Search

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 {\it HST} archival data. With a precise astrometry, we confirm that NGC 4861 X1 associated with an HII complex as reported by \cite{2014MNRAS.441.1841T} and we conclude that NGC 4861 X2 resides in a young star group with 400$\pm$80 M$\odot$. We also find that NGC 4449 X7 is associated with three optical candidates within an error radius of 0$\farcs$2 at the 90$\%$ confidence level. Absolute magnitudes (M$_{v}$) of these candidates are determined as $-$5.0 and $-$4.1. The age and mass values for the three candidates by using stellar evolutionary tracks are estimated as 40-50 Myr and $\sim$8 M$\odot$, respectively. The locations of optical candidates suggest possible association with a nearby group of stars. In addition, we analyzed the previously unused archival data of {\it XMM-Newton}, {\it Chandra} and {\it Swift} where the sources 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., Comment: 14 pages, 8 tables, 6 figures, Accepted for publication in MNRAS

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$\times$10$^{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$\times$10$^{38}$ erg s$^{-1}$. Present luminosity implies 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. If explained by a diskblackbody model, the mass of compact object is estimated 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 0.22" error circle at 95\% confidence level for X-7 using the archival HST/ACS and 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) located 6" away from X-7 has a (high) luminosity of $\sim$ 10$^{39}$ erg s$^{-1}$ with no visible optical candidate., Accepted for publication in MNRAS (updated according to the published version)

We present the results of a search for optical counterparts of ultraluminous X-ray source (ULX) X-1 in the nearby galaxy NGC 2500 by using archival images taken with the Hubble Space Telescope ({\it HST}) Wide Field Camera (WFC3)/UVIS. Four optical sources have been identified as possible counterparts within the 2$\sigma$ error radius of 0.3 arcsec in the images. However, only two of them were investigated as candidates for counterparts due to their point-like features and their identification in various filters. These two faint candidates have absolute magnitudes of $M_{\rm V}$ $\approx$ -3.4 and -3.7. Also possible spectral classes of them were determined as B type main sequence stars. The ages and the masses of the candidates from Color Magnitude Diagram (CMD) were estimated as 45 Myr and 7 $M_{\rm \odot}$, respectively. The spectral energy distributions (SEDs) of two candidates were modeled by a power-law spectrum with a photon index ($\alpha$) $\sim$1.5. The spectra with such slopes could be interpreted as an evidence of reprocessing of the X-rays in the outer part of the disk that generates optical emission., Comment: Submitted to MNRAS, 5 figures, 2 tables

We report the identification of the possible optical counterparts of five out of seven Ultraluminous X-ray Sources (ULXs) in NGC 4490/4485 galaxy pair. Using archival Hubble Space Telescope ({\it HST}) imaging data, we identified a single optical candidate for two ULXs (X-4 and X-7) and multiple optical candidates for the other three ULXs (X-2, X-3 and X-6) within $\sim$ $0\farcs2$ error radius at the 90\% confidence level. Of the two remaining ULXs, X-1 has no {\it HST} imaging data and photometry could not be performed due to the position of X-5 in NGC4490. Absolute magnitudes ($M_{V}$) of the optical candidates lie between $-5.7$ and $-3.8$. Color-Magnitude Diagrams (CMDs) have been used to investigate the properties of counterparts and their environments. The locations of the counterparts of X-2, X-4, and X-6 suggest possible association with nearby group of stars while others have no association with a star cluster or group of stars. For comparison purposes, we analyzed previously unused three archival XMM-Newton observations. The long-term X-ray light curves of the sources (except transient X-7) show variability by a factor of three in a time scale more than a decade. The use of disk blackbody model for the mass of the compact objects indicates that these objects might have masses most likely in the range 10$-$15 $M_{\sun}$., 20 pages, 13 figures, submitted to ApJ

