arXiv subjects

A. Zezas

Publications and source records attributed to A. Zezas.

At least 127 records · Page 7Linked to original sources

arXiv2007

Occultation measurement of the size of the X-ray emitting region in the Active Galactic Nucleus of NGC 1365

We present an occultation of the central X-ray emitting region in the Seyfert Galaxy NGC 1365. This extreme spectral variation (from Compton-thin to reflection-dominated and back to Compton-thin in four days) has been caught in a ten days Chandra monitoring campaign consisting of six short (15 ks) observations performed every two days. We discuss the implications of this occultation within the scenario of a Compton-thick cloud crossing the line of sight of the X-ray source. We estimate a source size R<10^14 cm and a distance of the cloud from the source D<10^16 cm. This direct measurement confirms the theoretical expectations of an extremely compact X-ray source, and shows that the Compton-thick circumnuclear gas is located at a distance from the center on the scale of the Broad Line Region.

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arXiv2007

Two SMC Symbiotic stars undergoing steady hydrogen burning

Two symbiotic stars in the Small Magellanic Cloud (SMC), Lin 358 and SMC 3, have been supersoft X-ray sources (SSS) for more than 10 years. We fit atmospheric and nebular models to their X-ray, optical and UV spectra obtained at different epochs. The X-ray spectra are extremely soft, and appear to be emitted by the white dwarf atmosphere and not by the nebula like in some other symbiotics. The white dwarf of SMC 3, the hottest of the two sources, had a constant effective temperature ~500,000 K at various epochs during 12 years. No nova-like outbursts of these systems have been recorded in the last 50 years, despite continuous optical monitoring of the SMC, and there are no indications of cooling of the white dwarf, expected after a thermonuclear flash. The bolometric luminosity of this system in March of 2003 was more than an order of magnitude lower than three years later, however the time of the observation is consistent with a partial eclipse of the white dwarf, previously found in ROSAT and optical observations. The red giant wind is either very asymmetric or very clumpy. The conpact object of Lin 358 has been at T>~180,000 K since 1993, perhaps with a moderate increase. Atmospheric fits are obtained with log(g)>=9, appropriate only for WD mass >1.18 M(sol). The two systems are probably accreting and burning hydrogen steadily at the high rate required for type Ia supernova progenitors.

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arXiv2006

Probing the Low-Luminosity XLF in Normal Elliptical Galaxies

We present the first low luminosity (LX > 5 - 10 1036 erg s-1) X-ray luminosity functions (XLFs) of low-mass X-ray binaries (LMXBs) determined for two typical old elliptical galaxies, NGC 3379 and NGC 4278. Because both galaxies contain little diffuse emission from hot ISM and no recent significant star formation (hence no high-mass X-ray binary contamination), they provide two of the best homogeneous sample of LMXBs. With 110 and 140 ks Chandra ACIS S3 exposures, we detect 59 and 112 LMXBs within the D25 ellipse of NGC 3379 and NGC 4278, respectively. The resulting XLFs are well represented by a single power-law with a slope (in a differential form) of 1.9 0.1. In NGC 4278, we can exclude the break at LX ~ 5 x 1037 erg s-1 that was recently suggested to be a general feature of LMXB XLFs. In NGC 3379 instead we find a localized excess over the power law XLF at ~4 x 1037 erg s-1, but with a marginal significance of ~1.6s. Because of the small number of luminous sources, we cannot constrain the high luminosity break (at 5 x 1038 erg s-1) found in a large sample of early type galaxies. While the optical luminosities of the two galaxies are similar, their integrated LMXB X-ray luminosities differ by a factor of 4, consistent with the relation between the X-ray to optical luminosity ratio and the globular cluster specific frequency.

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arXiv2006

X-ray number counts of normal galaxies

We use the number counts of X-ray selected normal galaxies to explore their evolution by combining the most recent wide-angle shallow and pencil-beam deep samples available. The differential X-ray number counts, dN/dS, for early and late-type normal galaxies are constructed separately and then compared with the predictions of the local X-ray luminosity function under different evolution scenarios. The dN/dS of early type galaxies is consistent with no evolution out to z~0.5. For late-type galaxies our analysis suggests that it is the sources with X-ray--to--optical flux ratio logfx/fopt>-2 that are evolving the fastest. Including these systems in the late-type galaxy sample yields evolution of the form ~(1+z)^{2.7} out to z~0.4. On the contrary late-type sources with logfx/fopt<-2 are consistent with no evolution. This suggests that the logfx/fopt>-2 population comprises the most powerful and fast evolving starbursts at moderate and high-z. We argue that although residual low-luminosity AGN contamination may bias our results toward stronger evolution, this is unlikely to modify our main conclusions.

