Identification of a New Ultraluminous X-ray Source in NGC 1316

S. Allak, A. Akyuz, N. Aksaker, M. Ozdogan Ela, S. Avdan, F. Soydugan

Published 2020-07-18Version 1

In this study, we report identification of a new ultraluminous X-ray source (ULX) which we named as X-7 in NGC 1316, with an unabsorbed luminosity of 2.10$\times$10$^{39}$ erg s$^{-1}$ using two recent {\it Chandra} archival observations. The X-7 was detected in the {\it Chandra} 2001 observation and it was included in the X-ray source list of the NGC 1316 as CXOUJ032240.8-371224 with luminosity of 5.7$\times$10$^{38}$ erg s$^{-1}$ implying a luminosity increase of a factor of $\sim$ 4. The best fit spectral model parameters indicate that X-7 has a relatively hot disk and hard spectra. The mass of compact object is obtained as $\sim$ 8 M$\odot$ which is in the range of a stellar-mass black hole. The X-7 shows a relatively long-term count rate variability while no short-term variability is observed. We also identified a unique optical candidate within $0.\arcsec22$ error circle at 95\% confidence level for X-7 using the archival {\it HST}/ACS and {\it HST}/WFC3 data. Absolute magnitude (M$_{V}$) of this candidate is -7.8 mag. Its spectral energy distribution is adequately fitted a blackbody model with a temperature of 3100 K indicating an M type supergiant, assuming the donor star dominates the optical emission. In addition, we identified a transient ULX candidate (XT-1) has a high luminosity of $\sim$ 10$^{39}$ erg s$^{-1}$ located 6 $\arcsec$ away from X-7 with no visible counterpart(s).