Carlo Stanghellini

What are the gigahertz peaked-spectrum radio sources?

The astrophysical implications of recent radio and optical observations of the powerful compact gigahertz peaked-spectrum (GPS) radio sources are discussed. Some tentative conclusions are presented and a scenario based on the existing data is suggested. It is argued that the spectrum is due to synchrotron self-absorption, which is assumed as the basis for the present inferences from the radio spectral shape. The finding that some GPS sources have a very narrow spectral shape is consistent with the hypothesis that there is a narrow range of size scales which dominate the radio luminosity. The highly inverted LF radio spectrum is consistent with the hypothesis that the radio source is tightly confined. The conclusion that the radio polarization of these sources is systematically low is consistent either with a very tangled magnetic field or very large Faraday rotation measures. It is suggested that GPS radio sources are formed when the radio plasma is confined on the scale of the narrow-line region by an unusually dense and clumpy ISM. The existing optical spectroscopic results are also consistent with the existence of a dense and dusty nuclear ISM.

Bright OB stars in the Galaxy - III. Constraints on the radial stratification of the clumping factor in hot star winds from a combined H

Context. Recent results strongly challenge the canonical picture of massive star winds: various evidence indicates that currently accepted mass-loss rates, u M, may need to be revised downwards, by factors extending to one magnitude or even more. This is because the most commonly used mass-loss diagnostics are affected by “clumping” (small-scale density inhomogeneities), influencing our interpretation of observed spectra and fluxes. Aims. Such downward revisions would have dramatic consequences for the evolution of, and feedback from, massive stars, and thus robust determinations of the clumping properties and mass-loss rates are urgently needed. We present a first attempt concerning this objective, by means of constraining the radial stratification of the so-called clumping factor. Methods. To this end, we have analyzed a sample of 19 Galactic O-type supergiants/giants, by combining our own and archival data for Hα, IR, mm and radio fluxes, and using approximate methods, calibrated to more sophisticated models. Clumping has been included into our analysis in the “conventional” way, by assuming the inter-clump matter to be void. Because (almost) all our diagnostics depends on the square of density, we cannot derive absolute clumping factors, but only factors normalized to a certain minimum. Results. This minimum was usually found to be located in the outermost, radio-emitting region, i.e., the radio mass-loss rates are the lowest ones, compared to u M derived from Hα and the IR. The radio rates agree well with those predicted by theory, but are only upper limits, due to unknown clumping in the outer wind. Hα turned out to be a useful tool to derive the clumping properties inside r < 3 ... 5R� . Our most important result concerns a (physical) difference between denser and thinner winds: for denser winds, the innermost region is more strongly clumped than the outermost one (with a normalized clumping factor of 4.1 ± 1.4), whereas thinner winds have similar clumping properties in the inner and outer regions. Conclusions. Our findings are compared with theoretical predictions, and the implications are discussed in detail, by assuming different scenarios regarding the still unknown clumping properties of the outer wind.

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Extended radio emission detected around a sample of GHz Peaked Spectrum (GPS) radio sources is discussed. Evidence for extended emission which is related to the GPS source is found in 6 objects out of 33. Three objects are associated with quasars with core-jet pc-scale morphology, and three are identified with galaxies with symmetric (CSO) radio morphology. We conclude that the core-jet GPS quasars are likely to be beamed objects with a continuous supply of energy from the core to the kpc scale. It is also possible that low surface brightness extended rad io emission is present in other GPS quasars but the emission is below our detection limit due to the high redshifts of the objects. On the other hand, the CSO/galaxies with extended large scale emission may be rejuvenated sources where the extended emission is the relic of previous activity. In general, the prese nce of large scale emission associated with GPS galaxies is uncommon, suggesting that in the context of the recurrent activity model, the time scale between subsequent bursts is in general longer than the radiative lifetime of the radio emission from the e arlier activity (� 10 8 yrs).