Bulgarian Air Traffic Services Authority;Bulgarian Nuclear Regulatory Agency;European Physical Society (EPS);Kozloduy Nuclear Power Plant;Ministry of Education and Science of Bulgaria;University of Sofia St. Clement of Ohrid 10th Jubilee Conference of the Balkan Physical Union, BPU 2018 --26 August 2018 through 30 August 2018 -- -- We present optical identification of counterparts of ULX-4 in spiral galaxy M51. Ultraluminous X-ray sources (ULXs) are variable, non-nuclear point-like X-ray sources with isotropic luminosities (Lx > 10 39 erg s -1 ) above the Eddington limit for a 10 M E black hole. The ULX-4 has observed with Hubble Space Telescope/Advanced Camera for Surveys (HST/ACS) in filters F438W (B), F555W (V) and F814W (I), their data are available in archive. Two optical sources have been identified as counterpart candidates within the error radius of 0.2 arcsec and their V magnitudes are ~23. The possible counterparts of ULX-4 were determined as B type supergiant stars, indicating that this system may be a high-mass X-ray binary. © 2019 Author(s). 117F115 Space Telescope Science Institute National Aeronautics and Space Administration: NAS 5-26555 This research was supported by the Scientific and Technological Research Council of Turkey (TUBITAK) through project number 117F115. We thank TUBITAK for a partial support in using RTT-150 (Russian–Turkish 1.5 m telescope in Antalya) with project number 18BRTT150-1367. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.

Turkish Physical Society 33rd International Physics Congress, TPS 2017 --6 September 2017 through 10 September 2017 -- -- The X-ray and optical properties of seven ultraluminous X-ray Sources (ULXs) in the interacting galaxies NGC 4490/4485 were examined in this study. The XMM-Newton and Chandra archival data were used for X-ray analyses. To study the optical counterparts of ULXs, the Hubble Space Telescope, (HST) archival data were used. After the relative astrometric correction between the Chandra and HST images, bright optical counterpart candidates were found for three ULXs (X-4, X-6 and X-7). © 2018 Author(s).

We study the X-ray and optical properties of the ultraluminous X-ray source (ULX) X-6 in the nearby galaxy NGC 4258 (M106) based on the archival {\it XMM-Newton}, {\it Chandra}, {\it Swift}, and {\it Hubble Space Telescope} ({\it HST}) observations. The source has a peak luminosity of $L_{\mathrm{X}} \sim 2 \times 10^{39}$ erg s$^{-1}$ in the {\it XMM-Newton} observation of 2004 June. Consideration of the hardness ratios and spectral model parameters shows that the source seems to exhibit possible spectral variations throughout the X-ray observations. In the images from the {\it HST}/Advanced Camera for Surveys (ACS), three optical sources have been identified as counterpart candidates within the 1$\sigma$ error radius of 0$\arcsec$.3. The brightest one has an absolute magnitude of $M_{V} \approx$ $-$7.0 and shows extended structure. The remaining two sources have absolute magnitudes of $M_{V} \approx$ $-$5.8 and $-$5.3 mag. The possible spectral types of the candidates from brightest to dimmest were determined as B6$-$A5, B0$-$A7, and B2$-$A3, respectively. The counterparts of the X-ray source possibly belong to a young star cluster. Neither the standard disk model nor the slim disk model provides firm evidence to determine the spectral characteristics of ULX X-6. We argue that the mass of the compact object lies in the range $10-15M_{\sun}$ indicating that the compact source is most likely a stellar-mass black hole., Comment: 10 pages, 8 figures, accepted for publication in ApJ

We identified two optical counterparts of brightest ultraluminous X-ray Sources (ULXs) in galaxies NGC5474 and NGC3627 (M66). The counterparts in Hubble Space Telescope images are very faint, their V magnitudes are 24.7 ($M_V \approx -4.5$) and 25.9 ($M_V \approx -4.2$), respectively. NGC5474 X-1 changes the X-ray flux more than two orders of magnitude, in its bright state it has $L_X \approx 1.6 \times 10^{40}$ erg/s, the spectrum is best fitted by an absorbed power-law model with a photon index $\Gamma \approx 0.94$. M66 X-1 varies in X-rays with a factor of ~2.5, its maximal luminosity being $2.0 \times 10^{40}$ erg/s with $\Gamma \approx 1.7$. Optical spectroscopy of the NGC5474 X-1 has shown a blue spectrum, which however was contaminated by a nearby star of 23 mag, but the counterpart has a redder spectrum. Among other objects captured by the slit are a background emission-line galaxy (z=0.359) and a new young cluster of NGC5474. We find that these two ULXs have largest X-ray-to-optical ratios of $L_X/L_{opt}$ ~ 7000 for NGC5474 X-1 (in its bright state) and 8000 for M66 X-1 both with the faintest optical counterparts ever measured. Probably their optical emission originates from the donor star. If they have super-Eddington accretion discs with stellar-mass black holes, they may also have the lowest mass accretion rates among ULXs such as in M81 X-6 and NGC1313 X-1., Comment: 5 pages, 3 figures, to appear in MNRAS Letters