arXiv subjects

A. Cavaliere

Publications and source records attributed to A. Cavaliere.

At least 37 records · Page 2Linked to original sources

arXiv2009

SSC radiation in BL Lac sources, the end of the tether

The synchrotron-self Compton (SSC) radiation process is widely held to provide a close representation of the double peaked spectral energy distributions from BL Lac Objects (BL Lacs), which are marked by non-thermal beamed radiations, highly variable on timescales of days or less. Their outbursts in the gamma ray relative to the optical/X rays might be surmised to be enhanced in BL Lacs as these photons are upscattered via the inverse Compton (IC) process. From the observed correlations among the spectral parameters during optical/X-ray variations we aim at predicting corresponding correlations in the gamma-ray band, and the actual relations between the gamma-ray and the X-ray variability consistent with the SSC emission process. We start from the homogeneous single-zone SSC source model, with log-parabolic energies distributions of emitting electron as required by the X-ray data of many sources. We find relations among spectral parameters of the IC radiation in both the Thomson (for Low energy BL Lacs) and the Klein-Nishina regimes (mainly for High energy BL Lacs) and we compute how variability is driven by a smooth increase of key source parameters, primarily the root mean square electron energy. The single component SSC source model in the Thomson regime turns out to be adequate for many LBL sources. However, the simple model meets its limits with the fast/strong flares recently reported for a few sources in the TeV range; these require sudden accelerations of emitting electrons in a second source component.

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arXiv2009

Supermodel Analysis of Galaxy Clusters

[abridged] We present the analysis of the X-ray brightness and temperature profiles for six clusters belonging to both the Cool Core and Non Cool Core classes, in terms of the Supermodel (SM) developed by Cavaliere, Lapi & Fusco-Femiano (2009). Based on the gravitational wells set by the dark matter halos, the SM straightforwardly expresses the equilibrium of the IntraCluster Plasma (ICP) modulated by the entropy deposited at the boundary by standing shocks from gravitational accretion, and injected at the center by outgoing blastwaves from mergers or from outbursts of Active Galactic Nuclei. The cluster set analyzed here highlights not only how simply the SM represents the main dichotomy Cool vs. Non Cool Core clusters in terms of a few ICP parameters governing the radial entropy run, but also how accurately it fits even complex brightness and temperature profiles. For Cool Core clusters like A2199 and A2597, the SM with a low level of central entropy straightforwardly yields the characteristic peaked profile of the temperature marked by a decline toward the center, without requiring currently strong radiative cooling and high mass deposition rates. Non Cool Core clusters like A1656 require instead a central entropy floor of a substantial level, and some like A2256 and even more A644 feature structured temperature profiles that also call for a definite floor extension; in such conditions the SM accurately fits the observations, and suggests that in these clusters the ICP has been just remolded by a merger event, in the way of a remnant cool core. The SM also predicts that dark matter halos with high concentration should correlate with flatter entropy profiles and steeper brightness in the outskirts; this is indeed the case with A1689.

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arXiv2009

Galaxy Clusters, a Novel Look at Diffuse Baryons Withstanding Dark Matter Gravity

[abridged] The equilibria of the intracluster plasma (ICP) and of the gravitationally dominant dark matter (DM) are governed by the hydrostatic and the Jeans equation. Jeans, with the DM `entropy' set to K ~ r^αand α~ 1.25 - 1.3 applying from groups to rich clusters, yields our radial α-profiles. In the ICP the entropy run k(r) is mainly shaped by shocks, as steadily set by supersonic accretion of gas at the cluster boundary, and intermittently driven from the center by merging events or by AGNs; the resulting equilibrium is described by the exact yet simple formalism constituting our ICP Supermodel. With a few parameters, this accurately represents the runs of density n(r) and temperature T(r) as required by recent X-ray data on surface brightness and spectroscopy for both cool core (CC) and non cool core (NCC) clusters; the former are marked by a middle temperature peak, whose location is predicted from rich clusters to groups. The Supermodel inversely links the inner runs of n(r) and T(r), and highlights their central scaling with entropy n_c ~ k_c^-1 and T_c ~ k_c^0.35, to yield radiative cooling times t_c ~ 0.3 (k_c/15 keV cm^2)^1.2 Gyr. We discuss the stability of the central values so focused either in CC and NCC clusters. From the Supermodel we derive as limiting cases the classic polytropic β-models, and the `mirror' model with T(r) ~ σ^2(r) suitable for NCC and CC clusters, respectively; these highlight how the ICP temperature T(r) tends to mirror the DM velocity dispersion σ^2(r) away from entropy injections. Finally, we discuss how the Supermodel connects information derived from X-ray and gravitational lensing observations.

