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I. Georgantopoulos

Publications and source records attributed to I. Georgantopoulos.

At least 163 records · Page 9Linked to original sources
arXiv2001

BeppoSAX Observations of the Seyfert-2 Galaxies NGC 7172 and ESO 103-G35

We investigate the X-ray spectra of the type-2 Seyfert galaxies NGC 7172 and ESO 103-G35, using BeppoSAX observations, separated by approximately one year. We find that the X-ray spectra of both NGC 7172 and ESO 103-G35 can be well fitted using a power-law model with an Fe K emission line at 6.4 keV. We did not find any statistically significant evidence for the existence of a reflection component in the X-ray spectra of these two galaxies. The continuum flux has decreased by a factor of approximately two during this period, in both objects. However, the spectral index as well as the absorption column have remained constant. We find weak evidence for the decrease of the normalization of the Fe K emission line in a similar manner to the continuum in NGC7172. We also report evidence for a broad Fe K confirming previous observations. In contrast, in the case of ESO 103-G35 the line flux does not change while its width remains unresolved.

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arXiv2000

RXTE observations of Seyfert-2 galaxies: evidence for spectral variability

We present a series of RXTE observations of the nearby obscured Seyfert galaxies ESO103-G35, IC5063, NGC4507 and NGC7172. The period of monitoring ranges from seven days for NGC7172 up to about seven months for ESO103-G035. The spectra of all galaxies are well fit with a highly obscured (N_H>10^23 cm-2) power-law and an Fe line at 6.4 keV. We find strong evidence for the presence of a reflection component in ESO103-G35 and NGC4507. The observed flux presents strong variability on day timescales in all objects. Spectral variability is also detected in the sense that the spectrum steepens with increasing flux similar to the behaviour witnessed in some Seyfert-1 galaxies.

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arXiv2000

The angular correlation function of the ROSAT All Sky Survey Bright Source Catalogue

We have derived the angular correlation function of a sample of 2096 sources detected in the ROSAT All Sky Survey Bright Source Catalogue, in order to investigate the clustering properties of AGN in the local Universe. Our sample is constructed by rejecting all known stars, as well as extended X-ray sources. Areas with |b|<30 deg. and declination <-30 deg. are also rejected due to the high or uncertain neutral hydrogen absorption. Cross-correlation of our sample with the Hamburg/RASS optical identification catalogue, suggests that the vast majority of our sources are indeed AGN. A 4.1 sigma correlation signal between 0 and 8 degrees was detected with w(theta<8 deg.)=0.025 +- 0.006. Assuming the usual power-law form of the 2-point correlation function we find an angular correlationlength of 0.062 degrees. Deprojection on 3 dimensions, using the Limber's equation, yields a spatial correlation length of 6.0+- 1.6 h^-1 Mpc. This is consistent with the AGN clustering results derived at higher redshifts in optical surveys and suggests a comoving model for the clustering evolution.

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arXiv2000

X-ray variability in a deep, flux limited sample of QSOs

We present an analysis of X-ray variability in a flux limited sample of QSOs. Selected from our deep ROSAT survey, these QSOs span a wide range in redshift ($0.1 0.5$) do not show the anti-correlation with luminosity seen in local AGN. We speculate on the implications of these results for physical models of AGN and their evolution. Finally, we find evidence for X-ray variability in an object classified as a narrow emission-line galaxy, suggesting the presence of an AGN.

