Ultraluminous X-ray Source NGC Research Articles

Simultaneous and observations of the ultraluminous source NGC 4190 ULX-1

We present an X-ray analysis of three different XMM-Newton observations together with simultaneous and observations of the ultraluminous X-ray source NGC 4190 ULX-1. Our goal is to constrain the structure of the accretion disk and the geometrical properties of the source. We performed temporal and spectral analyses in the 0.4--30 keV energy range in which the source is significantly detected in dedicated XMM-Newton and NuSTAR observations. The temporal analysis shows no flaring activity in the light curves. No pulsation is detected throughout. The source exhibits a typical ULX spectrum, which can be fitted with two thermal blackbody components plus a Comptonization tail at high energies. The luminosity-temperature ($L-T$) relation of each thermal spectral component is consistent with the $L $ relation expected from an advection-dominated supercritical disk. We interpret these results as a super-Eddington accreting black hole seen almost face-on. A dense wind ejected from the disk obscures the central source, and a hot electron plasma is evacuated through the funnel formed above the hole. Geometric beaming is responsible for the ULX soft emission, whereas the hard tail is the result of the Comptonization of soft photons by the electrons ejected through the funnel.

A multiband look at ultraluminous X-ray sources in NGC 7424

ABSTRACT We studied the multiband properties of two ultraluminous X-ray sources (2CXO J225728.9−410211 = X-1 and 2CXO J225724.7−410343 = X-2) and their surroundings, in the spiral galaxy NGC 7424. Both sources have approached X-ray luminosities LX ∼ 1040 erg s−1 at some epochs. Thanks to a more accurate astrometric solution (based on Australia Telescope Compact Array and Gaia data), we identified the point-like optical counterpart of X-1, which looks like an isolated B8 supergiant (M ≈ 9M⊙, age ≈30 Myr). Instead, X-2 is in a star-forming region (size of about 100 pc × 150 pc), near young clusters and ionized gas. Very large telescope long-slit spectra show a spatially extended region of He ii λ4686 emission around the X-ray position, displaced by about 50 pc from the brightest star cluster, which corresponds to the peak of lower ionization line emission. We interpret the He ii λ4686 emission as a signature of X-ray photoionization from the ULX, while the other optical lines are consistent with UV ionization in an ordinary He ii region. The luminosity of this He++ nebula puts it in the same class as other classical photoionized ULX nebulae such as those around Holmberg II X-1 and NGC 5408 X-1. We locate a strong (5.5 GHz luminosity $\nu \, L_{\nu } \approx 10^{35}$ erg s−1), steep-spectrum, unresolved radio source at the peak of the low-ionization lines, and discuss alternative physical scenarios for the radio emission. Finally, we use WISE data to obtain an independent estimate of the reddening of the star-forming clump around X-2.

Study of the ultraluminous X-ray sources in NGC 4382 and NGC 1399 with Chandra

Study of the ultraluminous X-ray sources in NGC 4382 and NGC 1399 with Chandra

Spectral and Timing Study of the Newly Detected Ultraluminous X-Ray Sources in NGC 3585 Using Different Chandra Observations.

The present work aims to study the previously unstudied Ultraluminous X-ray sources (ULXs) in the galaxy NGC 3585 at its various epochs of Chandra observation. We report here the detection of two new ULXs viz. CXOUJ111306.0-264825 (X-1) and CXOUJ111325.3-264732 (X-2) with their bolometric luminosity > 1039erg s−1 in its various Chandra observations. X-1 was found to be a spectrally hard ULX in both the epochs where it was detected. However in the ULX, X-2, a slight hardening of the spectra was observed within a period of 17 years. Assuming isotropic emission and explained by disk blackbody model, the spectrally softer epoch of X-2 with an inner disk temperature, kTin ∼ 0.79 keV and bolometric luminosity ∼ 2.51 × 1039erg s−1 implies for X-2 to be powered by a compact object, necessarily a black hole of mass, MBH ∼ 44.85+82.11−25.92M⊙ accreting at ∼ 0.42 times the Eddington limit. The Lightcurve of X-1 and X-2 binned at 500s, 1ks, 2ks and 4ks has shown no signature of short-term variability in both the ULXs in kilo-seconds time scales. Overall, both the detected ULXs seem to be almost static sources both in long-term (years) as well as short-term (kilo-seconds) time scales with the presently available Chandra Observations.

