Sites Of Particle Acceleration Research Articles

PARTICLE ACCELERATION IN THE EXPANDING BLAST WAVE OF η CARINA'S GREAT ERUPTION OF 1843

Non-thermal hard X-ray and high-energy (HE; 1 MeV < E < 100 GeV) gamma-ray emission in the direction of Eta Carina has been recently detected using the INTEGRAL, AGILE and Fermi satellites. So far this emission has been interpreted in the framework of particle acceleration in the colliding wind region between the two massive stars. However, the existence of a very fast moving blast wave which originates in the historical 1843 "Great Eruption" provides an alternative particle acceleration site in this system. Here we explore an alternate scenario and find that inverse Compton emission from electrons accelerated in the blast wave can naturally explain both the flux and spectral shape of the measured hard X-ray and HE gamma-ray emission. This scenario is further supported by the lack of significant variability in the INTEGRAL and Fermi measured fluxes.

3D simulations of wind-jet interaction in massive X-ray binaries

High-mass microquasars may produce jets that will strongly interact with surrounding stellar winds on binary system spatial scales. We study the dynamics of the collision between a mildly relativistic hydrodynamical jet of supersonic nature and the wind of an OB star. We performed numerical 3D simulations of jets that cross the stellar wind with the code Ratpenat. The jet head generates a strong shock in the wind, and strong recollimation shocks occur due to the initial overpressure of the jet with its environment. These shocks can accelerate particles up to TeV energies and produce gamma-rays. The recollimation shock also strengthens jet asymmetric Kelvin-Helmholtz instabilities produced in the wind/jet contact discontinuity. This can lead to jet disruption even for jet powers of several times $10^{36}$ erg s$^{-1}$. High-mass microquasar jets likely suffer a strong recollimation shock that can be a site of particle acceleration up to very high energies, but also eventually lead to the disruption of the jet.

X-ray polarization in relativistic jets

We investigate the polarization properties of Comptonized X-rays from relativistic jets in active galactic nuclei (AGN) using Monte Carlo simulations. We consider three scenarios commonly proposed for the observed X-ray emission in AGN: Compton scattering of blackbody photons emitted from an accretion disc; scattering of cosmic microwave background (CMB) photons and self-Comptonization of intrinsically polarized synchrotron photons emitted by jet electrons. Our simulations show that for Comptonization of disc and CMB photons, the degree of polarization of the scattered photons increases with the viewing inclination angle with respect to the jet axis. In both cases, the maximum linear polarization is ≈20 per cent. In the case of synchrotron self-Comptonization (SSC), we find that the resulting X-ray polarization depends strongly on the seed synchrotron photon injection site, with typical fractional polarizations P ≈ 10–20 per cent when synchrotron emission is localized near the jet base, while P ≈ 20–70 per cent for the case of uniform emission throughout the jet. These results indicate that X-ray polarimetry may be capable of providing unique clues to identify the location of particle acceleration sites in relativistic jets. In particular, if synchrotron photons are emitted quasiuniformly throughout a jet, then the observed degree of X-ray polarization may be sufficiently different for each of the competing X-ray emission mechanisms (synchrotron, SSC or external Comptonization) to determine which is the dominant process. However, X-ray polarimetry alone is unlikely to be able to distinguish between disc and CMB Comptonization.

Multifluid model of a one‐dimensional steady state magnetotail current sheet

The central current sheet of the magnetotail has been modeled frequently as a nearly one‐dimensional configuration that might have a significant role in magnetospheric physics as a site of particle acceleration. Yet comparisons of data with tail models have mainly used the Harris solution in spite of its deficiencies: No normal magnetic field, uniform cross‐tail drift, and vanishing asymptotic plasma density. We present a time‐independent one‐dimensional multifluid model that separates ions into groups coming from sources above and below the current sheet. Electrons are a separate group. A disadvantage is that equations of state are required for closure. The isotropic pressure equation of state is supported by observation and gives reasonable results. This model is more realistic; it encodes more information than a single fluid or an ion and electron fluid model. This fluid model can be easily used to compare with magnetotail data. Field and plasma moment data provide boundary conditions for a model calculation. We examine symmetric models that predict specific relations between various quantities, such as magnetic field or density. Solutions are obtained similar to Harris' model, but with non‐uniform out‐of‐plane ion flow and non‐vanishing asymptotic density. Sheet thickness has weak dependence on the normal magnetic field and modest dependence on electron‐to‐ion pressure ratio. The ratio of sheet center to asymptotic density decreases with asymptotic ion pressure. This one‐dimensional configuration of the current sheet, in Earth's reference frame, provides an example of conversion of electromagnetic energy into fluid energy.

