arXiv subjects

Lorenzo Sironi

Publications and source records attributed to Lorenzo Sironi.

143 records · Page 8Linked to original sources

arXiv2014

Non-Thermal Electron Acceleration in Low Mach Number Collisionless Shocks. I. Particle Energy Spectra and Acceleration Mechanism

Electron acceleration to non-thermal energies in low Mach number (M<5) shocks is revealed by radio and X-ray observations of galaxy clusters and solar flares, but the electron acceleration mechanism remains poorly understood. Diffusive shock acceleration, also known as first-order Fermi acceleration, cannot be directly invoked to explain the acceleration of electrons. Rather, an additional mechanism is required to pre-accelerate the electrons from thermal to supra-thermal energies, so they can then participate in the Fermi process. In this work, we use two- and three-dimensional particle-in-cell plasma simulations to study electron acceleration in low Mach number shocks. We focus on the particle energy spectra and the acceleration mechanism in a reference run with M=3 and a quasi-perpendicular pre-shock magnetic field. We find that about 15 percent of the electrons can be efficiently accelerated, forming a non-thermal power-law tail in the energy spectrum with a slope of p~2.4. Initially, thermal electrons are energized at the shock front via shock drift acceleration. The accelerated electrons are then reflected back upstream, where their interaction with the incoming flow generates magnetic waves. In turn, the waves scatter the electrons propagating upstream back toward the shock, for further energization via shock drift acceleration. In summary, the self-generated waves allow for repeated cycles of shock drift acceleration, similarly to a sustained Fermi-like process. This mechanism offers a natural solution to the conflict between the bright radio synchrotron emission observed from the outskirts of galaxy clusters and the low electron acceleration efficiency usually expected in low Mach number shocks.

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arXiv2014

Simulating the effect of the Sgr A* accretion flow on the appearance of G2 after pericenter

We study the dynamical interaction of the G2 cloud with the accretion flow around Sgr A* by means of three-dimensional, hydrodynamic simulations. We show the effects of the rotating accretion flow on the evolution of G2 by projecting the cloud density onto the plane of the sky, and extracting position-velocity diagrams. We study a number of possible orientations of the cloud orbit with respect to the disk. We find that once the center of mass of the cloud has crossed the pericenter, the differences between models becomes significant. Models with the cloud counter-rotating with respect to the disk are expected to reach higher blue-shifted line of sight velocities. The spatial extent of the emission depends strongly on the cloud-to-disk inclination angle. Future imaging and spectroscopy of G2 emission will shed light both on the structure of the Sgr A* disk and on the properties of the cloud.

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arXiv2014

Relativistic Reconnection: an Efficient Source of Non-Thermal Particles

In magnetized astrophysical outflows, the dissipation of field energy into particle energy via magnetic reconnection is often invoked to explain the observed non-thermal signatures. By means of two- and three-dimensional particle-in-cell simulations, we investigate anti-parallel reconnection in magnetically-dominated electron-positron plasmas. Our simulations extend to unprecedentedly long temporal and spatial scales, so we can capture the asymptotic state of the system beyond the initial transients, and without any artificial limitation by the boundary conditions. At late times, the reconnection layer is organized into a chain of large magnetic islands connected by thin X-lines. The plasmoid instability further fragments each X-line into a series of smaller islands, separated by X-points. At the X-points, the particles become unmagnetized and they get accelerated along the reconnection electric field. We provide definitive evidence that the late-time particle spectrum integrated over the whole reconnection region is a power-law, whose slope is harder than -2 for magnetizations sigma>10. Efficient particle acceleration to non-thermal energies is a generic by-product of the long-term evolution of relativistic reconnection in both two and three dimensions. In three dimensions, the drift-kink mode corrugates the reconnection layer at early times, but the long-term evolution is controlled by the plasmoid instability, that facilitates efficient particle acceleration, in analogy to the two-dimensional physics. Our findings have important implications for the generation of hard photon spectra in pulsar winds and relativistic astrophysical jets.

