Roberto Soria

Ultraluminous X-ray Sources in the Chandra and XMM-Newton Era

Abstract Ultraluminous X-ray sources (ULXs) are accreting black holes that may contain the missing population of intermediate mass black holes or reflect super-Eddington accretion physics. Ten years of Chandra and XMM-Newton observations of ULXs, integrated by multiband studies of their counterparts, have produced a wealth of observational data and phenomenological classifications. We review the properties of their host galaxies, list popular spectral models and implications for standard and supercritical accretion physics, demonstrate how X-ray timing of these objects places constraints on their masses. We also review multiwavelength studies of ULXs, including the optical emission of the binary system and nebulosity around them. We summarize that three classes of black holes could power ULXs: normal stellar mass black holes (∼10M⊙), massive stellar black holes (≲100M⊙), and intermediate mass black holes (102–104M⊙). We collect evidence for the presence of these three types of compact objects, including caveat of each interpretation, and briefly review their formation processes.

A COMPLETE SAMPLE OF ULTRALUMINOUS X-RAY SOURCE HOST GALAXIES

One hundred seven ultraluminous X-ray (ULX) sources with 0.3-10.0 keV luminosities in excess of 1e39 erg/s are identified in a complete sample of 127 nearby galaxies. The sample includes all galaxies within 14.5 Mpc above the completeness limits of both the Uppsala Galaxy Catalog and the Infrared Astronomical Satellite survey. The galaxy sample spans all Hubble types, a four decade range in mass and in star-formation rate. ULXs are detected in this sample at rates of one per 3.2e10 solar mass, one per 0.5 solar mass/year star-formation rate, and one per 57 cubic Mpc corresponding to a luminosity density of ~2e37 erg/s/Mpc3. At these rates we estimate as many as 19 additional ULXs remain undetected in fainter dwarf galaxies within the survey volume. An estimated 14 or 13%, of the 107 ULX candidates are expected to be background sources. The differential ULX luminosity function shows a power law slope of -1.2 to -2.0 with an exponential cutoff at 2e40 erg/s with precise values depending on the model and on whether the ULX luminosities are estimated from their observed numbers of counts or, for a subset of candidates, from their spectral shapes. Extrapolating the observed luminosity function predicts at most one very luminous ULX, L~1e41 erg/s, within a distance as small as 100 Mpc. The luminosity distribution of ULXs within the local universe cannot account for the recent claims of luminosities in excess of 2e41 erg/s requiring a new population class to explain these extreme objects.

Detection of the Tip of Red Giant Branc in NGC 5128

We present a color-magnitude diagram of more than 10,000 stars in the halo of the galaxy NGC 5128 (Centaurus A), based on WFPC2 images through V-band and I-band filters. The position of the red giant branch (RGB) stars is compared with the loci of the red giant branch in six well-studied globular clusters and in the dwarf elliptical galaxy NGC 185; the tip of the RGB is signalled by an observed turn-up in the luminosity function at I≃24.1 ± 0.1 mag; this yields a distance modulus (m - M)_0 = 27.8 ± 0.2 for NGC 5128 (i.e., a distance of 3.6 ± 0.2 Mpc), in agreement with previous determinations based on the planetary nebulae luminosity function and on the surface brightness fluctuations technique. The presence of an intermediate-age stellar population (~5 Gyr) is suggested by the luminosity function of the asymptotic giant branch stars, extending up to I= 22.6 mag (for V - I > 2) and M_(bot) ~ -5 mag; however, the number of these stars constrains the intermediate-age stellar population in the halo of NGC 5128 to be less than ~ 10% of the total. The color distribution at constant I magnitude, albeit affected by the completeness level of our sample, strongly suggests a mean value of [Fe/H] > -0.9 dex, possibly similar to the value found in M31 and higher than that observed in NGC 185. Like the M31 halo, the halo of NGC 5128 exhibits a broad range of levels of chemical enrichment.