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arXiv2006

The Eclipsing ULX in NGC 3379

We report recent Chandra observations of the ULX in the elliptical galaxy NGC3379 that clearly detect two flux variability cycles. Comparing these data with the Chandra observation of ~5yr ago, we measure a flux modulation with a period of ~12.6hr. Moreover, we find that the emission undergoes a correlated spectral modulation, becoming softer at low flux. We argue that our results establish this source as a ULX binary in NGC3379. Given the old stellar population of this galaxy, the ULX is likely to be a soft transient, however historical X-ray sampling suggests that the current on phase has lasted ~10yr. We discuss our results in terms of ADC and wind-feedback models. We constrain the donor mass and orbital period at the onset of mass transfer within 1.15-1.4M and 12.5-16hr, respectively. The duration of the mass-transfer phase so far is probably ~1Gyr and the binary has been a soft X-ray transient throughout this time. These constraints are insensitive to the mass of the accretor.

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arXiv2005

Chemical enrichment of the complex hot ISM of the Antennae Galaxies: II. Physical properties of the hot gas and supernova feedback

We investigate the physical properties of the interstellar medium (ISM) in the merging pair of galaxies known as The Antennae (NGC 4038/39), using the deep coadded ~411 ks Chandra ACIS-S data set. The method of analysis and some of the main results from the spectral analysis, such as metal abundances and their variations from ~0.2 to ~20-30 times solar, are described in Paper I (Baldi et al. submitted). In the present paper we investigate in detail the physics of the hot emitting gas, deriving measures for the hot-gas mass (~10^ M_sun), cooling times (10^7-10^8 yr), and pressure (3.5x10^-11-2.8x10^-10 dyne cm^-2). At least in one of the two nuclei (NGC 4038) the hot-gas pressure is significantly higher than the CO pressure, implying that shock waves may be driven into the CO clouds. Comparison of the metal abundances with the average stellar yields predicted by theoretical models of SN explosions points to SNe of Type II as the main contributors of metals to the hot ISM. There is no evidence of any correlation between radio-optical star-formation indicators and the measured metal abundances. Although due to uncertainties in the average gas density we cannot exclude that mixing may have played an important role, the short time required to produce the observed metal masses (<=2 Myr) suggests that the correlations are unlikely to have been destroyed by efficient mixing. More likely, a significant fraction of SN II ejecta may be in a cool phase, in grains, or escaping in hot winds. In each case, any such fraction of the ejecta would remain undetectable with soft X-ray observations.

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arXiv2005

Chemical enrichment of the complex hot ISM of the Antennae galaxies: I. Spatial and spectral analysis of the diffuse X-ray emission

We present an analysis of the properties of the hot interstellar medium (ISM) in the merging pair of galaxies known as The Antennae (NGC 4038/39), performed using the deep, coadded ~411 ks Chandra ACIS-S data set. These deep X-ray observations and Chandra's high angular resolution allow us to investigate the properties of the hot ISM with unprecedented spatial and spectral resolution. Through a spatially resolved spectral analysis, we find a variety of temperatures (from 0.2 to 0.7 keV) and Nh (from Galactic to 2x10^21 cm^-2). Metal abundances for Ne, Mg, Si, and Fe vary dramatically throughout the ISM from sub-solar values (~0.2) up to several times solar.

