New X-ray supernova remnants in NGC 7793

¹ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain
² Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels, (Barcelona), Spain
³ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA, 02138 USA

^⋆ Corresponding author: kopsacheili@ice.csic.es

Received: 7 March 2025
Accepted: 5 June 2025

Abstract

Context. This work focuses on the detection of X-ray supernova remnants (SNRs) in the galaxy NGC 7793 and the study of their properties.

Aims. X-ray SNRs in galaxies beyond the Local Group are rare, mainly due to the limited sensitivity of current X-ray instruments. Additionally, their identification requires an optical counterpart, making incomplete optical identification methods an extra challenge. Detecting X-ray SNRs in other galaxies is crucial to understanding their feedback in different evolutionary phases and gaining insights into their local interstellar medium (ISM). In NGC 7793, only one X-ray SNR was previously known, while a recent study reported nearly 240 optical SNRs. The discovery of a new, larger optical SNR sample motivated a re-examination of the X-ray SNR population by comparing optical SNRs with X-ray sources.

Methods. To identify X-ray SNRs, we utilised Chandra’s spatial resolution and analyzed all available archival data of NGC 7793, totaling 229.9 ks over 19 years. After data reduction, we performed source detection and analysis, searching for X-ray sources coinciding with optical SNRs. We also used XMM-Newton (1.1 Ms combined EPIC MOS) for a spectral analysis of the confirmed and candidate SNRs.

Results. We detected 58 X-ray sources down to an observed luminosity of ∼9 × 10³⁵ erg s⁻¹. Among them, five X-ray counterparts to optical SNRs were identified, all presenting soft emission (< 1.2 keV) with no short- or long-term variability. One corresponds to the previously known X-ray SNR, while four are newly detected. Spectral modelling of two SNRs shows thermal spectra exceeding 2.5 million K, with strong O VII, O VIII, and Ne IX emission lines. A correlation between density, X-ray luminosity, and source softness was observed. We also report X-ray emission from supernova 2008bk, refining its position, and suggest two candidate X-ray SNRs with soft, non-variable spectra, one resembling the identified X-ray SNRs.