Identification of a new ultraluminous X-ray source in NGC 1316

Abstract

In this study, we report identification of a new ultraluminous X-ray source (ULX) named as X-7 in NGC 1316, with an unabsorbed luminosity of 2.1× 1039 erg s−1 using the two recent Chandra archival observations. The X-7 was detected in the Chandra 2001 observation and was included in the source list of the NGC 1316 as CXOUJ032240.8-371224 with a luminosity of 5.7× 1038 erg s−1. Present luminosity implies a luminosity increase of a factor of ∼ 4. The best fit spectral model parameters indicate that X-7 has a relatively hot disk and hard spectra. If explained by a diskblackbody model, the mass of compact object is estimated as ∼ 8 M⊙ 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.″22 error circle at 95% confidence level for X-7 using the archival HST/ACS and HST/WFC3 data. Absolute magnitude (MV) 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) located 6 ″ away from X-7 has a (high) luminosity of ∼ 1039 erg s−1 with no visible optical candidate.

Three Ultraluminous X-ray Sources Hosted by Globular Clusters in NGC 1316

“…X-ray coordinates of the three ULXs and lower X-ray luminosity sources coincident with candidate globular clusters. These sources have been identified in Kim & Fabbiano (2003), Swartz et al (2004), andAllak et al (2020). Sources marked with 𝑜 were identified by Goudfrooij et al (2001b), sources with † were identified by Jordán et al (2016) tbabs*(diskbb+pegpwrlw) to the deep spectra, however, only GCULX2 could be fit by the two component model.…”

“…Two ULX sources, GCULX1 and GCULX2, match to globular cluster candidates in the Jordán et al (2016) survey. GCULX3's optical counterpart, identified in Allak et al (2020), also had characteristics typical of extragalactic globular clusters. We use DAOPHOT (Stetson 1987) to perform photometry on all three sources, as well as other known GCs in the field, using images that corresponds to the 𝑔, 𝑧 bands, and found that GCULX3's colour and magnitude are typical for a globular cluster.…”

The Transient Ultra-luminous X-ray Source, ULX-4, in M51

“…We used HST archival data (see Table 3 for details) to investigate the optical properties of ULX-4. For a relative astrometry between Chandra (ObsID 13816) and HST (ObsID J97C51R4Q) images, we followed a similar method that we used in our previous works (Allak et al 2020;Ozdogan Ela et al 2021). The tool in 7 and wavedetect tool in were used for source detection in HST and Chandra images, respectively.…”

“…This is likely to increase the discovery of new transit sources as well (e.g. Middleton et al 2012;Barnard et al 2013;Hu et al 2018;van Haaften et al 2019;Allak et al 2020;Walton et al 2021). PULXs are observed sources in this class with their high flux variability reaching up to factors of 50 in flux variation.…”

“…The absolute magnitudes (M V ) of ULXs are in the range of −3 < M V < −8 mag (Vinokurov et al 2018). There are hundreds of ULXs but only about 20 have a single optical counterpart (Tao et al 2011;Gladstone et al 2013;Avdan et al 2019;Allak et al 2020). Multiple optical counterparts have been detected for a number of ULXs (Soria et al 2005;Mucciarelli et al 2007;Avdan et al 2019;Aksaker et al 2019;Akyuz et al 2020).…”