Unveiling the nature of the donor stars of the ultraluminous X-ray sources in NGC 1559

¹ Space Science and Solar Energy Research and Application Center (UZAYMER), University of Çukurova, 01330 Adana, Türkiye
² Department of Physics, University of Çukurova, 01330 Adana, Türkiye

^⋆ Corresponding authors; 0417allaksinan@gmail.com; aakyuz@cu.edu.tr

Received: 17 October 2024
Accepted: 17 January 2025

Abstract

Context. X-ray data provide insights into accretion processes and the compact objects of ultraluminous X-ray sources (ULXs), while optical and IR observations help identify the donor star and surrounding environment. Together, these approaches shed light on the origins of the high X-ray luminosities observed in ULXs

Aims. This study examines the optical and IR properties of eight ULXs in NGC 1559 using archival data from the Hubble Space Telescope and the James Webb Space Telescope. To constrain the nature of the donor stars of the ULXs, photometric results were obtained from the temporal analysis, spectral energy distributions (SEDs), and color-magnitude diagrams (CMDs). Furthermore, the long-term and spectral characteristics of one ULX, X-1, were investigated.

Methods. ULX counterparts were determined from astrometric calculations. The long-term light curves and SEDs were constructed to interpret the origin of the optical and IR emissions. The masses and ages of donor star candidates were determined using CMDs. To constrain the mechanism of X-ray emission, the time-averaged spectrum and long-term light curve of X-1 were obtained.

Results. Unique optical and IR counterparts for ULXs X-14 and X-24 were determined, while only optical counterparts were detected for X-1 and X-18. Our findings indicate that the optical emission originates from the donor stars of X-14 and X-24, whereas for X-1 and X-18 it is dominated by the accretion disk. In addition, X-1 exhibits long-term X-ray variability spanning orders of magnitude.