Signatures Of Particle Acceleration Research Articles

Long-term Variability and Correlation Study of the Blazar 3C 454.3 in the Radio, NIR, and Optical Wavebands

Abstract We performed a long-term optical (B, V, R bands), infrared (J and K bands), and radio band (15, 22, 37 GHz band) study of the flat spectrum radio quasar, 3C 454.3, using data collected over a period of more than 8 yr (MJD 54500–57500). The temporal variability, spectral properties, and interwaveband correlations were studied by dividing the available data into smaller segments with more regular sampling. This helped us constrain the size and the relative locations of the emission regions for different wavebands. Spectral analysis of the source revealed the interplay between the accretion disk and jet emission. The source predominantly showed a redder-when-brighter trend, though we observed a bluer-when-brighter trend at high flux levels, which could be a signature of particle acceleration and radiative cooling. Significant correlations with near-zero lag were seen between various optical and infrared bands, indicating that these emission regions are cospatial. Correlations with a time lag of about 10–100 days are seen between the optical/infrared and radio bands indicating these emissions arise from different regions. We also observe the DCF peak lag change from year to year. We try to explain these differences using a curved jet model where the different emission regions have different viewing angles resulting in a frequency-dependent Doppler factor. This variable Doppler factor model explains the variability timescales and the variation in DCF peak lag between the radio and optical emissions in different segments. Lags of 6–180 days are seen between emissions in various radio bands, indicating a core-shift effect.

Three-dimensional magnetic reconnection in a collapsing coronal loop system

Context. Magnetic reconnection is believed to be the primary mechanism by which non-potential energy stored in coronal magnetic fields is rapidly released during solar eruptive events. Unfortunately, owing to the small spatial scales on which reconnection is thought to occur, it is not directly observable in the solar corona. However, larger scale processes, such as associated inflow and outflow, and signatures of accelerated particles have been put forward as evidence of reconnection. Aims. Using a combination of observations we explore the origin of a persistent Type I radio source that accompanies a coronal X-shaped structure during its passage across the disk. Of particular interest is the time range around a partial collapse of the structure that is associated with inflow, outflow, and signatures of particle acceleration. Methods. Imaging radio observations from the Nançay Radioheliograph were used to localise the radio source. Solar Dynamics Observatory (SDO) AIA extreme ultraviolet (EUV) observations from the same time period were analysed, looking for evidence of inflows and outflows. Further mpole magnetic reconstructions using SDO HMI observations allowed the magnetic connectivity associated with the radio source to be determined. Results. The Type I radio source was well aligned with a magnetic separator identified in the extrapolations. During the partial collapse, gradual (1 km s−1) and fast (5 km s−1) inflow phases and fast (30 km s−1) and rapid (80–100 km s−1) outflow phases were observed, resulting in an estimated reconnection rate of ∼0.06. The radio source brightening and dimming was found to be co-temporal with increased soft X-ray emission observed in both Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and Geostationary Operational Environmental Satellite (GOES). Conclusions. We interpret the brightening and dimming of the radio emission as evidence for accelerated electrons in the reconnection region responding to a gradual fall and rapid rise in electric drift velocity, in response to the inflowing and outflowing field lines. These results present a comprehensive example of 3D null-point reconnection.

OBSERVATIONAL EVIDENCE OF PARTICLE ACCELERATION ASSOCIATED WITH PLASMOID MOTIONS

ABSTRACT We report a strong association between the particle acceleration and plasma motions found in the 2010 August 18 solar flare. The plasma motions are tracked in the extreme ultraviolet (EUV) images taken by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory and the Extreme UltraViolet Imager (EUVI) on the Solar Terrestrial Relations Observatory spacecraft Ahead, and the signature of particle acceleration was investigated by using Nobeyama Radioheliograph data. In our previous paper, we reported that in EUV images many plasma blobs appeared in the current sheet above the flare arcade. They were ejected bidirectionally along the current sheet, and the blobs that were ejected sunward collided with the flare arcade. Some of them collided or merged with each other before they were ejected from the current sheet. We discovered impulsive radio bursts associated with such plasma motions (ejection, coalescence, and collision with the post flare loops). The radio bursts are considered to be the gyrosynchrotron radiation by nonthermal high energy electrons. In addition, the stereoscopic observation by AIA and EUVI suggests that plasma blobs had a three-dimensionally elongated structure. We consider that the plasma blobs were three-dimensional plasmoids (i.e., flux ropes) moving in a current sheet. We believe that our observation provides clear evidence of particle acceleration associated with the plasmoid motions. We discuss possible acceleration mechanisms on the basis of our results.

Studying astrophysical collisionless shocks with counterstreaming plasmas from high power lasers

Collisions of high Mach number flows occur frequently in astrophysics, and the resulting shock waves are responsible for the properties of many astrophysical phenomena, such as supernova remnants, Gamma Ray Bursts and jets from Active Galactic Nuclei. Because of the low density of astrophysical plasmas, the mean free path due to Coulomb collisions is typically very large. Therefore, most shock waves in astrophysics are “collisionless”, since they form due to plasma instabilities and self-generated magnetic fields. Laboratory experiments at the laser facilities can achieve the conditions necessary for the formation of collisionless shocks, and will provide a unique avenue for studying the nonlinear physics of collisionless shock waves. We are performing a series of experiments at the Omega and Omega-EP lasers, in Rochester, NY, with the goal of generating collisionless shock conditions by the collision of two high-speed plasma flows resulting from laser ablation of solid targets using ∼10 16 W/cm 2 laser irradiation. The experiments will aim to answer several questions of relevance to collisionless shock physics: the importance of the electromagnetic filamentation (Weibel) instabilities in shock formation, the self-generation of magnetic fields in shocks, the influence of external magnetic fields on shock formation, and the signatures of particle acceleration in shocks. Our first experiments using Thomson scattering diagnostics studied the plasma state from a single foil and from double foils whose flows collide “head-on”. Our data showed that the flow velocity and electron density were 10 8 cm/s and 10 19 cm −3, respectively, where the Coulomb mean free path is much larger than the size of the interaction region. Simulations of our experimental conditions show that weak Weibel mediated current filamentation and magnetic field generation were likely starting to occur. This paper presents the results from these first Omega experiments.