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arXiv2009

Structure and History of Dark Matter Halos Probed with Gravitational Lensing

We test with gravitational lensing data the dark matter (DM) halos embedding the luminous baryonic component of galaxy clusters; our benchmark is provided by their two-stage cosmogonical development that we compute with its variance, and by the related 'α-profiles' we derive. The latter solve the Jeans equation for the self-gravitating, anisotropic DM equilibria, and yield the radial runs of the density ρ(r) and the velocity dispersion σ_r^2(r) in terms of the DM 'entropy' K = σ_r^2/ρ^(2/3) ~ r^(α) highlighted by recent N-body simulations; the former constrains the slope to the narrow range α~ 1.25 - 1.3. These physically based α-profiles meet the overall requirements from gravitational lensing observations, being intrinsically flatter at the center and steeper in the outskirts relative to the empirical NFW formula. Specifically, we project them along the l.o.s. and compare with a recent extensive dataset from strong and weak lensing observations in and around the cluster A1689. We find an optimal fit at both small and large scales in terms of a halo constituted by an early body with α~ 1.25 and by recent extensive outskirts, that make up an overall mass 10^15 M_sun with a concentration parameter c ~ 10 consistent with the variance we compute in the \LambdaCDM cosmogony. The resulting structure corresponds to a potential well shallow in the outskirts as that inferred from the X rays radiated from the hot electrons and baryons constituting the intracluster plasma.

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arXiv2009

Dark Matter Equilibria in Galaxies and Galaxy Systems

[abridged] In the dark matter (DM) halos embedding galaxies and galaxy systems the `entropy' K = σ^2 / ρ^{2/3} (a quantity that combines the radial velocity dispersion σwith the density ρ) is found from intensive N-body simulations to follow a powerlaw run K ~ r^α throughout the halos' bulk, with αaround 1.25. Taking up from phenomenology just that α~ const applies, we cut through the rich analytic contents of the Jeans equation describing the self-gravitating equilibria of the DM; we specifically focus on computing and discussing a set of novel physical solutions that we name α-profiles, marked by the entropy slope αitself, and by the maximal gravitational pull κ_crit required for a viable equilibrium to hold. We then use an advanced semianalytic description for the cosmological buildup of halos to constrain the values of αto within the narrow range 1.25-1.29 from galaxies to galaxy systems. Our range of αapplies since the transition time that - both in oursemianalytic description and in state-of-the-art numerical simulations - separates two development stages: an early violent collapse that comprises a few major mergers and enforces dynamical mixing, followed by smoother mass addition through slow accretion. We also give an accurate analytic representation of the α-profiles with parameters derived from the Jeans equation. We finally stress how our findings and predictions as to αand κ_crit contribute to understand hitherto unsolved issues concerning the fundamental structure of DM halos.

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arXiv2008

The Blast Wave Model for AGN Feedback: Effects on AGN Obscuration

We compute the effect of the galactic absorption on AGN emission in a cosmological context by including a physical model for AGN feeding and feedback in a semi-analytic model of galaxy formation. This is based on galaxy interactions as triggers for AGN accretion, and on expanding blast waves as a mechanism to propagate outwards the AGN energy injected into the interstellar medium at the center of galaxies. We first test our model against the observed number density of AGNs with different intrinsic luminosity as a function of redshift. The model yields a ''downsizing'' behavior in close agreement with the observed one for z<2. At higher redshifts, the model predicts an overall abundance of AGNs (including Compton-thick sources) larger than the observed Compton-thin sources by a factor around 2 for z>2 and L_X < 10^{44} erg/s. Thus, we expect that at such luminosities and redshifts about 1/2 of the total AGN population is contributed by Compton-thick sources. We then investigate the dependence of the absorbing column density N_H associated to cold galactic gas (and responsible for the Compton-thin component of the overall obscuration) on the AGN luminosity and redshift. We find that the absorbed fraction of AGNs with N_H>10^{22} cm^{-2} decreases with luminosity for z<1; in addition, the total (integrated over luminosity) absorbed fraction increases with redshift up to z around 2, and saturates to the value around 0.8 at higher redshifts. Finally, we predict the luminosity dependence of the absorbed fraction of AGNs with L_X< 3 10^{44} erg/s to weaken with increasing redshift. We compare our results with recent observations, and discuss their implications in the context of cosmological models of galaxy formation.

