rHARM: Accretion and Ejection in Resistive GR-MHD

Turbulent magnetic diffusivity plays an important role for accretion disks and the launching of disk winds. We have implemented magnetic diffusivity, respective resistivity in the general relativistic MHD code HARM. This paper describes the theoretical background of our implementation, its numerical realization, our numerical tests and preliminary applications. The test simulations of the new code rHARM are compared with an analytic solution of the diffusion equation and a classical shock tube problem. We have further investigated the evolution of the magneto-rotational instability (MRI) in tori around black holes for a range of magnetic diffusivities. We find indication for a critical magnetic diffusivity (for our setup) beyond which no MRI develops in the linear regime and for which accretion of torus material to the black hole is delayed. Preliminary simulations of magnetically diffusive thin accretion disks around Schwarzschild black holes that are threaded by a large-scale poloidal magnetic field show the launching of disk winds with mass fluxes of about 50% of the accretion rate. The disk magnetic diffusivity allows for efficient disk accretion that replenishes the mass reservoir of the inner disk area and thus allows for long-term simulations of wind launching for more than 5000 time units.Comment: 21 pages, 43 figures, accepted by Ap

Dynamo action in thick disks around Kerr black holes: high-order resistive GRMHD simulations

We present the first kinematic study of an $\alpha\Omega$-dynamo in the General Relativistic Magneto-HydroDynamics (GRMHD) regime, applied to thick disks orbiting around Kerr black holes and using a fully covariant mean field dynamo closure for the Ohm law. We show that the $\alpha\Omega$-dynamo mechanism leads to a continuous exponential growth of the magnetic field within the disk and to the formation of dynamo waves drifting away or toward the equatorial plane. Since the evolution of the magnetic field occurs qualitatively in the same fashion as in the Sun, we present also butterfly diagrams that characterize our models and show the establishment of an additional timescale, which depends on the microscopic properties of the turbulent motions, possibly providing an alternative explanation to periodicities observed in many high-energy astrophysical sources where accretion onto a rotating black hole is believed to operate.Comment: 5 pages, 4 figures. Accepted for publication in MNRA

Fast reconnection in relativistic plasmas: the magnetohydrodynamics tearing instability revisited

Fast reconnection operating in magnetically dominated plasmas is often invoked in models for magnetar giant flares, for magnetic dissipation in pulsar winds, or to explain the gamma-ray flares observed in the Crab nebula, hence its investigation is of paramount importance in high-energy astrophysics. Here we study, by means of two dimensional numerical simulations, the linear phase and the subsequent nonlinear evolution of the tearing instability within the framework of relativistic resistive magnetohydrodynamics, as appropriate in situations where the Alfven velocity approaches the speed of light. It is found that the linear phase of the instability closely matches the analysis in classical MHD, where the growth rate scales with the Lundquist number S as S^-1/2, with the only exception of an enhanced inertial term due to the thermal and magnetic energy contributions. In addition, when thin current sheets of inverse aspect ratio scaling as S^-1/3 are considered, the so-called "ideal" tearing regime is retrieved, with modes growing independently on S and extremely fast, on only a few light crossing times of the sheet length. The overall growth of fluctuations is seen to solely depend on the value of the background Alfven velocity. In the fully nonlinear stage we observe an inverse cascade towards the fundamental mode, with Petschek-type supersonic jets propagating at the external Alfven speed from the X-point, and a fast reconnection rate at the predicted value R~(ln S)^-1.Comment: 14 pages, 9 figures, accepted for publication (MNRAS