Using X‐ray observations to identify the particle acceleration mechanisms in VHE SNRs and “dark” VHE sources

AbstractVery high energy (VHE) γ‐ray observations have proven to be very successful in localizing Galactic acceleration sites of VHE particles. Observations of shell‐type supernova remnants have confirmed that particles are accelerated to VHE energies in supernova blast waves; the interpretation of the γ‐ray data in terms of hadronic or leptonic particle components in these objects relies nevertheless strongly on input from X‐ray observations. The largest identified Galactic VHE source class consists of pulsar wind nebulae, as detected in X‐rays. Many of the remaining VHE sources remain however unidentified until now. With X‐ray observations of these enigmatic “dark” objects one hopes to solve the following questions: What is the astrophysical nature of these sources? Are they predominantly electron or hadron accelerators? And what is their contribution to the overall cosmic ray energy budget? The paper aims to provide an overview over the identification status of the Galactic VHE source population. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The STEREO Mission: An Introduction

The twin STEREO spacecraft were launched on October 26, 2006, at 00:52 UT from Kennedy Space Center aboard a Delta 7925 launch vehicle. After a series of highly eccentric Earth orbits with apogees beyond the moon, each spacecraft used close flybys of the moon to escape into orbits about the Sun near 1 AU. Once in heliospheric orbit, one spacecraft trails Earth while the other leads. As viewed from the Sun, the two spacecraft separate at approximately 44 to 45 degrees per year. The purposes of the STEREO Mission are to understand the causes and mechanisms of coronal mass ejection (CME) initiation and to follow the propagation of CMEs through the inner heliosphere to Earth. Researchers will use STEREO measurements to study the mechanisms and sites of energetic particle acceleration and to develop three-dimensional (3-D) time-dependent models of the magnetic topology, temperature, density and velocity of the solar wind between the Sun and Earth. To accomplish these goals, each STEREO spacecraft is equipped with an almost identical set of optical, radio and in situ particles and fields instruments provided by U.S. and European investigators. The SECCHI suite of instruments includes two white light coronagraphs, an extreme ultraviolet imager and two heliospheric white light imagers which track CMEs out to 1 AU. The IMPACT suite of instruments measures in situ solar wind electrons, energetic electrons, protons and heavier ions. IMPACT also includes a magnetometer to measure the in situ magnetic field strength and direction. The PLASTIC instrument measures the composition of heavy ions in the ambient plasma as well as protons and alpha particles. The S/WAVES instrument uses radio waves to track the location of CME-driven shocks and the 3-D topology of open field lines along which flow particles produced by solar flares. Each of the four instrument packages produce a small real-time stream of selected data for purposes of predicting space weather events at Earth. NOAA forecasters at the Space Environment Center and others will use these data in their space weather forecasting and their resultant products will be widely used throughout the world. In addition to the four instrument teams, there is substantial participation by modeling and theory oriented teams. All STEREO data are freely available through individual Web sites at the four Principal Investigator institutions as well as at the STEREO Science Center located at NASA Goddard Space Flight Center.

The luminous X-ray hotspot in 4C 74.26: synchrotron or inverse-Compton emission?

We report the discovery of an X-ray counterpart to the southern radio hotspot of the largest-known radio quasar 4C 74.26 (whose redshift is z= 0.104). Both XMM–Newton and Chandra images reveal the same significant (10 arcsec i.e. 19 kpc) offset between the X-ray hotspot and the radio hotspot imaged with MERLIN. The peak of the X-ray emission may be either due to synchrotron emission or due to inverse-Compton emission. If synchrotron emission, the hotspot represents the site of particle acceleration and the offset arises from either the jet exhibiting Scheuer's ‘dentist's drill’ effect or a fast spine having less momentum than the sheath surrounding it, which creates the radio hotspot. If the emission arises from the inverse-Compton process, it must be inverse-Compton scattering of the cosmic microwave background in a decelerating relativistic flow, implying that the jet is relativistic (Γ≥ 2) out to a distance of at least 800 kpc. Our analysis, including optical data from the Liverpool Telescope, rules out a background active galactic nucleus for the X-ray emission and confirms its nature as a hotspot, making it the most-X-ray-luminous hotspot detected at low redshift.