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arXiv2007

X-ray spectral evolution of TeV BL Lac objects: eleven years of observations with BeppoSAX, XMM-Newton and SWIFT satellites

Many of the extragalactic sources detected in $γ$ rays at TeV energies are BL Lac objects. In particular, they belong to the subclass of ``high frequency peaked BL Lacs" (HBLs), as their spectral energy distributions exhibit a first peak in the X-ray band. At a closer look, their X-ray spectra appear to be generally curved into a log-parabolic shape. In a previous investigation of Mrk 421, two correlations were found between the spectral parameters. One involves the height $S_p$ increasing with the position $E_p$ of the first peak; this was interpreted as a signature of synchrotron emission from relativistic electrons. The other involves the curvature parameter $b$ decreasing as $E_p$ increases; this points toward statistical/stochastic acceleration processes for the emitting electrons. We analyse X-ray spectra of several TeV HBLs to pinpoint their behaviours in the $E_p-S_p$ and $E_p-b$ planes and to compare them with Mrk 421. We perfom X-ray spectral analyses of a sample of 15 BL Lacs. We report the whole set of observations obtained with the \sax, \xmm and \swf satellites between 29/06/96 and 07/04/07. We focus on five sources (PKS 0548-322, 1H 1426+418, Mrk 501, 1ES 1959+650, PKS2155-304) whose X-ray observations warrant detailed searching of correlations or trends. Within our database, we find that four out of five sources, namely PKS 0548-322, 1H 1426+418, Mrk 501 and 1ES 1959+650, follow similar trends as Mrk 421 in the $E_p-S_p$ plane, while PKS 2155-304 differs. As for the $E_p-b$ plane, all TeV HBLs follow a similar behaviour. The trends exhibited by Mrk 421 appear to be shared by several TeV HBLs, such as to warrant discussing predictions from the X-ray spectral evolution to that of TeV emissions.

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arXiv2007

Missing Baryons, from Clusters to Groups of Galaxies

From clusters to groups of galaxies, the powerful bremsstrahlung radiation L_X emitted in X rays by the intracluster plasma is observed to decline sharply with lowering virial temperatures T (i.e., at shallower depths of the gravitational wells) after a steep local L_X-T correlation; this implies increasing scarcity of diffuse baryons relative to dark matter, well under the cosmic fraction. We show how the widely debated issue concerning these `missing baryons' is solved in terms of the thermal and/or dynamical effects of the kinetic (at low redshifts z) and radiative (at high z) energy inputs from central active galactic nuclei, of which independent evidence is being observed. From these inputs we compute shape and z-evolution expected for L_X-T correlation which agree with the existing data, and provide a predictive pattern for future observations.

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arXiv2007

Bimodal AGNs in Bimodal Galaxies

By their star content, the galaxies split out into a red and a blue population; their color index peaked around u-r=2.5 or u-r=1, respectively, quantifies the ratio of the blue stars newly formed from cold galactic gas, to the redder ones left over by past generations. On the other hand, upon accreting substantial gas amounts the central massive black holes energize active galactic nuclei (AGNs); here we investigate whether these show a similar, and possibly related, bimodal partition as for current accretion activity relative to the past. To this aim we use an updated semianalytic model; based on Monte Carlo simulations, this follows with a large statistics the galaxy assemblage, the star generations and the black hole accretions in the cosmological framework over the redshift span from z=10 to z=0. We test our simulations for yielding in close detail the observed split of galaxies into a red, early and a blue, late population. We find that the black hole accretion activities likewise give rise to two source populations: early, bright quasars and later, dimmer AGNs. We predict for their Eddington parameter $λ_E$ -- the ratio of the current to the past black hole accretions -- a bimodal distribution; the two branches sit now under $λ_E \approx 0.01$ (mainly contributed by low-luminosity AGNs) and around $λ_E \approx 0.3-1$. These not only mark out the two populations of AGNs, but also will turn out to correlate strongly with the red or blue color of their host galaxies.