A 300-parsec-long jet-inflated bubble around a powerful microquasar in the galaxy NGC 7793

Black-hole accretion states near or above the Eddington luminosity (the point at which radiation force outwards overcomes gravity) are still poorly known because of the rarity of such sources. Ultraluminous X-ray sources are the most luminous class of black hole (LX ≈ 1040 erg s−1) located outside the nuclei of active galaxies. They are likely to be accreting at super-Eddington rates, if they are powered by black holes with masses less than 100 solar masses. They are often associated with shock-ionized nebulae, though with no evidence of collimated jets. Microquasars with steady jets are much less luminous. Here we report that the large nebula S26 (ref. 4) in the nearby galaxy NGC 7793 is powered by a black hole with a pair of collimated jets. It is similar to the famous Galactic source SS433 (ref. 5), but twice as large and a few times more powerful. We determine a mechanical power of around a few 1040 erg s−1. The jets therefore seem 104 times more energetic than the X-ray emission from the core. S26 has the structure of a Fanaroff–Riley type II (FRII-type) active galaxy: X-ray and optical core, X-ray hot spots, radio lobes and an optical and X-ray cocoon. It is a microquasar where most of the jet power is dissipated in thermal particles in the lobes rather than relativistic electrons.

A study of Jupiter's aurorae with XMM-Newton

We present a detailed analysis of Jupiters X-ray (0.2−10 keV) auroral emissions as observed over two XMM- Newton revolutions in Nov. 2003 and compare it with that of an earlier observation in Apr. 2003. We discover the existence of an electron bremsstrahlung component in the aurorae, which accounts for essentially all the X-ray flux above 2 keV: its pr esence had been predicted but never detected for lack of sensitivit y of previous X-ray missions. This bremsstrahlung component varied significantly in strength and spectral shape over the 3.5 day s covered by the Nov. 2003 observation, displaying substantial hardening of the spectrum with increasing flux. This variabi lity may be linked to the strong solar activity taking place a t the time, and may be induced by changes in the acceleration mechanisms inside Jupiters magnetosphere. As in Apr. 2003, t he auroral spectra below 2 keV are best fitted by a superposition of line emission most likely originating from ion charge exchange, with OVII playing the dominant role. We still cannot resolve conclusively the ion species responsible for the lowest energy lines (around 0.3 keV), so the question of the origin of the ions (magnetospheric or solar wind) is still open. It is conceivable that both scenarios play a role in what is certainly a very complex planetary structure. High resolution spectra of the whole planet obtained with the XMM-NewtonReflection Grating Spectrometer in the range 0.5−1 keV clearly separate emission lines (mostly of iron) originating at low latitudes on Jupiter from the auroral lines due t o oxygen. These are shown to possess very broad wings which imply velocities of∼5000 km s −1 . Such speeds are consistent with the energies at which precipitating and charge exchanging oxygen ions are expected to be accelerated in Jupiters magnetos phere. Overall we find good agreement between our measurements and t he predictions of recently developed models of Jupiters auroral processes.

The star‐forming environment of an ultraluminous X‐ray source in NGC 4559: an optical study

We have studied the candidate optical counterparts and the stellar population in the star- forming complex around the bright ultraluminous X-ray source (ULX) in the western part of the spiral galaxy NGC 4559, using the HST Wide Field Planetary Camera 2 (WFPC2), XMM-Newton/Optical Monitor and ground-based data. We find that the ULX is located near a small group of OB stars, but is not associated with any massive young clusters nor with any extraordinary massive stars. The brightest point source in the Chandra error circle is consistent with a single blue supergiant (BSG) of mass ≈20 Mand age ≈10 Myr. A few other stars are resolved inside the error circle: mostly BSGs and red supergiants (RSGs) with inferred masses ≈10-15 Mand ages ≈20 Myr. This is consistent with the interpretation of this ULX as a black hole (BH) accreting from a high-mass donor star in its supergiant phase, with mass transfer occurring via Roche-lobe overflow. The observed optical colours and the blue-to-red supergiant ratio suggest a low metal abundance for the stellar population: 0.2 Z/Z� 0.4 (using the Padua tracks), or 0.05 Z/Z� 0.2 (using the Geneva tracks). The age of the star-forming complex is30 Myr. Hα images show that this star-forming region has a ring-like appearance. We propose that it is an expanding wave of star formation, triggered by an initial density perturbation, in a region where the gas was only marginally stable to gravitational collapse. We also suggest that the most likely trigger was a collision with a satellite dwarf galaxy going through the gas-rich outer disc of NGC 4559 less than 30 Myr ago. The culprit could be the dwarf galaxy visible a few arcsec north-west of the complex. If this is the case, this system is a scaled-down version of the Cartwheel galaxy. The X-ray data favour a BH more massive (M > 50 M� ) than typical Milky Way BH candidates. The optical data favour a young BH originating in the recent episode of massive star formation; however, they also rule out an association with young massive star clusters (none are present in the X7 field). We speculate that other mechanisms may lead to the formation of relatively massive BHs (perhaps M ∼ 50-100 M� ) from stellar evolution processes in low-metallicity environments, or when star formation is triggered by galactic collisions. Ke yw ords: accretion, accretion discs - black hole physics - galaxies: individual: NGC 4559 - X-rays: galaxies - X-rays: stars.