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arXiv2005

Highly ionized iron absorption lines from outflowing gas in the X-ray spectrum of NGC 1365

We present the discovery of four absorption lines in the X-ray spectrum of the Seyfert Galaxy NGC 1365, at energies between 6.7 and 8.3 keV. The lines are detected with high statistical confidence (from >20sigma for the strongest to \~4sigma for the weakest) in two XMM-Newton observations 60 ksec long. We also detect the same lines, with lower signal-to-noise (but still >2sigma for each line) in two previous shorter (~10 ksec) XMM observations. The spectral analysis identifies these features as FeXXV and FeXXVI Kalpha and Kbeta lines, outflowing with velocities varying between ~1000 to ~5000 km/s among the observations. These are the highest quality detections of such lines so far. The high equivalent widths (EW(Kalpha)~100 eV) and the Kalpha/Kbeta ratios imply that the lines are due to absorption of the AGN continuum by a highly ionized gas with column density N_H~5x10^23 cm^{-2}

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arXiv2005

The X-ray binary population in M33: I. Source list and luminosity function

In this paper we present the source list for 3 Chandra observations of the Local Group galaxy M33. The observations are centered on the nucleus and on the star forming region NGC 604. We detect a total of 261 sources in an area of \~0.2 square degree down to a flux limit of 3*10^{-16} erg/s/cm^2 which corresponds to a luminosity of ~2*10^34 erg/s at a distance of 840 kpc. From the source list we construct the luminosity functions of sources observed in M33. Taking into account background contamination the luminosity functions are consistent with those of other star forming galaxies. In addition, the combination of X-ray color analysis and the existence of ``blue'' optical counterparts strongly indicate that the X-ray point source population in M33 consists of young objects. Above 3*10^35 erg/s there are few X-ray sources in the locus of the X-ray hardness ratio diagram that is generally populated by LMXBs.

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arXiv2005

Extreme X-ray spectral variability in the Seyfert 2 Galaxy NGC 1365

We present multiple Chandra and XMM-Newton observations of the type 1.8 Seyfert Galaxy NGC 1365, which shows the most dramatic X-ray spectral changes observed so far in an AGN: the source switched from reflection dominated to transmission dominated and back in just 6 weeks. During this time the soft thermal component, arising from a ~1 kpc region around the center, remained constant. The reflection component is constant at all timescales, and its high flux relative to the primary component implies the presence of thick gas covering a large fraction of the solid angle. The presence of this gas, and the fast variability time scale, suggest that the Compton-thick to Compton thin change is due to variation in the line-of-sight absorber, rather than to extreme intrinsic emission variability. We discuss a structure of the circumnuclear absorber/reflector which can explain the observed X-ray spectral and temporal properties.

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arXiv2004

A Mass Measurement for the Missing Baryons in the Warm-Hot Intergalactic Medium via the X-ray Forest

Recent Cosmological measurements indicate that baryons comprise about four percent of the total mass-energy density of the Universe, which is in accord with the predictions arising from studies of the production of the lightest elements. It also is in agreement with the actual number of baryons detected at early times (redshifts>2). However, close to our own epoch (z<2), the number of baryons actually detected add up to just over half (~55 percent) of the number seen at z>2, meaning that about 45 percent are 'missing'. Here we report a determination of the mass-density of a previously undetected population of baryons, in the warm-hot phase of the intergalactic medium. We show that this mass-density is consistent, within the uncertainties, with the mass-density of the missing baryons.

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arXiv2004

XMM-Newton observations of the interacting galaxy pairs NGC7771/0 and NGC2342/1

We present XMM-Newton X-ray observations of the interacting galaxy pairs NGC7771/7770 and NGC2342/2341. In NGC7771, for the first time we are able to resolve the X-ray emission into a bright central source (L(X)~10^41 erg/s), plus two (L(X)>10^40 erg/s) ultraluminous X-ray sources (ULXs) located either end of the bar. The soft emission of the central source is well-modelled by a two-temperature thermal plasma with kT=0.4/0.7 keV. The hard emission is modelled with a flat absorbed power-law (Gamma~1.7, NH~10^22 cm^2), and this together with a low-significance (1.7 sigma) ~300 eV equivalent width emission line at ~6 keV are the first indications that NGC7771 may host a low-luminosity AGN. For the bar ULXs, a power-law fit to X-1 is improved at the 2.5 sigma level with the addition of a thermal plasma component (kT~0.3 keV), while X-2 is improved only at the 1.3 sigma level with the addition of a disc blackbody component with T(in)~0.2 keV. Both sources are variable on short time-scales implying that their emission is dominated by single accreting X-ray binaries (XRBs). The three remaining galaxies, NGC7770, NGC2342 and NGC2341, have observed X-ray luminosities of 0.2, 1.8 & 0.9x10^41 erg/s respectively (0.3-10 keV). Their integrated spectra are also well-modelled by multi-temperature thermal plasma components with kT=0.2-0.7 keV, plus power-law continua with slopes of Gamma=1.8-2.3 that are likely to represent the integrated emission of populations of XRBs as observed in other nearby merger systems. A comparison with other isolated, interacting and merging systems shows that all four galaxies follow the established correlations for starburst galaxies between X-ray, far-infrared and radio luminosities, demonstrating that their X-ray outputs are dominated by their starburst components.

