arXiv subjects

G. Fabbiano

Publications and source records attributed to G. Fabbiano.

At least 145 records · Page 8Linked to original sources

arXiv2002

X-ray Arc Structures in Chandra Images of NGC5128 (Centaurus A)

We describe results from our initial study of the diffuse X-ray emission structures in NGC 5128 (Cen A) using the Chandra X-ray Observatory observations. The high-angular resolution Chandra images reveal multi-scale X-ray struct ures with unprecedented detail and clarity. The large scale structures suggest complex symmetry, including a component possibly associated with the inner radio lobes, and a separate component with an orthogonal symmetry that may be associated with the galaxy as a whole. We detect arc-like soft X-ray structures, extending to ~8 kpc in the direction perpendicular to the jet. These arcs appear to be tracing an ellipse, or a ring seen in projection, and ar e enclosed between the dust and HI emitting gas in the central region of the galaxy, and the optical and HI shell fragments at a distance of about 8-10 kpc. The diffuse X-ray and the optical emission in the arcs could originate in a region of interaction (possibly a shock) between the infalling material from the outer regions of the galaxy, and the cool dust and HI emitting material in the center, or an equatorial outflow resulting from an outburst of nuclear activity ~10^7 years ago.

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arXiv2002

Chandra Observations of "The Antennae" Galaxies (NGC 4038/39): II. Detection and Analysis of Galaxian X-ray source

We report the detailed analysis of the X-ray properties of the discrete sources detected in a long (72 ks) \chandra ACIS-S observation of the Antennae galaxies. We detect 49 sources, down to a detection limit of ~ 10^{38} erg\s; 18 sources have Lx >10^{39} erg\s (Ultra Luminous X-ray sources; ULXs). Six of the 49 sources have an extended component. Two sources show evidence for variability during this observation, and three sources exhibit long term variability in timescales of a few years. The spectra of the discrete sources are diverse, suggesting different emission mechanisms, broadly correlated with the source luminosity: the most luminous sources exhibit harder emission, while the spectra of fainter sources appear softer. Spectra and variability suggest that the ULXs may be binary accretion sources; Supernova remnants or hot ISM in the Chandra beam may be responsible for some of the softer sources.

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arXiv2002

The X-ray Luminosity Function of "The Antennae" Galaxies (NGC4038/39) and the Nature of Ultra-Luminous X-ray Sources

We derive the X-ray luminosity function (XLF) of the X-ray source population detected in the Chandra observation of NGC4038/39 (the Antennae). We explicitly include photon counting and spectral parameter uncertainties in our calculations. The cumulative XLF is well represented by a flat power law ($α=-0.47$), similar to those describing the XLFs of other star-forming systems (e.g. M82, the disk of M81), but different from those of early type galaxies. This result associates the X-ray source population in the Antennae with young High Mass X-ray Binaries. In comparison with less actively star-forming galaxies, the XLF of the Antennae has a highly significant excess of sources with luminosities above 10^{39} erg\s (Ultra Luminous Sources; ULXs). We discuss the nature of these sources, based on the XLF and on their general spectral properties, as well as their optical counterparts discussed in Paper III. We conclude that the majority of the ULXs cannot be intermediate mass black-holes (M > 10-1000 \msun) binaries, unless they are linked to the remnants of massive Population III stars (the Madau & Rees model). Instead, their spatial and multiwavelength properties can be well explained by beamed emission as a consequence of supercritical accretion. Binaries with a neutron star or moderate mass black-hole (up to 20\msun), and B2 to A type star companions would be consistent with our data. In the beaming scenario, the XLF should exibit caracteristic breaks that will be visible in future deeper observations of the Antennae.

