The transient ultraluminous X-ray source, ULX-4, in M51

Abstract

We present the results of a temporal and spectral analysis of the transient source ULX-4 in the galaxy M51. The data used were drawn from Chandra, XMM-Newton and Swift-XRT archives, spanning the years 2000-2019. The X-ray flux of the source is seen to vary by two orders of magnitudes within a month but a short-term variability was not observed over the time intervals of 100-2000 second in the 0.3−10 keV energy band. We find some evidence for the existence of bi-modality feature in the flux distribution of ULX-4. We identified two optical sources as possible counterparts within an error radius of 0.″18 at 95${{\ \rm per\ cent}}$ confidence level for ULX-4 based on the archival HST/ACS and HST/WFC3 data. Blackbody fits of the spectral energy distributions (SEDs) indicate the spectral type to be B-type stars. One of these counterparts exhibits a low-amplitude optical periodicity of 264 ± 37 days in the F606W filter; if we assume this apparent periodicity is associated with the orbital motion of the donor, then it is more likely that the donor is a red supergiant satisfying the long periodicity and accretion via Roche lobe overflow. Consequently, the SED would then have to be interpreted as a superposition of emissions from a cold donor and a hot flow component, most likely from an accretion disk. If, on the other hand, the periodicity is super orbital in nature i.e. due to possible interactions of the compact object with a circumstellar disk, the donor could then be a Be/X star hosting a neutron star.

Possible optical counterparts of ULXs in NGC 1672

“…Chandra and HST images were used to obtain astrometric precision by following the method we used in our previous studies Allak et al (2022) (and references therein). We chose deep Chandra ACIS‐S (ObsID 5932) and HST ACS (ObsID j6n202010) images for astrometric corrections of the sources.…”

Optical counterparts of ULXs in NGC 1672

“…Determining the optical counterparts of the ULXs requires accurate source positions provided by astrometric corrections. Chandra and HST images were used to obtain astrometric precision by following the method we used in our previous studies Allak et al (2022) (and references therein). We chose deep Chandra ACIS-S (ObsID 5932) and HST ACS (ObsID j6n202010) images for astrometric corrections of the sources.…”

“…The occurrence of high flux variability in ULXs may provide some evidence for exhibiting bi-modal flux distributions as might be expected for sources related to propeller transitions. As discussed in several studies, probing some ULX transient systems with such a bi-modal flux distribution may be a way to identify a ULX hosts a NS without detecting pulsations (Earnshaw et al 2018;Song et al 2020;Allak et al 2022). However, we note that the statistics for the X5 and X9 sources are very low and the bi-modal nature is not evident in all the datasets we used.…”

“…Therefore, identification of counterparts often requires the use of the Hubble Space Telescope (HST 1 ) for optical imaging. In addition, precise astrometry between high resolution Chandra X-ray and HST images is also essential (Patruno & Zampieri 2010;Grisé et al 2011;Poutanen et al 2013;Egorov et al 2017;Urquhart et al 2018;Allak et al 2022). Identification of point-like (potential) counterparts of ULXs with blue colors are indicative of early-type, OB stars.…”

“…Moreover, some study of the long-term variability of ULXs has shown orbital or super-orbital variations (Brightman et al 2019). Thanks to Neil ★ E-mail:0417allaksinan@gmail.com Gehrels Swift Observatory X-ray Telescope (hereafter Swift-XRT) it is possible to obtain long and regularly sampled light curves of ULXs (La Parola et al 2015;Song et al 2020;Brightman et al 2022;Allak et al 2022a). Therefore, comparing the variability properties of Galactic XRBs which are well-known properties with ULXs gives us the chance to place strong constraints on compact objects.…”