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arXiv2007

Signatures of synchrotron emission and of electron acceleration in the X-ray spectra of Mrk 421

BL Lac objects undergo strong flux variations involving considerable changes in their spectral shapes. We specifically investigate the X-ray spectral evolution of Mrk 421 over a time span of about nine years. We aim at statistically describing and physically understanding the large spectral changes in X rays observed in Mrk 421 over this time span. We perform a homogeneous spectral analysis of a wide data set including archived observations with ASCA, BeppoSax, RXTE, as well as published and unpublished XMM-Newton data. The presence of uncertainties is taken into account in our correlation analysis. The significance of the correlations found and possible spurious effects are studied with Monte Carlo simulations. We find that the Mrk421 spectral energy distribution (SED) has a lower peak at energies that vary in the range, 0.1-10 keV while its X-ray spectrum is definitely curved. Parameterizing the X-ray spectra with a log-parabolic model, we find a positive correlation between the position and the height of the SED peak. In addition, we find a negative trend of the spectral curvature parameter vs. the SED peak energy. We show that these relations between the spectral parameters are consistent with statistical or stochastic acceleration of the emitting particles, and provide insight into the physical processes occurring in BL Lac nuclei.

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arXiv2006

Searching for AGN-driven Shocks in Galaxy Clusters

Shocks and blastwaves are expected to be driven driven into the intracluster medium filling galaxy groups and clusters by powerful outbursts of active galactic nuclei or quasars in the member galaxies; the first footprints of shock fronts have been tentatively traced out with X-ray imaging. We show how overpressures in the blasts behind the shock can prove the case and also provide specific marks of the nuclear activity: its strength, its current stage, and the nature of its prevailing output. We propose to detect these marks with the aimed pressure probe constituted by the resolved Sunyaev-Zel'dovich effect. We compute and discuss the outcomes to be expected in nearby and distant sources at different stages of their activity.

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arXiv2005

The Impact of Energy Feedback on Quasar Evolution and Black Hole Demographics

We investigate how accretion episodes onto massive black holes power quasars and AGN while they accumulate mass into the holes. We compute both the trend and the stochastic component to the trigger of the accretion events, as provided by structure buildup after the hierarchical paradigm. We base on host galaxy evolution proceeding from the protogalactic era at redshifts z > 2.5 dominated by major merging events in high density regions, to the subsequent era marked by galaxy-galaxy interactions in newly forming groups. These dynamical events perturb the gravitational equilibrium of the gas reservoir in the hosts, and trigger recurrent accretion episodes controlled by energy feedback from the very source emission onto the surrounding gas. Depletion of the latter by these events (adding to quiescent star formation) concurs with the slowing down of the clustering to cause a fast drop of the activity in dense regions. Meanwhile, in the "field" later and rarer events are triggered by interactions of still gas-rich galaxies, and eventually by captures of dwarf satellite galaxies; these are also included in our analytic model. Thus we compute the quasar and AGN luminosity functions; we find these to brighten and rise from z ~ 6 to z ~ 2.5, and then toward z ~ 0 to dim and fall somewhat, in detailed agreement with the observations. We predict that for z < 2.5 the mass distribution of the holes progressively rises and shifts rightwards; we compare our results with the local data. We also find that downward of z ~ 2.5 the Eddington ratios related to emitting, most massive holes drift below unity on average, with a widening scatter; meanwhile, some smaller holes flare up closer to the Eddington limit.

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arXiv2005

Intracluster Entropy from Joint X-ray and Sunyaev-Zel'dovich Observations

The temperature and density of the hot diffuse medium pervading galaxy groups and clusters combine into one significant quantity, the entropy. Here we express the entropy levels and profiles in model-independent forms by joining two observables, the X-ray luminosity and the change in the CMB intensity due to the Sunyaev-Zel'dovich (SZ) effect. Thus we present both global scaling relations for the entropy levels from clusters and groups, and a simple expression yielding the entropy profiles in individual clusters from resolved X-ray surface brightness and SZ spatial distributions. We propose that our approach provides two useful tools for comparing large data samples with models, in order to probe the processes that govern the thermal state of the hot intracluster medium. The feasibility of using such a diagnostic for the entropy is quantitatively assessed, based on current X-ray and upcoming SZ measurements.

