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.
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 (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 ∼ 0. 2 error radius at the 90% confidence level. Of the two remaining ULXs, X-1 has no 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 .
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.
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 XMM-Newton, Chandra, Swift, and Hubble Space Telescope (HST) observations. The source has a peak luminosity of L X ∼ 2 × 10 39 erg sin the 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 HST/Advanced Camera for Surveys (ACS), three optical sources have been identified as counterpart candidates within the 1σ error radius of 0 .3. The brightest one has an absolute magnitude of M V ≈ −7.0 and shows extended structure. The remaining two sources have absolute magnitudes of M V ≈ −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 indicating that the compact source is most likely a stellar-mass black hole.
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 (HST) Wide Field Camera (WFC3)/UVIS. Four optical sources have been identified as possible counterparts within the 2σ error radius of 0. ′′ 3 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 V ≈ -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 ⊙ , respectively. The spectral energy distributions (SEDs) of two candidates were modeled by a power-law spectrum with a photon index (α) ∼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.
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