On the Energy Dependence of Galactic Cosmic Ray Anisotropies in the Very Local Interstellar Medium
Romina Nikoukar, Matthew E. Hill, Lawrence Brown, Stamatios M. Krimigis, Robert B. Decker, Konstantinos Dialynas, Jozsef Kota, Edmond C. Roelof, Scott Lasley, D… 展开作者
Romina Nikoukar, Matthew E. Hill, Lawrence Brown, Stamatios M. Krimigis, Robert B. Decker, Konstantinos Dialynas, Jozsef Kota, Edmond C. Roelof, Scott Lasley, Douglas C. Hamilton, Vladimir Florinski, Joe Giacalone, John Richardson, Merav Opher
We report on the energy dependence of galactic cosmic rays (GCRs) in the very local interstellar medium (VLISM) as measured by the Low Energy Charged Particle (LECP) instrument on the Voyager 1 (V1) spacecraft. The LECP instrument includes a dual-ended solid state detector particle telescope mechanically scanning through 360 deg across eight equally-spaced angular sectors. As reported previously, LECP measurements showed a dramatic increase in GCR intensities for all sectors of the >=211 MeV count rate (CH31) at the V1 heliopause (HP) crossing in 2012, however, since then the count rate data have demonstrated systematic episodes of intensity decrease for particles around 90° pitch angle. To shed light on the energy dependence of these GCR anisotropies over a wide range of energies, we use V1 LECP count rate and pulse height analyzer (PHA) data from >=211 MeV channel together with lower energy LECP channels. Our analysis shows that while GCR anisotropies are present over a wide range of energies, there is a decreasing trend in the amplitude of second-order anisotropy with increasing energy during anisotropy episodes. A stronger pitch-angle scattering at the higher velocities is argued as a potential cause for this energy dependence. A possible cause for this velocity dependence arising from weak rigidity dependence of the scattering mean free path and resulting velocity-dominated scattering rate is discussed. This interpretation is consistent with a recently reported lack of corresponding GCR electron anisotropies.
The propagation of TeV-PeV cosmic rays (CR) in our Galaxy can be described as a diffusive process. We discuss here two effects, with important observational consequences, that cannot be predicted by the diffusion approximation in its usual form. First, we present an explanation for the CR anisotropies observed at small angular scales on the sky. We show that the local magnetic field configuration within a CR mean free path from Earth naturally results in CR flux anisotropies at small and medium scales. Second, we point out that TeV-PeV CRs should be expected to diffuse strongly anisotropically…
We search for anisotropies in the arrival directions of cosmic rays observed by the KASCADE-Grande air shower experiment. The analysis is based on public data of about 23.7 million events with reconstructed primary energies above 1 PeV. We apply a novel maximum-likelihood reconstruction method for the cosmic ray anisotropy, that compensates for spurious anisotropies induced by local detector effects. We find no evidence for a large-scale dipole anisotropy in the data, consistent with official results based on the conventional East-West derivative method. On the other hand, a subset of cosmic r…
Constraining the behavior of cosmic ray data observed at Earth requires a precise understanding of how the cosmic rays propagate in the interstellar medium. The interstellar medium is not homogeneous; although turbulent magnetic fields dominate over large scales, small coherent regions of magnetic field exist on scales relevant to particle propagation in the nearby Galaxy. Guided propagation through a coherent field is significantly different from random particle diffusion and could be the explanation of spatial anisotropies in the observed cosmic rays. We present a Monte Carlo code to propaga…
GX 301-2 is a high-mass X-ray binary (HMXB) with strong stellar outflows. The evolution of these binaries can be closely related with the interstellar environment due to strong wind interactions. We try to constrain the energy injected in the interstellar medium by GX 301-2 through stellar wind using HAWK-I and Herschel data. We analysed HAWK-I images in four different filters (Br$γ$, H$_2$, J, and Ks) and tried to retrieve signatures of the impact of GX 301-2 on its environment. We used Herschel data to outline the interstellar medium and the Gaia DR3 catalogue to infer the proper motion of G…
The interstellar medium is a multi-phase, magnetized, and highly turbulent medium. In this paper, we address both theoretical and observational aspects of plasma turbulence in the interstellar medium. We successively consider radio wave propagation through a plasma and radio polarized emission. For each, we first provide a theoretical framework in the form of a few basic equations, which enable us to define useful diagnostic tools of plasma turbulence; we then show how these tools have been utilized to detect and interpret observational signatures of plasma turbulence, and what astronomers hav…
Radioactive components of the interstellar medium provide an entirely-different and new aspect to the studies of the interstellar medium. Injected from sources of nucleosynthesis, unstable nuclei decay along their trajectories. Measurements can occur through characteristic gamma rays that are emitted with the decay, or in cosmic material samples through abundances of parent and daughter isotopes as they change with decay. The dynamics and material flows within interstellar medium are thus accessible to measurement, making use of the intrinsic clock that radioactive decay provides. We describe …
We describe the propagation of supernova shocks within the surrounding medium, which may be due to mass-loss from the progenitor star. The structure and density profile of the ejected material and surrounding medium are considered. Shock wave interaction with clouds and wind-bubbles, and issues relevant to cosmic rays are briefly discussed.
