arXiv subjects

Alexandre Lazarian

Publications and source records attributed to Alexandre Lazarian.

13 recordsLinked to original sources

arXiv2025

Cosmic Ray Transport and Gamma-Ray Signatures in the Interstellar Medium

The interaction of cosmic rays (CRs) with magnetic fields and the interstelar medium (ISM) leads to the production of nonthermal radiation. Although this has been a topic of study for many years, it still poses many challenges to the understanding of these processes. In this work we present a short review of recent advances in the understanding of CR propagation in magnetohydrodynamical (MHD) turbulence, in particular the process of mirror diffusion, and how it can help explain recent observational constraints for CR diffusion away from sources. We also present preliminary results from Monte Carlo simulations of CR cascading and propagation within a young massive stellar cluster (YMSC), aimed at probing the origin of very-high-energy (VHE) emission from these sources.

astro-ph.HE↗

arXiv2025

Optical design and polarimetric performance of a SmallSat UV polarimeter to study interstellar dust: PUFFINS

The Polarimetry in the Ultraviolet to Find Features in INterStellar dust (PUFFINS) is a SmallSat mission concept designed to obtain ultraviolet (UV) spectropolarimetric observations to probe the interstellar dust grain properties and to understand wavelength-dependent extinction and star formation. PUFFINS plans to observe 70 UV bright target stars at varying distances within a 180-320 nm wavelength range with 0.02% polarimetric accuracy. PUFFINS uses a simple telescope design with all reflective optics coated with protected aluminum to enhance reflectivity in the UV. The telescope and the spectropolarimeter, which consists of a Wollaston prism and a half-wave retarder, have been carefully selected to be greater than Technology Readiness Level 6 (TRL6). The telescope is designed to exhibit negligible instrumental polarization and crosstalk, significantly reducing the time needed for polarimetric calibration in orbit. The optimum and careful selection of the target stars will enable PUFFINS to observe an expanded and well-defined sample to test the predictions by interstellar grain alignment theory in the observation phase of 9 months. This paper outlines the details of the optical and optomechanical design and evaluates the polarimetric performance of PUFFINS.

astro-ph.IM↗

arXiv2024

Cosmic Ray Diffusion in the Turbulent Interstellar Medium: Effects of Mirror Diffusion and Pitch Angle Scattering

Cosmic rays (CRs) interact with turbulent magnetic fields in the intestellar medium, generating nonthermal emission. After many decades of studies, the theoretical understanding of their diffusion in the ISM continues to pose a challenge. This study numerically explores a recent prediction termed "mirror diffusion" and its synergy with traditional diffusion mechanism based on gyroresonant scattering. Our study combines 3D MHD simulations of star-forming regions with test particle simulations to analyze CR diffusion. We demonstrate the significance of mirror diffusion in CR diffusion parallel to the magnetic field, when the mirroring condition is satisfied. Our results support the theoretical expectation that the resulting particle propagation arising from mirror diffusion in combination with much faster diffusion induced by gyroresonant scattering resembles a Levy-flight-like propagation. Our study highlights the necessity to reevaluate the diffusion coefficients traditionally adopeted in the ISM based on gyroresonant scattering alone. For instance, our simulations imply a diffusion coefficient $\sim10^{27}cm^2/s$ for particles with a few hundred TeV within regions spanning a few parsecs around the source. This estimate is in agreement with gamma-ray observations, which shows the relevance of our results for understanding of diffuse gamma-ray emission in star-forming regions.

astro-ph.HE↗

arXiv2023

The JWST Galactic Center Survey -- A White Paper

The inner hundred parsecs of the Milky Way hosts the nearest supermassive black hole, largest reservoir of dense gas, greatest stellar density, hundreds of massive main and post main sequence stars, and the highest volume density of supernovae in the Galaxy. As the nearest environment in which it is possible to simultaneously observe many of the extreme processes shaping the Universe, it is one of the most well-studied regions in astrophysics. Due to its proximity, we can study the center of our Galaxy on scales down to a few hundred AU, a hundred times better than in similar Local Group galaxies and thousands of times better than in the nearest active galaxies. The Galactic Center (GC) is therefore of outstanding astrophysical interest. However, in spite of intense observational work over the past decades, there are still fundamental things unknown about the GC. JWST has the unique capability to provide us with the necessary, game-changing data. In this White Paper, we advocate for a JWST NIRCam survey that aims at solving central questions, that we have identified as a community: i) the 3D structure and kinematics of gas and stars; ii) ancient star formation and its relation with the overall history of the Milky Way, as well as recent star formation and its implications for the overall energetics of our galaxy's nucleus; and iii) the (non-)universality of star formation and the stellar initial mass function. We advocate for a large-area, multi-epoch, multi-wavelength NIRCam survey of the inner 100\,pc of the Galaxy in the form of a Treasury GO JWST Large Program that is open to the community. We describe how this survey will derive the physical and kinematic properties of ~10,000,000 stars, how this will solve the key unknowns and provide a valuable resource for the community with long-lasting legacy value.

