Searcharxiv⌕ Search

arXiv subjects

G. Fabbiano

Publications and source records attributed to G. Fabbiano.

At least 127 records · Page 7Linked to original sources
arXiv2004

XMM-Newton Spectroscopy of Bright ULX Sources in the Antennae Galaxies (NGC 4038/4039)

We report the results of spectral fits to four bright ultra-luminous X-ray sources (ULXs) in the Antennae galaxies (NGC 4038/4039) observed for 41 ksec with XMM-Newton. Although emission regions are not resolved as well as in prior Chandra observations, at least four ULXs (X-11, X-16, X-37, and X-44 in the Zezas and Fabbiano scheme) are sufficiently bright and well-separated with XMM-Newton that reliable extractions and spectral analyses are possible. We find that the single component multi-color disk blackbody models cannot describe any of the spectra. Sources X-11 and X-16 are acceptably fit with simple power-law models. A thermal bremsstrahlung model provides a better fit to the spectrum of X-44. Including a disk blackbody component to the spectrum of X-37 improves the fit and reveals an apparently cool disk (kT = 0.13 +/- 0.02 keV). This would suggest a parallel to cool disks recently found in other very luminous ULXs, which may contain intermediate mass black holes; however, the complex diffuse emission of the Antennae demands this finding be regarded cautiously.

astro-ph↗
arXiv2004

X-raying Chemical Evolution and Galaxy Formation in The Antennae

We present the integrated 411 ks {\it Chandra} ACIS-S exposure of the Antennae galaxies (NGC 4038/39). Besides a rich population of point-like sources, this spectacular image reveals a spatially and spectrally complex hot diffuse gaseous component. For the first time we detect intense line emission from Fe, Ne, Mg and Si in The Antennae, and obtain a detailed picture of spatially varied metal abundances in the hot interstellar medium (ISM) of a galaxy. In certain regions, the abundances of $α$-elements may be many times solar, while the Fe abundance is sub-solar or near-solar. The differences in the local metal enrichment of the hot ISM may be related to the local star formation rates and to the degree of confinement of the enriched hot ISM. We also report large-scale gaseous features, including two gigantic, $\sim$10-kpc-scale `loops' extending to the South of the merging disks, and a low-surface-brightness hot halo, extending out to $\sim$18 kpc. These features may be related to superwinds from the starburst in The Antennae or result from the merger hydrodynamics. Their long cooling times suggest that they may persist to form the hot X-ray halo of the emerging elliptical galaxy.

astro-ph↗
arXiv2003

Displacement of X-Ray Sources from Star Clusters in Starburst Galaxies

We examine the spatial offsets between X-ray point sources and star clusters in three starburst galaxies. We find that the X-ray sources are preferentially located near the star clusters. Because the star clusters are very good tracers of the star formation activity in the galaxies, this indicates that the X-ray sources are young objects associated with current star formation. We find significant displacements of the X-ray sources from the clusters. These displacements are likely due to motion of the X-ray sources and indicates that they are X-ray binaries. We find that brighter X-ray sources preferentially occur closer to clusters. The absence of very bright sources at large displacements from clusters may help constrain models of the sources.

astro-ph↗
arXiv2003

X-Ray Binary Populations: The Luminosity Function of NGC1569

Using the population synthesis code StarTrack we construct the first synthetic X-ray binary populations for direct comparison with the X-ray luminosity function (XLF) of NGC 1569 observed with Chandra. Our main goal is to examine whether it is possible to reproduce the XLF shape with our models, given the current knowledge for the star-formation history of this starburst galaxy. We thus produce hybrid models meant to represent the two stellar populations: one old, metal-poor with continuous star-formation for 1.5 Gyr and another recent and metal-rich population. To examine the validity of the models we compare XLFs calculated for varying ages of the populations and varying relative weights for the star-formation rates in the two populations. We find that, for typical binary evolution parameters, it is indeed possible to quite closely match the observed XLF shape. The robust match is achieved for an age of the young population and a ratio of star formation rates in the two populations that are within factors of 1.5 and 2, respectively, of those inferred from HST observations of NGC 1569. In view of this encouraging first step, we discuss the implications of our X-ray binary models and their potential as tools to study binary populations in galaxies.

