arXiv subjects

G. Brunetti

Publications and source records attributed to G. Brunetti.

321 records · Page 18Linked to original sources

arXiv2001

Chandra discovery of extended non-thermal emission in 3C 207 and the spectrum of the relativistic electrons

We report on the {\it Chandra} discovery of large scale non--thermal emission features in the double lobed SSRL quasar 3C 207 (z=0.684). These are: a diffuse emission well correlated with the western radio lobe, a bright one sided jet whose structure coincides with that of the eastern radio jet and an X-ray source at the tip of the jet coincident with the hot spot of the eastern lobe. The diffuse X-ray structure is best interpreted as inverse Compton (IC) scattering of the IR photons from the nuclear source and provides direct observational support to an earlier conjecture (Brunetti et al., 1997) that the spectrum of the relativistic electrons in the lobes of radio galaxies extends to much lower energies than those involved in the synchrotron radio emission. The X-ray luminous and spatially resolved knot along the jet is of particular interest: by combining VLA and {\it Chandra} data we show that a SSC model is ruled out, while the X-ray spectrum and flux can be accounted for by the IC scattering of the CMB photons (EIC) under the assumptions of a relatively strong boosting and of an energy distribution of the relativistic electrons as that expected from shock acceleration mechanisms. The X-ray properties of the hot spot are consistent with a SSC model. In all cases we find that the inferred magnetic field strength are lower, but close to the equipartition values. The constraints on the energy distribution of the relativistic electrons, imposed by the X-ray spectra of the observed features, are discussed. To this aim we derive in the Appendices precise semi--analytic formulae for the emissivities due to the SSC and EIC processes.

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arXiv2001

Chandra detection of the radio and optical double hot spot of 3C 351

In this letter we report a Chandra X-ray detection of the double northern hot spot of the radio quasar 3C 351. The hot spot has also been observed in the optical with the Hubble Space Telescope (R-band) and with the 3.5m. Telescopio Nazionale Galileo (B-band). The radio-to-optical and X-ray spectra are interpreted as the results of the synchrotron and synchrotron-self-Compton (SSC) mechanisms, respectively, with hot-spot magnetic field strengths ~3 times smaller than the equipartition values. In the framework of shock acceleration theory, we show that the requirement for such a relatively small field strength is in agreement with the fitted synchrotron spectral models and with the sizes of the hot spots. Finally, we show that the combination of a lower magnetic field strength with the high frequencies of the synchrotron cut-off in the fitted synchrotron spectra provides strong evidence for electron acceleration in the hot spots.

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arXiv2001

Hard X-ray emission from the galaxy cluster A3667

We report the results of a long BeppoSAX observation of Abell 3667, one of the most spectacular galaxy cluster in the southern sky. A clear detection of hard X-ray radiation up to ~ 35 keV is reported, while a hard excess above the thermal gas emission is present at a marginal level that should be considered as an upper limit to the presence of nonthermal radiation. The strong hard excesses reported by BeppoSAX in Coma and A2256 and the only marginal detection of nonthermal emission in A3667 can be explained in the framework of the inverse Compton model. We argue that the nonthermal X-ray detections in the PDS energy range are related to the radio index structure of halos and relics present in the observed clusters of galaxie.

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arXiv2001

Anisotropic inverse Compton scattering in powerful radio galaxies: the case of 3C 295

Inverse Compton (IC) scattering of nuclear photons with relativistic electrons in the lobes of powerful radio galaxies and quasars can give detectable extended X-ray emission from the radio lobes if relativistic electrons with a Lorentz factor gamma about 300 are present (Brunetti, Setti, Comastri 1997). In general these electrons are not detected since they emit synchrotron radiation at frequencies below the radio band, so that the study of this effect provides a unique tool to measure the energy distribution of the electron population in the radio lobes at gamma lower than 1000. In this paper we reanalyze the Chandra observation of the powerful and compact radio galaxy 3C 295 for which the IC scattering of nuclear photons is expected to be an important mechanism. We find strong evidence for extended and asymmetrical X-ray emission associated with the radio lobes in the energy band 0.1--2 keV. We show that both the luminosity and morphology of the extended X-ray emission associated with the radio lobes, not compatible with other X-ray mechanisms, can be best interpreted by the IC scattering with nuclear photons. We also show that the relativistic electron energy distribution obtained from the synchrotron radio emission can be extrapolated down to gamma about 100 thus providing a first direct evidence on the electron spectrum in the lobes down to lower energies.