A relativistic spacecraft of the type envisioned by the Breakthrough Starshot initiative will inevitably get charged through collisions with interstellar particles and UV photons. Interstellar magnetic fields would, therefore, deflect the trajectory of the spacecraft. We calculate the expected deflection for typical interstellar conditions. We also find that the charge distribution of the spacecraft is asymmetric, producing an electric dipole moment. The interaction between the moving electric dipole and the interstellar magnetic field is found to produce a large torque, which can result in fa…
E(A+M)PEC traces the ionization structure, cooling and emission spectra of plasmas. It is written in OpenCL, runs in NVIDIA Graphics Processor Units and can be coupled to any HD or MHD code to follow the dynamical and thermal evolution of any plasma in, e.g., the interstellar medium (ISM).
Several cosmic-ray observatories have provided a high accuracy map of the sky at TeV--PeV energies. The data reveals an O(0.1%) deficit from north galactic directions that peaks at 10 TeV and then evolves with the energy, together with other anisotropies at smaller angular scales. Using Boltzmann's equation we derive expressions for the cosmic-ray flux that fit most of these features. The anisotropies depend on the local interstellar magnetic field B_{IS}, on the average galactic field B_{R} in our vicinity, and on the distribution of dominant cosmic-ray sources. We show that the initial dipol…
Analysis is performed on ultra-high resolution large-scale cosmological radiation-hydrodynamic simulations to, for the first time, quantify the physical environment of long-duration gamma-ray bursts (GRBs) at the epoch of reionization. We find that, on parsec scales, 13% of GRBs remain in high density ($\ge 10^4$cm$^{-3}$) low-temperature star-forming regions, whereas 87% of GRBs occur in low-density ($\sim 10^{-2.5}$cm$^{-3}$) high temperature regions heated by supernovae. More importantly, the spectral properties of GRB afterglows, such as the neutral hydrogen column density, total hydrogen …
We examine the annihilation of positrons on polycyclic aromatic hydrocarbon (PAH) molecules in interstellar medium conditions. We estimate the annihilation rates of positrons on PAHs by a semi-empirical approach. We show that PAHs can play a significant role in the overall galactic positron annihilation picture and use the annihilation rates and INTEGRAL galactic emission measurements to constrain the amount of PAHs present in the ISM. We find an upper limit of 4.6 x 10^-7 for the PAH abundance.