General Relativistic Magnetohydrodynamics Mean-Field Dynamos

Large-scale, ordered magnetic fields in several astrophysical sources are supposed to be originated, and maintained against dissipation, by the combined amplifying action of rotation and small-scale turbulence. For instance, in the solar interior, the so-called α−Ω mean-field dynamo is known to be responsible for the observed 22-years magnetic cycle. Similar mechanisms could operate in more extreme environments, like proto neutron stars and accretion disks around black holes, for which the physical modelling needs to be translated from the regime of magnetohydrodynamics (MHD) and Newtonian gravity to that of a plasma in a general relativistic curved spacetime (GRMHD). Here we review the theory behind the mean field dynamo in GRMHD, the strategies for the implementation of the relevant equations in numerical conservative schemes, and we show the most important applications to the mentioned astrophysical compact objects obtained by our group in Florence. We also present novel results, such as three-dimensional GRMHD simulations of accretion disks with dynamo and the application of our dynamo model to a super massive neutron star, remnant of a binary neutron star merger as obtained from full numerical relativity simulations

VP6-SUMO Self-Assembly as Nanocarriers for Gastrointestinal Delivery

High proteolytic degradation and poor absorption through epithelial barriers are major challenges to successful oral delivery of therapeutics. Nanoparticle platforms can enhance drug stability and extend the residence time in gastrointestinal (GI) tract. However, drug delivery systems are often inactivated in acidic environment of stomach or suffer poor absorption from intestinal cells due to the mucus layer. To overcome these issues we developed a drug delivery system constituted by a protein construct made by a Rotavirus capsid protein (VP6) and the small ubiquitin-like modifier SUMO. This chimeric construct allows specificity towards intestinal cells, the Rotavirus natural target, combined by an enhanced stability given by the eukaryotic protein transporter SUMO. Furthermore SUMO can act as a molecular switch that facilitates import/export of its ligand to the nucleus, the hypersensitive subcellular site target of many cell killing therapies. In this paper we show that SUMO-VP6 constructs self-assembly into stable nanocarriers. SUMO-VP6 nanocarriers display ideal features for drug delivery: a small size and high monodispersity, a high stability in different pH conditions and a high uptake in the nuclear and cytoplasmic compartment of intestinal cells. These features make SUMO-VP6 nanocarriers a promising novel system for oral delivery of poorly soluble drugs

Analisi di un ambiente umido confinato, finalizzata alla conservazione: i graffiti di Grotta Romanelli

ItGrotta Romanelli (Castro-Le) è una delle principali stazioni italiane del paleolitico superiore. Il sito, che si ritiene essere stato frequentato anche da Homo neanderthalensis, ha restituito, tra l'altro, materiale paleontologico, paletnologico, una pietra dipinta, arte mobiliare e parietale graffita riferita al paleolitico superiore. Lo scavo di trincee interne, per lo studio stratigrafico, e lo svuotamento della maggior parte dei depositi, avvenuti tra gli ultimi anni del 1800 (STASI) ed il 1970 (CARDINI), ha messo a nudo la gran parte delle pareti della grotta. In questi ultimi anni, l'esposizione all'aria, la costituzione litica e le caratteristiche ambientali, hanno favorito l'insistente proliferare, sulle superfici graffite ed a danno di queste ultime, di specie biologiche aggressive. L'assenza di interventi di manutenzione e difesa, protrattasi per circa 200 anni, sta provocando il lento ed inesorabile deterioramento dei graffiti.Il presente lavoro, dopo aver brevemente presentato l'ambiente ed i contenuti di grotta Romanelli, propone una prima analisi degli agenti aggressivi presenti sulle pareti interne (Licheni), finalizzata alla conservazione dei materiali lapidei di supporto e quindi dei graffiti stessi.EnThe Romanelli cave (Castro-Le) is one of the main italian sites of the superior paleolithic era. It is believed to have been frequented by Homo neanderthalensis, and among other finds there are paleonthological and paletnological material, a painted stone, mobiliar art and pariethal graffiti from the superior paleolithic era. The escavation of internal trenches in late 1800 by STASI and in 1970 by CARDINI has freed most of the walls of the cave. Now the exposure to air, the lithic consistency and the environmental characteristics have favoured the prolification of aggressive biological species on the graffiti with the risk of ruining them. The lack of maintenence and defence is causing a slow and unrelenting deterioration of the graffiti. This study, after a brief description of the environment and contents of the Romanelli cave, proposes an analysis of the aggressive agents present on the internal walls (lichens) finalized to the conservation of the stone walls and hence of the graffiti