astro-ph↗
arXiv2003

First results from a Chandra survey of the 'Bar' region of the SMC

We present the first results from a Chandra survey of the central region of the Small Magellanic Cloud. We detect a total of 122 sources down to a limiting luminosity of ~4.3x10^{33} erg\s (corrected for Galactic N_{H}), which is ~10 times lower than in any previous survey of the SMC. We identify 20 candidate transient sources: eighteen previously known sources in this area which are not detected in our observations, and two new bright sources. The spectral parameters of the brightest sources indicate that they are X-ray binary pulsars. The high spatial resolution of Chandra allows us to initially identify optical counterparts for 35 sources, 13 of which are new identifications.

astro-ph↗
arXiv2003

NGC 4261 and NGC 4697: rejuvenated elliptical galaxies

Chandra images present evidence for a non-uniform spatial distribution of discrete X-ray sources in the elliptical galaxy NGC 4261. This non-uniform distribution is inconsistent with the optical morphology of NGC 4261 at greater than a 99.9% confidence level. Similar evidence is seen in one more elliptical galaxy (NGC 4697; 98% confidence level) out of five cases we investigated. NGC 4261 and NGC 4697 have old stellar populations (9-15 Gyrs) and fine structure parameters of 1 and 0 respectively, suggesting no recent merging activity. On the basis of simulations of galaxy interactions, we propose that the X-ray sources responsible for the non-uniform distribution are associated with young stellar populations, related to the rejuvenating fall-back of material in tidal tails onto a relaxed merger remnant, or shock induced star-formation along the tidal tails.

astro-ph↗
arXiv2003

Long term X-ray spectral variability of the nucleus of M81

We have analysed the soft X-ray emission from the nuclear source of the nearby spiral galaxy M81, using the available data collected with ROSAT, ASCA, BeppoSAX and Chandra. The source flux is highly variable, showing (sometimes dramatic: a factor of 4 in 20 days) variability at different timescales, from 2 days to 4 years, and in particular a steady increase of the flux by a factor of >~ 2 over 4 years, broken by rapid flares. After accounting for the extended component resolved by Chandra, the nuclear soft X-ray spectrum (from ROSAT/PSPC, BeppoSAX/LECS and Chandra data) cannot be fitted well with a single absorbed power-law model. Acceptable fits are obtained adding an extra component, either a multi-color black body (MCBB) or an absorption feature. In the MCBB case the inner accretion disk would be far smaller than the Schwartzchild radius for the 3-60X 10^6 solar masses nucleus requiring a strictly edge-on inclination of the disk, even if the nucleus is a rotating Kerr black hole. The temperature is 0.27 keV, larger than expected from the accretion disk of a Schwartzchild black hole, but consistent with that expected from a Kerr black hole. In the power-law + absorption feature model we have either high velocity (0.3 c) infalling C_v clouds or neutral C_i absorption at rest. In both cases the C:O overabundance is a factor of 10.

astro-ph↗
arXiv2003

The Multi-Colored Hot Interstellar Medium of "The Antennae" Galaxies (NGC 4038/39)

We report the results of the analysis of the extended soft emission discovered in the Chandra ACIS pointing at the merging system NGC 4038/39 (the Antennae). We present a `multi-color' X-ray image that suggests both extensive absorption by the dust in this system, peaking in the contact region, as well as variations in the temperature of different emitting regions of the hot interstellar medium (ISM). Spectral fits to multi-component thermal emission models confirm this picture and give a first evaluation of the parameters of the hot plasma. We compare the diffuse X-ray emission with radio continuum (6cm), HI, CO, and H$α$ images to take a first look at the multi-phase ISM of the Antennae galaxies. We find that the hot (X-ray) and cold (CO) gas have comparable thermal pressures in the two nuclear regions. We also conclude that the displacement between the peak of the diffuse X-ray emission in the north of the galaxy system, towards the inner regions of the northern spiral arm (as defined by H$α$, radio continuum and HI), could result from ram pressure of infalling HI clouds.