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arXiv2013

Relativistic Pair Beams from TeV Blazars: A Source of Reprocessed GeV Emission rather than IGM Heating

The interaction of TeV photons from blazars with the extragalactic background light produces a relativistic beam of electron-positron pairs streaming through the intergalactic medium (IGM). The fate of the beam energy is uncertain. By means of two- and three-dimensional particle-in-cell simulations, we study the non-linear evolution of dilute ultra-relativistic pair beams propagating through the IGM. We explore a wide range of beam Lorentz factors gamma_b>>1 and beam-to-plasma density ratios alpha<<1, so that our results can be extrapolated to the extreme parameters of blazar-induced beams (gamma_b~10^6 and alpha~10^(-15), for the most powerful blazars). For cold beams, we show that the oblique instability governs the early stages of evolution, but its exponential growth terminates - due to self-heating of the beam in the transverse direction - when only a negligible fraction ~(alpha/gamma_b)^(1/3)~10^(-7) of the beam energy has been transferred to the IGM plasma. Further relaxation of the beam proceeds through quasi-longitudinal modes, until the momentum dispersion in the direction of propagation saturates at DeltaP_{b,parallel}/gamma_b m_e c ~ 0.2. This corresponds to a fraction ~10% of the beam energy being ultimately transferred to the IGM plasma, irrespective of gamma_b or alpha. If the initial dispersion in beam momentum satisfies DeltaP_{b,parallel}/gamma_b m_e c > 0.2 (as typically expected for blazar-induced beams), the fraction of beam energy deposited into the IGM is much smaller than ~10%. It follows that at least ~90% of the beam energy is still available to power the GeV emission produced by inverse Compton up-scattering of the Cosmic Microwave Background by the beam pairs.

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arXiv2013

A Late-Time Flattening of Light Curves in Gamma-Ray Burst Afterglows

The afterglow emission from Gamma-Ray Bursts (GRBs) is usually interpreted as synchrotron radiation from relativistic electrons accelerated at the GRB external shock, that decelerates from ultra-relativistic to non-relativistic speeds as it sweeps up the surrounding medium. We investigate the temporal decay of the emission from GRB afterglows at late times, when the bulk of the shock-accelerated electrons are non-relativistic. For a uniform circumburst medium, we show that such "deep Newtonian phase" begins at t_{DN} ~ 3 epsilon_{e,-1}^{5/6} t_{ST}, where t_{ST} marks the transition of the blast wave to the non-relativistic spherically-symmetric Sedov-Taylor solution, and epsilon_e = 0.1 epsilon_{e,-1} quantifies the amount of shock energy transferred to the post-shock electrons. For typical parameters, the deep Newtonian stage starts ~ 0.5-several years after the GRB. The radio flux in this phase decays as F_nu ~ t^{-3(p+1)/10}, with decay slope between -0.9 and -1.2, for a power-law distribution of shock-accelerated electrons with index 2 < p < 3. This is shallower than the commonly assumed scaling F_nu ~ t^{-3(5p-7)/10} (with decay slope between -0.9 and -2.4) derived by Frail et al. (2000), which only applies if the GRB shock is non-relativistic, but the electron distribution still peaks at ultra-relativistic energies (a regime that is relevant for a narrow time interval, and only if t_{DN} > t_{ST}, or epsilon_e > 0.03). We discuss how the deep Newtonian phase can be reliably used for GRB calorimetry, and we comment on the good detection prospects of trans-relativistic blast waves at 0.1-10 GHz with EVLA and LOFAR.

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arXiv2013

The S2 star as a probe of the accretion disk of Sgr A*

How accretion proceeds around the massive black hole in the Galactic center and other highly sub-Eddington accretors remains poorly understood. The orbit of the S2 star in the Galactic center passes through the accretion disk of the massive black hole and any observational signature from such interaction may be used as an accretion probe. Because of its early stellar type, S2 is expected to possess a fairly powerful wind. We show here that the ram pressure of the accretion disk shocks the stellar wind fairly close to the star. The shocked fluid reaches a temperature of ~ 1 keV and cools efficiently through optically thin, thermal bremsstrahlung emission. The radiation from the shocked wind peaks around the epoch of the pericenter passage of the star at a luminosity potentially comparable to the quiescent emission detected from Sgr A*. Detection of shocked wind radiation can constrain the density of the accretion disk at a distance of several thousands of gravitational radii from the black hole.

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arXiv2013

Photon Feedback: Screening and the Eddington Limit

Bright star-forming galaxies radiate well below their Eddington Limit. The value of the flux-mean opacity that mediates the radiation force onto matter is orders of magnitude smaller than the UV or optical dust opacity. On empirical grounds, it is shown that high-redshift ULIRGs radiate at two orders of magnitude below their Eddington Limit, while the local starbursters M82 and Arp 220 radiate at a few percent of their Eddington Limit. A simple model for the radiative transfer of UV and optical light in dust-rich environments is considered. Radiation pressure on dust does not greatly affect the large-scale gas dynamics of star-forming galaxies.