Probable intermediate-mass black holes in NGC 4559: XMM–Newton spectral and timing constraints

We have examined X-ray and optical observations of two ultraluminous X-ray sources, X7 and X10 in NGC 4559, using XMM-Newton, Chandra and the Hubble Space Telescope (HST). The ultraviolet/X-ray luminosity of X7 exceeds 2.1 x 10(40) erg s(-1) in the XMM-Newton observation, and that of X10 is > 1.3 x 10(40) erg s(-1). X7 has both thermal and power-law spectral components, The characteristic temperature of the thermal component is 0.12 keV. The power-law components in the two sources both have slopes with photon index similar or equal to2.1. A timing analysis of X7 indicates a break frequency at 28 MHz in the power spectrum, while that for X10 is consistent with an unbroken power law. The luminosity of the blackbody component in the X-ray spectrum of X7 and the nature of its time-variability provides evidence that this object is an intermediate-mass black hole accreting at sub-Eddington rates, but other scenarios which require high advection efficiencies from a hollowed-out disc might be possible. The emission from X10 can be characterized by a single power-law. This source can be interpreted either as an intermediate-mass black hole, or as a stellar mass black hole with relativistically beamed Comptonized emission. There are four optical counterparts in the error circle of X7. No counterparts are evident in the error circle for X10.

The star-forming environment of a ULX in NGC 4559: an optical study

We have studied the candidate optical counterparts and the stellar population in the star- forming complex around the bright ultraluminous X-ray source (ULX) in the western part of the spiral galaxy NGC 4559, using the HST Wide Field Planetary Camera 2 (WFPC2), XMM-Newton/Optical Monitor and ground-based data. We find that the ULX is located near a small group of OB stars, but is not associated with any massive young clusters nor with any extraordinary massive stars. The brightest point source in the Chandra error circle is consistent with a single blue supergiant (BSG) of mass ≈20 Mand age ≈10 Myr. A few other stars are resolved inside the error circle: mostly BSGs and red supergiants (RSGs) with inferred masses ≈10-15 Mand ages ≈20 Myr. This is consistent with the interpretation of this ULX as a black hole (BH) accreting from a high-mass donor star in its supergiant phase, with mass transfer occurring via Roche-lobe overflow. The observed optical colours and the blue-to-red supergiant ratio suggest a low metal abundance for the stellar population: 0.2 Z/Z� 0.4 (using the Padua tracks), or 0.05 Z/Z� 0.2 (using the Geneva tracks). The age of the star-forming complex is30 Myr. Hα images show that this star-forming region has a ring-like appearance. We propose that it is an expanding wave of star formation, triggered by an initial density perturbation, in a region where the gas was only marginally stable to gravitational collapse. We also suggest that the most likely trigger was a collision with a satellite dwarf galaxy going through the gas-rich outer disc of NGC 4559 less than 30 Myr ago. The culprit could be the dwarf galaxy visible a few arcsec north-west of the complex. If this is the case, this system is a scaled-down version of the Cartwheel galaxy. The X-ray data favour a BH more massive (M > 50 M� ) than typical Milky Way BH candidates. The optical data favour a young BH originating in the recent episode of massive star formation; however, they also rule out an association with young massive star clusters (none are present in the X7 field). We speculate that other mechanisms may lead to the formation of relatively massive BHs (perhaps M ∼ 50-100 M� ) from stellar evolution processes in low-metallicity environments, or when star formation is triggered by galactic collisions. Ke yw ords: accretion, accretion discs - black hole physics - galaxies: individual: NGC 4559 - X-rays: galaxies - X-rays: stars.