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arXiv2002

The X-ray Source Population of "The Antennae" Galaxies: X-ray Properties from Chandra and Multiwavelength Associations

[abridged] We investigate the nature of the luminous X-ray source population detected in a (72 ks) Chandra ACIS-S observation of NGC 4038/39, the Antennae galaxies. We derive the average X-ray spectral properties of sources in different luminosity ranges, and we correlate the X-ray positions with radio, IR, and optical HST data. The X-ray sources are predominantly associated with young stellar clusters, indicating that they belong to the young stellar population. Based on both their co-added X-ray spectrum, and on the lack of associated radio emission, we conclude that the Ultra Luminous X-ray sources (ULXs), with Lx>10^{39} erg\s, are not young compact Supernova Remnants (SNR), but accretion binaries. While their spectrum is consistent with those of ULXs studied in nearby galaxies, and interpreted as the counterparts of intermediate mass black-holes (M >10-1000 \msun), comparison with the position of star-clusters suggests that some of the ULXs may be runaway binaries, thus suggesting lower-mass binary systems. The co-added spectrum of the sources in the 3x10^{38}-10^{39} erg\s luminosity range is consistent with those of Galactic black-hole candidates. These sources are also on average displaced from neighboring star clusters. The softer spectrum of the less luminous sources suggests the presence of SNRs or of hot interstellar medium (ISM) in the Chandra source extraction area. Comparison with HI and CO observations shows that most sources are detected in the outskirts of large concentrations of gas. The absorbing columns inferred from these observations would indeed absorb X-rays up to 5 keV, so there may be several hidden X-ray sources. Associated with these obscured regions we find 6 sources with heavily absorbed X-ray spectra and absorption-corrected luminosities in the ULX range.

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arXiv2001

Nuclear and global X-ray properties of LINER galaxies: Chandra and BeppoSAX results for Sombrero and NGC 4736

We report on the 0.1--100 keV BeppoSAX observations of two nearby LINER galaxies, Sombrero and NGC 4736. Chandra ACIS-S observations supplement this broad-beam spectral study with a high resolution look into the nuclear region, and show a dominating central point source in Sombrero and a complex X-ray binary dominated/starburst region in NGC 4736. A compact non-thermal radio source, present in the nucleus of both galaxies, coincides with the central source in Sombrero, while in NGC 4736 its X-ray counterpart is a much fainter point source, not the brightest of the central region. On the basis of these and other results, we conclude that the LINER activity is linked to the presence of a low luminosity AGN in Sombrero and to a recent starburst in NGC 4736, and that Chandra's spectroscopic capabilities coupled to high resolution imaging are essential to establish the origin of the nuclear activity.

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arXiv2001

Observations of Galaxies with Chandra

This talk discusses the growing impact of Chandra observations on the study of galaxies. In particular, we address progress in the study of X-ray source populations in diverse galaxies and in the study of the hot ISM.

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arXiv2001

Deep ROSAT-HRI observations of the NGC 1399/NGC 1404 region: morphology and structure of the X-ray halo

We present the analysis of a deep (167 ks) ROSAT HRI observation of the cD galaxy NGC 1399 in the Fornax cluster. Using both HRI and, at larger radii, archival PSPC data, we find that the radial behavior of the X-ray surface brightness profile is not consistent with a simple Beta model and suggests instead three distinct components. We use a multi-component bidimensional model to study in detail these three components that we identify respectively with the cooling flow region, the galactic and the cluster halo. From these data we derive a binding mass distribution in agreement with that suggested by optical dynamical indicators, with an inner core dominated by luminous matter and an extended dark halo differently distributed on galactic and cluster scales. The HRI data and a preliminary analysis of Chandra public data, allow us to detect significant density fluctuations in the halo. We discuss possible non-equilibrium scenarios to explain the hot halo structure, including tidal interactions with neighboring galaxies, ram stripping from the intra-cluster medium and merging events. In the innermost region of NGC 1399, the comparison between the X-ray and radio emission suggests that the radio emitting plasma is displacing and producing shocks in the hot X-ray emitting gas. We found that the NGC 1404 halo is well represented by a single symmetric Beta model and follows the stellar light profile within the inner 8 kpc. The mass distribution is similar to the `central' component of the NGC 1399 halo. At larger radii ram pressure stripping from the intra-cluster medium produces strong asymmetries in the gas distribution. Finally we discuss the properties of the point source population finding evidence of correlation between the source excess and NGC 1399.

