Spectral study of the X-ray sources in NGC 4472

The recent evidence of a peculiar distribution of X-ray sources in the elliptical galaxy NGC 4261 reported by Zezas et al. has prompted us to study this galaxy combining archive X-ray and optical observations, from Chandra, INT, and HST. We find that a sizable fraction of the X-ray sources has a globular cluster as optical counterpart. This together with the shape of the luminosity function of the X-ray sources suggest that they are accreting low-mass binaries. We further show a remarkable similarity in the anisotropy of the projected spatial distributions of the optical and X-ray sources, which leads us to conclude that the spatial anisotropy of the X-ray sources in NGC 4261 is due to the anisotropy of the globular cluster population.

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.

We report on the discovery of an absorption line at keV detected with a significance of >3.3σ in the NuSTAR and XMM-Newton spectra of a newly discovered hyperluminous X-ray source (L X > 1041 erg s−1) in the galaxy NGC 4045 at a distance of 32 Mpc. The source was first discovered serendipitously in a Swift/XRT observation of the galaxy, and Swift monitoring reveals a highly variable source changing by over an order of magnitude from maximum to minimum. The origin of the absorption line appears likely to be from highly ionized iron with a blueshift of 0.19c, indicating an ultrafast outflow. However, the large equivalent width of the line ( keV) paired with the lack of other absorption lines detected is difficult to reconcile with models. An alternative explanation is that the line is due to a cyclotron resonance scattering feature produced by the interaction of X-ray photons with the powerful magnetic field of a neutron star.

We report on the discovery of a new, transient ultraluminous X-ray source (ULX) in the galaxy NGC 7090. This new ULX, which we refer to as NGC 7090 ULX3, was discovered via monitoring with Swift during 2019–2020, and to date has exhibited a peak luminosity of LX ∼ 6 × 1039 erg s−1. Archival searches show that, prior to its recent transition into the ULX regime, ULX3 appeared to exhibit a fairly stable luminosity of LX ∼ 1038 erg s−1. Such strong long-time-scale variability may be reminiscent of the small population of known ULX pulsars, although deep follow-up observations with XMM–Newton and NuSTAR do not reveal any robust X-ray pulsation signals. Pulsations similar to those seen from known ULX pulsars cannot be completely excluded, however, as the limit on the pulsed fraction of any signal that remains undetected in these data is ≲20 per cent. The broad-band spectrum from these observations is well modelled with a simple thin disc model, consistent with sub-Eddington accretion, which may instead imply a moderately large black hole accretor (MBH ∼ 40 M⊙). Similarly, though, more complex models consistent with the super-Eddington spectra seen in other ULXs (and the known ULX pulsars) cannot be excluded given the limited signal-to-noise ratio of the available broad-band data. The nature of the accretor powering this new ULX therefore remains uncertain.

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.

Two ultraluminous X-ray sources (ULXs) in the nearby Sb galaxy NGC 1313, named X-1 and X-2, were observed with Suzaku on 2005 September 15. During the observation for a net exposure of 28 ks (but over a gross time span of 90 ks), both objects varied in intensity by about 50%. The 0.4–10 keV X-ray luminosities of X-1 and X-2 were measured as $2.5 \times 10^{40} \,\mathrm{erg} \,\mathrm{s}^{-1}$ and $5.8 \times 10^{39} \,\mathrm{erg} \,\mathrm{s}^{-1}$, respectively, with the former exhibiting the highest ever reported for this ULX. The spectrum of X-1 can be explained by the sum of a strong and variable power-law component with a high-energy cutoff, and a stable multicolor blackbody with an innermost disk temperature of $\sim 0.2 \,\mathrm{keV}$. These results suggest that X-1 was in a “very high” state, where disk emission is strongly Comptonized. The absorber within NGC 1313 toward X-1 is suggested to have a subsolar oxygen abundance. The spectrum of X-2 is best represented, in its fainter phase, by a multicolor blackbody model with an innermost disk temperature of 1.2–1.3 keV, and becomes flatter as the source becomes brighter. Hence, X-2 is interpreted to be in a slim-disk state. These results suggest that the two ULXs have black hole masses of some dozens to a few hundred of solar masses.

