after the peak was about 5500 km/s.

Timing is important. The new ejecta flow appeared in the image simultaneously with the detection of high-energy gamma rays by NASA’s Fermi Gamma-ray Space Telescope. The collision of streams of different speeds formed a powerful shock wave emitting gamma rays.

The V1405 results were even more surprising. The first two observations during the peak period showed only a bright central light source and a few surrounding ejections. The diameter of the central region was about 0.99 milliarcsecond, which when converted to distance corresponds to a radius of about 0.85 astronomical unit (au stands for astronomical unit, the distance between the Earth and the Sun).