Yazar "Vinokurov, A." seçeneğine göre listele

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  • Küçük Resim
    Öğe
    Optical counterparts of ULXs in two dwarf galaxies: NGC 4861 and NGC 4449
    (Oxford University Press, 2021) Özdoğan Ela, M.; Akyüz, A.; Aksaker, N.; Avdan, S.; Oralhan, I. Akkaya; Vinokurov, A.; Allak, S.
    We present the results of a search for optical candidates of ultraluminous X-ray sources (ULXs) in two dwarf galaxies, NGC 4861 and NGC 4449, using Hubble Space Telescope (HST) archival data. With precise astrometry, we confirm that NGC 4861 X1 is associated with an H II complex and we conclude that NGC 4861 X2 resides in a young star group of mass 400 ± 80 M☉. We also find that NGC 4449 X7 is associated with three optical candidates within an error radius of 0.2 arcsec at the 90 per cent confidence level. Absolute magnitudes (Mv) of these candidates are determined as −5.0 and −4.1. The age and mass values for the three candidates are estimated as 40–50 Myr and ∼8 M☉, respectively, using stellar evolutionary tracks. The locations of optical candidates suggest a possible association with a nearby group of stars. In addition, we analysed previously unused archival data of XMM–Newton, Chandra, and Swift where the sources were detected. Although the X-ray spectral data do not allow us to discriminate between physical models, long-term data at hand are consistent with the sources being in luminous hard states.
  • [ X ]
    Öğe
    The transient ultraluminous X-ray source, ULX-4, in M51
    (Oxford Univ Press, 2022) Allak, S.; Akyuz, A.; Oralhan, I. Akkaya; Avdan, S.; Aksaker, N.; Vinokurov, A.; Soydugan, Faruk
    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 s 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 per cent confidence level for ULX-4 based on the archival Hubble Space Telescope (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 d 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 disc. If, on the other hand, the periodicity is superorbital in nature i.e. due to possible interactions of the compact object with a circumstellar disc, the donor could then be a Be/X star hosting a neutron star.