arXiv subjects

Paola Grandi

Publications and source records attributed to Paola Grandi.

28 records · Page 2Linked to original sources

arXiv2006

Broad Line Radio Galaxies: Jet Contribution to the nuclear X-Ray Continuum

It is shown that, for Broad Line Radio Galaxies the strength of the non-thermal beamed radiation, when present, is always smaller than the accretion flow by a factor < 0.7 in the 2-10 keV band. The result has been obtained using the procedure adopted for disentangling the Flat Spectrum Radio Quasar 3C 273 (Grandi & Palumbo 2004). Although this implies a significantly smaller non-thermal flux in Radio Galaxies when compared to Blazars, the jet component, if present, could be important at very high energies and thus easily detectable with GLAST.

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arXiv2005

BeppoSAX view of radio-loud Active Galactic Nuclei

A systematic analysis of a large sample of radio-loud Active Galactic Nuclei available in the BeppoSAX public archive has been performed. The sample includes 3 Narrow Line Radio Galaxies (NLRG), 10 Broad Line Radio Galaxies (BLRG), 6 Steep Spectrum Radio Quasars (SSRQ), and 16 Flat Spectrum Radio Quasars (FSRQ). According to the unified models, these classes correspond to objects with increasing viewing angles. As expected, the presence of a non-thermal beamed component emerges clearly in FSRQ. This class shows in fact a featureless continuum (with the exception of 3C273), and a significantly flatter average spectral slope. On the contrary, traces of a non-thermal Doppler enhanced radiation are elusive in the other classes. We find that the iron line equivalent widths (EW) are generally weaker in radio- loud AGN than in Seyfert 1 galaxies, and confirm the presence of an X-ray Baldwin effect, i.e. a decrease of EW with the 2--10 keV luminosity (L) from Seyferts to BLRG and quasars. Since the EW--L anti-correlation is present also in radio-quiet AGN alone, this effect cannot be ascribed entirely to a strongly beamed jet component. Possible alternative interpretations are explored.

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arXiv2004

1WGA J2223.7-0206: a Narrow-Line Quasi-Stellar Object in the XMM-Newton field of view of 3C445

We report the discovery of a Narrow Line QSO located at about 1.3' from the Broad Line Radio Galaxy 3C445. The source,1WGA J2223.7-0206, although already revealed by ROSAT has never been optically identified previously. An XMM-Newton observation of 3C445 has allowed, for the first time, an accurate X-ray spectral study of 1WGA J2223.7-0206, revealing an ultra-soft spectrum and fast flux variations typical of Narrow Line AGN. The 0.2-10 keV spectrum is well represented by a power law (Gamma=2.5) plus a black body component (kT = 117 eV) absorbed by Galactic NH. About 80% of the X-ray flux is emitted below 2 keV. The 0.2-2 keV flux is observed to decrease by about a factor 1.6 in about 5000 s. The optical observations, triggered by the X-ray study, confirm the Narrow Line AGN nature of this source. The continuum is blue with typical AGN emission lines, pointing to a redshift z=0.46. The full width half maximum of H_beta is 2000 km/secand the flux rat io [OIII}]/H_beta=0.21. The optical luminosity (M_R=-23.2) and the point-like appearance in the optical images identify 1WGA J2223.7-0206 as a Narrow Line QSO. From the optical-UV-X-ray Spectral Energy Distribution we obtain a lower limit of the bolometric luminosity of 1WGA J2223.7-0206 (L_bol > 3 10^{45} erg/sec) implying, for accretion rates close to the Eddington limit, a black hole mass M_BH > 2.4 x 10^{7} M_{\odot}

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arXiv2003

Confirmation of non-thermal hard X-ray excess in the Coma cluster from two epoch observations

We report the hard X-ray spectrum of the Coma cluster obtained using the PDS data of two independent BeppoSAX observations performed with a time interval of about three years. In both the spectra a non thermal excess with respect to the thermal emission is present at a confidence level of ~ 3.4 sigma. The combined spectrum obtained by adding up the two spectra allows a measurement of the excess at the level of ~ 4.8 sigma at energies above 20 keV. The analysis of the full BeppoSAX data set provides a revised non-thermal X-ray flux which is slightly lower than that previously estimated (Fusco-Femiano et al. 1999) and in agreement with that measured by two RXTE observations. The analysis of the offset fields in our Coma observations provides a possible flux determination of the BL Lac object 1ES 1255+244.

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arXiv2002

Detection of X-ray Emission from the Eastern Radio Lobe of PICTOR A

The XMM-Newton satellite has revealed extended X-ray emission from the eastern radio lobe of the Fanaroff-Riley II Radio Galaxy Pictor A. The X-ray spectrum, accumulated on a region covering about half the entire radio lobe, is well described by both a thermal model and a power law. The X-ray emission could be thermal and produced by circum-galactic gas shocked by the expanding radio lobe or, alternatively, by Inverse Compton (IC) of cosmic microwave background photons by relativistic electrons in the lobe. The latter possibility seems to be supported by the good agreement between the lobe-average synchrotron radio index and the X-ray energy slope. However, if this is the case, the magnetic field, as deduced from the comparison of the IC X-ray and radio fluxes, is more than a factor 2 below the equipartition value estimated in the same X-ray region.

