Search

Al centro della maggior parte delle galassie risiedono buchi neri massicci quiescenti. Talvolta questi oggetti possono accrescere materiale dall’ambiente circostante e diventare attivi. Un contributo alla loro accensione è dato dalla distruzione mareale di stelle che gli orbitano intorno. Infatti, le stelle di un cluster nucleare galattico interagiscono stocasticamente una con l’altra, aumentando la probabilità per una di queste di essere scatterata abbastanza vicino al buco nero centrale da risentire in modo significativo della sua influenza mareale. Essenzialmente, se il pericentro della stella attorno al buco nero è minore del cosiddetto raggio mareale, la stella viene completamente distrutta. Una frazione dei detriti stellari prodotti circolarizza e accresce sul buco nero alimentando un’emissione caratteristica. Nonostante la loro rarità e la generale sparsità delle loro osservazioni, sono stati osservati circa 70 candidati, principalmente nelle bande ottica, ultravioletta e X soffice, permettendo così di rivelare buchi neri altrimenti quiescenti. La scoperta di nuovi candidati risulta importante anche per verificare le teorie relative agli eventi di distruzione mareale attraverso le osservazioni. Circa due anni fa ho identificato XMMSL1J063045.9-603110 come nuovo candidato. La sua peculiarità consiste nell’essere associato probabilmente ad una galassia nana molto debole o addirittura ad un globular cluster molto luminoso che ospita un buco nero di massa intermedia. I buchi neri di massa intermedia sono attualmente oggetto di studio come collegamento tra i buchi neri di massa stellare e i buchi neri massicci e come materia prima per la formazione dei buchi neri massicci, pertanto poterli rivelare sarebbe estremamente importante. Principalmente, la letteratura riguardante gli eventi di distruzione mareale tratta le distruzioni mareali totali, ma il profilo dei tidal disruption flares ci si aspetta che dipenda da se la stella viene distrutta totalmente o parzialmente. Per come è definito il raggio mareale, è stato necessario innanzitutto determinare l’effettiva linea di demarcazione tra le distruzioni totali e quelle parziali. Guillochon e Ramirez-Ruiz (2013; 2015) l’hanno valutata attraverso simulazioni idrodinamiche di incontri mareali tra stelle modellate come politropi e buchi neri, basate su un codice a griglia. Seguendo il loro lavoro, ho indagato la stessa questione confrontando codici di simulazione diversi, dotati di differenti vantaggi e limiti. La demarcazione dipendeva in maniera significativa dall’indice politropico scelto (ovvero, dalla struttura interna della stella), ma solo lievemente dal metodo di simulazione utilizzato, al di sopra però di una risoluzione minima. Contrariamente a stelle singole, un gran numero di stelle si trova in sistemi binari. Mandel e Levin (2015) hanno dimostrato che in determinate condizioni entrambe le componenti di una binaria potrebbero essere distrutte marealmente in sequenza subito dopo la rottura mareale della binaria, alimentando così un’emissione totale caratteristica. Da distruzioni mareali doppie ci si aspettano curve di luce a doppio picco. Attraverso simulazioni idrodinamiche di distruzioni mareali doppie, ho dimostrato che possono essere più facilmente osservate curve di luce con una doppia pendenza, piuttosto che a doppio picco, diminuendo l’intensità della distruzione e aumentando la differenza di massa tra le componenti della binaria. Ho identificato la prima evidenza osservativa di questi eventi in PS16dtm. La rivelazione di un flare con doppia pendenza può anticipare l’insorgere di flares periodici se una delle componenti della binaria, solo parzialmente distrutta, rimane legata al buco nero dopo la separazione della binaria. Quiescent massive black holes live at the centre of most galaxies. Sometimes they can accrete matter from the surroundings and become active. A contribution to the black hole turning on is given by the tidal disruption of stars orbiting around them. Indeed, stars in a galactic nuclear cluster stochastically interact with each other, increasing the probability for one of them to be scattered close enough to the central black hole to significantly feel its tidal influence. Basically, if the pericentre of the star around the black hole is less than about the so-called tidal radius, the star is completely disrupted. A fraction of the produced stellar debris circularises and accretes onto the black hole powering a characteristic flare. Despite their scarcity and general sparseness in observations, about 70 candidates have been observed, mainly in the optical, UV and soft X-ray energy bands, thus allowing the detection of otherwise quiescent black holes. The discovery of new candidates is also important to check the theories about tidal disruption events through observations. About two years ago I classified XMMSL1J063045.9-603110 as a new candidate. Its peculiarity is to be likely associated with a very dim dwarf galaxy or even a very bright globular cluster hosting an intermediate-mass black hole. Intermediate-mass black holes are currently under study as the connecting bridge between stellar-mass and massive black holes and the raw material for massive black holes, thus their detection would be extremely important. Mostly, the literature of tidal disruption events deals with total tidal disruptions, but the appearance of tidal disruption flares is expected to depend on whether the star is fully or partially disrupted. Given the tidal radius definition, it first followed the need to define the effective demarcation line between total and partial disruptions. Guillochon & Ramirez-Ruiz (2013; 2015) evaluated it through grid-based hydrodynamical simulations of tidal encounters between polytropic stars and black holes. Following their work, I investigated the same problem by comparing different simulation codes, which have different advantages and limits. The demarcation distance depended significantly on the chosen polytropic index (i.e. on the stellar internal structure), but only weakly on the adopted simulation method, provided a minimum resolution threshold. As opposed to single stars, a great number of field stars are in binaries. Mandel & Levin (2015) demonstrated that under certain conditions both stars in a binary might be tidally disrupted in sequence immediately after the tidal binary break-up, thus powering a peculiar total accretion flare. Double-peaked light curves are expected to rise from double tidal disruptions. Via hydrodynamical simulations of double tidal disruptions, I demonstrated that kneed light curves, rather than double-peaked ones, can be more easily observed when decreasing the strength of the disruption and when elevating the mass difference between the binary components. I identified the first observational evidence of such events in PS16dtm. The detection of a knee can anticipate the onset of periodic flares if one of the binary components, only partially disrupted, remains bound to the black hole after binary separation.

