The restless population of bright X-ray sources of NGC 3621

¹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA, 02138 USA
² Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, via della Ricerca Scientifica 1, I-00133 Roma, Italy
³ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, I-00078 Monteporzio Catone, Italy
⁴ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate (LC), Italy
⁵ Department of Physics, Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, OX1 3RH Oxford, UK
⁶ Scuola Universitaria Superiore IUSS Pavia, Palazzo del Broletto, piazza della Vittoria 15, I-27100 Pavia, Italy
⁷ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, Via A. Corti 12, I-20133 Milano, Italy
⁸ Department of Electronics, Information and Bioengineering, Politecnico di Milano, Via G. Ponzio, 34, I-20133 Milan, Italy
⁹ INAF – IASF Palermo, Via Ugo La Malfa, 153, I-90146 Palermo, Italy

Received: 10 April 2024
Accepted: 27 June 2024

Abstract

We report on the multi-year evolution of the population of X-ray sources in the nuclear region of NGC 3621 based on Chandra, XMM-Newton, and Swift observations. Among these, two sources, X1 and X5, after their first detection in 2008, seem to have faded below the detectability threshold, a most interesting fact as X1 is associated with the active galactic nucleus (AGN) of the galaxy. Two other sources, X3 and X6, are presented for the first time, the former showing a peculiar short-term variability in the latest available dataset, suggesting an egress from eclipse, and hence belonging to the handful of known eclipsing ultra-luminous X-ray sources. One source, X4, previously known for its heartbeat (i.e. a characteristic modulation in its signal with a period of ≈1 h), shows a steady behaviour in the latest observation. Finally, the brightest X-ray source in NGC 3621, here labelled X2, shows steady levels of flux across all the available datasets, but a change in its spectral shape, reminiscent of the behaviours of Galactic disc-fed X-ray binaries.