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arXiv2013

The Maximum Energy of Accelerated Particles in Relativistic Collisionless Shocks

The afterglow emission from gamma-ray bursts (GRBs) is usually interpreted as synchrotron radiation from electrons accelerated at the GRB external shock, that propagates with relativistic velocities into the magnetized interstellar medium. By means of multi-dimensional particle-in-cell simulations, we investigate the acceleration performance of weakly magnetized relativistic shocks, in the magnetization range 0<sigma<1e-1. The pre-shock magnetic field is orthogonal to the flow, as generically expected for relativistic shocks. We find that relativistic perpendicular shocks propagating in electron-positron plasmas are efficient particle accelerators if the magnetization is sigma<1e-3. For electron-ion plasmas, the transition to efficient acceleration occurs for sigma<3e-5. Here, the acceleration process proceeds similarly for the two species, since the electrons enter the shock nearly in equipartition with the ions, as a result of strong pre-heating in the self-generated upstream turbulence. In both electron-positron and electron-ion shocks, we find that the maximum energy of the accelerated particles scales in time as t^(1/2). This scaling is shallower than the so-called (and commonly assumed) Bohm limit, and it naturally results from the small-scale nature of the Weibel turbulence generated in the shock layer. In magnetized plasmas, the energy of the accelerated particles increases until it reaches a saturation value that scales with the magnetization as sigma^(-1/4). Further energization is prevented by the fact that the self-generated turbulence is confined within a finite region of thickness proportional to sigma^(-1/2) around the shock. Our results can provide physically-grounded inputs for models of non-thermal emission from a variety of astrophysical sources, with particular relevance to GRB afterglows.

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arXiv2012

Radio Synchrotron Emission from a Bow Shock around the Gas Cloud G2 Heading toward the Galactic Center

A dense ionized cloud of gas has been recently discovered to be moving directly toward the supermassive black hole, Sgr A*, at the Galactic Center. In June 2013, at the pericenter of its highly eccentric orbit, the cloud will be approximately 3100 Schwarzschild radii from the black hole and will move supersonically through the ambient hot gas with a velocity of v_p ~ 5400 km/s. A bow shock is likely to form in front of the cloud and could accelerate electrons to relativistic energies. We estimate via particle-in-cell simulations the energy distribution of the accelerated electrons and show that the non-thermal synchrotron emission from these electrons might exceed the quiescent radio emission from Sgr A* by a factor of several. The enhanced radio emission should be detectable at GHz and higher frequencies around the time of pericentric passage and in the following months. The bow shock emission is expected to be displaced from the quiescent radio emission of Sgr A* by ~33 mas. Interferometric observations could resolve potential changes in the radio image of Sgr A* at wavelengths < 6 cm.

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arXiv2012

Particle-in-cell simulations of shock-driven reconnection in relativistic striped winds

By means of two- and three-dimensional particle-in-cell simulations, we investigate the process of driven magnetic reconnection at the termination shock of relativistic striped flows. In pulsar winds and in magnetar-powered relativistic jets, the flow consists of stripes of alternating magnetic field polarity, separated by current sheets of hot plasma. At the wind termination shock, the flow compresses and the alternating fields annihilate by driven magnetic reconnection. Irrespective of the stripe wavelength "lambda" or the wind magnetization "sigma" (in the regime sigma>>1 of magnetically-dominated flows), shock-driven reconnection transfers all the magnetic energy of alternating fields to the particles, whose average Lorentz factor increases by a factor of sigma with respect to the pre-shock value. In the limit lambda/(r_L*sigma)>>1, where r_L is the relativistic Larmor radius in the wind, the post-shock particle spectrum approaches a flat power-law tail with slope around -1.5, populated by particles accelerated by the reconnection electric field. The presence of a current-aligned "guide" magnetic field suppresses the acceleration of particles only when the guide field is stronger than the alternating component. Our findings place important constraints on the models of non-thermal radiation from Pulsar Wind Nebulae and relativistic jets.

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arXiv2011

Acceleration of Particles at the Termination Shock of a Relativistic Striped Wind

The relativistic wind of obliquely-rotating pulsars consists of toroidal stripes of opposite magnetic field polarity, separated by current sheets of hot plasma. By means of two- and three-dimensional particle-in-cell simulations, we investigate particle acceleration and magnetic field dissipation at the termination shock of a relativistic striped wind. At the shock, the flow compresses and the alternating fields annihilate by driven magnetic reconnection. Irrespective of the stripe wavelength "lambda" or the wind magnetization "sigma" (in the regime sigma>>1 of magnetically-dominated flows), shock-driven reconnection transfers all the magnetic energy of alternating fields to the particles, whose average Lorentz factor increases by a factor of sigma with respect to the pre-shock value. The shape of the post-shock spectrum depends primarily on the ratio lambda/(r_L*sigma), where r_L is the relativistic Larmor radius in the wind. The spectrum becomes broader as the value of lambda/(r_L*sigma) increases, passing from a relativistic Maxwellian to a flat power-law tail with slope around -1.5, populated by particles accelerated by the reconnection electric field. Close to the equatorial plane of the wind, where the stripes are symmetric, the highest energy particles resulting from magnetic reconnection can escape ahead of the shock, and be injected into a Fermi-like acceleration process. In the post-shock spectrum, they populate a power-law tail with slope around -2.5, that extends beyond the flat component produced by reconnection. Our study suggests that the spectral break between the radio and the optical band in Pulsar Wind Nebulae can be a natural consequence of particle acceleration at the termination shock of striped pulsar winds.