NGC 5474 X-1: A neutron star ultraluminous X-ray source in an old stellar cluster?

ABSTRACT We present an optical and X-ray study of the ultraluminous X-ray source (ULX) NGC 5474 X-1. The X-ray spectrum taken during the bright state of the source (LX ∼ 2 × 1040 erg s–1) shows signatures of a broad absorption line at ≃ 8 keV, which may be a cyclotron resonant scattering feature. This implies that this system may host a neutron star with a magnetic field of ∼1012 G. The first observation of this area with the Hubble Space Telescope (HST), carried out 14 months later, revealed that the source was bright in the optical range as well. Subsequent observations have shown that the source faded in both ranges (more than 2.8 mag in the U band and by a factor of 50–100 in the X-ray) and has never become bright again. Deeper HST observations made it possible to impose constraints on the donor star spectral class and mass ($\lt \, 7$ M⊙), as well as to identify a stellar cluster of about 1 Gyr, the centre of which is located at a projected distance of ~2 pc from NGC 5474 X-1. The ULX could be a member of this old cluster; however, the presence of stars with ages of ∼10 Myr within 300 pc of the ULX means that it is possible that this is just an accidental projection.

Discovery of an X-Ray Photoionized Optical Nebula and a Radio Nebula Associated with the ULX NGC 4861 X-1

We have conducted long-slit spectroscopic observations and analyzed archival radio data for the ultraluminous X-ray source (ULX) NGC 4861 X-1. Our spectral line analysis unveils NGC 4861 X-1 as the fourth ULX situated within an X-ray photoionized nebula, following three previous findings made approximately two decades ago. Remarkably, we discover NGC 4861 X-1 also possesses a radio nebula emitting optically thin synchrotron radiation, which contradicts its X-ray photoionization and raises doubts about the four ULXs being a mere coincidence. Instead of gradually accumulating from different bands bit by bit, our multiband discovery is made all at once. Moreover, we tentatively perceive a faint continuum spectrum of the optical nebula. Further observations are needed to ascertain its radio structures and verify the optical continuum.

Cover Picture: Astron. Nachr. 04/2023

Visualization of the results of a loop of spectral fits onto the time-averaged XMM-Newton / RGS spectrum of the Ultraluminous X-ray source NGC 247 ULX-1. The parameter space explored by the spectral fits covers a range of wind velocities (as constrained by the Doppler shift of the absorption lines) and photoionization parameters 𝜉 (an indicator of the wind temperature measured through the relative line ratios). The color code of the 2D map indicates, for each individual spectral fit, the improvement provided by the wind plasma component on the broadband continuum model. The black contours show the (2.0, 2.5, … 5.0 sigma) confidence levels, suggesting that the wind blows at about 17% of the speed of light. These results provide one of the most convincing detections of a hot plasma outflow in ultraluminous X-ray sources, the very first in the subclass of the supersoft sources. The implications of this discovery are discussed in the contribution of C. Pinto and P. Kosec, this issue, e20220134

Correction to: Super-Eddington accretion on to a stellar mass ultraluminous X-ray source NGC 4190 ULX1

Correction to: Super-Eddington accretion on to a stellar mass ultraluminous X-ray source NGC 4190 ULX1

Identification of a Helium Donor Star in NGC 247 ULX-1

With Very Large Telescope Multi Unit Spectroscopic Explorer (MUSE) observations, we detected highly variable helium emission lines from the optical counterpart of the supersoft ultraluminous X-ray source (ULX) NGC 247 ULX-1. No Balmer lines can be seen in the source spectrum. This is the first evidence for the presence of a helium donor star in ULXs, consistent with a prediction that helium donor stars may be common in ULXs. The helium lines with an FWHM of about 200 km s−1 are likely produced on the outer accretion disk. Their strong variation implies that the central X-ray source can be significantly obscured to the outer disk. Also, a ring or a double-ring structure is revealed in the MUSE image. It is unknown whether or not it is related to the progenitor of the ULX binary.

