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Anisotropic emission of gravitational waves during the coalescence of supermassive black hole binaries can deliver a large “kick” velocity to the new BH (v~10^₃ km/s). N-body simulations predict that, for kick velocities larger than 40% of the galactic escape velocity, the BH may experience long lived (few Gyr) oscillations with amplitudes comparable with the size of the galactic core (~10^2 pc), suggesting that offset BHs may be common, even in nearby ellipticals. In order to search for such offsets, we perform a photometric analysis of a sample of 14 “core” elliptical galaxies using archival HST observations. Typical recovered offsets are within 1% of the core radius. In contrast, statistical arguments based on N-body simulations show that the typical probability to observe an offset larger than 0.1 core radii (~10 pc) is of order of 70% for a single galaxy. This is derived under the assumption that the BH binary coalescence produced a moderately large kick (v ~ 250 km/s) at the time of the last merger. Assuming that all galaxies in the sample experienced at least one such merger during their lifetime, the probability to observe no offsets larger than 1% the core radius over the sample is of order of 10^-8.

Nearby star-forming regions are ideal laboratories to study high-energy emission of different stellar populations, from very massive stars to brown dwarfs. NGC 2023 is a reflection nebula situated to the south of the Orion B. In this work, we present a comprehensive study of X-ray emitters in the region of NGC 2023 and its surroundings. We combine optical and infrared data to determine physical properties (mass, temperature, luminosity, presence of accretion disks) of the stars detected in an XMM-Newton observation. This study has allowed us to analyze spectral energy distribution of these stars for the first time and determine their evolutionary stage. Properties of the X-ray emitting plasma of these stars are compared to those found in other nearby star-forming regions. The results indicate that the stars that are being formed in this region have characteristics, in terms of physical properties and luminosity function, similar to those found in the Taurus-Auriga molecular complex. In addition we will present the first results for the study of Orion B molecular cloud carried out with Infrared and X-ray photometry, where we found 604 source in the X-ray observations.

Activity cycles are of major interest to understand the solar physics and the influence of the Sun in the Earth’s climate. The effects of solar activity in the Earth are mainly produced by the high energy photons and particles. The solar coronal cycle varies as much as a factor of 50, as measured in terms of Lx. But there is little known on the amplitude of this cycle in the past. So far there were only three stars others than the Sun with a known coronal cycle, all of them in rather old K dwarfs with low activity levels. In a recent paper we have found the coronal cycle of Iota Horologii, the star with the shortest chromospheric cycle known to date. This star represents the first coronal cycle in an active star, in a G dwarf, and in a young star. Iota Hor is a solar-like star with an age of ~600 Myr, the age at which the life appeared on Earth. This cycle may represent the first coronal cycles in the evolution of a solar-like star.

La corona solare visibile nelle bande X ed Estremo UltraVioletto (EUV) \`e costituita per lo pi\`u da plasma confinato dal campo magnetico in tubi di flusso curvi e ancorati sulla superficie, gli archi coronali. Sebbene ultimamente i telescopi spaziali solari abbiano portato a notevoli progressi nella conoscenza dei meccanismi che strutturano e riscaldano la corona, il dibattito \`e ancora aperto sulle cause che convertono l’energia magnetica in riscaldamento. Il riscaldamento \`e graduale o impulsivo? Che strutturazione fine hanno gli archi? Che ruolo gioca la cromosfera? Questo lavoro attacca lo studio della corona solare su due fronti complementari. Una prima parte \`e dedicata all’indagine sul ruolo di meccanismi di riscaldamento impulsivo. Quest’indagine \`e imperniata sulla rivelazione di eventuali piccole componenti di plasma molto pi\`u calde della media nelle regioni attive. Evidenza di queste componenti \`e stata trovata sia con strumenti a immagine, sia in dati spettroscopici, e qui si vuole completare l’informazione attraverso la ricostruzione dell’intera struttura termica lungo la linea di vista. Si sono cos\`i analizzati dati spettroscopici ottenuti con lo strumento Hinode/EIS in parti di una regione attiva precedentemente identificate come molto diverse termicamente, confrontandone i risultati. Il presente studio ha portato ad una conferma, sia pure non conclusiva, della presenza di piccole ma significative componenti molto calde, in favore di un riscaldamento impulsivo almeno in alcune zone delle regioni attive. Lo studio ha anche portato diverse nuove e interessanti informazioni sull’analisi spettrale dei dati. La seconda parte del lavoro \`e dedicata invece all’indagine sul ruolo della cromosfera nelle dinamiche e nel riscaldamento della corona. Di interesse sono flussi veloci dalla cromosfera che, incanalati negli archi coronali, potrebbero portare a effetti visibili nella banda EUV, detti spicole di tipo II. \`E stata trovata evidenza di una corrispondenza tra flussi cromosferici ed emissione nella banda EUV e ci si pone la domanda sulla relazione causa/effetto. Qui abbiamo usato un modello magnetoidrodinamico di una regione con un arco coronale per descrivere in dettaglio e capire l’effetto di un flusso dalla cromosfera sul plasma coronale. Il modello \`e molto complesso, richiede simulazioni numeriche su calcolatori multi-processore, e il codice utilizzato \`e di punta nel panorama internazionale. Lo studio ha permesso di valutare l’effetto della propagazione di fronti d’urto lungo l’intero arco, che vengono innescati dai flussi cromosferici. Il risultato d’interesse \`e che i fronti d’urto provocano l’addensamento e il riscaldamento del plasma all’interno dell’arco, e quindi potrebbero spiegare parte dell’evidenza osservativa.