Flares from tidal disruption events are unique tracers of quiescent black holes at the centre of galaxies. The appearance of these flares is very sensitive to whether the star is totally or partially disrupted, and in this paper we seek to identify the critical distance of the star from the black hole (r_d) that enables us to distinguish between these two outcomes. We perform here Mesh-free Finite Mass, traditional, and modern Smoothed Particle Hydrodynamical simulations of star-black hole close encounters, with the aim of checking if the value of r_d depends on the simulation technique. We find that the critical distance (or the so-called critical disruption parameter beta_d) depends only weakly on the adopted simulation method, being beta_d=0.92\pm 0.02 for a gamma=5/3 polytrope and beta_d=2.01\pm 0.01 for a gamma=4/3 polytrope., 8 pages, 8 figures, 2 tables; accepted to A&A

In a galactic nucleus, a star on a low angular momentum orbit around the central massive black hole can be fully or partially disrupted by the black hole tidal field, lighting up the compact object via gas accretion. This phenomenon can repeat if the star, not fully disrupted, is on a closed orbit. Because of the multiplicity of stars in binary systems, also binary stars may experience in pairs such a fate, immediately after being tidally separated. The consumption of both the binary components by the black hole is expected to power a double-peaked flare. In this paper, we perform for the first time, with GADGET2, a suite of smoothed particle hydrodynamics simulations of binary stars around a galactic central black hole in the Newtonian regime. We show that accretion luminosity light curves from double tidal disruptions reveal a more prominent knee, rather than a double peak, when decreasing the impact parameter of the encounter and when elevating the difference between the mass of the star which leaves the system after binary separation and the mass of the companion. The detection of a knee can anticipate the onset of periodic accretion luminosity flares if one of the stars, only partially disrupted, remains bound to the black hole after binary separation. Thus knees could be precursors of periodic flares, which can then be predicted, followed up and better modelled. Analytical estimates in the black hole mass range $10^5-10^8 \rm M_{\rm \odot}$ show that the knee signature is enhanced in the case of black holes of mass $10^6-10^7 \rm M_{\rm \odot}$., Comment: 14 pages, 6 figures, 8 tables. Accepted by MNRAS

Black holes at the centre of quiescent galaxies can be switched on when they accrete gas that is gained from stellar tidal disruptions. A star approaching a black hole on a low angular momentum orbit may be ripped apart by tidal forces, which triggers raining down of a fraction of stellar debris onto the compact object through an accretion disc and powers a bright flare. In this paper we discuss XMMSL1J063045.9-603110 as a candidate object for a tidal disruption event. The source has recently been detected to be bright in the soft X-rays during an XMM-Newton slew and later showed an X-ray flux decay by a factor of about 10 in twenty days. We analyse XMM-Newton and Swift data. XMMSL1J063045.9-603110 shows several features typical of tidal disruption events: the X-ray spectrum shows the characteristics of a spectrum arising from a thermal accretion disc, the flux decay follows a t^-5/3 law, and the flux variation is > 350. Optical observations testify that XMMSL1J063045.9-603110 is probably associated with an extremely small galaxy or even a globular cluster, which suggests that intermediate-mass black holes are located in the cores of (at least) some of them., 8 pages, 4 figures; accepted to A&A

Aims: We present a polarimetric and spectroscopic study of the persistent ultra compact X-ray binary 4U 0614+091 aimed at searching for the emission of a relativistic particle jet and at unveiling the orbital period of the system. Methods: We obtained r-band polarimetric observations with the Telescopio Nazionale Galileo (TNG) equipped with the PAOLO polarimeter and with the Nordic Optical Telescope (NOT) equipped with the ALFOSC instrument, covering ~ 2 hours and ~ 0.5 hours observations, respectively. We carried out low resolution spectroscopy of the system using the ESO Very Large Telescope equipped with FORS1 for ~ 1.5 hours (16 spectra covering the range 430-800 nm). Results: The polarimetric analysis performed starting from the TNG dataset revealed a polarisation degree in the r-band of 3 % +/- 1 %. From the NOT dataset, due to the lower S/N ratio, we could obtain only a 3 sigma upper limit of 3.4 %. From the joining of a spectroscopic and photometric analysis, through the study of the equivalent width variations of the CII 724 nm line and the r-band light curve, we could find a hint of a ~ 45 min periodicity. Conclusions: A polarisation degree P of ~ 3 % in the r-band is consistent with the emission of a relativistic particle jet, which is supposed to emit intrinsically linearly polarised synchrotron radiation. Since no variations of P with time have been detected, and the accretion disc of the system does not contain ionised hydrogen, scattering by free electrons in the accretion disc has been rejected. The period of ~ 45 min obtained through the analysis of the system light curve and of the equivalent width variations of the selected spectral line is probably linked to the presence of a hot spot or a superhump in the accretion disc, and lead to an orbital period > 1 hour for the binary system., 8 pages, 6 figures, 4 tables. Accepted for publication in Astronomy & Astrophysics (Section 7)