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arXiv2001

Weak Reprocessed Features in the Broad Line Radio Galaxy 3C382

We present a detailed X-ray study of the Broad Line Radio Galaxy 3C382, observed with the BeppoSAX satellite in a very bright state. The continuum emission is well modeled with a power law that steepens at high energies, with an e-folding energy of about 120 keV. At soft energies a clear excess of emission is detected, which can not be explained solely by the extended thermal halo seen in a ROSAT HRI image. A second, more intense soft X-ray component, possibly related to an accretion disk, is required by the data. Both a reflection component (R=0.3) and an iron line (EW \sim 50) are detected, at levels much weaker than in Seyfert galaxies, suggesting a common origin. Combining our measurements with results from the literature we find that the iron line has remained approximately constant over 9 years while the continuum varied by a factor of 5. Thus the fluorescent gas does not respond promptly to the variations of the X-ray primary source, suggesting that the reprocessing site is located away, likely at parsec distances. While the continuum shape indicates that X-rays derive from a thermal Comptonization process, the weakness of other spectral features implies that either the upper layers of the optically thick accretion disk are completely ionized or the corona above the disk is outflowing with mildly relativistic velocity.

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arXiv2000

BeppoSAX Observations of Bright Radio Galaxies

BeppoSAX observations of Broad Line Radio Galaxies (BLRGs) have shown that they have a considerable variety of spectral properties and important differences with respect to their radio-quiet counter-part Seyfert 1s. In radio galaxies the soft photons are often absorbed by cold material. In contrast, in Seyfert 1s the soft photons are generally absorbed by warm gas. The iron lines, always detected in Seyfert 1s, are not always present in BLRGs and generally are weak. In addition, small iron line equivalent widths seem to correspond to weak reflection components in radio galaxies. The emerging picture of BLRGs is complex. Probably several X-ray components, jet, accretion flow and molecular torus, mixed in different way in different objects, contribute to the production of their X-ray spectrum and determine the observed variety. The weakness of the reprocessed features can be explained either by a dilution of the Seyfert-like continuum from non-thermal (jet) radiation or by an accretion gas that is hot and geometrically thick close to the black hole and cold geometrically thin (i.e. able to reprocess the primary X-ray radiation) at larger radii.

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arXiv2000

X-rays from Radio-Galaxies: BeppoSAX Observations

We briefly review BeppoSAX observations of X-ray bright radio-galaxies. Their X-ray spectra are quite varied, and perhaps surprisingly, any similarity between radio-loud AGN and Seyfert galaxies is the exception rather than the rule. When detected, reprocessing features (iron line and reflection) are generally weak, suggesting two possible scenarios: either: (1) non-thermal (jet?) radiation dilutes the X-ray emission from the disk in radio-loud objects, or (2) the solid angle subtended by the X-ray reprocessing material is smaller in radio-loud than in radio-quiet AGN due to different characteristics of the accretion disk itself.

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arXiv1997

Rosat, Asca and Osse Observations of the Broad Line Radio Galaxy 3C120

We present simultaneous observations of the superluminal radio galaxy 3C120 performed with the ASCA and GRO (OSSE) satellites on February-March 1994, as well as an analysis of all the ROSAT archival data. The ASCA spectrum of this object can be described by an absorbed (N_H=1.6e^21 cm^-2) power law with photon index Gamma_ASCA=2 and a very broad sigma>0.8 keV intense iron line (EW>400 eV) at ~ 6 keV. The combined ASCA--OSSE data do not exclude the presence of a narrower (sigma=0.4 keV) and less intense (EW<300 eV) iron line plus a hard component, corresponding either to reflection from an accretion disk or to a flatter power law from a jet. However a single power law plus broad Fe line is preferred from a statistical point of view by the ASCA data. The ROSAT data yield a column density in excess of the Galactic value. The spectral slopes, ranging from Gamma_ROSAT=2.5 to 3.3, are steeper than that measured by ASCA, suggesting the presence of a soft excess. The 0.1-2 keV power-law slope is variable and softer at higher intensity. These results show that the combined soft and hard X-ray spectrum of 3C120 is rather complex. The intrinsic absorption, the soft excess, and the iron line indicate that the X-ray emission from this blazar-like radio galaxy is dominated by a Seyfert-like component, at least in the 0.1-10keV energy band. The jet contribution, if present, becomes important only at higher energies.

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arXiv1997

Multiwavelength Monitoring of the BL Lacertae Object PKS 2155-304 in May 1994. II. The IUE Campaign

PKS 2155-304, the brightest BL Lac object in the ultraviolet sky, was monitored with the IUE satellite at ~1 hour time-resolution for ten nearly uninterrupted days in May 1994. The campaign, which was coordinated with EUVE, ROSAT, and ASCA monitoring, along with optical and radio observations from the ground, yielded the largest set of spectra and the richest short time scale variability information ever gathered for a blazar at UV wavelengths. The source flared dramatically during the first day, with an increase by a factor ~2.2 in an hour and a half. In subsequent days, the flux maintained a nearly constant level for ~5 days, then flared with ~35% amplitude for two days. The same variability was seen in both short- and long-wavelength IUE light curves, with zero formal lag (~<2 hr), except during the rapid initial flare, when the variations were not resolved. Spectral index variations were small and not clearly correlated with flux. The flux variability observed in the present monitoring is so rapid that for the first time, based on the UV emission alone, the traditional Delta L/Delta t limit indicating relativistic beaming is exceeded. The most rapid variations, under the likely assumption of synchrotron radiation, lead to a lower limit of 1 G on the magnetic field strength in the UV emitting region. These results are compared with earlier intensive monitoring of PKS 2155-304 with IUE in November 1991, when the UV flux variations had completely different characteristics.

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