Astronomers stumble on X-ray ‘fireball’ of massive stellar explosion

Astronomers just stumbled across one of the biggest explosions in the universe, confirming a theory about how dead stars can temporarily, and dramatically, burst back to life.

Astronomers using the space-based erosita X-ray telescope observed a single bright flash of X-rays on 7 July 2020, which was never seen before or since. In a new paper published in the journal Nature, the researchers detail how they associated this X-ray flash with the classical Nova YZ Reticuli, an exploding white dwarf star some 8,000 light years away, observing for the first time the predicted “fireball phase” of a nova.

A white dwarf star is a dim, cool, but very dense remnant of a larger star that has exhausted its nuclear fuel — a dead star that can no longer generate the thermonuclear reactions that sustain living stars. But as a very dense object with powerful gravity, white dwarfs with a larger, still living companion star sometimes siphon hydrogen from their companion’s outer shell.

When enough hydrogen accumulates on the surface of a white dwarf, the pressure and heat increase to the point that a runaway thermonuclear reaction takes place — the hydrogen is fused into helium, and a colossal explosion, a classical nova, results. This is a different type of stellar explosion than a supernova, which results from the death throes of a still living, very massive star.

As described by Stony Brook University astronomer and author of a commentary appearing in the same issue of Nature, Frederick Walter, a prediction from 1990 held that a classical nova would shine at its brightest at the moment the explosion reaches the outer layer of the white dwarf, the photosphere — a nova “fireball.” This fireball would be so hot and so bright, the thinking went, that it would only be visible in X-rays until hours or days later when the material ejected from the white dwarf during the nova cooled enough to shine in the visible portion of the spectrum.

The erosita researchers were able to piece together the X-ray burst they observed and the visible outburst of the YZ Reticule Nova seen on 15 July, and realize they had managed to capture both fireball and Nova, confirming the prediction from 1990.

As Dr Walter wrote in his commentary, it was a serendipitous discovery.

The erosita instrument flies on the Spektr-RG space telescope, a partnership between Germany and Russia. It takes erosita six months to complete a scan of the sky, and it completed just four such scans before the Russian invasion of Ukraine interrupted the mission. Luckily, one of those scans captured the fireball of the YZ Reticuli nova.