The cosmic detective story began with a quiet flash in the optical band. AT2019wzc slowly brightened and faded like a smoldering ember. But hidden inside was something more energetic — repeating soft X-ray bursts that astronomers call quasi-periodic eruptions.
A team led by scientists from the XMM-Newton project aimed an X-ray spectrograph at AT2019wzc. In the high-resolution spectrum, a narrow absorption line appeared — the signature of ionized nitrogen (N VI), blueshifted. That means the gas is moving away from us. But here's the odd part: lines of other elements with similar ionization states (carbon, oxygen) are barely visible or absent altogether. Such an imbalance is only possible if nitrogen is anomalously abundant in the gas — about 11.6 times more than in the Sun.
Where does that excess come from? A star like our Sun, but evolved off the main sequence, had accumulated nitrogen in its core over billions of years of nuclear reactions. When a supermassive black hole tore it apart with tidal forces, the nitrogen-rich innards spilled out. The stream of matter collided with itself, producing an outflowing cloud: that's what we see as absorption. Thus spectroscopy turned AT2019wzc from a suspect into the key witness of a tidal disruption event.
The smoldering ember turned out to be the ash of a dead star. The nitrogen fingerprint links two phenomena into one story: from the first fatal gravitational pull to the repeating flares that will long remind us of the catastrophe.
Nitrogen, the seventh element of the periodic table, is born in stars from carbon and oxygen in the CNO cycle. By old age, a star has accumulated hundreds of times more nitrogen than it had at birth.
In Peter Watts's novel Blindsight, alien probes analyze the spectra of stellar remnants to reconstruct the history of civilizations — much like astronomers use the nitrogen line to reconstruct the fate of AT2019wzc.