, IR and radio analysis

In this paper we present a new sample of 87 Compact Steep Spectrum radio sources (CSS) with flux density0.8 Jy at 0.4 GHz. This sample has been selected from the B3-VLA sample with the aid of new VLA observations at 4.9 and 8.5 GHz which allowed to clean an earlier selection based on VLA 1.5 GHz data. Redshifts, either spectroscopic or photometric, are known for 62% of the sources. About 75% of the sources are resolved or slightly resolved in the new observations. The range of measured linear sizes is from 20 h 1 kpc (selection upper limit) down to0.5 h 1 kpc, which corresponds to our resolution limit. The Largest Linear Size (LLS) distribution is well represented by the power law dN=d(LLS) / LLS 0:6 , consistent with earlier results. The majority of the radio sources smaller than 0.5 h 1 kpc shows a marked spectral flattening at low frequencies. The four frequency spectra computed in the range 0.4{8.5 GHz display a signicant steepening at high frequencies in the largest majority of cases. A considerable fraction of source components are polarized, with median values of their fractional polarization6% and4% at 8.5 and 4.9 GHz respectively.

Extended emission around GPS radio sources

VLA radio observations and optical imaging and spectroscopy of the Einstein radio ring 0218+357 are presented. The ring is detected at 22.4 GHz and shows a basically similar structure at 5, 15, and 22.4 GHz. The B component has varied and was about 15 percent brighter in the 8.4 GHz data than in the data of Patnaik et al. (1992). The ring is highly polarized. A weak jetlike feature extending out roughly 2 arcsec to the southeast of component A is detected at 6 cm. The source has amorphous radio structure extending out to about 11 arcsec from the core. For an adopted redshift of 0.68, the extended radio emission is very powerful. The optical spectrum is rather red and shows no strong features. A redshift of about 0.68 is obtained. The identification is a faint compact m(r) about 20 galaxy which extends to about 4.5 arcsec (about 27 kpc). As much as 50 percent of the total light may be due to a central AGN. The observed double core and ring may be produced by an off-center radio core with extended radio structure.

Multi{frequency VLA observations of a new sample of CSS/GPS radio sources

VLBA observations at 18 cm are presented for 28 Compact Steep-spectrum radio Sources (CSSs) from the B3-VLA CSS sample. These sources were unresolved in previous VLA observations at high frequencies or their brightness distribution was dominated by an unresolved steep spectrum component. More than half of them also showed a low frequency turnover in their radio spectrum. The VLBA images display in most cases a compact symmetric structure. Only in a minority of cases complex morphologies are present.

Radio and optical observations of 0218+357 - The smallest Einstein ring?

We report the XMM-Newton discovery of the first Compton-thick obscured AGN in a Broad Line Radio Galaxy, the Gigahertz Peaked-Spectrum source Mkn668 (OQ+208). The remarkably flat 2-10 keV X-ray spectrum (observed photon index, \Gamma ~ 0.7), alongside with a prominent iron K-alpha fluorescent emission line, is a clear signature of a Compton-reflection dominated spectrum. Mkn688 represents a remarkable example of discrepancy between X-ray spectral properties and optical classification, as its optical spectrum is characterized by broad and asymmetric Balmer lines. The obscuring matter is constrained to be located within the radio hotspots, in turn separated by about 10 pc. If the jets are piercing their way through a Compton-thick medium pervading the nuclear environment, one could be largely underestimating the radio activity dynamical age determined from the observed hotspot recession velocity. The soft X-ray spectrum is dominated by a much steeper component, which may be due to nuclear continuum electron scattering, or inverse Compton of the - remarkably large - far infrared emission. Soft X-rays are suppressed by a further Compton-thin (N_H ~ 10^21/cm/cm) absorbing system, that we identify with matter responsible for free-free absorption of the radio lobes.We report the XMM-Newton discovery of the first Compton-thick obscured AGN in a Broad Line Radio Galaxy, the Gigahertz Peaked-Spectrum source Mkn 668 (OQ+208). The remarkably flat 2-10 keV X-ray spectrum (observed photon index, Γ � 0.7), with a prominent iron Kα fluorescent emission line, is a clear signature of a Compton-reflection dominated spectrum. Mkn 688 represents a remarkable example of discrepancy between X-ray spectral properties and optical classification, as its optical spectrum is characterized by broad and asymmetric Balmer lines. The obscuring matter is constrained to be located within the radio hotspots, in turn separated by about 10 pc. If the jets are piercing their way through a Compton-thick medium pervading the nuclear environment, one could be largely underestimating the radio activity dynamical age determined from the observed hotspot recession velocity. The soft X-ray spectrum is dominated by a much steeper component, which may be due to nuclear continuum electron scattering, or inverse Compton of the - remarkably large - far infrared emission. Soft X-rays are suppressed by a further Compton-thin (NH ∼ 10 21 cm −2 ) absorbing system, that we identify with matter responsible for free-free absorption of the radio lobes.