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arXiv2000

A deep ROSAT survey XV. the average QSO spectrum and its evolution

Using a sample of 165 X-ray selected QSOs from seven deep ROSAT fields we investigate the X-ray spectral properties of an ``average''radio-quiet broad line QSO as a function of redshift. The QSO stacked spectra, in the observers 0.1-2 keV band, in five redshift bins over the range 0.1 1.0) the single power-law model gives an acceptable fit to the data over the full energy band, at lower redshifts the spectra need a second component at low energies, a `soft excess'. Inclusion of a simple model for the soft excess, i.e. a black-body component (kT~100 eV), results in a significant improvement to the model fit, and yields power-law slopes of 1.8-1.9, for all redshift bins. This power-law is not inconsistent, within the error bars, with those of nearby AGN in the 2-10 keV band, suggesting that the same intrinsic power-law slope may continue from 10 keV down to below 0.5 keV. We caution that there is a possibility that the spectral upturn observed may not represent a real physical component but could be due to co-adding spectra with a large dispersion in spectral indices. Regardless of the origin of the soft excess, the average QSO spectrum has important consequences for the origin of the X-ray background: the average spectra of the typical, faint, high redshift QSO are significantly steeper than the spectrum of the X-ray background extending the spectral paradox into the soft 0.1-2 keV X-ray band.

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arXiv1999

X-ray observations of the Ultraluminous infrared galaxy IRAS19254-7245 (The Superantennae)

We present ROSAT HRI and ASCA observations of the well known ULIRG IRAS19254-7245 (the Superantennae). The object is not detected by ROSAT yielding a 3σupper limit of L_x ~8x10^{41} erg/s in the 0.1-2 keV band. However, we obtain a clear detection by ASCA yielding a luminosity in the 2-10 keV band of 2 \times 10^{42}erg/s. Its X-ray spectrum is very hard, equivalent to a photon index of Gamma=1.0+-0.35. We therefore, attempt to model the X-ray data with a "scatterer" model in which the intrinsic X-ray emission along our line of sight is obscured by an absorbing screen while some fraction, f, is scattered into our line of sight by an ionized medium; this is the standard model for the X-ray emission in obscured (but non Compton-thick) Seyfert galaxies. We obtain an absorbing column of 2x10^{23}cm^{-2} for a power-law photon index of Gamma=1.9, an order of magnitude above the column estimated on the basis of optical observations; the percentage of the scattered emission is high (~20%). Alternatively, a model where most of the X-ray emission comes from reflection on a Compton thick torus (N_H>10^{24} cm^{-2}) cannot be ruled out. We do not detect an Fe line at 6.4 keV; however, the upper limit (90%) to the equivalent width of the 6.4 keV line is high (~3 keV). All the above suggest that most of the X-ray emission originates in an highly obscured Seyfert-2 nucleus.

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arXiv1999

X-ray observations of the'composite' Seyfert/star-forming galaxy IRAS00317-2142

I present ASCA observations of IRAS00317-2142, the most luminous (Lx~10^43 erg s-1 0.1-2 keV) of the 'composite' class of galaxies. This enigmatic class of objects presents narrow-emission line optical spectra classifying these galaxies as star-forming on the basis of the diagnostic emission line ratios; yet, the presence of weak Halpha broad wings also suggests the presence of a weak or obscured AGN. The ASCA spectrum can be represented with a power-law with a photon index of 1.76+-0.08. Strong variability is detected (by about a factor of three) between the ROSAT and ASCA 1-2 keV flux. These characteristics clearly suggest an AGN origin for the X-ray emission. However, the precise nature of this AGN remains still uncertain. There is no evidence for a high absorbing column density above the Galactic. Moreover, there is no strong evidence for an Fe line at 6.4 keV, with the 90 per cent upper limit on the equivalent width being 0.9 keV. Thus the X-ray spectrum is consistent with an unobscured Seyfert-1 interpretation. This discrepancy with the optical spectrum, may be explained by either a strong star-forming component or a 'dusty' ionised absorber. Finally, the possibility that IRAS00317-2142 may harbour a heavily obscured AGN where the X-ray emission is mainly due to scattered light, appears less plausible due to the high value of the fx/f[OIII] ratio which is more indicative of unobscured Seyfert-1 type AGN.

