Radio Emission Mechanism Research Articles

Self-consistent particle-in-cell simulations of fundamental and harmonic plasma radio emission mechanisms

Aims. The simulation of three-wave interaction based plasma emission, thought to be the underlying mechanism for Type III solar radio bursts, is a challenging task requiring fully-kinetic, multi-dimensional models. This paper aims to resolve a contradiction in past attempts, whereby some studies indicate that no such processes occur. Methods. We self-consistently simulate three-waved based plasma emission through all stages by using 2D, fully kinetic, electromagnetic particle-in-cell simulations of relaxing electron beams using the EPOCH2D code. Results. Here we present the results of two simulations; Run 1 (nb/n0 = 0.0057, vb/{\Delta}vb = vb/Ve = 16) and Run 2 (nb/n0 = 0.05, vb/{\Delta}vb = vb/Ve = 8), which we find to permit and prohibit plasma emission respectively. We show that the possibility of plasma emission is contingent upon the frequency of the initial electrostatic waves generated by the bump-in-tail instability, and that these waves may be prohibited from participating in the necessary three-wave interactions due to frequency conservation requirements. In resolving this apparent contradiction through a comprehensive analysis, in this paper we present the first self-consistent demonstration of fundamental and harmonic plasma emission from a single-beam system via fully kinetic numerical simulation. We caution against simulating astrophysical radio bursts using unrealistically dense beams (a common approach which reduces run time), as the resulting non-Langmiur characteristics of the initial wave modes significantly suppresses emission. Comparison of our results also indicates that, contrary to the suggestions of previous authors, an alternative plasma emission mechanism based on two counter-propagating beams is unnecessary in an astrophysical context.

Radio Pulsars

Almost 50 years after radio pulsars were discovered in 1967, our understanding of these objects remains incomplete. On the one hand, within a few years it became clear that neutron star rotation gives rise to the extremely stable sequence of radio pulses, that the kinetic energy of rotation provides the reservoir of energy, and that electromagnetic fields are the braking mechanism. On the other hand, no consensus regarding the mechanism of coherent radio emission or the conversion of electromagnetic energy to particle energy yet exists. In this review, we report on three aspects of pulsar structure that have seen recent progress: the self-consistent theory of the magnetosphere of an oblique magnetic rotator; the location, geometry, and optics of radio emission; and evolution of the angle between spin and magnetic axes. These allow us to take the next step in understanding the physical nature of the pulsar activity.

COORDINATED X-RAY, ULTRAVIOLET, OPTICAL, AND RADIO OBSERVATIONS OF THE PSR J1023+0038 SYSTEM IN A LOW-MASS X-RAY BINARY STATE

The PSR J1023+0038 binary system hosts a neutron star and a low-mass, main-sequence-like star. It switches on year timescales between states as an eclipsing radio millisecond pulsar and a low-mass X-ray binary (LMXB). We present a multi-wavelength observational campaign of PSR J1023+0038 in its most recent LMXB state. Two long XMM-Newton observations reveal that the system spends ~70% of the time in a ≈3 × 10^(33) erg s^(−1) X-ray luminosity mode, which, as shown in Archibald et al., exhibits coherent X-ray pulsations. This emission is interspersed with frequent lower flux mode intervals with ≈5 x 10^(32) erg s^(−1) and sporadic flares reaching up to ≈10^(34) erg s^(−1), with neither mode showing significant X-ray pulsations. The switches between the three flux modes occur on timescales of order 10 s. In the UV and optical, we observe occasional intense flares coincident with those observed in X-rays. Our radio timing observations reveal no pulsations at the pulsar period during any of the three X-ray modes, presumably due to complete quenching of the radio emission mechanism by the accretion flow. Radio imaging detects highly variable, flat-spectrum continuum radiation from PSR J1023+0038, consistent with an origin in a weak jet-like outflow. Our concurrent X-ray and radio continuum data sets do not exhibit any correlated behavior. The observational evidence we present bears qualitative resemblance to the behavior predicted by some existing propeller and trapped disk accretion models although none can account for key aspects of the rich phenomenology of this system.

