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Search Results: 1 - 10 of 462001 matches for " A. Zezas "
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The X-ray Luminosity Function of "The Antennae" Galaxies (NGC4038/39) and the Nature of Ultra-Luminous X-ray Sources
A. Zezas,G. Fabbiano
Physics , 2002,
Abstract: 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 ($\alpha=-0.47$), 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.
Discovery of X-ray pulsations in the Be/X-ray binary IGR J21343+4738
P. Reig,A. Zezas
Physics , 2014, DOI: 10.1093/mnras/stu898
Abstract: We report on the discovery of X-ray pulsations in the Be/X-ray binary IGR J21343+4738 during an XMM-Newton observation. We obtained a barycentric corrected pulse period of 320.35+-0.06 seconds. The pulse profile displays a peak at low energy that flattens at high energy. The pulse fraction is 45+-3$% and independent of energy within the statistical uncertainties. The 0.2-12 keV spectrum is well fit by a two component model consisting of a blackbody with kT=0.11+-0.01 keV and a power law with photon index Gamma=1.02+-0.07. Both components are affected by photoelectric absorption with a equivalent hydrogen column density NH=(1.08+-0.15)x 10^{22} cm^{-2} The observed unabsorbed flux is 1.4x10^{-11} erg cm^{-2} s^{-1} in the 0.2-12 keV energy band. Despite the fact that the Be star's circumstellar disc has almost vanished, accretion continues to be the main source of high energy radiation. We argue that the observed X-ray luminosity (LX~10^{35} erg s^{-1}) may result from accretion via a low-velocity equatorial wind from the optical companion.
Chandra observations of NGC4698: a Seyfert-2 with no absorption
I. Georgantopoulos,A. Zezas
Physics , 2003, DOI: 10.1086/377120
Abstract: We present Chandra ACIS-S observations of the enigmatic Seyfert-2 galaxy NGC4698. This object together with several other bona-fide Seyfert-2 galaxies show no absorption in the low spatial resolution ASCA data, in contrast to the standard unification models. Our Chandra observations of NGC4698 probe directly the nucleus allowing us to check whether nearby sources contaminate the ASCA spectrum. Indeed, the Chandra observations show that the ASCA spectrum is dominated by two nearby AGN. The X-ray flux of NGC4698 is dominated by a nuclear source with luminosity L(0.3-8 keV) ~ 10^39, erg s-1 coincident with the radio nucleus. Its spectrum is well represented by a power-law, ~ 2.2, obscured by a small column density of 5x10^20 cm-2 suggesting that NGC4698 is an atypical Seyfert galaxy. On the basis of its low luminosity we then interpret NGC4698 as a Seyfert galaxy which lacks a broad-line region.
Disc-loss episode in the Be shell optical counterpart to the high-mass X-ray binary IGR J21343+4738
P. Reig,A. Zezas
Physics , 2013, DOI: 10.1051/0004-6361/201321408
Abstract: The main goal of this work is to determine the properties of the optical counterpart to the INTEGRAL source IGR J21343+4738, and study its long-term optical variability. We present optical photometric BVRI and spectroscopic observations covering the wavelength band 4000-7500 A. We find that the optical counterpart to IGR J21343+4738 is a V=14.1 B1IVe shell star located at a distance of ~8.5 kpc. The Halpha line changed from an absorption dominated profile to an emission dominated profile, and then back again into absorption. In addition, fast V/R asymmetries were observed once the disc developed. Although the Balmer lines are the most strongly affected by shell absorption, we find that shell characteristics are also observed in He I lines. The optical spectral variability of IGR J21343+4738 is attributed to the formation of an equatorial disc around the Be star and the development of an enhanced density perturbation that revolves inside the disc. We have witnessed the formation and dissipation of the circumstellar disc. The strong shell profile of the Halpha and He I lines and the fact that no transition from shell phase to a pure emission phase is seen imply that we are seeing the system near edge-on.