Possible Suzaku detection of non-thermal X-ray signals from a rotating magnetized white dwarf

Although rotating neutron stars (NSs) have been regarded as being textbook examples of astrophysical particle acceleration sites for decades, details of the acceleration mechanism remain a mystery; for example, we cannot yet observationally distinguish “polar cap” models from “outer gap” models. To solve the model degeneracy, it is useful to study similar systems with much different physical parameters. Strongly magnetized white dwarfs (WDs) are ideal for this purpose, because they have essentially the same system geometry as NSs, but differ largely from NSs in the system parameters, including the size, magnetic field, and the rotation velocity, with the induced electric field expected to reach 10 13–10 14 eV. Based on this idea, the best candidate among WDs, AE Aquarii, was observed with the fifth Japaneses X-ray satellite, Suzaku. The hard X-ray detector (HXD) on-board Suzaku has the highest sensitivity in the hard X-ray band over 10 keV. A marginal detection in the hard X-ray band was achieved with the HXD, and was separated from the thermal emission. The flux corresponds to about 0.02% of its spin-down energy. If the signal is real, this observation must be a first case of the detection of non-thermal emission from WDs.

Oblique Shocks in the Magnetic Reconnection Jet in Solar Flares

Abstract Strong radio emission of energetic electrons is observed in some solar flares. The origin of energetic electrons is, however, not fully known. In this letter we suggest that oblique shocks are created in reconnection jets in solar flares, and that energetic electrons are accelerated by shocks. We examine 2D MHD simulations of magnetic reconnection with high spacial resolution by assuming an anomalous resistivity model. As a result, magnetic reconnection is found to occur after a secondary tearing instability at the current sheet. We find that, during nonsteady Petschek reconnection, oblique shocks are created by an even mode of the Kelvin–Helmholtz-like instability in the reconnection jet when we assume an anomalous resistivity model. Furthermore, bursty, time-dependent reconnection ejects many plasmoids from the diffusion region, and creates shocks. We suggest that these shocks can be possible sites of particle acceleration in solar flares.

Fast collisionless reconnection in electron-positron plasmas

Magnetic reconnection in electron-positron (pair) plasmas is studied by two-dimensional, electromagnetic, particle-in-cell simulations. In pair plasmas, the Hall current does not appear because there is no scale separation between ion and electron motion. Simulations show that fast reconnection is realized in both nonrelativistic and relativistic regimes. It is demonstrated that the divergence of the electron and positron pressure tensors essentially plays the role of an effective collisionless resistivity in realizing fast reconnection. Since there is no Hall current in pair plasmas, there is no quadrupolar structure in the out-of-plane magnetic field. Particle acceleration by reconnection is investigated. Major sites of particle acceleration are the regions in the vicinity of X lines, where ultrarelativistic particles are generated by the reconnection electric field.

Theoretical overview on high-energy emission in microquasars

Microquasar (MQ) jets are sites of particle acceleration and synchrotron emission. Such synchrotron radiation has been detected coming from jet regions of different spatial scales, which for the instruments at work nowadays appear as compact radio cores, slightly resolvedradio jets, or (very) extended structures (e.g. Mirabel and Rodríguez, 1999; Fender, 2001; Corbel et al., 2002). Because of the presence of relativistic particles and dense photon, magnetic and matter fields, these outflows are also the best candidates to generate the very high-energy (VHE) gamma-rays detected coming from two of these objects, LS 5039 and LS I +61 303 (Aharonian, 2005; Aharonian et al., 2006a; and Albert, 2006, respectively), and may be contributing significantly to the X-rays emitted from the MQ core (e.g. Markoff et al., 2001; Bosch-Ramon et al., 2005a). In addition, beside electromagnetic radiation, jets at different scales are producing some amount of leptonic and hadronic cosmic rays (CR), and evidences of neutrino production in these objects may be eventually found. In this work, we review on the different physical processes that may be at work in or related to MQ jets. The jet regions capable to produce significant amounts of emission at different wavelengths have been reduced to the jet base, the jet at scales of the order of the size of the system orbital semi-major axis, the jet middle scales (the resolved radio jets), and the jet termination point. The surroundings of the jet could be sites of multiwavelength emission as well, deserving also an insight. We focus on those scenarios, either hadronic or leptonic, in which it seems more plausible to generate both photons from radio to VHE and high-energy neutrinos. We briefly comment as well on the relevance of MQ as possible contributors to the galactic CR in the GeV–PeV range.