astro-ph.GA↗

arXiv2023

Outlook on Magnetohydrodynamical Turbulence and its Astrophysical Implications

Magnetohydrodynamical (MHD) turbulence is ubiquitous in magnetized astrophysical plasmas, and it radically changes a great variety of astrophysical processes. In this review, we give the concept of MHD turbulence and explain the origin of its scaling. We consider the implications of MHD turbulence to various problems: dynamo in different types of stars, flare activity, solar and stellar wind from different stars, propagation of cosmic rays, and star formation. We also discuss how the properties of MHD turbulence provide a new way of tracing magnetic fields in interstellar and intracluster media.

astro-ph.SR↗

arXiv2022

A Lunar Backup Record of Humanity

The risk of a catastrophic or existential disaster for our civilization is increasing this century. A significant motivation for a near-term space settlement is the opportunity to safeguard civilization in the event of a planetary-scale disaster. A catastrophic event could destroy the significant cultural, scientific, and technological progress on Earth. However, early space settlements can preserve records of human activity by maintaining a backup data storage system. The backup can also store information about the events leading up to the disaster. The system would improve the ability of early space settlers to recover our civilization after collapse. We show that advances in laser communications and data storage enable the development of a data storage system on the lunar surface with a sufficient uplink data rate and storage capacity to preserve valuable information about the achievements of our civilization and the chronology of the disaster.

physics.soc-ph↗

arXiv2017

Strong Coupling of Alfvén and Fast Modes in Compressible Relativistic Magnetohydrodynamic Turbulence in Magnetically-Dominated Plasmas

In this paper, we report our detailed analysis of the new strong-coupling regime between Alfvén and fast modes in Poynting-dominated plasma turbulence, reported in our previous work Takamoto & Lazarian (2016), which is an important effect for many relativistic plasma phenomena, and calls for new theories of Poynting-dominated MHD turbulence. We performed numerical simulations of relativistic MHD turbulence in isothermal plasmas, and analyzed the ratio of fast to Alfvén mode energy. We found that the increase of the fast mode with the background $σ$-parameter can be observed even in isothermal plasma, showing that such a phenomena is universal in trans-Alfvénic turbulence in Poynting-dominated plasmas. To study the detailed energy conversion process, we also performed a series of simulations of decaying turbulence injecting pure Alfvén, fast, and slow modes, respectively, and investigated the development of the mode conversion from the each mode. We also found that the mode conversion between Alfvén and fast modes is nearly insensitive to the background temperature. Finally, we report a result of a simulation with initially fast mode dominated turbulence. It developed into a temporally strong-coupling regime, which is a strong evidence for the existence of our suggesting strong-coupling regime of fast and Alfvén modes. Our result suggests that the strong turbulence in Poynting-dominated plasma is very different from that in the non-relativistic plasma. It will also give an important guidance to studies of particle acceleration and non-thermal photon emission from Poynting-dominated plasma.

astro-ph.HE↗

arXiv2017

Radiative grain alignment in protoplanetary disks: Implications for polarimetric observations

We apply the theory of radiative torque (RAT) alignment for studying protoplanetary disks around a T-Tauri star and perform 3D radiative transfer calculations to provide the expected maps of polarized radiation to be compared with observations, such as with ALMA. We revisit the issue of grain alignment for large grains expected in the protoplanetary disks and find that mm-sized grains at midplane do not align with the magnetic field as the Larmor precession timescale for such large grains becomes longer than the gaseous damping timescale. Hence, for these grains the RAT theory predicts that the alignment axis is determined by the grain precession with respect to the radiative flux. As a result, we expect that the polarization will be in the azimuthal direction for a face-on disk. It is also shown that if dust grains have superparamagnetic inclusions, magnetic field alignment is possible for (sub-)micron grains at the surface layer of disks, and this can be tested by mid-infrared polarimetric observations.

astro-ph.EP↗

arXiv2016

Compressible Relativistic Magnetohydrodynamic Turbulence in Magnetically-Dominated Plasmas And Implications for A Strong-Coupling Regime