astro-ph↗
arXiv2003

An XMM-Newton and Chandra investigation of the nuclear accretion in the Sombrero Galaxy (NGC4594)

We present an analysis of the XMM-Newton and Chandra ACIS-S observations of the LINER nucleus of the Sombrero galaxy and we discuss possible explanations for its very sub-Eddington luminosity by complementing the X-ray results with high angular resolution observations in other bands. The X-ray investigation shows a hard (Gamma=1.89) and moderately absorbed (N_H=1.8 10^21 cm^-2) nuclear source of 1.5 10^40 erg s^-1 in the 2-10 keV band, surrounded by hot gas at a temperature of \sim 0.6 keV. The bolometric nuclear luminosity is at least \sim 200 times lower than expected if mass accreted on the supermassive black hole, that HST shows to reside at the center of this galaxy, at the rate predicted by the spherical and adiabatic Bondi accretion theory and with the high radiative efficiency of a standard accretion disc. The low luminosity, coupled to the observed absence of Fe-K emission in the nuclear spectrum, indicates that such a disc is not present. This nucleus differs from bright unobscured AGNs also for the lack of high flux variability and of prominent broad Halpha emission. However, it is also too faint for the predictions of simple radiatively inefficient accretion taking place at the Bondi rate; it could be too radio bright, instead, for radiatively inefficient accretion that includes strong mass outflows or convection. This discrepancy could be solved by the possible presence of nuclear radio jets. An alternative explanation of the low luminosity, in place of radiative inefficiency, could be unsteady accretion.

astro-ph↗
arXiv2003

Ultra-Luminous X-ray Source Populations in Normal Galaxies: a Preliminary Survey with Chandra

We present results of a Chandra survey of the ultra-luminous X-ray sources (ULX) in 13 normal galaxies, in which we combine source detection with X-ray flux measurement. 22 ULX were detected, i.e. with L_x > 1 x 10^{39} erg s^{-1} (L_10), and 39 other sources were detected with L_x > 5 x 10^{38} erg s^{-1} (L_5). We also use radial intensity profiles to remove extended sources from the sample. The majority of sources are not extended, which for a typical distance constrains the emission region size to less than 50 pc. X-ray colour-colour diagrams and spectral fitting results were examined for indicators of the ULX nature. In the case of the brighter sources, spectral fitting generally requires two-component models. In only a few cases do colour-colour diagrams or spectral fitting provide evidence of black hole nature. We find no evidence of a correlation with stellar mass, however there is a strong correlation with star formation as indicated by the 60 micron flux as found in previous studies.

astro-ph↗
arXiv2003

A Variable Ultraluminous Supersoft X-ray Source in "The Antennae": Stellar-Mass Black Hole or White Dwarf?

The chandra monitoring observations of The Antennae (NGC 4038/39) have led to the discovery of a variable, luminous, supersoft source (SSS). This source is only detected at energies below 2 keV and, in 2002 May, reached count rates comparable to those of the nine ultraluminous X-ray sources (ULXs) detected in these galaxies. Spectral fits of the SSS data give acceptable results only for a $\sim$100--90 eV blackbody spectrum with an intrinsic absorption column of $N_{\rm H} \sim 2-3 \times 10^{21} \rm cm^{-2}$. For a distance of 19 Mpc, the best-fit observed luminosity increases from 1.7$\times 10^{38}$\ergs in 1999 December to 8.0$\times 10^{38}$\ergs in 2002 May. The intrinsic, absorption-corrected best-fit luminosity reaches 1.4$\times 10^{40}$ \ergs in 2002 May. The assumption of unbeamed emission would suggest a black hole of $\ga$100\msun. However, if the emission is blackbody at all times, as suggested by the steep soft spectrum, the radiating area would have to vary by a factor of $\sim10^3$, inconsistent with gravitational energy release from within a few Schwarzschild radii of a black hole. Viable explanations for the observed properties of the SSS are provided by anisotropic emission from either an accreting nuclear-burning white dwarf or an accreting stellar-mass black hole.