Location of Decimetric Pulsations in Solar Flares

This work investigates the spatial relation between coronal X-ray sources and coherent radio emissions, both generally thought to be signatures of particle acceleration. Two limb events were selected during which the radio emission was well correlated in time with hard X-rays. The radio emissions were of the type of decimetric pulsations as determined from the spectrogram observed by Phoenix-2 of ETH Zurich. The radio positions were measured from observations with the Nancay Radioheliograph between 236 and 432 MHz and compared to the position of the coronal X-ray source imaged with RHESSI. The radio pulsations originated at least 30 - 240 Mm above the coronal hard X-ray source. The altitude of the radio emission increases generally with lower frequency. The average positions at different frequencies are on a line pointing approximately to the coronal hard X-ray source. Thus, the pulsations cannot be caused by electrons trapped in the flare loops, but are consistent with emission from a current sheet above the coronal source.

MULTIWAVELENGTH SIGNATURES OF MAGNETIC ACTIVITY FROM YOUNG STELLAR OBJECTS IN THE LkHα 101 CLUSTER

[abridged] We describe the results of our multi-wavelength observing campaign on the young stellar objects in the LkH$\alpha$101 cluster. Our simultaneous X-ray and multi-frequency radio observations are unique in providing simultaneous constraints on short-timescale variability at both wavelengths as well as constraints on the thermal or nonthermal nature of radio emission from young stars. Focussing in on radio-emitting objects and the multi-wavelength data obtained for them, we find that multi-frequency radio data indicate nonthermal emission even in objects with infrared evidence for disks. We find radio variability on timescales of decades, days and hours. About half of the objects with X-ray and radio detections were variable at X-ray wavelengths, despite lacking large-scale flares or large variations. Variability appears to be a bigger factor affecting radio emission than X-ray emission. A star with infrared evidence for a disk, [BW88]~3, was observed in the decay phase of a radio flare. In this object and another ([BW88]~1), we find an inverse correlation between radio flux and spectral index which contrasts with behavior seen in the Sun and active stars. We interpret this behavior as the repopulation of the hardest energy electrons due to particle acceleration. A radio and X-ray source lacking an infrared counterpart, [BW88]~1, may be near the substellar limit; its radio properties are similar to other cluster members, but its much higher radio to X-ray luminosity ratio is reminiscent of behavior in nearby very low mass stars/brown dwarfs. We find no correspondence between signatures of particle acceleration and those of plasma heating, both time-averaged and time-variable. The lack of correlated temporal variability in multi-wavelength behavior, the breakdown of multi-wavelength correlations of...

High‐Energy Emission and Cosmic Rays from Gamma‐Ray Bursts

This paper is devoted to the analysis of particle acceleration in gamma-ray bursts (GRBs) and its radiative consequences. Therefore, we get on one hand constraints on the physics and on the other hand possible signatures of particle acceleration that could be recorded by new gamma-ray instruments. In a recent paper we have shown that UHECRs can be generated in GRBs even with conservative assumptions about the magnetic field and the scattering capability of its perturbations, provided that a suitable relativistic Fermi process is at work during the so-called internal shock phase. In this paper we extend the analysis of the consequences of these assumptions to the whole prompt emission of both electrons and protons. Indeed, assuming that the magnetic field decays as 1/r2 and that the scattering time of particles is longer than the Bohm assumption, in particular, with a rule derived from Kolmogorov scaling, we show that the four following events naturally happen with no other parameter adaptation than the intensity of the magnetic field, which turns out to be subequipartition: (1) UHECRs can be generated with a sufficient flux (≃1 km-2 yr-1) within the GZK sphere to account for the CR spectrum at the ankle. (In the previous paper, we showed that the associated pγ-neutrino emission is tiny.) (2) A thermal component below the so-called Epeak is often unavoidable and even amplified when the shocks start before the photosphere. (3) The CRs could radiate gamma rays around 67 MeV (in the comoving frame, which implies ≃20 GeV for the observer) due to π0 decay and a low-energy neutrino emission (around 0.2 GeV) associated with neutron decay and also neutrinos of energy between 5 and 150 GeV due to muon decay (as predicted in the previous paper). (4) The UHECRs radiate high-energy gamma rays between a few hundred MeV and 10 GeV (taking the pair creation process into account) due to their synchrotron emission, with a sufficient flux to be observable.

Signatures of particle acceleration at SN 1987A

At least eight experiments are currently monitoring SN 1987A for photons or neutrinos with energies in the TeV range or higher. Observation of such signals would indicate that the supernova is the site of acceleration of charged particles to even higher energies. The way that the acceleration might occur is discussed, and the prospects for detection of such signals in light of current limits on power in the supernova from sources other than the nickel-cobalt-iron decay chain are evaluated. It is pointed out that signals of particle acceleration may show up in continuum MeV gamma rays as well as in the 100 MeV range. 57 refs.