New Insights into an Ultramuminous X-ray Source in NGC 628

Researchers Hasan Avdan and Senay Avdan have conducted a detailed study on an ultraluminous X-ray source (ULX) in the galaxy NGC 628. Utilizing multi-epoch archival X-ray data from telescopes such as XMM-Newton, Chandra, and Swift, the team analyzed the spectral and temporal properties of the ULX over a period of 22 years. The study aimed to constrain the nature of the compact object in the system, which could either be a black hole or a neutron star.

The X-ray flux of the source was observed to change by a factor of approximately 200 throughout the observations. The researchers confirmed a previously detected quasi-periodic signal in the range of 0.1 to 0.4 mHz using Lomb-Scargle periodograms. Additionally, two optical counterpart candidates were identified using data from the Hubble Space Telescope's Wide Field Camera 3.

The spectral energy distributions in the optical band for both candidates suggest that the optical emission is dominated by the irradiation of the accretion disc. Based on the best-fit model parameters of the multi-colour disc black-body model, the mass of the black hole in the system is estimated to be between 3 and 16 solar masses. However, the long-term variability and spectral transitions in the hardness-luminosity diagram make it challenging to rule out the possibility of a neutron star.

These findings are significant as they contribute to the understanding of the nature and behavior of ULXs, which are among the most luminous X-ray sources in the universe. The study provides valuable insights into the physical characteristics of these enigmatic objects and their potential to host either black holes or neutron stars.