Since the past decade, the knowledge of high-energy processes in stars has experienced large advance. However, there are still some issues about physical processes taking place in stars that remain uncertain and need a more accurate study. In particular, to deep into some aspects of X-ray coronal emission, we need new observational methods and/or new instrumentation. An example of it is the poor constraints done to physical parameters of flaring loops in solar-type stars. Several theoretical and observational studies have been carried out but there is still controversy of some particular results, including the geometry of the magnetic field and the loop’s length. On the one hand, some authors believe that flaring processes in late-type stars are scaled-up versions of those taking place in the Sun and that the geometrical aspect of loops must be similar in both cases. The models used by these authors to obtain parameters from the observation of flares are so-called scaling laws. This scenario predicts stronger magnetic fields in stars than in the Sun in many cases. On the other hand, other models are based on physical laws and assume aspect rations of the flaring loops similar to that of the Sun, obtaining much longer loop lengths and magnetic fields with values more similar to those observed for the Sun. To distinguish between both scenarios, we need to use independent methods that allow us to determine the same parameters for the flare. A powerful technique to determine some parameters of the flaring loop is the wavelet analysis of light curves. I present a preliminary study of this technique using Montecarlo analysis and apply it to several stars.

Accretion processes onto classical T Tauri stars (CTTSs) are believed to generate shocks at the stellar surface due to the impact of supersonic downflowing plasma. Although current models of accretion streams provide a plausible global picture of this process, several aspects are still unclear. For example, the observed X-ray luminosity in accretion shocks is, in general, well below the predicted value. A possible explanation discussed in the literature is in terms of significant absorption of the emission due to the thick surrounding medium. Here we consider a 2D MHD model describing an accretion stream propagating through the atmosphere of a CTTS and impacting onto its chromosphere. The model includes all the relevant physics, namely the gravity, the thermal conduction, and the radiative cooling, and a realistic description of the unperturbed stellar atmosphere (from the chromosphere to the corona). From the model results, we synthesize the X-ray emission emerging from the hot slab produced by the accretion shock, exploring different configurations and strengths of the stellar magnetic field and different density profiles of the accretion stream (accounting also for non uniform streams). The synthesis includes the local absorption by the thick surrounding medium and the Doppler shift of lines due to the component of plasma velocity along the line-of-sight. We explore the effects of absorption on the emerging X-ray spectrum, considering different inclinations of the accretion stream with respect to the observer. We also investigate the detectability of line shift due to Doppler effect under different physical conditions. Finally we compare our results with the observations.

Characterizing UV variability and accretion in the young open cluster NGC 2264
I will present the results of an extensive UV/optical variability survey of the young open cluster NGC 2264 (3 Myr), performed at CFHT/MegaCam as a part of a wide project of simultaneous multi-wavelength (X-rays to IR) monitoring aimed at unambiguously characterizing YSO variability (the Coordinated Synoptic Investigation of NGC 2264). A complete u,g,r,i photometric dataset has been obtained for more than 700 young stars, ranging in mass from 0.2 to 2 MSun, and their u-band and r-band variability monitored over two full weeks. The u-band observations offer a direct access to the accretion features, hence providing a unique clue to the accretion dynamics throughout the region. I investigate the photometric properties of different stellar groups on various color-color and color-magnitude diagrams and infer a straightforward characterization based on accretion properties. I analyze the u-band variability of T Tauri stars on week timescales and probe the color signatures of different physical processes, showing that well-distinguished behaviors are specific to processes of different nature. Based on the UV excess diagnostics, I derive a dynamical picture of accretion in NGC 2264. I investigate the dependence of the inferred mass accretion rates on stellar mass and discuss the large spread in values detected at each mass. I explore the variability of the mass accretion rates on a timescale of weeks, resulting from the geometric effects linked with stellar rotation and from the intrinsic accretion variability, and show that this variability cannot explain the observed spread.

We study the rotation-activity relationship for low-mass members of the young cluster h Persei (~13 Myr). h Per, thanks to its age, allows us to link the rotation-activity relation observed for main-sequence stars to the puzzling case of very young PMS stars. We constrained the activity levels of h~Per members by analyzing a deep Chandra/ACIS-I observation pointed to the central field of h Per. Considering also the catalog of h Per members with measured rotational period, presented by Moraux et al. (2013), we obtained a final catalog of 202 h Per members with measured X-ray luminosity and rotational period. We investigate the rotation-activity relation of h Per members considering different mass ranges. We find that stars with 1.3 Msun < M < 1.4 Msun show significant evidence of supersaturation for short periods. This phenomenon is instead not observed for lower mass stars.