Bubble in the Whale: Identifying the Optical Counterparts and Extended Nebula for the Ultraluminous X-Ray Sources in NGC 4631

We present a deep optical imaging campaign on the starburst galaxy NGC 4631 with CFHT/MegaCam. By supplementing the HST/ACS and Chandra/ACIS archival data, we search for the optical counterpart candidates of the five brightest X-ray sources in this galaxy, four of which are identified as ultraluminous X-ray sources (ULXs). The stellar environments of the X-ray sources are analyzed using the extinction-corrected color–magnitude diagrams and the isochrone models. We discover a highly asymmetric bubble nebula around X4 that exhibits different morphology in the Hα and [O iii] images. The [O iii]/Hα ratio map shows that the Hα-bright bubble may be formed mainly via the shock ionization by the one-sided jet/outflow, while the more compact [O iii] structure is photoionized by the ULX. We constrain the bubble expansion velocity and interstellar medium density with the MAPPINGS V code and hence estimate the mechanical power injected into the bubble as P w ∼ 5 × 1040 erg s−1 and the corresponding bubble age as ∼7 × 105 yr. Relativistic jets are needed to provide such a level of mechanical power with a mass-loss rate of ∼10−7 M ⊙ yr−1. Besides the accretion, the black hole spin is likely an additional energy source for the super-Eddington jet power.

Optical counterparts of two candidate ultraluminous X-ray sources in NGC 4536

ABSTRACT Archival XMM–Newton, Chandra, and Hubble Space Telescope(HST) data have been used to study the X-ray and optical properties of two candidate ultraluminous X-ray sources in NGC 4536. In order to search for potential optical counterparts, relative astrometry between Chandra and HST was improved, and as a result, optical counterparts were detected for both X-ray sources. To complement our findings (based on the archival data), ground-based optical spectra of the counterparts were obtained with the 6-m BTA (Big Telescope Alt-azimuth) Telescope located at the Special Astrophysical Observatory. The calculated redshift (z = 0.4391 ± 0.0010) for one of the sources (X-3) indicates that the source is, in fact, a background active galactic nucleus. Two possible optical counterparts (s1 and s2) were found for X-2. Whether s1 is point-like or an extended source is unclear: If it is point-like and the emission is dominated by the donor, its spectral type indicates O–B star. The second source (s2) is point-like and is consistent with the colours and absolute magnitudes of a red supergiant.

Multiwavelength study of NGC 1365: The obscured active nucleus and off-nuclear compact X-ray sources

ABSTRACTWe present a multiwavelength study of the active nucleus and the off-nuclear X-ray sources in the nearby spiral galaxy, NGC 1365 using three simultaneous UV/X-ray observations by AstroSat over a two months period and archival IR observations performed with Spitzer and Herschel. Utilizing the data from the Soft X-ray Telescope (SXT) onboard AstroSat, we find spectral variability mainly caused by the variation in the X-ray column density, (NH ∼ 1022–1023 cm−2). With the accurate spatial resolution of the UVIT onboard AstroSat, we separate the intrinsic AGN flux from the host galaxy emission and then correct for the Galactic and the internal reddening. We detect no significant variation in the NUV emission over the observation period. The AGN in FUV band is undetectable due to heavy intrinsic extinction. Further, the multiwavelength IR/UV/X-ray AGN SED reveals that the AGN is in a low-luminosity phase with accretion rate ∼0.01 LEdd. The steady UV emission and strong X-ray absorption variability suggest that the obscuring clouds are likely compact and affect the compact X-ray source only and do not possibly cover the extended UV emitting region. In addition, the UVIT is able to resolve two bright spots at a radius of 7 arcsec (∼6.3 Kpc) from the central nucleus in the South-West (SW) direction. In the UVIT image of the entire galaxy, we identify UV counterparts to four Chandra identified bright X-ray sources. One well-known ultra-luminous X-ray source (ULX) NGC 1365 X2 is identified with its UV counterpart at 86 arcsec from the nucleus in the North-East (NE) direction from the active nucleus.

Optical and X-ray follow-up to a globular cluster ultraluminous X-ray source in NGC 4472

ABSTRACT NGC 4472 is home to five ultraluminous X-ray sources hosted by globular clusters. These sources have been suggested as good black hole candidates in extragalactic globular clusters—a highly sought after population that may provide observational information regarding the progenitors of merging black hole binaries. In this body of work, we present X-ray and optical follow-up to one of these sources, CXOUJ1229410+075744 (GCU1). We find no evidence of [OIII] optical emission in GCU1 which indicates a lack of significant evidence for super-Eddington outflows, unlike what is seen in a handful of ULXs in extragalactic GCs. X-ray monitoring from 2019 to 2021 shows no detected X-ray emission above a few × 1038 erg/s. Comparisons of the multiwavelength properties to disc-dominated, near Eddington Galactic black hole low-mass X-ray binaries (GRS 1915+105 and XTEJ1817-330) suggests that GCU1 may show similar behaviour to GRS 1915+105 in terms of X-ray variability and similar relationships between LX and kT, with GCU1 showing maximum X-ray luminosities on order of higher magnitude.