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arXiv2004

Chandra discovery of activity in the quiescent nuclear black hole of NGC821

We report the results of the Chandra ACIS-S observations of the elliptical galaxy NGC 821, which harbors a supermassive nuclear black hole (of 3.5x10^7 solar masses), but does not show sign of AGN activity. A small, 8.5" long (~1 kpc at the galaxy's distance of 23 Mpc), S-shaped, jet-like feature centered on the nucleus is detected in the 38 ksec ACIS-S integrated exposure of this region. The luminosity of this feature is L_X ~ 2.6 x 10^39 erg/s (0.3-10 keV), and its spectrum is hard (described by a power-law of Gamma = 1.8^{+0.7}_{-0.6}; or by thermal emission with kT > 2 keV). We discuss two possibilities for the origin of this feature: (1) a low-luminosity X-ray jet, or (2) a hot shocked gas. In either case, it is a clear indication of nuclear activity, detectable only in the X-ray band. Steady spherical accretion of the mass losses from the central stellar cusp within the accretion radius, when coupled to a high radiative efficiency, already provides a power source exceeding the observed radiative losses from the nuclear region.

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arXiv2004

The Ionized Nuclear Environment in NGC 985 as seen by Chandra and BeppoSAX

(Abridged) We investigate the ionized environment of the Seyfert 1 galaxy NGC 985 with a new Chandra-HETGS observation and an archival BeppoSAX observation. Two absorption components are clearly required to fit absorption features observed in the Chandra spectrum. The components have a difference of 29 in ionization parameter and 3 in column density. The presence of the low ionization component is evidenced by an Fe M-shell unresolved transition array (UTA) produced by charge states VII to XIII. The high ionization phase is required by the presence of broad absorption features arising from several blends of Fe L-shell transitions (Fe XVII-XXII). A third highly ionized component might also be present, but the data does not allow to constrain its properties. An X-ray luminosity variation by a factor 2.3 is observed between the BeppoSAX and Chandra observations (separated by almost 3 years). Variability in the opacity of the absorbers is detected in response to the continuum variation, but while the colder component is consistent with a simple picture of photoionization equilibrium, the ionization state of the hotter component seems to increase while the continuum flux drops. The most striking result in our analysis is that during both the Chandra and the BeppoSAX observations, the two absorbing components appear to have the same pressure. Thus, we suggest that the absorption arises from a multi-phase wind. Such scenario can explain the change in opacity of both absorption components during the observations, but requires that a third hotter component is pressure-confining the two phases. Hence, our analysis points to a 3-phase medium similar to the wind found in NGC 3783, and further suggests that such a wind might be a common characteristic in AGN.

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arXiv2004

High Resolution Imaging of the HeII lambda 4686 Emission Line Nebula Associated with the Ultraluminous X-Ray Source in Holmberg II

We present Hubble Space Telescope images of the He{\sc iii} region surrounding the bright X-ray source in the dwarf irregular galaxy Holmberg II. Using {\it Chandra}, we find a position for the X-ray source of $α=$ 08h 19m 28.98s, $δ=$ +70\arcdeg 42\arcmin 19.\arcsec3 (J2000) with an uncertainty of 0.6$''$. We identify a bright, point-like optical counterpart centered in the nebula with the X-ray source. The optical magnitude and color of the counterpart are consistent with a star with spectral type between O4V and B3 Ib at a distance of 3.05 Mpc or reprocessed emission from an X-ray illuminated accretion disk. The nebular He{\sc ii} luminosity is $2.7 \times 10^{36} \rm erg s^{-1}$. The morphology of the He{\sc ii}, H$β$, and [O{\sc i}] emission are consistent with being due to X-ray photoionization and are inconsistent with narrow beaming of the X-ray emission. A spectral model consisting of a multicolor disk blackbody with inverse-Compton emission from a hot corona gives a good fit to X-ray spectra obtained with XMM-Newton. Using the fitted X-ray spectrum, we calculate the relation between the He{\sc ii} and X-ray luminosity and find that the He{\sc ii} flux implies a lower bound on the X-ray luminosity in the range 4 to $6 \times 10^{39} \rm erg s^{-1}$ if the extrapolation of the X-ray spectrum between 54 eV and 300 eV is accurate. A compact object mass of at least 25 to $40 M_{\odot}$ would be required to avoid violating the Eddington limit.