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arXiv2000

Particle injection and reacceleration in clusters of galaxies and the EUV excess: the case of Coma

We calculate the energy distribution of the relativistic particles injected in the ICM during a phase of reacceleration of the relativistic particles in the cluster volume. We apply our results to the case of the Coma cluster in which recent merging activity and the presence of the radio halo may suggest that reacceleration processes are efficient. We find that the electron population injected in the central part of the cluster by the head-tail radio galaxy NGC 4869 may account for a large fraction, if not all, of the detected EUV excess via inverse Compton scattering of the cosmic microwave background (CMB) photons. If radio haloes are powered by reacceleration mechanisms active in the cluster volume, moderate non-thermal EUV excesses (of order of $\sim 1-5 \cdot 10^{42}$erg s^{-1}) should be a common feature of clusters containing powerful head-tail radio galaxies and/or AGNs.

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arXiv2000

Anisotropic inverse Compton scattering in radio galaxies and particle energy distribution

In this talk we briefly review the inverse Compton (IC) scattering of nuclear photons in the lobes of radio galaxies and quasars. We concentrate on the possibility to test this model with the Chandra observatory and to constrain the electron energy distribution by measuring the X-ray fluxes produced by this effect. We also discuss the evidence for IC scattering of nuclear photons in the powerful radio galaxies.

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arXiv2000

Non thermal emission from galaxy clusters: radio halos

The number of diffuse radio halos in clusters of galaxies has grown in recent years, making it possible to derive statistical properties of these sources and of the hosting clusters. We show that diffuse sources are associated with X-ray luminous clusters which have undergone recent merger processes. The radio and X-ray structures are often similar, and correlations are found between radio and X-ray parameters. A model for the formation and maintenance of these sources is suggested, including a first phase of relativistic particle injection by past major merger events, starbust and AGN activity and a second phase of reacceleration by the energy supplied from shocks and turbulence in a recent merger event.

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arXiv2000

Particle reacceleration in Coma cluster: radio properties and hard X-ray emission

The radio spectral index map of the Coma halo shows a progressive steepening of the spectral index with increasing radius. Such a steepening cannot be simply justified by models involving continuous injection of fresh particles in the Coma halo or by models involving diffusion of fresh electrons from the central regions. We propose a {\it two phase} model in which the relativistic electrons injected in the Coma cluster by some processes (starbursts, AGNs, shocks, turbulence) during a {\it first phase} in the past are systematically reaccelerated during a {\it second phase} for a relatively long time ($\sim$ 1 Gyr) up to the present time. We show that for reacceleration time scales of $\sim 0.1$ Gyr this hypothesis can well account for the radio properties of Coma C. For the same range of parameters which explain Coma C we have calculated the expected fluxes from the inverse Compton scattering of the CMB photons finding that the hard X-ray tail discovered by BeppoSAX may be accounted for by the stronger reacceleration allowed by the model. The possibility of extending the main model assumptions and findings to the case of the other radio haloes is also discussed, the basic predictions being consistent with the observations.

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arXiv2000

Extended X-ray emission from FRIIs and RL quasars

We review the evidence that detectable fluxes of X-rays are produced by inverse Compton scattering of nuclear photons with the relativistic electrons in the radio lobes of strong FRII radio galaxies within the FRII-RL quasar unification scheme. We report here on the possible detection of this effect in two steep spectrum RL quasars. This may have important implications on the physics and evolution of powerful radio galaxies.

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arXiv1999

Anisotropic inverse Compton scattering from the trans-relativistic to the ultra-relativistic regime and application to the radio galaxies

The problem of the anisotropic Inverse Compton scattering between a monochromatic photon beam and relativistic electrons is revisited and formally solved without approximations. Solutions are given for the single scattering with an electron beam and with a population of electrons isotropically distributed, under the assumption that the energy distribution of the relativistic particles follows a simple power law as it is the case in many astrophysical applications. Both the Thomson approximation and the Klein-Nishina regime are considered for the scattering of an unpolarized photon beam. The equations are obtained without the ultra-relativistic approximation and are compared with the ultra-relativistic solutions given in the literature. The main characteristics of the power distribution and spectra of the scattered radiation are discussed for relevant examples. In the Thomson case for an isotropic electron population simple formulae holding down to mildly-relativistic energies are given. As an application the formulae of the anisotropic inverse Compton scattering are used to predict the properties of the X and $γ$-ray spectra from the radio lobes of strong FR II radio galaxies due to the interaction of the relativistic electrons with the incoming photons from the nucleus. The dependence of the emitted power on the relativistic electron energy distribution and on its evolution with time is discussed.