In this Letter, we report compressible mode effects on relativistic magnetohydrodynamic (RMHD) turbulence in Poynting-dominated plasmas using 3-dimensional numerical simulations. We decomposed fluctuations in the turbulence into 3 MHD modes (fast, slow, and Alfvén) following the procedure mode decomposition in (Cho & Lazarian 2002), and analyzed their energy spectra and structure functions separately. We also analyzed the ratio of compressible mode to Alfvén mode energy with respect to its Mach number. We found the ratio of compressible mode increases not only with the Alfvén Mach number but with the background magnetization, which indicates a strong coupling between the fast and Alfvén modes and appearance of a new regime of RMHD turbulence in Poynting-dominated plasmas where the fast and Alfvén modes strongly couples and cannot be separated, different from the non-relativistic MHD case. This finding will affect particle acceleration efficiency obtained by assuming Alfvénic critical balance turbulence, and will change the resulting photon spectra by non-thermal electrons.

astro-ph.HE↗

arXiv2016

Turbulence-induced magnetic fields in shock precursors

Galactic cosmic rays are believed to be mostly accelerated at supernova shocks. However, the interstellar magnetic field is too weak to efficiently accelerate galactic cosmic rays up to the highest energies, i.e. $10^{15}$ eV. A stronger magnetic field in the pre-shock region could provide the efficiency required. Bell's cosmic-ray nonresonant streaming instability has been claimed to be responsible for the amplification of precursor magnetic fields. However, an alternative mechanism has been proposed in which the cosmic-ray pressure gradient forms the shock precursor and drives turbulence, amplifying the magnetic field via the small-scale dynamo. A key ingredient for the mechanism to operate are the inhomogeneities present in the interstellar medium (ISM). These inhomogeneities are the consequence of turbulence. In this work we explore the magnetic field amplification in different ISM conditions through 3D MHD numerical simulations.

astro-ph.HE↗

arXiv2015

Turbulent Reconnection in Relativistic Plasmas And Effects of Compressibility

We report turbulence effects on magnetic reconnection in relativistic plasmas using 3-dimensional relativistic resistive magnetohydrodynamics simulations. We found reconnection rate became independent of the plasma resistivity due to turbulence effects similarly to non-relativistic cases. We also found compressible turbulence effects modified the turbulent reconnection rate predicted in non-relativistic incompressible plasmas; The reconnection rate saturates and even decays as the injected velocity approaches to the Alfvén velocity. Our results indicate the compressibility cannot be neglected when compressible component becomes about half of incompressible mode occurring when the Alfvén Mach number reaches about $0.3$. The obtained maximum reconnection rate is around $0.05$ to $0.1$, which will be able to reach around $0.1$ to $0.2$ if injection scales are comparable to the sheet length.

astro-ph.HE↗

arXiv2013

Topics in microphysics of relativistic plasmas

Astrophysical plasmas can have parameters vastly different from the more studied laboratory and space plasmas. In particular, the magnetic fields can be the dominant component of the plasma, with energy-density exceeding the particle rest-mass energy density. Magnetic fields then determine the plasma dynamical evolution, energy dissipation and acceleration of non-thermal particles. Recent data coming from astrophysical high energy missions, like magnetar bursts and Crab nebula flares, point to the importance of magnetic reconnection in these objects. In this review we outline a broad spectrum of problems related to the astrophysical relevant processes in magnetically dominated relativistic plasmas. We discuss the problems of large scale dynamics of relativistic plasmas, relativistic reconnection and particle acceleration at reconnecting layers, turbulent cascade in force-fee plasmas. A number of astrophysical applications are also discussed.

astro-ph.HE↗

arXiv2009

Stringent Limits on the Polarized Submillimeter Emission from Protoplanetary Disks

We present arcsecond-resolution Submillimeter Array (SMA) polarimetric observations of the 880 um continuum emission from the protoplanetary disks around two nearby stars, HD 163296 and TW Hydrae. Although previous observations and theoretical work have suggested that a 2-3% polarization fraction should be common for the millimeter continuum emission from such disks, we detect no polarized continuum emission above a 3-sigma upper limit of 7 mJy in each arcsecond-scale beam, or <1% in integrated continuum emission. We compare the SMA upper limits with the predictions from the exploratory Cho & Lazarian (2007) model of polarized emission from T Tauri disks threaded by toroidal magnetic fields, and rule out their fiducial model at the ~10-sigma level. We explore some potential causes for this discrepancy, focusing on model parameters that describe the shape, magnetic field alignment, and size distribution of grains in the disk. We also investigate related effects like the magnetic field strength and geometry, scattering off of large grains, and the efficiency of grain alignment, including recent advances in grain alignment theory, which are not considered in the fiducial model. We discuss the impact each parameter would have on the data and determine that the suppression of polarized emission plausibly arises from rounding of large grains, reduced efficiency of grain alignment with the magnetic field, and/or some degree of magnetic field tangling (perhaps due to turbulence). A poloidal magnetic field geometry could also reduce the polarization signal, particularly for a face-on viewing geometry like the TW Hya disk. The data provided here offer the most stringent limits to date on the polarized millimeter-wavelength emission from disks around young stars.

astro-ph.SR↗