Spectral variability in NGC 1042 ULX1

ABSTRACT We report X-ray spectral variability in an ultraluminous X-ray source NGC 1042 ULX1, using archival XMM-Newton and recent NuSTAR observations. In long-term evolution, the source has shown a trend of variation in spectral hardness. The variability in different XMM-Newton observations is prominent above ∼1 keV. Cool thermal disc component with a characteristic temperature of ∼0.2 keV manifests that the spectral state of NGC 1042 ULX1 in all epochs is similar to that of the ultraluminous state sources. An apparent anti-correlation between luminosity and power-law index demonstrates that the source becomes spectrally harder when it is in a brighter state. That is conceivably related to variation in accretion rate, strength of Comptonization, wind/outflow in the system or a manifestation of varying disc occultation. Typical hard ultraluminous-type spectra indicate that NGC 1042 ULX1 is a low inclination system in general. Spectral properties suggest that, like many other ULXs which show spectral curvature around ∼6–10 keV, NGC 1042 ULX1 could be another stellar-mass super-Eddington accretor.

Unveiling the disc structure in ultraluminous X-ray source NGC 55 ULX-1

ABSTRACT Ultraluminous X-ray sources (ULXs) are the most extreme among X-ray binaries in which the compact object, a neutron star or a black hole, accretes matter from the companion star, and exceeds a luminosity of $10^{39} \ \rm erg \, s^{-1}$ in the X-ray energy band alone. Despite two decades of studies, it is still not clear whether ULX spectral transitions are due to stochastic variability in the wind or variations in the accretion rate or in the source geometry. The compact object is also unknown for most ULXs. In order to place constraints on to such scenarios and on the structure of the accretion disc, we studied the temporal evolution of the spectral components of the variable source NGC 55 ULX-1. Using recent and archival data obtained with the XMM-Newton satellite, we modelled the spectra with two blackbody components which we interpret as thermal emission from the inner accretion flow and the regions around or beyond the spherization radius. The luminosity–temperature (L–T) relation of each spectral component agrees with the L ∝ T4 relationship expected from a thin disc model, which suggests that the accretion rate is close to the Eddington limit. However, there are some small deviations at the highest luminosities, possibly due to an expansion of the disc and a contribution from the wind at higher accretion rates. Assuming that such deviations are due to the crossing of the Eddington or supercritical accretion rate, we estimate a compact object mass of 6–14 M⊙, favouring a stellar-mass black hole as the accretor.

A transient ultraluminous X-ray source in NGC 55

ABSTRACT Ultraluminous X-ray sources (ULXs) are a class of accreting compact objects with X-ray luminosities above 1039 erg s−1 . The average number of ULXs per galaxy is still not well-constrained, especially given the uncertainty on the fraction of ULX transients. Here, we report the identification of a new transient ULX in the galaxy NGC 55 (which we label as ULX-2), thanks to recent XMM–Newton and the Neil Gehrels Swift Observatory observations. This object was previously classified as a transient X-ray source with a luminosity around a few 1038 erg s−1 in a 2010 XMM–Newton observation. Thanks to new and deeper observations (∼130 ks each), we show that the source reaches a luminosity peak >1.6 × 1039 erg s−1. The X-ray spectrum of ULX-2 is much softer than in previous observations and fits in the class of soft ULXs. It can be well-described using a model with two thermal components, as often found in ULXs. The time-scales of the X-ray variability are of the order of a month and are likely driven by small changes in the accretion rate or due to super-orbital modulations, attributed to precession of the accretion disc, which is similar to other ULXs.