An ultraluminous X-ray source (ULX) in NGC 5252 has been known as a strong candidate for an off-nuclear intermediate-mass black hole (IMBH). We present near-infrared imaging data of the ULX obtained with the William Herschel Telescope. Using this data we estimate a stellar mass associated with the ULX of ≈107.9 ± 0.1$\, \mathrm{M}_\odot$, suggesting that it could be (the remnant of) a dwarf galaxy that is in the process of merging with NGC 5252. Based on a correlation between the mass of the central black hole (BH) and host galaxy, the ULX is powered by a 105 $\, \mathrm{M}_\odot$ BH. Alternatively, if the BH mass is ≈106$\, \mathrm{M}_\odot$ or larger, the host galaxy of the ULX must have been heavily stripped during the merger. The ULX Ks-band luminosity is two orders of magnitude smaller than that expected from an ordinary active galactic nucleus with the observed [O iii] luminosity, which also suggests the ULX lacks a dusty torus. We discuss how these findings provide suggestive evidence that the ULX is hosting an IMBH.

We have studied a highly variable ultraluminous X-ray source (ULX) in the Fornax galaxy NGC 1365, with a series of 12 Chandra and XMM-Newton observations between 2002 and 2006. In 2006 April, the source peaked at a luminosity ≈3 x 10 40 erg s -1 in the 0.3-10 keV band (similar to the maximum luminosity found by ASCA in 1995), and declined on an e-folding time-scale ≈3 d. The X-ray spectrum is always dominated by a broad power-law-like component. When the source is seen at X-ray luminosities ≈10 40 erg s -1 , an additional soft thermal component (which we interpret as emission from the accretion disc) contributes ≈1/4 of the X-ray flux; when the luminosity is higher, ≈3 x 10 40 erg s -1 , the thermal component is not detected and must contribute <10 per cent of the flux. At the beginning of the decline, ionized absorption is detected around ∼0.5-2 keV; it is a possible signature of a massive outflow. The power law is always hard, with a photon index r ≈1.7 (and even flatter at times), as is generally the case with bright ULXs. We speculate that this source and perhaps most other bright ULXs are in a high/hard state: as the accretion rate increases well above the Eddington limit, more and more power is extracted from the inner region of the inflow through non-radiative channels, and is used to power a Comptonizing corona, jet or wind. The observed thermal component comes from the standard outer disc; the transition radius between outer standard disc and Comptonizing inner region moves further out and to lower disc temperatures as the accretion rate increases. This produces the observed appearance of a large, cool disc. Based on X-ray luminosity and spectral arguments, we suggest that this accreting black hole has a likely mass ∼50-150M ⊙ (even without accounting for possible beaming).

Aims. In this work, we study the X-ray spectral and temporal properties of an ultraluminous X-ray source (ULX) in NGC 628 by using multi-epoch archival X-ray data. The physical parameters were estimated in each epoch in order to constrain the nature of the compact object in the system. Also, the optical counterpart candidates of the ULX were examined using the archival Hubble Space Telescope (HST)/Wide Field Camera 3 (WFC3) data. Methods.XMM-Newton, Chandra, and Swift data were used to create the long-term light curve (which covers a period of 22 years) and perform the spectral analysis. Lomb-Scargle periodograms of the source were constructed to examine the short-term variability in each epoch. In order to search for an optical counterpart in the HST/WFC3 images, a relative astrometric correction was initially applied to the Chandra and HST/WFC3 images. Results. The X-ray flux of the source changes by a factor of ∼200 throughout the observations. The previously detected quasi-periodic signal (in the range of 0.1−0.4 mHz) was confirmed by using the Lomb-Scargle method. After astrometric correction, two optical counterpart candidates were detected for the source. The obtained spectral energy distributions in the optical band for both candidates indicate that the optical emission is dominated by the irradiation of the accretion disc. Considering the best-fit model parameters of the multi-colour disc black-body model, we derived the mass of the black hole in the system as being in the range of (5−28) M⊙. Nonetheless, the long-term variability and the spectral transitions in the hardness–luminosity diagram make it difficult to rule out the neutron star scenario.

