arXiv subjects

C. Vignali

Publications and source records attributed to C. Vignali.

423 records · Page 24Linked to original sources

arXiv2000

The BeppoSAX High Energy Large Area Survey (HELLAS): a progress Report

The integrated emission of highly obscured AGN is expected to provide a major contribution to the X-ray energy density in the Universe: the X-ray background (XRB). The study of these objects is possible only at energies where the effects of absorption are less severe. For this reason we have carried out the BeppoSAX High Energy LLarge Area Survey in the hardest band (5-10 keV) accessible so far with imaging X-ray instruments. The source surface density at the survey limiting flux accounts for a significant fraction (20-30%) of the hard XRB. The X-ray data complemented by multiwavelength follow--up observations suggest that a large fraction of the hard sources are AGN and that X-ray absorption with column densities in the range 10^22-23.5 cm-2 is common among them. The great diversity in their optical-near-IR properties suggests that the optical appearance of obscured sources is a function of the X-ray luminosity. We briefly discuss the implications of these findings for the XRB models.

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arXiv1998

Probing the hard X-ray properties of high-redshift Radio-Quiet Quasars with ASCA

This paper reports the X-ray spectral analysis of 5 high-redshift (z \ge 2) radio-quiet quasars observed by ASCA. A simple power law continuum plus cold Galactic absorption model well fits all the spectra (tipically between $\sim$ 2-30 keV in the sources frame). Neither the X-ray spectral hardening, attributed to a reflection component and observed in Seyfert galaxies, nor the excess absorption previously detected in high-redshift radio-loud quasars, have been revealed. Only a marginal evidence of a neutral or mildly ionized FeKαline is found in one of the quasars. The average spectral slope in the observed 0.7--10 keV energy range, $<Γ> = 1.67 \pm{0.11}$ (dispersion $σ\sim 0.07$), appears to be flatter than that of low-z radio-quiet quasars ($Γ\simeq 1.9-2$) and slightly steeper, but consistent with $Γ$ = 1.61 \pm{0.04} (σ\sim 0.10)$ of high-z radio-loud quasars.

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arXiv1998

First ASCA results for high-redshift Radio-Quiet Quasars

Here we report the X-ray spectral analysis of 5 high-redshift ($z \geq 2$) radio-quiet quasars (RQQs) observed with the ASCA satellite. A simple power law with Galactic absorption model well fits the data. There is no evidence of a reflection component. The average spectral slope in the observed 0.5--10 keV energy range, $< Γ> = 1.68\pm{0.09}$ (dispersion $σ\sim 0.11$), places the high-z objects of the present sample in the lower-Γtail of the RQQs spectral indices distribution.

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arXiv1997

ASCA detection of the FeK edge in the spectrum of the Seyfert 2 galaxy IRAS 04575-7537: a sign of a complex absorber

ASCA X-ray spectral analysis of the Seyfert 2 galaxy IRAS 04575-7537 is presented. The main result is the presence of a significant iron edge at a rest energy of 7.13^+0.21_-0.16 keV. The spectrum is flat (Gamma = 1.5) with a substantial absorption Nh = 10^22 cm^-2 and does not require any reflection component. There is also evidence of a narrow Fe Kalpha emission line, whose rest energy (E = 6.35^+0.08_ -0.03 keV) and equivalent width (EW = 130+-50 eV) suggest that the line originates from thick and cold matter. The intensity of the line and, in particular, the depth of the iron edge are too strong to be explained by simple transmission through the measured absorption column density. This strongly suggests that a model more complex than a single absorbed power law is needed. We propose an absorption model that we call "leaky-total absorber" which can explain the spectrum flatness, the iron emission line and the edge absorption feature and has also the advantage of having a straightforward physical interpretation in the framework of Unified models. In this model a thick absorber (Nh = 10^23 cm^-2), possibly associated with broad line region (BLR) clouds, partially (36 %) covers the source continuum with Gamma fixed to 1.9 as observed in Sey 1 galaxies. The escaping radiation is then absorbed by a column density Nh = 10^22 cm^-2, which can be attributed to the torus, seen through its rim.