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arXiv1999

X-ray luminous star-forming galaxies

We present results from the cross-correlation of the spectroscopic atlas of Ho etal (1995) with the ROSAT All-Sky Survey Bright Source Catalogue, in an attempt to understand the X-ray emission mechanisms in nearby galaxies. The resulting sample of 45 galaxies consists predominantly of AGN. However, there are several starforming galaxies spanning a wide range of X-ray luminosities (~10^{38} - 10^{42} erg s^{-1}). We have analyzed ROSAT and ASCA data for the two most luminous star-forming galaxies, namely NGC3310 and NGC3690. We find that their 0.1-10 keV X-ray spectra can be fitted by a soft thermal plasma of kT$\sim0.8$ keV and a harder component with kT~10-15 keV or a power-law with Gamma~1.6. These are very similar to the spectra of the archetypal star-forming galaxies NGC253 and M82.

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arXiv1999

ROSAT observations of the dwarf starforming galaxy Holmerg II (UGC 4305)

We present ROSAT PSPC and HRI observations of the dwarf irregular galaxy Holmberg II (UGC4305). This is one of the most luminous dwarf galaxies (Lx~ 10^{40} erg s^{-1} cm^{-2}) detected in the ROSAT All-Sky Survey. The X-ray emission comes from a single unresolved point source, coincident with a large HII region which emits intense radio emission. The source is variable on both year and day timescales, clearly favouring accretion into a compact object rather than a supernova remnant or a superbubble interpretation for the origin of the X-ray emission. However, its X-ray spectrum is well-fit by a a Raymond-Smith spectrum with kT~0.8 keV, lower than the temperature of X-ray binaries in nearby spiral galaxies.

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arXiv1999

RXTE monitoring observations of Markarian 3

We present Rossi X-ray Timing Explorer, monitoring observations of the Seyfert 2 galaxy Markarian 3 spanning a 200 day period during which time the source flux varied by a factor $\sim 2$ in the 4-20 keV bandpass. In broad agreement with earlier Ginga results, the average spectrum can be represented in terms of a simple spectral model consisting of a very hard power-law continuum ($Γ\approx 1.1$) modified below $\sim 6$ keV by a high absorbing column ($N_H\sim 6\times 10^{23}$ \cunits) together with a high equivalent width Fe-K emission feature at 6.4 keV. The abnormally flat spectral index is probably the signature of a strong reflection component and we consider two models incorporating such emission. In the first the reflected signal suffers the same absorption as the intrinsic continuum, whereas in the second the reflection is treated as an unabsorbed spectral component. In the former case, we require a very strong reflection signal ($R ~< 3$) in order to match the data; in addition variability of both the intrinsic power-law and the reflection component is required. The unabsorbed reflection model requires a somewhat higher line-of-sight column density to the nuclear source ($\sim 10^{24}$ \cunits), but in this case the reflected signal remains constant whilst the level of the intrinsic continuum varies. The latter description is consistent with the reflection originating from the illuminated far inner wall of a molecular torus, the nearside of which screens our direct view of the central continuum source.

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arXiv1999

The X-ray luminosity function of local galaxies

We present an estimate of the local X-ray luminosity function and emissivity for different subsamples of galaxies namely Seyferts, LINERS, star-forming and passive (no-emission-line) galaxies. This is performed by convolving their optical luminosity function, as derived from the Ho et al. spectroscopic sample of nearby galaxies with the corresponding L_x/L_B relation. The local galaxy emissivity is about 1.6 X 10^{39} h erg/sec Mpc^3 in agreement with the results of Lahav et al. derived from cross-correlation techniques of the X-ray background with optical and infrared galaxy catalogues. From our analysis, it becomes evident that the largest fraction of the galaxy emissivity comes from galaxies associated with AGN (Seyferts but also LINERS) while the contribution of star-forming and passive galaxies is small. This independently supports the view that most of the yet unidentified X-ray sources in deep \rosat fields which are associated with faint optical galaxies, do harbour an AGN.

