Exploring the nature of an ultraluminous X-ray source in NGC 628

TL;DR

This study analyzes the spectral and temporal X-ray properties of an ultraluminous X-ray source in NGC 628 over 22 years, exploring its nature and potential optical counterparts, and estimating the black hole mass.

Contribution

It provides a detailed multi-epoch analysis of the ULX, including spectral modeling, variability study, and optical counterpart identification, offering new insights into its nature.

Findings

X-ray flux varies by a factor of ~200 over 22 years.

Confirmed quasi-periodic oscillations in the 0.1-0.4 mHz range.

Black hole mass estimated between 3 and 16 solar masses.

Abstract

In this work, we study the X-ray spectral and temporal properties of an ultraluminous X-ray source (ULX) in NGC 628 by using multi-epoch archival X-ray data. The physical parameters were estimated in each epoch in order to constrain the nature of the compact object in the system. Also, the optical counterpart candidates of the ULX were examined using the archival {\it Hubble} Space Telescope (HST)/Wide Field Camera 3 (WFC3) data. {\it XMM-Newton}, {\it Chandra}, and {\it Swift} data were used to create the long-term light curve (which covers a period of 22 years) and perform the spectral analysis. Lomb-Scargle periodograms of the source were constructed to examine the short-term variability in each epoch. In order to search for an optical counterpart in the HST/WFC3 images, a relative astrometric correction was initially applied to the {\it Chandra} and HST/WFC3 images. The X-ray flux of the source changes by a factor of $\sim 200$ throughout the observations. The previously detected quasi-periodic signal (in the range of 0.1$-$0.4 mHz) was confirmed by using the Lomb-Scargle method. After astrometric correction, two optical counterpart candidates were detected for the source. The obtained spectral energy distributions in the optical band for both candidates indicate that the optical emission is dominated by the irradiation of the accretion disc. Considering the best-fit model parameters of the multi-colour disc black-body model, we derived the mass of the black hole in the system as being in the range of (3$-$16) $M_{\odot}$. Nonetheless, the long-term variability and the spectral transitions in the hardness-luminosity diagram make it difficult to rule out the neutron star scenario.