Numerical simulations of mass loading in the tails of Bow Shock Pulsar Wind Nebulae

When a pulsar is moving through a partially ionized medium, a fraction of neutral Hydrogen atoms penetrate inside the pulsar wind and can be photo-ionized by the nebula UV radiation. The resulting protons remains attached to the magnetic field of the light leptonic pulsar wind enhancing its inertia and changing the flow dynamics of the wind. We present here the first numerical simulations of such effect in the tails of bow shock nebulae. We produce a set of different models representative of pulsars moving in the interstellar medium with different velocities, from highly subsonic to supersonic, by means of 2D hydrodynamic relativistic simulations. We compare the different tail morphologies with results from theoretical models of mass loading in bow shocks. As predicted by analytical models we observe a fast sideways expansion of the tail with the formation of secondary shocks in the ISM. This effect could be at the origin of the head-and-shoulder morphology observed in many BSPWNe.Comment: 7 pages, 5 figures, 1 tabl

From young to old: the evolutionary path of Pulsar Wind Nebulae

Pulsar wind nebulae are fascinating systems, and archetypal sources for high-energy astrophysics in general. Due to their vicinity, brightness, to the fact that they shine at multi-wavelengths, and especially to their long-living emission at gamma-rays, modelling their properties is particularly important for the correct interpretation of the visible Galaxy. A complication in this respect is the variety of properties and morphologies they show at different ages. Here we discuss the differences among the evolutionary phases of pulsar wind nebulae, how they have been modeled in the past and what progresses have been recently made. We approach the discussion from a phenomenological, theoretical (especially numerical) and observational point of view, with particular attention to the most recent results and open questions about the physics of such intriguing sources.Comment: 29 pages, 12 figures, 2 table

Constraints on particle acceleration sites in the Crab Nebula from relativistic MHD simulations

The Crab Nebula is one of the most efficient accelerators in the Galaxy and the only galactic source showing direct evidence of PeV particles. In spite of this, the physical process behind such effective acceleration is still a deep mystery. While particle acceleration, at least at the highest energies, is commonly thought to occur at the pulsar wind termination shock, the properties of the upstream flow are thought to be non-uniform along the shock surface, and important constraints on the mechanism at work come from exact knowledge of where along this surface particles are being accelerated. Here we use axisymmetric relativistic MHD simulations to obtain constraints on the acceleration site(s) of particles of different energies in the Crab Nebula. Various scenarios are considered for the injection of particles responsible for synchrotron radiation in the different frequency bands, radio, optical and X-rays. The resulting emission properties are compared with available data on the multi wavelength time variability of the inner nebula. Our main result is that the X-ray emitting particles are accelerated in the equatorial region of the pulsar wind. Possible implications on the nature of the acceleration mechanism are discussed.Comment: 12 pages, 7 figures, 2 table

Magnetars and Gamma Ray Bursts

In the last few years, evidences for a long-lived and sustained engine in Gamma Ray Bursts (GRBs) have increased the attention to the so called millisecond-magnetar model, as a competitive alternative to the standard collapsar scenario. I will review here the key aspects of the {\it millisecond magnetar} model for Long Duration Gamma Ray Bursts (LGRBs). I will briefly describe what constraints, present observations put on any engine model, both in term of energetic, outflow properties, and the relation with the associated Supernova (SN). For each of these I will show how the millisecond magnetar model satisfies the requirements, what are the limits of the model, how can it be further tested, and what observations might be used to discriminate against it. I will also discuss numerical results that show the importance of the confinement by the progenitor star in explaining the formation of a collimated outflow, how a detailed model for the evolution of the central engine can be built, and show that a wide variety of explosive events can be explained by different magnetar parameters. I will conclude with a suggestion that magnetars might be at the origin of the Extended Emission (EE) observed in a significant fraction of Short GRBs.Comment: 8 pages; to appear in Proceedings of IAU 279 "Death of Massive Stars: Supernovae and Gamma-ray Bursts

X-Ray polarimetry of Pulsar Wind Nebulae with XIPE

Le nebulose da pulsar sono fra gli oggetti più spettacolari dell’astronomia nei raggi X. La loro emissione X non è altro che la radiazione di sincrotrone del vento ultrarelativistico della pulsar, rallentato e compresso dal mezzo circostante. Siccome l’angolo di polarizzazione della radiazione è perpendicolare alla direzione del campo magnetico nel punto di emissione, e il grado di polarizzazione dipende dalla distribuzione in energia degli elettroni emettenti, misure di polarizzazione risolte spazialmente possono dare informazioni sulla topologia del campo magnetico e la sua connessione con gli elementi morfologici della nebulosa, offrendo un’opportunità unica di studiare processi fisici altrimenti inaccessibili. Missioni future come il satellite XIPE saranno cruciali a tale riguardo.Pulsar Wind Nebulae are among the most spectacular targets of X-ray astronomy. Their X-ray emission is the synchrotron radiation of the ultrarelativistic pulsar wind shocked in the ambient medium. Since the polarization position angle is perpendicular to the direction of the magnetic field at the site of emission, and the degree of polarization depends on the energy spectrum of emitting electrons, spatially resolved polarization measurements will probe the magnetic field topology and its connection with the PWN morphological elements, providing a unique opportunity to investigate physical processes not otherwise accessible. Future missions such as XIPE will be crucial in this respect

Vedi alla voce Franco Pacini

In occasione dei cinquanta anni dall’uscita su Nature dell’articolo “Energy Emission from a Neutron Star”, la Biblioteca dell’Osservatorio Astrofisico di Arcetri ha terminato il lavoro di ricerca e di raccolta delle pubblicazioni di Franco Pacini ora identificabili in Polvere di stelle: il portale dei beni culturali INAF.To celebrate the fiftieth anniversary of the publication in Nature of the article “Energy Emission of a Neutron Star”, the Arcetri Astrophysical Observatory Library has completed the work of research and collection of Franco Pacini’s publications, now listed in Polvere di Stelle (“Stardust”): INAF’s cultural heritage portal

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