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arXiv1998

The ROSAT X-ray Background Dipole

We estimate the dipole of the diffuse 1.5 keV X-ray background from the ROSAT all-sky survey map of Snowden et al (1995). We first subtract the diffuse Galactic emission by fitting to the data an exponential scale height, finite radius, disk model. We further exclude regions of low galactic latitudes, of local X-ray emission (eg the North Polar Spur) and model them using two different methods. We find that the ROSAT X-ray background (XRB) dipole points towards $(l,b) ~ (288, 25) \pm 19 degrees within 30 degrees of the CMB dipole. Its direction is also in good agreement with the HEAO1 X-ray dipole at harder energies. The normalised amplitude of the ROSAT XRB dipole is about 1.7 per cent. Subtracting from the ROSAT map the expected X-ray background dipole due to the reflex motion of the observer with respect to the cosmic rest frame (Compton-Geting effect) we find the large-scale dipole of the X-ray emitting extragalactic sources having an amplitude 90% of the total XRB, in general agreement with the predictions of Lahav et al (1997). We finally estimate that the Virgo cluster is responsible for about 20 per cent of the total measured XRB dipole amplitude.

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arXiv1998

ASCA observations of deep ROSAT fields IV.IR and hard X-ray observations of an obscured high-redshift QSO

We use UKIRT and ASCA observations to determine the nature of a high redshift (z=2.35) narrow-line AGN, previously discovered by Almaini et al. (1995). The UKIRT observations show a broad Halpha line while no Hbeta line is detected. This together with the red colour (B-K=5.4) suggest that our object is a moderately obscured QSO (Av>3), in optical wavelengths. The ASCA data suggest a hard spectrum, probably due to a large obscuring column, with a photon index of 1.93 (+0.62, -0.46), NH~10^23. The combined ASCA and ROSAT data again suggest a heavily obscured spectrum (NH~10^23 or Av~100). In this picture, the ROSAT soft X-ray emission may arise from electron scattering, in a similar fashion to local Seyfert 1.9. Then, there is a large discrepancy between the moderate reddening witnessed by the IR and the large X-ray absorbing column. This could be possibly explained on the basis of e.g. high gas metallicities or assuming that the X-ray absorbing column is inside the dust sublimation radius. An alternative explanation can be obtained when we allow for variability between the ROSAT and ASCA observations. Then the best fit spectrum is still flat, 1.35 (+0.16, -0.14), but with low intrinsic absorption in better agreement with the IR data, while the ROSAT normalization is a factor of two below the ASCA normalization. This object may be one of the bright examples of a type-II QSO population at high redshift, previously undetected in optical surveys. The hard X-ray spectrum of this object suggests that such a population could make a substantial contribution to the X-ray background.

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arXiv1998

ROSAT and ASCA Observations of X-ray Luminous starburst Galaxies : NGC3310 and NGC3690

We present ROSAT (HRI and PSPC) and ASCA observations of the two luminous (L_x ~10^{41-42} erg s^{-1}) star-forming galaxies NGC3310 and NGC3690. The HRI shows clearly that the sources are extended with the X-ray emission in NGC3690 coming from at least three regions. The combined 0.1-10 keV spectrum of NGC3310 can be described by two components, a Raymond-Smith plasma with temperature kT=0.81^{+0.09}_{-0.12} keV and a hard power-law, Gamma=1.44^{+0.20}_{-0.11}, (or alternatively a harder Raymond-Smith plasma with kT ~15 keV), while there is no substantial excess absorption above the Galactic. The soft component emission is probably due to a super-wind while the nature of the hard emission is more uncertain with likely origins, X-ray binaries, inverse Compton scattering of IR photons, an AGN or a very hot gas component (~10^8 K). The spectrum of NGC3690 is similar, with kT=0.83^{+0.02}_{-0.04} keV and Gamma=1.56^{+0.11}_{-0.11}. We also employ more complicated models such as a multi-temperature thermal plasma, a non-equilibrium ionization code or the addition of a third softer component which improve the fit but not at a statistically significant level (<2sigma). These results are similar to recent results on the archetypal star-forming galaxies M82 and NGC253.