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arXiv2010

Particle Acceleration in Relativistic Magnetized Collisionless Electron-Ion Shocks

We investigate shock structure and particle acceleration in relativistic magnetized collisionless electron-ion shocks by means of 2.5D particle-in-cell simulations with ion-to-electron mass ratios (m_i/m_e) ranging from 16 to 1000. We explore a range of inclination angles between the pre-shock magnetic field and the shock normal. In "subluminal" shocks, where relativistic particles can escape ahead of the shock along the magnetic field lines, ions are efficiently accelerated via a Fermi-like mechanism. The downstream ion spectrum consists of a relativistic Maxwellian and a high-energy power-law tail, which contains ~5% of ions and ~30% of ion energy. Its slope is -2.1. Upstream electrons enter the shock with lower energy than ions, so they are more strongly tied to the field. As a result, only ~1% of the incoming electrons are Fermi-accelerated at the shock before being advected downstream, where they populate a steep power-law tail (with slope -3.5). For "superluminal" shocks, where relativistic particles cannot outrun the shock along the field, the self-generated turbulence is not strong enough to permit efficient Fermi acceleration, and the ion and electron downstream spectra are consistent with thermal distributions. The incoming electrons are heated up to equipartition with ions, due to strong electromagnetic waves emitted by the shock into the upstream. Thus, efficient electron heating (>15% of the upstream ion energy) is the universal property of relativistic electron-ion shocks, but significant nonthermal acceleration of electrons (>2% by number, >10% by energy, with slope flatter than -2.5) is hard to achieve in magnetized flows and requires weakly magnetized shocks (magnetization <1e-3). These findings place important constraints on the models of AGN jets and Gamma Ray Bursts that invoke particle acceleration in relativistic magnetized electron-ion shocks.

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arXiv2009

The Eddington Limit in Cosmic Rays: An Explanation for the Observed Lack of Low-Mass Radio-Loud Quasars and the M_{BH}-M_{Bulge} Relation

We present a feedback mechanism for supermassive black holes and their host bulges that operates during epochs of radio-loud quasar activity. In the radio cores of relativistic quasar jets, internal shocks convert a fraction of ordered bulk kinetic energy into randomized relativistic ions, or in other words cosmic rays. By employing a phenomenologically-motivated jet model, we show that enough 1-100 GeV cosmic rays escape the radio core into the host galaxy to break the Eddington limit in cosmic rays. As a result, hydrostatic balance is lost and a cosmic ray momentum-driven wind develops, expelling gas from the host galaxy and thus self-limiting the black hole and bulge growth. Although the interstellar cosmic ray power is much smaller than the quasar photon luminosity, cosmic rays provide a stronger feedback than UV photons, since they exchange momentum with the galactic gas much more efficiently. The amount of energy released into the host galaxy as cosmic rays, per unit of black hole rest mass energy, is independent of black hole mass. It follows that radio-loud jets should be more prevalent in relatively massive systems since they sit in galaxies with relatively deep gravitational potentials. Therefore, jet-powered cosmic ray feedback not only self-regulates the black hole and bulge growth, but also provides an explanation for the lack of radio-loud activity in relatively small galaxies. By employing basic known facts regarding the physical conditions in radio cores, we approximately reproduce both the slope and the normalization of the M_{BH}-M_{Bulge} relation.

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arXiv2009

Synthetic Spectra from PIC Simulations of Relativistic Collisionless Shocks

We extract synthetic photon spectra from first-principles particle-in-cell simulations of relativistic shocks propagating in unmagnetized pair plasmas. The two basic ingredients for the radiation, namely accelerated particles and magnetic fields, are produced self-consistently as part of the shock evolution. We use the method of Hededal & Nordlund (2005) and compute the photon spectrum via Fourier transform of the electric far-field from a large number of particles, sampled directly from the simulation. We find that the spectrum from relativistic collisionless shocks is entirely consistent with synchrotron radiation in the magnetic fields generated by Weibel instability. We can recover the so-called "jitter'' regime only if we artificially reduce the strength of the electromagnetic fields, such that the wiggler parameter K = qB lambda/mc^2 becomes much smaller than unity ("B" and "lambda" are the strength and scale of the magnetic turbulence, respectively). These findings may place constraints on the origin of non-thermal emission in astrophysics, especially for the interpretation of the hard (harder than synchrotron) low-frequency spectrum of Gamma-Ray Bursts.