astro-ph↗
arXiv2003

X-ray Spectroscopic Evidence for Intermediate Mass Black Holes: Cool Accretion Disks in Two Ultra--Luminous X-ray Sources

We have analyzed an XMM-Newton observation of the nearby spiral galaxy NGC 1313, which contains two "ultra-luminous" X-ray (ULX) sources. We measure isotropic luminosities of L_X = 2.0 * 10^(40) erg/s and L_X = 6.6 * 10^(39) erg/s for NGC 1313 X-1 and X-2 (0.2-10.0 keV, assuming a distance of 3.7 Mpc). The spectra statistically require soft and hard spectral components to describe the continuum emission; some prior studies of ULXs have claimed cool soft components with lower statistics. The improvement over several single-component models exceeds the 8 sigma level of confidence for X-1; the improvement for X-2 is significant at the 3 sigma level. The soft components in these ULX spectra are well-fit by multi-color disk blackbody models with color temperatures of kT = 150 eV. This temperature differs markedly from those commonly measured in the spectra of stellar-mass (10 M_sun) black holes in their brightest states (kT ~ 1 keV). It is expected that the temperature of an accretion disk orbiting a black hole should decrease with increasing black hole mass. If the soft components we measure are due to emission from the inner region of an accretion disk, and disks extend close to the innermost stable circular orbit at the accretion rates being probed, the low color temperatures may be interpreted as spectroscopic evidence of black holes with intermediate masses: M_BH ~ 10^(3) M_sun. Simple Eddington scaling arguments suggest a mimum mass of M_BH ~ 10^(2) M_sun. NGC 1313 X-1 and X-2 are found in optical nebulae (Pakull and Mirioni 2002), which may indicate that anisotropic emission geometries are unlikely to account for the fluxes observed.

astro-ph↗
arXiv2003

The X-ray-faint Emission of the Supermassive Nuclear Black Hole of IC1459

Chandra observations of the supermassive black hole in the nucleus of IC1459 show a weak [L_X=8x10(40)erg s(-1), 0.3-8keV], unabsorbed nuclear X-ray source, with a slope Gamma=1.88+/-0.09, and no strong Fe-K line at 6.4keV (EW$<382 eV). This describes a normal AGN X-ray spectrum, but lies at 3x10(-7) below the Eddington limit. The SED of the IC1459 nucleus is extremely radio loud compared to normal radio-loud quasars. The nucleus is surrounded by hot ISM (kT~0.5-0.6 keV) with an average density of 0.3cm(-3), within the central ~180 pc radius, which is comparable to the gravitational capture radius, r_A~140 pc. We estimate that for a standard AGN efficiency of 10%, the Bondi accretion would correspond to a luminosity of ~6x10(44)erg s(-1), nearly four orders of magnitude higher than L_X. ADAF solutions can explain the X-ray spectrum, but not the high radio/X-ray ratio. A jet model fits the radio-100micron and X-ray spectra well. The total power in this jet is ~10% of L(Bondi), implying that accretion close to the Bondi rate is needed.

astro-ph↗
arXiv2002

The Time-Variable Ultra-Luminous X-ray Sources of "The Antennae"

We report the first results of the Chandra temporal monitoring of the ultra-luminous X-ray sources (ULXs) in the Antennae galaxies (NGC 4038/39). Observations at four different epochs, covering time scales of 2 years to 2 months, show variability in seven out of nine ULXs, confirming that they are likely to be accreting compact X-ray binaries (XRBs). The seven variable ULXs exhibit a variety of temporal and spectral behaviors: one has harder X-ray colors with decreasing luminosity, similar to the black hole binary Cyg X-1, but four other ULXs show the opposite behavior. We suggest that the latter may be black-hole binaries accreting at very high rates.