The Variability Behavior of NGC 925 ULX-3

We report the results of a 2019–2021 monitoring campaign with Swift and associated target-of-opportunity observations with XMM-Newton and NuSTAR, examining the spectral and timing behavior of the highly variable ultraluminous X-ray source (ULX) NGC 925 ULX-3. We find that the source exhibits a 127–128-day periodicity, with fluxes typically ranging from 1 × 10−13 to 8 × 10−13 erg s−1 cm−2. We do not find strong evidence for a change in period over the time that NGC 925 ULX-3 has been observed, although the source may have been in a much lower flux state when first observed with Chandra in 2005. We do not detect pulsations, and we place an upper limit on the pulsed fraction of ∼40% in the XMM-Newton band, consistent with some previous pulsation detections at low energies in other ULXs. The source exhibits a typical ULX spectrum that turns over in the NuSTAR band and can be fitted using two thermal components. These components have a high temperature ratio that may indicate the lack of extreme inner disk truncation by a magnetar-level magnetic field. We examine the implications for a number of different models for superorbital periods in ULXs, finding that a neutron star with a magnetic field of ∼1012 G may be plausible for this source. The future detection of pulsations from this source would allow for the further testing and constraining of such models.

A multi-wavelength view of distinct accretion regimes in the pulsating ultraluminous X-ray source NGC 1313 X-2

ABSTRACTNGC 1313 X-2 is one of the few known pulsating ultraluminous X-ray sources (PULXs), and so is thought to contain a neutron star that accretes at highly super-Eddington rates. However, the physics of this accretion remains to be determined. Here, we report the results of two simultaneous XMM–Newton and HST observations of this PULX taken to observe two distinct X-ray behaviours as defined from its Swift light curve. We find that the X-ray spectrum of the PULX is best described by the hard ultraluminous regime during the observation taken in the lower flux, lower variability amplitude behaviour; its spectrum changes to a broadened disc during the higher flux, higher variability amplitude epoch. However, we see no accompanying changes in the optical/UV fluxes, with the only difference being a reduction in flux in the near-infrared (NIR) as the X-ray flux increased. We attempt to fit irradiation models to explain the UV/optical/IR fluxes but they fail to provide meaningful constraints. Instead, a physical model for the system leads us to conclude that the optical light is dominated by a companion O/B star, albeit with an IR excess that may be indicative of a jet. We discuss how these results may be consistent with the precession of the inner regions of the accretion disc leading to changes in the observed X-ray properties, but not the optical, and whether we should expect to observe reprocessed emission from ULXs.

A Long-Term Study of Ultraluminous X-ray Sources in NGC 891

We perform empirical fits to the Chandra and XMM-Newton spectra of three ultraluminous X-ray sources (ULXs) in the edge-on spiral galaxy NGC 891, monitoring the region over a 17-year time window. One of these sources was visible since the early 1990s with ROSAT and was observed multiple times with Chandra and XMM-Newton. Another was visible since 2011. We build upon prior analyses of these sources by analyzing all available data at all epochs. Where possible Chandra data is used, since its superior spatial resolution allows for more effective isolation of the emission from each individual source, thus providing a better determination of their spectral properties. We also identify a new transient ULX, CXOU J022230.1+421937, which faded from view over the course of a two month period from Nov 2016 to Jan 2017. Modeling of each source at every epoch was conducted using six different models ranging from thermal bremsstrahlung to accretion disk models. Unfortunately, but as is common with many ULXs, no single model yielded a much better fit than the others. The two known sources had unabsorbed luminosities that remained fairly consistent over five or more years. Various possibilities for the new transient ULX are explored.

Spectral and temporal properties of ultra-luminous X-ray source NGC 55 ULX1

ABSTRACT We investigate the spectral and temporal properties of ultra-luminous X-ray source (ULX) NGC 55 ULX1 using Swift, XMM-Newton and NuSTAR observations conducted during 2013–2021. In these observations, the source flux varies by a factor of ∼5–6, and we identify the source mainly in the soft ultraluminous (SUL) state of ULXs. We fit the X-ray spectra with a two thermal component model consisting of a blackbody (for the soft component) and a disc (for the hard component), and the soft component dominates in these observations. The soft component in the SUL state shows properties similar to that of ultraluminous supersoft sources, for example, an anticorrelation between the characteristic radius and temperature of the blackbody component. In addition, we observe a positive correlation between the blackbody and inner disc temperatures when the X-ray spectra are fitted with the two-thermal component model. The source exhibits marginal evidence of X-ray flux dips in the Swift and XMM-Newton observations at different intensity levels. We explain the observed spectral and temporal properties of the source by invoking the supercritical radiatively driven outflow mechanism.