Cosmic rays of kinetic energies below ~1 GeV per nucleon are thought to play a key role in the chemistry and dynamics of the interstellar medium. They are also thought to be responsible for nucleosynthesis of the light elements Li, Be, and B. However, very little is known about the flux and composition of low-energy cosmic rays since the solar modulation effect makes impossible a direct detection of these particles near Earth. We first discuss the information that the light elements have brought to cosmic-ray studies. We then discuss the prospects for detection of nuclear gamma-ray line emissi…
A model is introduced, in which the irregularity spectrum of the Galactic magnetic field beyond the dissipation length scale is first a Kolmogorov spectrum $k^{-5/3}$ at small scales $λ\, = \, 2 π/k$ with $k$ the wave-number, then a saturation spectrum $k^{-1}$, and finally a shock-dominated spectrum $k^{-2}$ mostly in the halo/wind outside the Cosmic Ray disk. In an isotropic approximation such a model is consistent with the Interstellar Medium (ISM) data. With this model we discuss the Galactic Cosmic Ray (GCR) spectrum, as well as the extragalactic Ultra High Energy Cosmic Rays (UHECRs), th…
The propagation of cosmic rays (CRs) in turbulent interstellar magnetic fields is typically described as a spatial diffusion process. This formalism predicts only a small deviation from an isotropic CR distribution in the form of a dipole in the direction of the CR density gradient or relative background flow. We show that the existence of a global CR dipole moment necessarily generates a spectrum of higher multipole moments in the local CR distribution. These "anomalous" anisotropies are a direct consequence of Liouville's theorem in the presence of a local turbulent magnetic field. We show t…
Massive stars drive strong winds that impact the surrounding interstellar medium, producing parsec-scale bubbles for isolated stars and superbubbles around young clusters. These bubbles can be observed across the electromagnetic spectrum, both the wind itself and the swept up interstellar gas. Runaway massive stars produce bow shocks that strongly compresses interstellar gas, producing bright infrared, optical and radio nebulae. With the detection of non-thermal radio emission from bow shocks, particle acceleration can now also be investigated. I review research on wind bubbles and bow shocks …
We present a comprehensive study of interstellar X-ray extinction using the extensive Chandra supernova remnant archive and use our results to refine the empirical relation between the hydrogen column density and optical extinction. In our analysis, we make use of the large, uniform data sample to assess various systematic uncertainties in the measurement of the interstellar X-ray absorption. Specifically, we address systematic uncertainties that originate from (i) the emission models used to fit supernova remnant spectra, (ii) the spatial variations within individual remnants, (iii) the physi…
Supernova remnants (SNRs) are one of the most energetic astrophysical events and are thought to be the dominant source of Galactic cosmic rays (CRs). A recent report on observations from the Fermi satellite has shown a signature of pion decay in the gamma-ray spectra of SNRs. This provides strong evidence that high-energy protons are accelerated in SNRs. The actual gamma-ray emission from pion decay should depend on the diffusion of CRs in the interstellar medium. In order to quantitatively analyse the diffusion of high-energy CRs from acceleration sites, we have performed test particle numeri…
Propagation of Galactic cosmic rays (CR) in the interstellar medium (ISM) is among the unsolved problems in particle astrophysics. Interpretation of CR spectrum and composition measurements and their possible link to dark matter crucially relies on our understanding of CR propagation in the Galaxy. Several air shower experiments have measured a significant anisotropy of CRs in the TeV to PeV energy range. These observations hint to a complicated overlap of more than one cause: from the distribution of the CR sources in the Milky Way to the nature of such sources, from the turbulence properties…
We present an X-ray absorption model for the interstellar medium, to be referred to as ISMabs, that takes into account both neutral and ionized species of cosmically abundant elements, and includes the most accurate atomic data available. Using high-resolution spectra from eight X-ray binaries obtained with the Chandra High Energy Transmission Grating Spectrometer, we proceed to benchmark the atomic data in the model particularly in the neon K-edge region. Compared with previous photoabsorption models, which solely rely on neutral species, the inclusion of ions leads to improved spectral fits.…
We investigate the ionization of the diffuse interstellar medium by cosmic rays by modeling their propagation along the wandering magnetic fields using a Monte Carlo method. We study how low-energy cosmic rays propagate in turbulent, translucent molecular clouds, and how they regulate the ionization and both lose and gain energy from the medium. As a test case, we use high spatial resolution (0.03 pc) CO maps of a well-studied high latitude translucent cloud, MBM 3, to model turbulence. The propagation problem is solved with a modified Monte Carlo procedure that includes trapping, energization…