We have studied an ultra-luminous X-ray source (ULX) in the dwarf galaxy NGC 5408 with a series of XMM-Newton observations, between 2001 July and 2003 January. We find that its X-ray spectrum is best fitted with a power law of photon index Γ ≈ 2.6−2.9 and a thermal component with blackbody temperature kTbb ≈ 0.12−0.14 keV. These spectral features, and the inferred luminosity ≈ 10 40 erg s −1 in the 0.3−12 keV band, are typical of bright ULXs in nearby dwarf galaxies. The blackbody plus power-law model is a significantly better fit than either a simple power law or a broken power law (although the latter model is also acceptable at some epochs). Doppler-boosted emission from a relativistic jet is not required, although we cannot rule out this scenario. Our preliminary timing analysis shows flaring behaviour which we interpret as variability in the power- law component, on timescales of ∼10 2 s. The hard component is suppressed during the dips, while the soft thermal component is consistent with being constant. The power density spectrum is flat at low frequencies, has a break at νb ≈ 2.5 mHz, and has a slope ≈− 1 at higher frequencies. A comparison with the power spectra of Cyg X-1 and of a sample of other BH candidates and AGN suggests a mass of ∼10 2 M� . It is also possible that the BH is at the upper end of the stellar-mass class (M ∼ 50 M� ), in a phase of moderately super-Eddington accretion. The formation of such a massive BH via normal stellar evolution may have been favoured by the very metal-poor environment of NGC 5408.

We have re-examined the most luminous X-ray sources in the starburst galaxy NGC 4631, using XMM-Newton, Chandra and ROSAT data. The most interesting source is a highly variable supersoft ULX. We suggest that its bolometric luminosity ~ a few 10^{39} erg/s in the high/supersoft state: this is an order of magnitude lower than estimated in previous studies, thus reducing the need for extreme or exotic scenarios. Moreover, we find that this source was in a non-canonical low/soft (kT ~ 0.1-0.3 keV) state during the Chandra observation. By comparing the high and low state, we argue that the spectral properties may not be consistent with the expected behaviour of an accreting intermediate-mass black hole. We suggest that recurrent super-Eddington outbursts with photospheric expansion from a massive white dwarf (M_{wd} >~ 1.3 M_{sun}), powered by non-steady nuclear burning, may be a viable possibility, in alternative to the previously proposed scenario of a super-Eddington outflow from an accreting stellar-mass black hole. The long-term average accretion rate required for nuclear burning to power such white-dwarf outbursts in this source and perhaps in other supersoft ULXs is ~ 5-10 x 10^{-6} M_{sun}/yr: this is comparable to the thermal-timescale mass transfer rate invoked to explain the most luminous hard-spectrum ULXs (powered by black hole accretion). The other four most luminous X-ray sources in NGC 4631 (three of which can be classified as ULXs) appear to be typical accreting black holes, in four different spectral states: high/soft, convex-spectrum, power-law with soft excess, and simple power-law. None of them requires masses >~ 50 M_{sun}.

Quasi-periodic oscillations and X-ray spectroscopy are powerful probes of black hole masses and accretion disks, and here we apply these diagnostics to an ultraluminous X-ray source (ULX) in the spiral galaxy NGC 628 (M74). This object was observed four times over 2 years with the Chandra X-Ray Observatory and XMM-Newton, with three long observations showing dramatic variability, distinguished by a series of outbursts with a quasi period of 4000-7000 s. This is unique behavior among ULXs and Galactic X-ray binaries because of the combination of its burstlike peaks and deep troughs, its long quasi periods, its high variation amplitudes of >90%, and its substantial variability between observations. The X-ray spectra is fitted by an absorbed accretion disk plus a power-law component, suggesting the ULX was in a spectral state analogous to the low/hard state or the very high state of Galactic black hole X-ray binaries. A black hole mass of ~(2-20) × 103 M☉ is estimated from the fb-M• scaling relation found in the Galactic X-ray binaries and active galactic nuclei.

