Yazar "Allak, S." seçeneğine göre listele
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Öğe Detection of 125.5-day optical periodic modulation of the neutron star M51 ULX-8(Oxford Univ Press, 2022) Allak, S.Studying Ultraluminous X-ray sources (ULXs) in the optical wavelengths provides important clues about the accretion mechanisms and the evolutionary processes of X-ray binary systems. In this study, three (C1, C2, and C3) possible optical counterparts were identified for well-known neutron star (NS) candidate M51 ULX-8 through advanced astrometry based on the Chandra and Hubble Space Telescope (HST) observations, as well as the GAIA optical source catalogue. Optical periodic modulation of 125.5 d with an amplitude of 0.14 mag was determined for C3, which has evidence to represent the optical nature of ULX-8 using one-year (2016-2017) 34 HST Advanced Camera for Surveys (ACS)/Wide Field Camera (WFC) observations. Moreover, surprisingly, the observed optical fluxes of C3 exhibit a bi-modal distribution. This could mean that there is a possible correlation between the optical and the X-ray flux variabilities of the ULX-8. The possible scenarios which are frequently mentioned in the literature proposed for the nature of optical emission and optical super-orbital period. The most probable scenario is that the optical emission could have originated from the accretion disc of the ULX-8.Öğe Identification of a new ultraluminous X-ray source in NGC 1316(Oxford Univ Press, 2020) Allak, S.; Akyuz, A.; Aksaker, N.; Ela, M. Ozdogan; Avdan, S.; Soydugan, FarukIn this study, we report identification of a newultraluminous X-ray source (ULX) named as X-7 inNGC1316, with an unabsorbed luminosity of 2.1 x 10(39) 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 x 10(38) erg s(-1). Present luminosity implies a luminosity increase of a factor of similar to 4. The best-fitting spectral model parameters indicate that X-7 has a relatively hot disc and hard spectra. If explained by a disc blackbody model, the mass of compact object is estimated as similar to 8 M-circle dot 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 arcsec error circle at 95 per cent confidence level for X-7 using the archival HST/ACS (Hubble Space Telescope/Advanced Camera for Surveys) and HST/WFC3 (The Wide Field Camera 3) 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) located 6 arcsec away from X-7 has a (high) luminosity of similar to 10(39) erg s(-1) with no visible optical candidate.Öğe New times of minima of some eclipsing binary stars(Hungarian Academy of Sciences (Konkoly Observatory), 2017) Bulut, I.; Kabaş, A.; Nehir, Ç.; Allak, S.; Neslihan, K.; Yilan, E.; Doğan, M.[No abstract available]Öğe Optical counterparts of ULXs in NGC 1672(Oxford Univ Press, 2022) Allak, S.; Akyuz, A.; Sonbas, E.; Dhuga, K. S.In this work, we deploy archival data from Hubble Space Telescope, Chandra, XMM-Newton, and Swift-XRT, to probe the nature of nine candidate ultraluminous X-ray sources (ULXs) in NGC 1672. Specifically, our study focuses on using the precise source positions obtained via improved astrometry based on Chandra and Hubble Space Telescope observations to search for and identify potential optical counterparts for these ULXs. Unique optical counterparts are identified for two of the ULX candidates, i.e. X2 and X6; for three of the candidates, i.e. X1, X5, and X7, we found two potential counterparts for each source within the respective error radii. No optical counterparts were found for the remaining four sources. The spectral energy distribution of X2 is fitted to a blackbody spectrum with a temperature of similar to 10(4) K and the spectral class of the source is determined to be B7-A3, a supergiant donor star. We used colour-magnitude diagrams to investigate ages of the counterparts. Of all the sources studied, X9 exhibits the most variability whereby the X-ray flux varies by a factor of similar to 50 over a time period spanning 2004-2019, and also traces a partial q-curve-like feature in the hardness-intensity diagram, hinting at possible spectral transitions.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.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, FarukWe 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.Öğe The ULX source X-3 in NGC 4258: a search for its X-ray and optical properties(Oxford Univ Press, 2020) Akyuz, A.; Avdan, S.; Allak, S.; Aksaker, N.; Oralhan, I. Akkaya; Balman, S.We present the results of a search for the nature of ultraluminous X-ray source (ULX) X-3 in the nearby galaxy NGC 4258. We use archival data from XMM-Newton, Chandra, NuSTAR, and Hubble Space Telescope (HST) observations. Total X-ray data were analysed to find that the model parameters of the system are indicative of a stellar-mass black hole, similar to 10 M-circle dot, as the central compact object. Furthermore, analyses of the optical data from HST reveal two optical candidates at 90 per cent confidence level within an error radius of 0.28 arcsec. Assuming the optical emission is dominated by the donor star, both of these candidates are found to have spectral types that lie between B3 and F1 with absolute magnitudes of M-V approximate to -6.4. Moreover, the age and mass estimates for the candidates are found to be 10 and 18 Myr and 13 and 20 M-circle dot, respectively.