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arXiv2005

Intracluster and Intragroup Entropy from Quasar Activity

We investigate how the hierarchical merging of dark matter halos, the radiative cooling of baryons, and the energy feedback from supernovae and active galactic nuclei or quasars combine to govern the amount and the thermal state of the hot plasma pervading groups and clusters of galaxies. We show that, by itself, supernova preheating of the external gas flowing into clusters falls short of explaining the observed X-ray scaling relations of the plasma luminosity L_X or the plasma entropy K versus the X-ray temperature T. To account for the scaling laws from rich to poor clusters takes preheating enhanced by the energy input from active galactic nuclei. In groups, on the other hand, the internal impacts of powerful quasars going off in member galaxies can blow some plasma out of the structure. So they depress L_X and raise K to the observed average levels; meanwhile, the sporadic nature of such impulsive events generates the intrinsic component of the wide scatter apparent in the data. The same quasar feedback gives rise in groups to entropy profiles as steep as observed, a feature hard to explain with simple preheating schemes. Finally, we argue a close connection of the L_X-T or the K-T relation with the M_{\bullet}-σcorrelation between the host velocity dispersion and the masses of the black holes, relics of the quasar activity.

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arXiv2004

X-Ray Evolution of Active Galactic Nuclei and Hierarchical Galaxy Formation

We have incorporated the description of the X-ray properties of Active Galactic Nuclei (AGNs) into a semi-analytic model of galaxy formation, adopting physically motivated scaling laws for accretion triggered by galaxy encounters. Our model reproduces the level of the cosmic X-ray background at 30 keV; we predict that the largest contribution (around 2/3) comes from sources with intermediate X-ray luminosity 10^{43.5}< L_X/erg/s <10^{44.5}, with 50 % of the total specific intensity produced at z<2. The predicted number density of luminous X-ray AGNs (L_X>10^{44.5} erg/s in the 2-10 keV band) peaks at z around 2 with a decline of around 3 dex to z=0; for the low luminosity sources (10^{43}<L_X/erg/s <10^{44}) it has a broaderand less pronounced maximum around z 1.5. The comparison with the data shows a generally good agreement. The model predictions slightly exceed the observed number of low-luminosity AGNs at z around 1.5, with the discrepancy progressively extending to intermediate-luminosity objects at higher redshifts; we discuss possible origins for the mismatch. Finally, we predict the source counts and the flux distribution at different redshifts in the hard (20-100 keV) X-ray band for the sources contributing to the X- ray background.

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arXiv2004

The Sunyaev-Zeldovich effect as a probe of the galaxy formation process

The Sunyaev-Zeldovich (SZ) effect has proven to be an extremely powerful tool to study the physical and evolutionary properties of rich clusters of galaxies. Upcoming SZ experiments, with their much improved sensitivity and angular resolution, will provide unique information also on phases of galaxy evolution characterized by the presence of large amounts of hot proto-galactic gas. We present a preliminary analysis of the SZ signals that can be expected at the collapse of the proto-galaxy when, according to the standard scenario, the gas is heated at its virial temperature, and during episodes of strong energy injections from the active nucleus. The contributions of such signals to excess power on arc-minute scales recently found by CBI and BIMA experiments are briefly discussed.

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arXiv2003

Early Hierarchical Formation of Massive Galaxies Triggered By Interactions

To address the problem concerning the early formation of stars in massive galaxies, we present the results of a semi-analytic model of galaxy formation which includes a physical description of starbursts triggered by galaxy interactions. These originate from the destabilization of cold galactic gas occurring in galaxy encounters, which in part feeds the accretion onto black holes powering quasars, and in part drives circumnuclear starsbursts at redshifts $z\approx 2-4$, preferentially in massive objects. This speeds up the formation of stars in massive galaxies at high redshifts without altering it in low mass galactic halos. Thus, at intermediate $z\approx 1.5-2$ we find that a considerable fraction of the stellar content of massive galaxies is already in place, at variance with the predictions of previous hierarchical models. The resulting high-$z$ star formation rate and B-band luminosity functions, and the luminosity and redshift distribution of galaxies in K-band at $z\lesssim 2$ are all in good agreement with the existing observations concerning the bright galaxy population.

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arXiv2003

Sunyaev-Zel'dovich Effects from Quasars in Galaxies and Groups

The energy fed by active galactic nuclei to the surrounding diffuse baryons changes their amount, temperature, and distribution; so in groups and in member galaxies it affects the X-ray luminosity and also the Sunyaev-Zel'dovich effect. Here we compute how the latter is enhanced by the transient blastwave driven by an active quasar, and is depressed when the equilibrium is recovered with a depleted density. We constrain such depressions and enhancements with the masses of relic black holes in galaxies and the X-ray luminosities in groups. We discuss how all these linked observables can tell the quasar contribution to the thermal history of the baryons pervading galaxies and groups.

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