I present the analysis results from XMM-Newton observations of the old open stellar cluster NGC 188, which has an age of about 7 Gyr and a near solar metallicity. 58 X-ray sources were detected in the field of view of the EPIC MOS and pn cameras, and 46 sources are new X-ray detections. Visible counterparts were found for 20 sources including the variable star WV 28, the W UMa-type binaries V371 Cep and V372 Cep, and the red giant V11. 9 X-ray sources are identified with probable cluster non-members, while 43 X-ray sources are of unknown membership. X-ray emission was detected from 6 stars with high membership probability above a luminosity threshold of 10 30 erg s -1 . This indicates the presence of very active latetype stars in NGC 188 in spite of its old age. The HR diagram positions of two of these stars just above the main sequence are reminiscent of those for W Ursae Majoris-type contact binaries. Two other sources could be either members of close binary systems or the product of the coalescence of W UMa type binaries into single stars. One X-ray source in NGC 188 is located at the bottom of the red giant branch in an evolutionary status similar to that of an FK Comae-type star. Another X-ray source detected in NGC 188 has the HR diagram position of an M type star. Its X-ray to bolometric luminosity ratio, greater than the canonical 10 -3 saturation level, suggests that the star was flaring during XMM-Newton observations. M stars are most likely the most numerous X-ray sources in NGC 188 at lower X-ray luminosity thresholds.

We present Chandra X-ray Observatory ACIS-S3 X-ray imaging observations and VLT/FORS2 and Hubble Space Telescope optical observations of two low-density Galactic globular clusters; NGC 6366 and M 55. We detect 16 X-ray sources with 0.5–6.0 keV luminosities above LX = 4 × 10 30 erg s −1 within the half-mass radius of M 55, of which 8 or 9 are expected to be background sources, and 5 within the half-mass radius of NGC 6366, of which 4 are expected to be background sources. Optical counterparts are identified for several X-ray sources in both clusters and from these we conclude that 3 of the X-ray sources in M 55 and 2 or 3 of the X-ray sources in NGC 6366 are probably related to the cluster. Combining these results with those for other clusters, we find the best fit for a predicted number of X-ray sources in a globular cluster μc = 1.2 Γ+ 1.1 Mh ,w hereΓ is the collision number and Mh is (half of) the cluster mass, both normalized to the values for the globular cluster M4. Some sources tentatively classified as magnetically active binaries are more luminous in X-rays than the upper limit of LX � 0.001 Lbol of such binaries in the solar neighbourhood. Comparison with XMM and ROSAT observations lead us to conclude that the brightest X-ray source in M 55, a dwarf nova, becomes fainter in X-rays during the optical outburst, in accordance with other dwarf novae. The brightest X-ray source in NGC 6366 is a point source surrounded by a slightly offset extended source. The absence of galaxies and Hα emission in our optical observations argues against a cluster of galaxies and against a planetary nebula, and we suggest that the source may be an old nova.

We present the results of a study of discrete X-ray sources in NGC 6946 using a deep Chandra ACIS observation. Based on the slope of the log N- log S distribution and the general correlation of sources with the spiral arms, we infer that the overall discrete source sample in NGC 6946 is dominated by high-mass X-ray binaries, in contrast to the source distributions in M31 and the Milky Way. This is consistent with the higher star formation rate in NGC 6946 than in those galaxies. We find that the strong X-ray sources in the region of the galactic center do not correlate in detail with images of the region in the near-IR, although one of them may be coincident with the galactic center. The noncentral ultraluminous X-ray source in NGC 6946, previously identified with a supernova remnant, has an X-ray spectrum and luminosity that is inconsistent with either a traditional pulsar wind nebula or a blast wave remnant.