The Localization of Particle Acceleration Sites in Solar Flares and CMES

We review the particular aspect of determining particle acceleration sites in solar flares and coronal mass ejections (CMEs). Depending on the magnetic field configuration at the particle acceleration site, distinctly different radiation signatures are produced: (1) If charged particles are accelerated along compact closed magnetic field lines, they precipitate to the solar chromosphere and produce hard X-rays, gamma rays, soft X-rays, and EUV emission; (2) if they are injected into large-scale closed magnetic field structures, they remain temporarily confined (or trapped) and produce gyrosynchrotron emission in radio and bremsstrahlung in soft X-rays; (3) if they are accelerated along open field lines they produce beam-driven plasma emission with a metric starting frequency; and (4) if they are accelerated in a propagating CME shock, they can escape into interplanetary space and produce beam-driven plasma emission with a decametric starting frequency. The latter two groups of accelerated particles can be geo-effective if suitably connected to the solar west side. Particle acceleration sites can often be localized by modeling the magnetic topology from images in different wavelengths and by measuring the particle velocity dispersion from time-of-flight delays.

Astrophysical Jets of Blazars and Microquasars

Some recent developments in the study of relativistic jets in active galactic nuclei and microquasars are reviewed. While it has been well established for some time that extragalactic jets found in radio galaxies, quasars, and BL Lac objects are the site of ultrarelativistic particle acceleration, the recent identification of the Galactic jet source and microquasar LS 5039 as a source of very-high-energy gamma-ray emission has underlined the striking similarity between the two types of astrophysical jet sources. In this paper, I will present an overview of the dominant radiation and particle acceleration processes and observational tests to distinguish between such processes. The wide-ranging analogies between Galactic and extragalactic jets, but also their distinct differences, in particular those caused by the presence of the companion star in Galactic microquasar systems, will be exposed.

Structure in the radio counterpart to the 2004 December 27 giant flare from SGR 1806–20

Abstract On 2004 December 27, the magnetar SGR 1806-20 underwent an enormous outburst resulting in the formation of an expanding, moving, and fading radio source. We report observations of this radio source with the Multi-Element Radio-Linked Interferometer Network and the Very Long Baseline Array. The observations confirm the elongation and expansion already reported based on observations at lower angular resolutions, but suggest that at early epochs the structure is not consistent with the very simplest models such as a smooth flux distribution. In particular, there appears to be significant structure on small angular scales, with ∼10 per cent of the radio flux arising on angular scales ⩽100 milliarcsec. This structure may correspond to localized sites of particle acceleration during the early phases of expansion and interaction with the ambient medium.

High-energy radiation generated by winds and shocks: SNRs and superbubbles

We discuss populations of X-ray and -ray sources in star-forming regions (SFR). Interacting winds of massive stars and high supernova activity in SFRs can be powerful sources of high energy emission. Models of nonthermal particle acceleration in the vicinity of active SFRs are reviewed. A class of hard emission sources where a fast wind from a massive star collides with a supernova shell is described. Stellar winds of massive stars and core collapsed supernova explosions with great energy release in the form of multiple interacting shock waves inside the superbubbles are argued as favorable sites of nonthermal particle acceleration. Young stellar objects and supernova activity in the dense environment of starforming regions produce an another potentially abundant class of hard faint X-ray sources due to interaction of fast moving knots with the dense ambient medium. The knots could have very different physical nature, e.g. supernova ejecta fragments or Herbig-Haro-like objects. We argue that the sources may have rather steep logN—logS distribution and can contribute substantially to the galactic diffuse emission including the both low-ionized 6.4 keV and He-like Fe lines.