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arXiv1998

The broad-band X-ray spectrum of Mrk 3

We have used non-simultaneous Ginga, ASCA and ROSAT observations to investigate the complex X-ray spectrum of the Seyfert 2 galaxy Mrk 3. We find that the composite spectrum can be well described in terms of a heavily cut-off hard X-ray continuum, iron K_alpha emission and a soft X-ray excess, with spectral variability confined to changes in the continuum normalisation and the flux in the iron line. Previous studies have suggested that the power-law continuum in Mrk 3 is unusually hard. We obtain a canonical value for the energy index of the continuum (i.e. alpha approx 0.7) when a warm absorber (responsible for an absorption edge observed near 8 keV) is included in the spectral model. Alternatively, the inclusion of a reflection component yields a comparable power-law index. The soft-excess flux cannot be modelled solely in terms of pure electron scattering of the underlying power-law continuum. However, a better fit to the spectral data is obtained if we include the effects of both emission and absorption in a partially photoionized scattering medium. In particular the spectral feature prominent at $\sim 0.9$ keV could represent O VIII recombination radiation produced in a hot photoionized medium. We discuss our results in the context of other recent studies of the soft X-ray spectra of Seyfert 2 galaxies.

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arXiv1998

ASCA observations of Deep ROSAT fields - III. The Discovery of an Obscured Type 2 AGN at z=0.67

We report on the discovery of a narrow-emission-line object at z=0.672 detected in a deep ASCA survey. The object, AXJ0341.4-4453, has a flux in the 2--10keV band of 1.1+/-0.27 E-13 erg/s/cm**2, corresponding to a luminosity of 1.8E44 erg/s (q0=0.5, $H0 = 50km/s/Mpc. It is also marginally detected in the ROSAT 0.5-2keV band with a flux 5.8E-15 erg/s/cm**2. Both the ASCA data alone and the combined ROSAT/ASCA data show a very hard X-ray spectrum, consistent with either a flat power law (alpha < 0.1) or photoelectric absorption with a column of nH > 4E22 cm**-2 (alpha = 1). The optical spectrum shows the high-ionisation, narrow emission lines typical of a Seyfert 2 galaxy. We suggest that this object may be typical of the hard sources required to explain the remainder of the X-ray background at hard energies.

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arXiv1997

ASCA observations of Deep ROSAT fields - II. The 2-10keV AGN luminosity function

We present additional optical spectroscopic identifications of sources identified in three deep ASCA GIS fields which also form part of a deep ROSAT survey. In total, 26 ASCA sources have been detected down to a 2-10keV flux limit of S = 5e-14 erg/cm/s. LDSS observations have increased the spectroscopic completeness of the survey to between 65 and 85 per cent, with identifications for up to 13 QSOs with broad emission lines and 6 objects with narrow emission lines. Combining these objects with the AGN identified in the LMA/HEAO-1 sample by Grossan, we find evidence for significant cosmological evolution in the 2-10keV band ( =0.63+/-0.03), fit by a pure luminosity evolution model; L \propto (1+z)^k, k=2.04^{+0.16}_{-0.22}$. The present-epoch 2-10keV AGN X-ray luminosity function, Φ(L_X), is best represented by a two power law function: Φ(L_X) \propto L^{-3.0}, L*>10^{44.1}erg/s; $Φ(L_X) \propto L^{-1.9}$, L*<10^{44.1}erg/s. Depending on the extent to which we extrapolate the z=0 AGN luminosity function, we predict a total contribution to the 2-10keV X-ray background from QSOs and narrow-emission-line galaxies which ranges from 48 per cent (for AGN with L>10^{42}erg/s) to 80 per cent (L>10^{39}erg/s).

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