Normally in astronomy, we don’t see things going completely away. There are lots of phenomena that flare up and fade away, but generally, there is something still there when that fade is complete.
One particularly interesting kind of flare and fade system is the symbiotic nova V407 Cygni. This system consists of a Mira variable – a giant red star that pulsates over a period of hundreds of days – and a compact white dwarf star. The Mira’s hold on its outer atmosphere is tenuous at best, and the star has massive stellar winds. These effects join forces to allow the white dwarf to gravitationally pull matter that was part of the Mira onto the white dwarf’s surface where it builds up until it detonates in massive nuclear reactions.
In 2010, the Fermi gamma-ray telescope observed gamma-ray emission coincident with this system, giving us a sense of how energetic these outbursts and their related phenomena can be. Now, looking at this system in the radio, astronomers have been able to observe the expanding shockwave from the nova as it interacts with the material in its surroundings and impacts the atmosphere of the Mira variable. These highly detailed observations, which are published in Astronomy & Astrophysics by Marcello Giroletti and company, revealed the expanding shock of an outburst over the first 203 days of an outburst’s evolution, and also caught the echo of a past outburst. They allow astronomers to understand the system’s alignment and understand what was going on back in 2010 when gamma rays were discovered. Based on these new data, the system was observed along the equatorial plane and from behind the Mira. How that generated the observed gamma rays is a mystery that waits for additional publications.
“Very Long Baseline Interferometry Imaging of the Advancing Ejecta in the First Gamma-Ray Nova V407 Cyg,” Marcello Giroletti et al., 2020 June 29, Astronomy & Astrophysics (Preprint on arxiv.org)