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arXiv2009

Polarized Infrared Emission by Polycyclic Aromatic Hydrocarbons resulting from Anisotropic Illumination

We study the polarized infrared emission by Polycyclic Aromatic Hydrocarbons (PAHs), when anisotropically illuminated by UV photons. PAH molecules are modeled as planar disks with in-plane and out-of-plane vibrational dipoles. As first pointed out by Leger (1988), infrared emission features resulting from in-plane and out-of-plane modes should have orthogonal polarization directions. We show analytically how the degree of polarization depends on the viewing geometry and the molecule's internal alignment between principal axis of inertia and angular momentum, which gets worse after photon absorption. Longer wavelength features, emitted after better internal alignment is recovered, should be more strongly polarized. The degree of polarization for uni-directional illumination (e.g., by a star) is larger than for diffuse illumination (e.g., by a disk galaxy), all else being equal. For PAHs in the Cold Neutral Medium, the predicted polarization is probably too small to distinguish from the contribution of linear dichroism by aligned foreground dust. The level of polarization predicted for PAH emission from the Orion Bar is only ~0.06% at 3.3 microns; Sellgren et al. (1988) report a much larger value, 0.86+-0.28%, which suggests that the smallest PAHs may have moderately suprathermal rotation rates. Future observations of (or upper limits on) the degree of polarization for the Orion Bar or for dust above edge-on galaxies (e.g., NGC 891 or M82) may constrain the internal alignment of emitting PAHs, thus providing clues to their rotational dynamics.

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arXiv2009

Particle Acceleration in Relativistic Magnetized Collisionless Pair Shocks: Dependence of Shock Acceleration on Magnetic Obliquity

We investigate shock structure and particle acceleration in relativistic magnetized collisionless pair shocks by means of 2.5D and 3D particle-in-cell simulations. We explore a range of inclination angles between the pre-shock magnetic field and the shock normal. We find that only magnetic inclinations corresponding to "subluminal" shocks, where relativistic particles following the magnetic field can escape ahead of the shock, lead to particle acceleration. The downstream spectrum in such shocks consists of a relativistic Maxwellian and a high-energy power-law tail with exponential cutoff. For increasing magnetic inclination in the subluminal range, the high-energy tail accounts for an increasing fraction of particles (from ~1% to ~2%) and energy (from ~4% to ~12%). The spectral index of the power law increases with angle from -2.8+-0.1 to -2.3+-0.1. Particle energization is driven by the Diffusive Shock Acceleration process for nearly parallel shocks, and switches to Shock-Drift Acceleration for larger subluminal inclinations. For "superluminal" shocks, the downstream particle spectrum does not show any significant suprathermal tail. As seen from the upstream frame, efficient acceleration in relativistic (Lorentz factor gamma0 > 5) magnetized (sigma > 0.03) flows exists only for a very small range of magnetic inclination angles (< 34/gamma0 degrees), so relativistic astrophysical pair shocks have to be either nearly parallel or weakly magnetized to generate nonthermal particles. These findings place constraints on the models of AGN jets, Pulsar Wind Nebulae and Gamma Ray Bursts that invoke particle acceleration in relativistic magnetized shocks. (Abridged)

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arXiv2007

Production of magnetic energy by macroscopic turbulence in GRB afterglows

Afterglows of gamma-ray bursts are believed to require magnetic fields much stronger than that of the compressed pre-shock medium. As an alternative to microscopic plasma instabilities, we propose amplification of the field by macroscopic turbulence excited by the interaction of the shock with a clumpy pre-shock medium, for example a stellar wind. Using a recently developed formalism for localized perturbations to an ultra-relativistic shock, we derive constraints on the lengthscale, amplitude, and volume filling factor of density clumps required to produce a given magnetic energy fraction within the expansion time of the shock, assuming that the energy in the field achieves equipartion with the turbulence. Stronger and smaller-scale inhomogeneities are required for larger shock Lorentz factors. Hence it is likely that the magnetic energy fraction evolves as the shock slows. This could be detected by monitoring the synchrotron cooling frequency if the radial density profile ahead of the shock, smoothed over clumps, is known.

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