# Discovery of an old nova remnant in the Galactic globular cluster M 22

## TL;DR

This paper reports the discovery of an ancient nova remnant in the globular cluster M 22, confirmed through spectroscopic analysis, potentially making it one of the oldest recorded extrasolar events.

## Contribution

It presents the first identification of a nova remnant in a Galactic globular cluster, linking it to a historical transient observed in 48 BCE.

## Key findings

- Confirmed the nebula as a nova remnant via spectral analysis.
- Estimated the nebula's mass to be between 1 and 17 x 10^{-5} solar masses.
- Linked the nebula to a historical 'guest star' observed in 48 BCE.

## Abstract

A nova is a cataclysmic event on the surface of a white dwarf in a binary system that increases the overall brightness by several orders of magnitude. Although binary systems with a white dwarf are expected to be overabundant in globular clusters (GCs) compared to the Galaxy, only two novae from Galactic globular clusters have been observed. We present the discovery of an emission nebula in the Galactic globular cluster M 22 (NGC 6656) in observations made with the integral-field spectrograph MUSE. We extract the spectrum of the nebula and use the radial velocity determined from the emission lines to confirm that the nebula is part of NGC 6656. Emission-line ratios are used to determine the electron temperature and density. It is estimated to have a mass of 1 to $17 \times 10^{-5}$ solar masses. This mass and the emission-line ratios indicate that the nebula is a nova remnant. Its position coincides with the reported location of a 'guest star', an ancient Chinese term for transients, observed in May 48 BCE. With this discovery, this nova may be one of the oldest confirmed extrasolar events recorded in human history.