Cosmic ray anisotropy has been observed in a wide energy range and at different angular scales by a variety of experiments over the past decade. However, no comprehensive or satisfactory explanation has been put forth to date. The arrival distribution of cosmic rays at Earth is the convolution of the distribution of their sources and of the effects of geometry and properties of the magnetic field through which particles propagate. It is generally believed that the anisotropy topology at the largest angular scale is adiabatically shaped by diffusion in the structured interstellar magnetic field…
Pulsar Timing Arrays use a set of millisecond pulsars in an attempt to directly detect nanohertz gravitational waves. For this purpose, high precision timing of the pulsars is essential and ultimately a precision of the order of ~100 ns is required. Propagation effects in the interstellar medium cause the radio emission from a pulsar to be dispersed and scattered, introducing time variable delays of the pulses on their way to Earth. If these delays are not properly corrected for, they may cause significant errors in the timing analysis of a pulsar. These proceedings will review the effects of …
Recent measurements of cosmic gamma ray intensities up to TeV energies have been used to estimate the spectral shape of the parent cosmic ray particles present in the interstellar medium. The case is made for the particle spectrum in the Inner Galaxy being flatter than locally and in the Outer Galaxy. Of various possible explanations we make the case for the propagation of the particles being different in the more turbulent interstellar medium of the Inner Galaxy. The characteristic parameter alpha for the so called anomalous diffusion is expected to be less in the Inner Galaxy than that local…
Turbulence in the weakly collisional intracluster medium of galaxies (ICM) is able to generate strong thermal velocity anisotropies in the ions (with respect to the local magnetic field direction), if the magnetic moment of the particles is conserved in the absence of Coulomb collisions. In this scenario, the anisotropic pressure magnetohydrodynamic (AMHD) turbulence shows a very different statistical behaviour from the standard MHD one and is unable to amplify seed magnetic fields, in disagreement with previous cosmological MHD simulations which are successful to explain the observed magnetic…
Gravitational wave (GW) detection with pulsar timing arrays (PTAs) requires accurate noise characterization. The noise of our Galactic-scale GW detector has been systematically evaluated by the Noise Budget and Interstellar Medium Mitigation working groups within the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration. Intrinsically, individual radio millisecond pulsars (MSPs) used by NANOGrav can have some degree of achromatic red spin noise, as well as white noise due to pulse phase jitter. Along any given line-of-sight, the ionized interstellar medium contr…
An analysis of the energy spectra of cosmic rays and particularly the precise data from the AMS-02 experiment support the view about the important role of the Local Bubble in the nearby interstellar medium. It is suggested that the bulk of cosmic rays below about 200 GV of rigidity (momentum/charge ratio) comes from the modest number of supernova remnants in the Local Bubble which appear to have occurred some $10^6$ years ago and contributed to its formation. At higher rigidities the contribution from a 'Local Source', a single supernova remnant generated some $10^5$ years ago seems to dominat…
The radio pulse from a pulsar can be temporally broadened by multipath scattering in the interstellar medium and by instrumental effects within the radio telescope. The observed pulse shape is a convolution of the intrinsic one with the impulse responses of the scattering medium and instrumentation. Until recently, common methods used to model the observed shape make assumptions regarding the intrinsic pulse shape and impulse responses, compute the convolution numerically, and solve for the pulse width and scattering timescale iteratively. An analytical solution is shown to exist for the speci…
One way in which the Canadian Galactic Plane Survey has made an important contribution to the understanding of the Galactic interstellar medium is through its polarization surveys. Investigation of these data has enabled a big step in the study of magnetic fields in the interstellar medium and a range of discrete, extended, interstellar objects. In this review, I will discuss the role that the magnetic field plays in the interstellar medium, summarizing the ways in which magnetic field interacts with the other components in the Milky Way. Magnetic fields in the Galactic halo are discussed, and…
We study the streaming instability of GeV$-100~$GeV cosmic rays (CRs) and its damping in the turbulent interstellar medium (ISM). We find that the damping of streaming instability is dominated by ion-neutral collisional damping in weakly ionized molecular clouds, turbulent damping in the highly ionized warm medium, and nonlinear Landau damping in the Galactic halo. Only in the Galactic halo, is the streaming speed of CRs close to the Alfvén speed. Alfvénic turbulence plays an important role in both suppressing the streaming instability and regulating the diffusion of streaming CRs via magnetic…