The B3–VLA CSS sample - II. VLBA images at 18 cm

Aims. We investigate the spectral characteristics of 51 candidate High Frequency Peakers (HFPs), from the “bright” HFP sample, in order to determine the nature of each object, and to obtain a smaller sample of genuine young radio sources. Methods. Simultaneous multi-frequency VLA observations carried out at various epochs have been used to detect flux density and spectral shape variability in order to pinpoint contaminant objects, since young radio sources are not expected to be significantly variable on such a short time-scale. Results. From the analysis of the spectral variability we find 13 contaminant objects, 11 quasars, 1 BL Lac, and 1 unidentified object, which we have rejected from the sample of candidate young radio sources. The ∼6 years elapsed between the first and latest observing run are not enough to detect any substantial evolution of the overall spectrum of genuine, non variable, young radio sources. If we also consider the pc-scale information, we find that the total radio spectrum we observe is the result of the superposition of the spectra of different regions (lobes, hot-spots, core, jets), instead of a single homogeneous radio component. This indicates that the radio source structure plays a relevant role in determining the spectral shape also in the rather common case in which the morphology appears unresolved even on high-resolution scales.

XMM-Newton discovery of a Compton-thick AGN in the GPS galaxy Mkn 668

We present pc-scale images of ten Compact Symmetric Objects (CSO) candidates observed with the European VLBI network (EVN). Five radio sources have been observed at 1.6 GHz, and five more at 2.3/8.4 GHz, the latter subsample with the inclusion in the VLBI array of 3 antennae normally used for geodesy. These objects were selected from existing samples of GHz Peaked Spectrum (GPS) radio sources with the purpose to find and/or confirm the CSO classification. These new VLBI observations allow us to confirm the classification of two CSO candidates, and to find a few new ones. The association of GPS radio galaxies with a CSO morphology is strengthened by our findings, and this result suggests an efficient way to increase the number of known CSOs by means of VLBI observations of compact radio galaxies showing a convex radio spectrum.

Constraining the nature of high frequency peakers The spectral variability

In this paper we present the results of a 4-year (1996{1999) radio flux density monitoring program for as ample of X{ and{ray loud blazars. Our program started in January 1996 and was carried out on a monthly basis at the frequencies of 5 GHz and 8.4 GHz with the 32-m antennas located at Medicina (Bologna, Italy) and Noto (Siracusa, Italy). 22 GHz data collected at Medicina from January 1996 to June 1997 will also be presented. The sample of selected sources comprises most radio-loud blazars with 10 characterised by emission in the X{ and {ray regimes, and target sources for the BeppoSAX X{ray mission. All sources in the sample, except J1653+397 (MKN 501), are variable during the four years of our monitoring program. We classied the type of variability in each source by means of a structure-function analysis. We also computed the spectral index 8:4 5 for all epochs with nearly simultaneous observations at these two frequencies, and found that 8:4 5 starts flattening at the very beginning of a radio flare, or flux density increase.