ChandraObservation of the Shell of Nova Persei 1901 (GK Persei): Detection of Localized Nonthermal X‐Ray Emission from a Miniature Supernova Remnant

I present data on the shell of classical Nova Persei (1901) obtained by the Advanced CCD Imaging Spectrometer S3 detector on board the Chandra X-Ray Observatory. The X-ray nebula is affected mostly by the complex interstellar medium around the nova and has not developed a regular shell. The X-ray nebula is lumpy and asymmetric, with the bulk of the emission coming from the southwestern quadrant. The brightest X-ray emission is detected as an arc that covers the region from the west to the south of the central source. Part of this feature, which is cospatial with the brightest nonthermal radio emission region, is found to be a source of nonthermal (synchrotron) X-ray emission with a power-law photon index of 2.3 and α = 0.68 at about a flux of 1.7 × 10-13 ergs cm-2 s-1. This confirms that the shell is a site of particle acceleration, mainly in the reverse shock zone. There are strong indications for nonlinear diffusive shock acceleration occurring in the forward shock/transition zone with an upper limit on the nonthermal X-ray flux of 1.0 × 10-14 ergs cm-2 s-1. The total X-ray spectrum of the nebula consists of two prominent components of emission (other than the resolved synchrotron X-ray emission). The component dominant below 2 keV is most likely a nonequilibrium ionization thermal plasma of kTs = 0.1-0.3 keV with an X-ray flux of 1.6 × 10-11 ergs cm-2 s-1. There is also a higher temperature, kTs = 0.5-2.6 keV, embedded, NH = (4.0-22.0) × 1022 cm-2 emission component prominent above 2 keV. The unabsorbed X-ray flux from this component is 1.5 × 10-10 ergs cm-2 s-1. The X-ray-emitting plasma is of solar composition except for enhancement in the elemental abundances (mean abundances over the remnant) of Ne/Ne☉ and N/N☉ in the ranges 13-21 and 1-5, respectively. A distinct emission line of neon, He-like Ne IX, is detected, which reveals a distribution of several emission knots/blobs and shows a conelike structure with wings extending toward the northwest and southeast at expansion velocities ~2600 km s-1 in the X-ray wavelengths. The emission measures yield an average electron density in the range 0.6-11.2 cm-3 for both of the components (filling factor = 1). The electron density increases to higher values ~300 cm-3 if the filling factor is decreased substantially. The mass in the X-ray-emitting nebula is (2.1-38.5) × 10-4 M☉. The X-ray luminosity of the forward shock ~4.3 × 1032 ergs s-1 indicates that it is adiabatic. The shocked mass, the X-ray luminosity, and comparisons with other wavelengths suggest that the remnant has started cooling and most likely is in a Sedov phase.

Internal Shocks in the Magnetic Reconnection Jet in Solar Flares: Multiple Fast Shocks Created by the Secondary Tearing Instability

Space solar missions such as Yohkoh and RHESSI observe the hard X- and gamma-ray emission from energetic electrons in impulsive solar flares. Their energization mechanism, however, is unknown. In this Letter, we suggest that the internal shocks are created in the reconnection jet and that they are possible sites of particle acceleration. We examine how magnetic reconnection creates the multiple shocks by performing two-dimensional resistive magnetohydrodynamic simulations. In this Letter, we use a very small grid to resolve the diffusion region. As a result, we find that the current sheet becomes thin due to the tearing instability, and it collapses to a Sweet-Parker sheet. The thin sheet becomes unstable to the secondary tearing instability. Fast reconnection starts by the onset of anomalous resistivity immediately after the secondary tearing instability. During the bursty, time-dependent magnetic reconnection, the secondary tearing instability continues in the diffusion region where the anomalous resistivity is enhanced. As a result, many weak shocks are created in the reconnection jet. This situation produces turbulent reconnection. We suggest that multiple fast shocks are created in the jet and that the energetic electrons can be accelerated by these shocks.