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arXiv1997

On the nature of the X-ray absorption in the Seyfert 2 galaxy NGC 4507

We present results of the ASCA observation of the Seyfert 2 galaxy NGC 4507. The 0.5-10 keV spectrum is rather complex and consists of several components: (1) a hard X-ray power law heavily absorbed by a column density of about 3 10^23 cm^-2, (2) a narrow Fe Kalpha line at 6.4 keV, (3) soft continuum emission well above the extrapolation of the absorbed hard power law, (4) a narrow emission line at about 0.9 keV. The line energy, consistent with highly ionized Neon (NeIX), may indicate that the soft X-ray emission derives from a combination of resonant scattering and fluorescence in a photoionized gas. Some contribution to the soft X-ray spectrum from thermal emission, as a blend of Fe L lines, by a starburst component in the host galaxy cannot be ruled out with the present data.

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arXiv1997

The First High-Energy X-Ray Spectrum of a Z$>2$ Radio-Quiet Quasar: Q1101-264

Results of the X-ray spectral analysis of the high-redshift radio-quiet quasar Q1101-264 are presented. The ASCA spectrum suggests a marginal evidence of a FeK$α$ emission line at about 2 keV (observer's frame). Both the ASCA and ROSAT spectra are well fitted by a power law with spectral slope $Γ$$\sim$1.9. This is the first 0.3-30 keV spectrum (rest frame) of a z $>$ 2 radio-quiet quasar.

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arXiv1997

ASCA and ROSAT X-ray Spectra of IC 5063 and NGC 3998

The results of the 0.1-10 keV spectral analysis of IC 5063 (Narrow Line Radio Galaxy) and NGC 3998 (LINER) are presented. In both cases there is evidence of a FeK$α$ emission line, but its width is poorly constrained. The highly absorbed spectrum of IC 5063 is well described by a double power law model. The X-ray luminosity of a few $\times$ 10$^{43}$ ergs s$^{-1}$ reveals a Sey 1 nucleus seen nearly edge-on, as confirmed by optical and IR studies. NGC 3998 is well fitted by a power law model ($Γ$$\sim$1.9) without reflection and weak absorption. The weak X-ray emission detected by ASCA and ROSAT suggests a low-luminosity AGN or an active nucleus in a low-accretion period.

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arXiv1997

Highly Ionized Neon in the Spectrum of the Sey 2 Galaxy NGC 4507 ?

Results of an ASCA X-ray observation of NGC 4507 are presented. The spectrum is best parameterized by a double power law (or a partial covering) model plus a narrow FeK$α$ emission line likely due to the line-of-sight absorption matter. The data require also an emission line at $\sim$ 0.9 keV consistent with NeIX, which may indicate that the soft X-ray emission derives from a combination of resonant scattering and fluorescence of the radiation by photoionized gas. Moreover, complex absorption (or other, unresolved, emission lines) is required by the data below 3 keV. Similarities between the X-ray spectra of NGC 4507 and NGC 4151 are also stressed.

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arXiv1996

ASCA and ROSAT X-ray Spectra of High-Redshift Radio-Loud Quasars

Results are presented on the X-ray properties of 9 high-redshift (1.2 < z < 3.4) radio-loud quasars (RLQs) observed by ASCA (10 observations) and ROSAT (11 observations, for a subset of 6 quasars). New ASCA observations of S5 0014+81 (z = 3.38) and S5 0836+71 (z = 2.17) and ROSAT observations of PKS 2126-158 for which results were never presented elsewhere, are included. A simple model consisting of a power law plus cold, uniform absorption gives acceptable fits to the spectra of all sources. The ASCA spectra of the 6 brightest objects show evidence for absorption in excess of the Galactic value at a << 99% confidence level. Comparison with the ROSAT data suggests that absorption has significantly varied ($ΔN_{H} \sim 8$ $\times$ 10$^{20}$ cm$^{-2}$) in the case of S5 0836+71, on a time-scale of $\sim$ 0.8 yr in the quasar frame. For the remaining 5 sources for which ROSAT spectra were available, the two instruments gave consistent results and the data were combined yielding unprecedent spectral coverage (typically $\sim$ 0.4-40 keV in the quasar frame) for high-z quasars. This allows to put severe limits on several different descriptions of the continuum (e.g. broken power law, bremsstrahlung, reflection component). No Fe K$α$ emission line is detected in any of the ASCA spectra. An absorption edge consistent with Fe K$α$ at the quasar redshift is marginally detected in S5 0014+81. Possible origins for the observed low energy absorption are discussed. In particular, contributions from the molecular clouds and dust present in our Galaxy (usually disregarded) are carefully considered. In the light of the new results for S5 0836+71 and S5 0014+81, absorption intrinsic to the quasars is considered and discussed. The average

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