New insights into the Be/X-ray binary system MXB 0656-072
E. Nespoli,P. Reig,A. Zezas
Physics , 2012, DOI: 10.1051/0004-6361/201219586
Abstract: The X-ray transient MXB 0656-072 is a poorly studied member of high-mass X-ray binaries. Based on the transient nature of the X-ray emission, the detection of pulsations, and the early-type companion, it has been classified as a Be X-ray binary (Be/XRB). However, the flaring activity covering a large fraction of a giant outburst is somehow peculiar. Our goal is to investigate the multiwavelength variability of the high-mass X-ray binary MXB 0656-072. We carried out optical spectroscopy and analysed all RXTE archive data, performing a detailed X-ray-colour, spectral, and timing analysis of both normal (type-I) and giant (type-II) outbursts from MXB 0656-072 This is the first detailed analysis of the optical counterpart in the classification region (4000-5000 A). From the strength and ratio of the elements and ions, we derive an O9.5Ve spectral type, in agreement with previous classification. This confirms its Be nature. The characterisation of the Be/XRB system relies on Balmer lines in emission in the optical spectra, long-term X-ray variability, and the orbital period vs. spin period and EW(H\alpha) relation. The peculiar feature that distinguishes the type-II outburst is flaring activity, which occurs during the whole outburst peak, before a smoother decay. We interpret it in terms of magneto-hydrodynamic instability. Colour and spectral analysis reveal a hardening of the spectrum as the flux increases. We explored the aperiodic X-ray variability of the system for the first time, finding a correlation of the central frequency and rms of the main timing component with luminosity, which extends up to a "saturation" flux of 1E-8 erg/cm^2/s. A correlation between timing and spectral parameters was also found, pointing to an interconnection between the two physical regions responsible for both phenomenologies.
Spatial Structures In the Globular Cluster Distribution of the Ten Brightest Virgo Galaxies
R. D'Abrusco,G. Fabbiano,A. Zezas
Physics , 2015, DOI: 10.1088/0004-637X/805/1/26
Abstract: We report the discovery of significant localized structures in the projected two-dimensional (2D) spatial distributions of the Globular Cluster (GC) systems of the ten brightest galaxies in the Virgo Cluster. We use catalogs of GCs extracted from the HST ACS Virgo Cluster Survey (ACSVCS) imaging data, complemented, when available, by additional archival ACS data. These structures have projected sizes ranging from $\sim\!5$ arcsec to few arc-minutes ($\sim\!1$ to $\sim\!25$ kpc). Their morphologies range from localized, circular, to coherent, complex shapes resembling arcs and streams. The largest structures are preferentially aligned with the major axis of the host galaxy. A few relatively smaller structures follow the minor axis. Differences in the shape and significance of the GC structures can be noticed by investigating the spatial distribution of GCs grouped by color and luminosity. The largest coherent GC structures are located in low-density regions within the Virgo cluster. This trend is more evident in the red GC population, believed to form in mergers involving late-type galaxies. We suggest that GC over-densities may be driven by either accretion of satellite galaxies, major dissipationless mergers or wet dissipation mergers. We discuss caveats to these scenarios, and estimate the masses of the potential progenitors galaxies. These masses range in the interval $10^{8.5}\!-\!10^{9.5}$ solar masses, larger than those of the Local Group dwarf galaxies.
On the need for simultaneity between X-ray and radio observations in Doppler factor estimates
I. Liodakis,A. Zezas,V. Pavlidou
Physics , 2015,
Abstract: We use archival X-ray and radio VLBA data to calculate inverse Compton Doppler factors for four high-power radio, $\gamma$-loud Flat Spectrum Radio Quasars frequently monitored by the F-GAMMA project. We explore the effect of the non-simultaneity between X-ray and radio observations by calculating Doppler factors for simultaneous and non-simultaneous observations. By comparing the newly re-calculated values from this work and archival values with variability Doppler factors, we show that simultaneous/quasi-simultaneous X-ray and radio observations can provide a reliable estimate of the true Doppler factor in blazar jets. For the particular case of PKS0528+134 we find that a time-difference of up to 1 week provides inverse Compton Doppler factor estimates consistent with the variability Doppler factor of this source at the 19% percent level. In contrast, time differences of more than 30 days between radio and X-ray observations result to discrepancies from 100% to more than a factor of 4.