The Efficient Acceleration of Thermal Protons by Perpendicular Shocks

The physics of charged-particle acceleration from near-thermal to much higher energies by collisionless shocks is investigated using large-scale self-consistent plasma simulations. The focus here is on acceleration at shocks that move normal to the average magnetic field. It is shown that a fraction of thermal protons incident on a perpendicular shock are readily accelerated to energies much higher than the ram energy of the incident plasma. This indicates that there is not an injection problem at perpendicular shocks. It is found that some (initially) thermal protons are reflected by the shock and move upstream along magnetic field lines that are multiply connected to other locations on the shock. This leads to efficient acceleration and results in a distribution function, averaged over a large spatial region downstream of the shock, having a high-energy tail that originates directly from the thermal population. It can be concluded from our results that perpendicular shocks are important sites of particle acceleration in a wide variety of astrophysical plasmas.

High‐Energy Gamma Rays from the Massive Black Hole in the Galactic Center

Accreting black holes are believed to be sites of possible particle acceleration with favorable conditions also for effective gamma-ray production. However, because of photon-photon pair production, only low energy (MeV) gamma-rays can escape these compact objects with typically very large compactness parameter, given that in most cases the accretion disks within 10 Schwarzschild radii radiate with a power exceeding 10 percent of the Eddington luminosity. Therefore, the high-energy gamma-ray emission of these objects (both stellar mass and supermassive BHs) is generally suppressed, and consequently the unique information on possible particle acceleration process near the event horizon of the BH is essentially lost. Fortunately, this is not the case for the super-massive BH located at the dynamical center of our Galaxy (Sgr A*) which, thanks to its extraordinary low bolometric luminosity is transparent fo gamma-rays up to very high energies, about 10 TeV. We discuss different scenarios of gamma-ray production in Sgr A* and show that for a reasonable set of parameters one can expect detectable gamma-ray fluxes of both hadronic and electronic origin. Some of these scenarios are applicable not only for the TeV gamma-ray emission recently reported from the direction of Galactic Center, but may have broader implications relevant to highly variable nonthermal emission of Sgr A* in radio, IR and X-ray bands.

The STEREO mission: an overview

In February 2006, NASA will launch the twin STEREO spacecraft from Kennedy Space Center aboard a Delta 7925 launch vehicle. After a series of highly eccentric Earth orbits with apogees beyond the moon, each spacecraft will use close flybys of the moon to escape into heliocentric orbits at 1 AU, with one spacecraft trailing Earth and the other leading Earth. As viewed from the Sun, the two spacecraft will separate at approximately 45° per year. The purposes of the STEREO Mission are to understand the causes and mechanisms of coronal mass ejection initiation and to follow the propagation of CMEs through the heliosphere. Additionally, STEREO will study the mechanisms and sites of energetic particle acceleration and develop 3D time-dependent models of the magnetic topology, temperature, density and velocity of the solar wind between the Sun and Earth. To accomplish these goals, each STEREO spacecraft will be equipped with a set of optical, radio and in situ particles and fields instruments. The SECCHI suite of instruments includes two white light coronagraphs covering the range from 1.4 to 15 solar radii, an extreme ultra violet imager covering the chromosphere and inner corona, and two heliospheric white light imagers covering the outer corona from 12 solar radii to 1 AU. The IMPACT suite of instruments will measure in situ solar wind electrons in the energy range from essentially 0 to 100 keV, energetic electrons to 6 MeV, and protons and heavier ions to 100 MeV/nucleon. IMPACT also contains a magnetometer to measure the in situ magnetic field strength and direction. The PLASTIC instrument will measure the composition of heavy ions as well as protons and alpha particles. The SWAVES instrument will use radio waves to track the location of CME-driven shocks and the 3D topology of open field lines along which energetic particles flow. Additionally, SWAVES will measure in situ plasma waves to provide an independent estimate of the local plasma density and temperature. Each of the four instruments will produce a small real-time stream of selected data for purposes of predicting space weather events at Earth, primarily for NOAA forecasters. In addition to the four instrument teams, there will be substantial participation by modeling and theory oriented teams. All STEREO data will be freely available through individual Web sites at the four Principal Investigator institutions as well as at the STEREO Science Center located at NASA Goddard Space Flight Center.