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arXiv2001

Chandra Observations of the Stellar Populations and Diffuse Gas in Nearby Galaxies

We present Chandra observations of two star-forming galaxies (M82 and the Antennae) and three starburst/AGN composite galaxies (NGC1808, NGC6240, NGC7331). In both star-forming galaxies we detect a large number of sources with diverse properties. Some of them can be identified as X-ray binaries, based on their variability and spectra. However, there is a significant number of very soft and/or extended sources which could be supernova remnants. These observations confirm previous indications that there is a population of sources with X-ray luminosities much higher than the Eddington limit for a neutron star, suggesting that these objects are abundant in star-forming galaxies. We find the the X-ray luminosity functions of the discrete sources in these two galaxies are very similar. In the case of the composite galaxies we find that the AGN does not dominate their X-ray emission. A significant fraction of the emission from these objects is extended but there are also X-ray sources associated with circumnuclear star-formation.

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arXiv2001

A Multivariate Statistical Analysis of Spiral Galaxy Luminosities. II. Morphology-Dependent Multiwavelength Emission Properties

This is the second of two papers based on a systematic multi-wavelength (X, B, H, 12$μ$m, FIR, 6cm) statistical analysis of the {\it Einstein Observatory Galaxy Catalog} sample of 234 `normal' S0/a-Irr galaxies. This sample is representative of spiral galaxies (Paper I), and its wide wavelength coverage provides a unique opportunity for a systematic exploration of the relations among different emission bands, that can give novel insight on the different emission processes and their relation to galaxian components, as a function of galaxy morphology.

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arXiv2001

Observations of galaxies with future X-ray observatories

Normal galaxies are faint and complex X-ray sources that provide very powerful probes for fundamental astrophysical questions. Examples include: the study of populations of X-ray emitting sources; the study of the entire spectrum of black-hole phenomena; and galaxy formation and evolution in interaction with the surrounding environment. While exciting, all these fields {\bf require} Chandra sub-arcsecond resolution, and at least comparable spectral capabilities. They also require much large collecting areas. I argue that this is the direction we must plan for future X-ray observatories.

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arXiv2001

A multivariate statistical analysis of spiral galaxy luminosities. I. Data and results

We have performed a multiparametric analysis of luminosity data for a sample of 234 normal spiral and irregular galaxies observed in X-rays with the {\it Einstein Observatory}. This sample is representative of S and Irr galaxies, with a good coverage of morphological types and absolute magnitudes. In addition to X-ray and optical da ta, we have compiled H-band magnitudes, IRAS near- and far-infrared, and 6cm radio co ntinuum observations for the sample from the literature. We have also performed a carefu l compilation of distance estimates. We have explored the effect of morphology by dividing the sample into early (S0/a-Sab), intermediate (Sb-Sbc), and late-type (Sc-Irr) subsamples. The data were analysed with bivariate and multiv ariate survival analysis techniques that make full use of all the information available in both detections and limits.

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arXiv2001

Deep ROSAT-HRI observation of the elliptical galaxy NGC 1399

We present the preliminary results of a deep (167 ks) ROSAT HRI observation of the cD galaxy NGC1399 in the Fornax cluster. We find, in agreement with previous observations, an extended (41 Kpc adopting a distance of 19 Mpc) gaseous halo with a luminosity of L_X=(4.41\pm 0.04)x10^{41} erg/s. The 5 arcsec resolution of the data allows us to detect a very complex and asymmetric structure of the halo with respect to the optical galaxy. Moreover the analysis of the radial structure reveals the presence of a multi-component profile not consistent with a simple King model over the whole 40 Kpc. We do not detect the presence of a central source and pose an upper limit to the luminosity of a possible active nucleus. Due to the length of the observation, comparable to that of a deep survey, we detect a large number of sources within the HRI FOV, in slight excess with respect to the estimates based on previous surveys. We study the flux distribution of the sources, their temporal behaviour and their spatial distribution with respect to the central galaxy.