The optical counterpart to IGR J06074+2205: a Be/X-ray binary showing disc loss and V/R variability
P. Reig,A. Zezas,L. Gkouvelis,cat
Physics , 2010, DOI: 10.1051/0004-6361/201014788
Abstract: Present X-ray missions are regularly discovering new X/gamma-ray sources. The identification of the counterparts of these high-energy sources at other wavelengths is important to determine their nature. In particular, optical observations are an essential tool in the study of X-ray binary populations in our Galaxy. The main goal of this work is to determine the properties of the optical counterpart to the INTEGRAL source IGR J06074+2205, and study its long-term optical variability. Although its nature as a high-mass X-ray binary has been suggested, little is known about its physical parameters. We have been monitoring IGR J06074+2205 since 2006 in the optical band. We present optical photometric BVRI and spectroscopic observations covering the wavelength band 4000-7000 A. The blue spectra allow us to determine the spectral type and luminosity class of the optical companion; the red spectra, together with the photometric magnitudes, were used to derive the colour excess E(B-V) and estimate the distance. We have carried out the first detailed optical study of the massive component in the high-mass X-ray binary IGR J06074+2205. We find that the optical counterpart is a V=12.3 B0.5Ve star located at a distance of ~4.5 kpc. The monitoring of the Halpha line reveals V/R variability and an overall decline of its equivalent width. The Halpha line has been seen to revert from an emission to an absorption profile. We attribute this variability to global changes in the structure of the Be star's circumstellar disc which eventually led to the complete loss of the disc. The density perturbation that gives rise to the V/R variability vanishes when the disc becomes too small.
The quiescent state of the accreting X-ray pulsar SAX J2103.5+4545
P. Reig,V. Doroshenko,A. Zezas
Physics , 2014, DOI: 10.1093/mnras/stu1840
Abstract: We present an X-ray timing and spectral analysis of the Be/X-ray binary SAX J2103.5+4545 at a time when the Be star's circumstellar disk had disappeared and thus the main reservoir of material available for accretion had extinguished. In this very low optical state, pulsed X-ray emission was detected at a level of L_X~10^{33} erg/s. This is the lowest luminosity at which pulsations have ever been detected in an accreting pulsar. The derived spin period is 351.13 s, consistent with previous observations. The source continues its overall long-term spin-up, which reduced the spin period by 7.5 s since its discovery in 1997. The X-ray emission is consistent with a purely thermal spectrum, represented by a blackbody with kT=1 keV. We discuss possible scenarios to explain the observed quiescent luminosity and conclude that the most likely mechanism is direct emission resulting from the cooling of the polar caps, heated either during the most recent outburst or via intermittent accretion in quiescence.
A multi-wavelength study of Supernova Remnants in six nearby galaxies. II. New optically selected Supernova Remnants
I. Leonidaki,P. Boumis,A. Zezas
Physics , 2012, DOI: 10.1093/mnras/sts324
Abstract: We present results from a study of optically emitting Supernova Remnants (SNRs) in six nearby galaxies (NGC 2403, NGC 3077, NGC 4214, NGC 4395, NGC 4449 and NGC 5204) based on deep narrow band H{\alpha} and [SII] images as well as spectroscopic observations. The SNR classification was based on the detected sources that fulfill the well-established emission line flux criterion of [SII]/H{\alpha} > 0.4. This study revealed ~400 photometric SNRs down to a limiting H{\alpha} flux of 10^(-15) erg sec^(-1) cm^(-2). Spectroscopic observations confirmed the shock-excited nature of 56 out of the 96 sources with ([SII]/H{\alpha})$_{phot}$> 0.3 (our limit for an SNR classification) for which we obtained spectra. 11 more sources were spectroscopically identified as SNRs although their photometric [SII]/H{\alpha} ratio was below 0.3. We discuss the properties of the optically-detected SNRs in our sample for different types of galaxies and hence different environments, in order to address their connection with the surrounding interstellar medium. We find that there is a difference in [NII]/H{\alpha} line ratios of the SNR populations between different types of galaxies which indicates that this happens due to metallicity. We cross-correlate parameters of the optically detected SNRs ([SII]/H{\alpha} ratio, luminosity) with parameters of coincident X- ray emitting SNRs, resulted from our previous studies in the same sample of galaxies, in order to understand their evolution and investigate possible selection effects. We do not find a correlation between their H{\alpha} and X-ray luminosities, which we attribute to the presence of material in a wide range of temperatures. We also find evidence for a linear relation between the number of luminous optical SNRs (10^(37) erg sec^(-1)) and SFR in our sample of galaxies.
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