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arXiv2004

X-ray luminous galaxies II. Chandra and XMM-Newton Observations of the 'composite' galaxy IRAS20051-1117

We present Chandra and XMM-Newton observations of the composite star-forming/Seyfert galaxy IRAS20051-1117. The X-ray imaging and spectral properties reveal the presence of an active nucleus. The Chandra image shows a strong nuclear point source (L(2-10 keV) ~ 4x10^{42} erg s-1). The nuclear X-ray source coincides with a bright, alsoun-resolved, UV source which appears in the xmm Optical Monitor images. The xmm and chandra spectrum is well represented by a power-law with a photon index of ~1.7-1.9 and a thermal component with a temperature of 0.2 keV. We also detect an Fe line at 6.4 keV with an equivalent width of ~0.3 keV, typical of the iron lines that have been detected in the X-ray spectra of "classical" AGN. We find no evidence for short-term variability in the X-ray light curves, while we detect no variations between the xmm and chandra observations which are separated by about 20 days. Optical spectroscopic observations which were performed ~2.5 months after the xmm observation show that the optical spectrum is dominated by a star-forming galaxy component, although a weak broad Halpha component is present, in agreement with the results from past observations. The lack of strong AGN signatures in the optical spectrum of the source can be explained by the dilution of the nuclear AGN emission by the nuclear star-forming component and the strong emission of the underlying, bright host galaxy.

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arXiv2004

Chandra's Discovery of Activity in the Quiescent Nuclear Black Hole of NGC 821

We report the results of the {\it Chandra} ACIS-S observations of the elliptical galaxy NGC 821, which harbors a supermassive nuclear black hole (of $3.5 \times 10^7 M_{\odot}$), but does not show sign of AGN activity. A small, 8.5$^{\prime\prime}$ long ($\sim 1$ kpc at the galaxy's distance of 23 Mpc), S-shaped, jet-like feature centered on the nucleus is detected in the 38 ksec ACIS-S integrated exposure of this region. The luminosity of this feature is $L_X \sim 2.6 \times 10^{39} \rm ergs s^{-1}$ (0.3-10 keV), and its spectrum is hard (described by a power-law of $Γ= 1.8^{+0.7}_{-0.6}$; or by thermal emission with $kT >2$ keV). We discuss two possibilities for the origin of this feature: (1) a low-luminosity X-ray jet, or (2) a hot shocked gas. In either case, it is a clear indication of nuclear activity, detectable only in the X-ray band. Steady spherical accretion of the mass losses from the central stellar cusp within the accretion radius, when coupled to a high radiative efficiency, already provides a power source exceeding the observed radiative losses from the nuclear region.

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arXiv2004

XMM-Newton observations of the starburst merger galaxies NGC 3256 & NGC 3310

We present XMM-Newton EPIC observations of the two nearby starburst merger galaxies NGC 3256 & NGC 3310. The broad-band (0.3-10 keV) integrated X-ray emission from both galaxies shows evidence of multi-phase thermal plasmas plus an underlying hard non-thermal power-law continuum. NGC 3256 is well-fit with a model comprising two MEKAL components (kT=0.6/0.9 keV) plus a hard power-law (Gamma=2), while NGC 3310 has cooler MEKAL components (kT=0.3/0.6 keV) and a harder power-law tail (Gamma=1.8). Chandra observations of these galaxies both reveal the presence of numerous discrete sources embedded in the diffuse emission, which dominate the emission above ~2 keV and are likely to be the source of the power-law emission. The thermal components show a trend of increasing absorption with higher temperature, suggesting that the hottest plasmas arise from supernova-heated gas within the disks of the galaxies, while the cooler components arise from outflowing galactic winds interacting with the ambient interstellar medium (ISM). We find no strong evidence for an active galactic nucleus (AGN) in either galaxy.

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