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arXiv2001

Ultraluminous X-Ray Sources in External Galaxies

We investigate models for the class of ultraluminous non-nuclear X-ray sources (ULXs) seen in a number of galaxies and probably associated with star-forming regions. Models where the X-ray emission is assumed to be isotropic run into several difficulties. In particular formation of sufficient numbers of the required ultramassive black-hole X-ray binaries is problematic, and the likely transient behaviour of the resulting systems is not in good accord with observation. The assumption of mild X-ray beaming suggests instead that ULXs may represent a shortlived but extremely common stage in the evolution of a wide class of X-ray binaries. The best candidate for this is the phase of thermal-timescale mass transfer inevitable in many intermediate and high-mass X-ray binaries. This in turn suggests a link with the Galactic microquasars. The short lifetimes of high-mass X-ray binaries would explain the association of ULXs with episodes of star formation. These considerations still allow the possibility that individual ULXs may contain extremely massive black holes.

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arXiv2001

The Ultra-luminous M81 X-9 source: 20 years variability and spectral states

The source X-9 was discovered with the {\it Einstein Observatory} in the field of M81, and is located in the dwarf galaxy Holmberg IX. X-9 has a 0.2-4.0 keV luminosity of $\sim 5.5\times 10^{39}$ ergs~s$^{-1}$, if it is at the same distance as Holmberg IX (3.4 Mpc). This luminosity is above the Eddington luminosity of a 1~$M_{\odot}$ compact accreting object. Past hypotheses on the nature of this Super-Eddington source included a SNR or supershell, an accreting compact object and a background QSO. To shed light on the nature of this source, we have obtained and analyzed archival data, including the {\it Einstein} data, 23 ROSAT observations, Beppo-SAX and ASCA pointings. Our analysis reveals that most of the emission of X-9 arises from a point-like highly-variable source, and that lower luminosity extended emission may be associated with it. The spectrum of this source changes between low and high intensity states, in a way reminiscent of the spectra of galactic Black Hole candidates. Our result strongly suggest that X-9 is not a background QSO, but a bonafide `Super-Eddington' source in Ho IX, a dwarf companion of M81.

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arXiv2001

Chandra Observations of "The Antennae" Galaxies (NGC 4038/39)

We report the results of a deep Chandra ACIS pointing at the merging system NGC 4038/39. We detect an extraordinarily luminous population of X-ray sources, with luminosity well above that of XRBs in M31 and the Milky Way. If these sources are unbeamed XRBs, our observations may point to them being 10-100Mo black hole counterparts. We detect an X-ray bright hot ISM, with features including bright superbubbles associated with the actively star-forming knots, regions where hot and warm (H$α$) ISM intermingle, and a large-scale outflow.

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arXiv2001

Spectra and evolution of two X-ray sources in M81

We analyzed the spectral and temporal features of X-ray sources in M81 using data from the relevant ROSAT-PSPC/HRI observations. We discuss the main features of the point-like nucleus and of the second brightest source in the field (X-9), whose identification is still unclear.

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arXiv2001

Spectral and temporal properties of X-ray emission from the ultra-luminous source X-9 in M81

We have analysed the spectra and the variability of individual X-ray sources in the M-81 field using data from the available ROSAT-PSPC and ROSAT-HRI observations of this nearby spiral galaxy. Here we present the results on the second brightest source in the field (X-9 - Fabbiano, 1988 ApJ 325 544), whose identification and interpretation is still unclear. Our work includes the study of the shape of X-9 from HRI data, the light curve and hardness ratio evolution, and the spectral analysis.

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arXiv2000

Broad band properties of medium and low Lx/Lb Early Type Galaxies

We have measured the spectral properties of five galaxies of low to intermediate Lx/Lb ratios with BeppoSAX and ASCA. A hard component (kT ~4-10 keV) is observed in all galaxies. In NGC1553 the BeppoSAX data show that this component is extended, and suggest an origin for the emission in the evolved stellar population. In NGC3115, a point-like source appears embedded in an extended component morphologically similar to the stellar body, suggesting an almost equal contribution from the nuclear region and the binary population. A large central mass concentration and low level optical activity in NGC3379 argue for a contribution from the nucleus also in this object. However, for both NGC3115 and NGC3379 the nuclear emission is at a level well below that observed in other galaxies who host similar nuclear black holes. A second soft (kT ~0.3-0.7 keV) spectral component is needed to fit the data of NGC1407, NGC1553 and NGC4125 over the entire energy range probed (about 0.2-10 keV), best represented by a thermal component with line emission. We discuss possible interpretations of the origin of this component, which however will be better defined only with higher quality data.

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