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arXiv1999

Particle reacceleration in the Coma cluster and the radio, EUV and hard X-ray emissions

The radio spectral index map of the Coma halo shows a progressive steepening of the spectral index with increasing radius. Such a steepening cannot be simply justified by models involving continuous injection of fresh particles in the Coma halo or by models involving diffusion of fresh electrons from the central regions. We propose a scenario in which the relativistic particles injected in the Coma cluster by some processes (merger, accretion shocks, turbulence) are systematically reaccelerated for a relatively long time ($\sim$ 1Gyr). We show that such a scenario can account for the synchrotron radio spectral index distribution, for the radio halo size at different frequencies, and for the total (integrated) radio spectrum of the Coma halo. We also show that for a suitable choice of the parameters the model can also account for the hard X-ray flux observed by BeppoSAX via the inverse Compton scattering of the cosmic microwave background radiation by the synchrotron electrons. The possibility to account for the EUV flux detected with the EUVE satellite by the inverse Compton emission is also discussed.

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arXiv1999

X-ray emission from FRII's radio lobes and the relativistic particle content

It is pointed out that copius X-rays from the lobes of FRII radio galaxies are expected as a natural consequence of the unification linking FRIIs and radio-loud quasars. The detection of extended X-ray emission from 3C 219, likely due to Inverse Compton scattering of the IR photons from a hidden quasar with the relativistic electrons in the lobes, provides the first observational evidence supporting this conjecture. The X-ray fluxes detected in the directions of other five pwerful FRIIs with large redshifts may be similarly accounted for (an alternative to a thermal origin from hot intracluster gas). Much less energetic electrons than those producing the synchrotron radio emission are involved in the process. Then the equipartition conditions imply stronger fields, and significantly stronger internal pressures of the relativic plasmas, than those derived with conventional equipartition equations. The importance of this effect in the pressure balance with the confining intergalactic gas is pointed out.

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arXiv1999

The contribution of AGN to the X-ray background: the effect of iron features

The contribution of the iron emission line, commonly detected in the X-ray spectra of Seyfert (Sey) galaxies, to the cosmic X-ray background (XRB) spectrum is evaluated in the framework of the XRB synthesis models based on AGN unification schemes. To derive the mean line properties, we have carried out a search in the literature covering a sample of about 70 AGN. When adopting line parameters in agreement with the observations, it turns out that the maximum contribution of the iron line to the XRB is less than 7% at a few keV. This is still below the present uncertainties in the XRB spectrum measurements.

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arXiv1998

Evidence for inverse Compton emission in the powerful radio galaxy 3C 219

Spectral analysis of ROSAT PSPC and ASCA archive data and a recent ROSAT HRI observation of the powerful FRII radio galaxy 3C 219 reveal an absorbed point-like source, coincident with the nucleus, and a non-thermal extended component aligned with the radio structure. The point-like source can be readily interpreted as a hidden quasar in the nucleus, giving further support to the unification scheme of FRII radio galaxies and radio loud quasars. The coincidence between the X-ray (0.1-10 keV) and radio spectral index suggest that most of the emission associated with the extended component is due to the inverse Compton process in the radio lobes. The extended circumnuclear emission can be understood as Compton scattering of the IR-optical radiation emitted by the hidden quasar and surrounding dusty/molecular torus. This is the first observational evidence supporting the existence of this effect, which also probes the relativistic electron spectrum at energies much lower than those involved in the synchrotron radio emission. The observed X-ray flux can be matched by assuming that the energy density of the relativistic particles exceed the equipartition value by about a factor of 10. At larger distances from the nucleus ($\geq 70$kpc) the inverse Compton scattering with the cosmic microwave background photons becomes more important and may explain the observed X-ray features if positive fluctuations in the column densities of relativistic electrons are present. Alternatively, one cannot exclude with the present data a thermal contribution by hot clumpy gas surrounding the radio lobes.

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arXiv1997

Inverse Compton X-rays from strong FRII radio-galaxies

In the framework of the quasar-radio galaxies unification the radio emitting lobes of FRII radio galaxies are pervaded by an intense quasar radiation field.Inverse Compton (IC) scattering between the relativistic electrons and the IR-optical photons from a hidden quasar may provide an important contribution to the X-ray emission of these radio galaxies. The soft X-ray emission properties of six strong, high redshift FRIIs (3C 277.2, 280, 294, 324, 356, 368) are compared with our model expectations,taking into account also the contribution from the IC-scattering of the CMB photons with the radio electrons. We find that the soft X-ray luminosities and spectra of five out six sources can be satisfactorily explained by our model with the exception of 3C 324 whose X-ray emission is probably dwarfed by that of the galaxies'cluster of which this sources is a member. One important effect is the prediction that the observed X-ray emission associated with the two radio lobes would be asymmetric if the radio axis is inclined with respect to the plane of the sky, the far-away lobe being the more luminous.The ratio between the X-ray luminosities of the two lobes is derived for several values of the inclination of the radio axis.However, the predicted angular sizes of these distant radio galaxies are small and difficult to be resolved with present X-ray facilities.

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