Properties of discrete X-ray sources in the starburst spiral galaxy M 83

We have identified 127 discrete sources in a Chandra ACIS observation of M83, with a detection limit of approximate to3 x 10(36) erg s(-1) in the 0.3-8.0 keV band. We discuss the individual X-ray spectral and time-variability properties of approximate to20 bright sources with luminosities greater than or similar to10(38) erg s(-1), and the statistical properties of the whole sample. About one third of the bright sources show X-ray spectra with a blackbody component at temperatures less than or similar to1 keV, plus a powerlaw component with Gamma approximate to 2.5, typical of X-ray binaries in a soft state; another third have powerlaw spectra with Gamma approximate to 1.5, consistent with X-ray binaries in a hard state. Two bright sources show emission lines on a hard powerlaw continuum, and are probably X-ray binaries surrounded by a photo-ionized nebula or stellar wind. Among the other bright sources, we also identified two supernova remnant candidates, with optically-thin thermal plasma spectra at temperatures similar to0.5 keV. The two brightest supersoft sources have blackbody temperatures kT approximate to 70 eV and luminosities similar to10(38) erg s(-1). Two candidate X-ray pulsars are detected with periods approximate to200 s. One X-ray source corresponds to the core of a background FRII radio galaxy. The discrete sources can be divided into three groups, based on their spatial, color and luminosity distributions. The first group comprises supersoft sources with no detected emission above 1 keV and blackbody spectra at temperatures <100 eV. The second group consists of soft sources with little or no detected emission above 2 keV. They are strongly correlated with H alpha emission in the spiral arms and starburst nucleus, tracing a young population. Their relative abundance depends on the current level of star-forming activity in the galaxy. Most of them are likely to be supernova remnants. The sources in the third group are mostly X-ray binaries, reaching higher X-ray luminosities than sources in the other two groups. Being a mixture of old low-mass and young high-mass systems, the whole group appears to be of intermediate age when correlated with the H alpha emission. The color-color diagrams allow us to distinguish between sources in a soft and hard state.

Accretion and Nuclear Activity of Quiescent Supermassive Black Holes. II. Optical Study and Interpretation

Our X-ray study of the nuclear activity in a new sample of six quiescent early-type galaxies, and in a larger sample from the literature, confirmed (Soria et al., Paper I) that the Bondi accretion rate of diffuse hot gas is not a good indicator of the supermassive black hole (SMBH) X-ray luminosity; in fact, the two quantities appear uncorrelated. Here we suggest that a more reliable estimate of the accretion rate must include the gas released by the stellar population inside the sphere of influence of the SMBH (generally too small to be probed by Chandra), in addition to the Bondi inflow of hot gas across that surface. We use optical surface-brightness profiles, from archival HST images, to estimate the mass-loss rate from stellar winds in the nuclear region: we show that for our sample of galaxies it is an order of magnitude higher (typical values of ∼ 10 −4 –10 −3 M⊙ yr −1 ) than the Bondi inflow rate of hot gas, as estimated from X-ray observations (Paper I). Only by taking into account both sources of fuel can we constrain the true accretion rate, the accretion efficiency, and the power budget. Radiatively efficient accretion is ruled out, for quiescent SMBHs. For typical radiatively inefficient flows, the observed X-ray luminosities of the SMBHs imply accretion fractions ∼ 1–10% (i.e., ∼ 90– 99% of the available gas does not reach the SMBH) for at least five of our six target galaxies, and most of the other galaxies with known SMBH masses. We discuss the conditions for mass conservation inside the sphere of influence of a SMBH, so that the total gas injection is balanced by accretion across the event horizon plus outflows. We show that a fraction of the total accretion power (mechanical plus radiative) would be sufficient to sustain a self-regulating,slow outflow which removes from the nuclear region all the gas that does not sink into the BH (“BH feedback”). The rest of the accretion power may be carried out in a jet, or advected, depending on the details of the radiative-inefficient accretion solutions. We also discuss possible scenarios that would lead to an intermittent nuclear activity, such as transitions in the outflow rate or in the efficiency. Subject headings: accretion, accretion disks — galaxies: nuclei — galaxies: individual (NGC821, NGC3377, NGC4486B, NGC4564, NGC4697, NGC5845) — galaxies: structure — X-ray: galaxies