astro-ph↗
arXiv2002

ROSAT-HRI investigation of the NGC 507 X-ray halo

We present an X-ray investigation of the elliptical galaxy NGC 507. We make use of archival Rosat HRI, PSPC and Chandra data, to connect the large-scale structure of the halo to the core morphology. Our analysis shows that the halo core (r<2-3 r_e) and the external halo (r>3 r_e) are characterised by different dynamical properties and suggests a different origin of the two components. The halo core has a complex morphology with a main X-ray emission peak, coincident with the center of the optical galaxy, and several secondary peaks. The spatial and spectral analysis of the central peak shows that this feature is produced by denser hot gas in the galaxy core. Our data support the scenario where the gas is kinetically heated by stellar mass losses. Comparison with previously published studies suggest that the core of X-ray extended galaxies is associated to the stellar distribution and has similar properties to the X-ray halo of compact galaxies. The secondary peaks are due instead to interactions between the radio-emitting plasma and the surrounding ISM. We found that the energy input by the central radio source in the ISM may be large enough to prevent gas from cooling. The total mass profile shows the presence of a dark matter halo extending on cluster scales which is likely to be associated with the whole cluster rather than with NGC 507. This structure is typical of many X-ray bright Early-Type galaxies and may explain the spatial and spectral differences with X-ray compact galaxies largely debated in literature. Finally we discuss the nature of the point sources detected in the HRI FOV.

astro-ph↗
arXiv2002

The nuclear accretion in the FR I radio galaxy IC4296 from CHANDRA and VLBA observations

A high angular resolution study of the nucleus of the FR I galaxy IC4296 using Chandra ACIS-S and VLBA observations is presented, with the aim of studying the nature of the accretion process. Pointlike and hard X-ray emission is found, well described by a moderately absorbed power law of Gamma=1.48^{+0.42}_{-0.34}; no iron fluorescence line from cold material is detected. The 0.3-10 keV luminosity is 2.4\times 10^{41} erg/s, that is \sim 400 times lower than the accretion luminosity resulting from the estimated Bondi mass accretion rate and a radiative efficiency of 10%. On the parsec scale a jet and a counter-jet extend out from a central unresolved ``core'' in the 8.4 GHz image. Their orientation is in good agreement with that of the large scale jets and their bulk speed is relativistic. The parsec scale spectrum is convex over 2-22 GHz. The observed nuclear luminosity is not likely to be reconciled with the accretion luminosity by assuming that Compton thick material surrounds the nucleus. Low radiative efficiency accretion flow models (ADAF and its variants) cannot account for the observed emission and spectral shape of the nucleus in the radio band. The power in the jets accounts for a sizable fraction (\ga 10%) of the accretion luminosity; therefore the mass accretion rate needs not to be much different from the estimate obtained using Bondi's theory. A jet dominated origin also for the observed radiative losses of the nucleus is suggested. This could also explain the finding that a nuclear luminosity orders of magnitude lower than that of ``normal'' radio loud AGNs is accompanied by a relatively higher radio emission.

astro-ph↗
arXiv2002

Spectral variability of the nucleus of M33 in a Chandra-ACIS observation

We have analyzed a 90 ksec long observation of the bright nucleus of M33 made with Chandra/ACIS. We detected low-amplitude (~sim10%) highly significant variability on timescales of ~5000 sec. We also find associated spectral variability. The two main spectral components (a power-law with Γ~2 and a multicolor disk -MCD- with kT~0.9-1.2 keV) vary in relative flux. The MCD temperature also increases with increasing MCD flux. The pattern of variability is reminiscent of (but not identical to) galactic black hole binaries. A ~5 M$_{\odot}$ accreting black hole may explain this source.

astro-ph↗