Axisymmetric force-free magnetosphere of a pulsar – II. Transition from the self-consistent two-fluid model

The self-consistent two-fluid model of the pulsar magnetosphere is considered. We concentrate on the case of vanishingly small inertia of the particles. Our approach allows to obtain the realistic particle distributions sustaining the force-free magnetic field configuration of a monopolar structure. The result differs substantially from the customary picture of the radial speed-of-light motion of massless particles. In our case, the electron and positron constituents follow slightly curved trajectories and are characterized by definite number densities and distinct velocities. The velocity shift is determined by the first-order longitudinal electric field, which appears the necessary ingredient of the self-consistent two-fluid model and implies the parallel conductivity of the order of the inverse particle mass. Our model is believed to be a proper context to describe radiation processes in the pulsar magnetosphere, including both the radio and high-energy emissions. The velocity shift is suggestive of the two-stream instability which may underlie the pulsar radio emission mechanism. The differential rotation of the particle flow may cause the diochotron instability expected to be responsible for the radio subpulse phenomenon. The connection between the radio and high-energy emissions of pulsars is predicted as well.

Absorption of Magnetosonic Waves in the Crust of Neutron Stars. Radio Emission from Pulsars

The kinetic equation for plasma in the crust of neutron star is solved in the relaxation time approximation. Values of the kinetic coefficients are obtained for crust plasma? with densities in the range of 102- 1014 g/cm3. The propagation of magnetosonic waves in the crust of neutron star is examined with various mechanisms for the dissipation of the wave energy taken into account. It is shown that the crust of neutron star are transparent for magnetosonic waves with frequencies ω ≤ 1011 Hz and that the wave absorption is lower for stars with higher central densities. A mechanism for radio emission by pulsars is proposed in which the source is on the star’s surface and has dimensions equal to the cross section of the beam of magnetosonic waves. Some features of the radio emission (spectral index and pulsar cutoff) are explained.

Method for high precision reconstruction of air showerXmaxusing two-dimensional radio intensity profiles

The mass composition of cosmic rays contains important clues about their origin. Accurate measurements are needed to resolve long-standing issues such as the transition from Galactic to extragalactic origin, and the nature of the cutoff observed at the highest energies. Composition can be studied by measuring the atmospheric depth of the shower maximum Xmax of air showers generated by high-energy cosmic rays hitting the Earth's atmosphere. We present a new method to reconstruct Xmax based on radio measurements. The radio emission mechanism of air showers is a complex process that creates an asymmetric intensity pattern on the ground. The shape of this pattern strongly depends on the longitudinal development of the shower. We reconstruct Xmax by fitting two-dimensional intensity profiles, simulated with CoREAS, to data from the LOFAR radio telescope. In the dense LOFAR core, air showers are detected by hundreds of antennas simultaneously. The simulations fit the data very well, indicating that the radiation mechanism is now well-understood. The typical uncertainty on the reconstruction of Xmax for LOFAR showers is 17 g/cm^2.

The mechanism of coherent radio emission in some classes of pulsar

Radio luminosities have been estimated from published data for a well-defined homogeneous set of 29 normal pulsars. The radio-frequency energies per unit charge in the primary accelerated particle beam are given for each pulsar and form a distribution more than two orders of magnitude in width. The values found show that pulsars are extremely efficient generators of radiation below 1 - 10 GHz given the kinematic constraints which are obtained here in the cases of electron-positron and ion-proton plasmas. Our conclusion is that only the ion-proton plasma source is consistent with the spectra of normal and millisecond pulsars and we list and describe the factors which together support this conclusion.

The importance of source positions during radio fine structure observations

The measurement of positions and sizes of radio sources in the observations of the fine structure of solar radio bursts is a determining factor for the selection of the radio emission mechanism. The identical parameters describing the radio sources for zebra structures (ZSs) and fiber bursts confirm there is a common mechanism for both structures. It is very important to measure the size of the source in the corona to determine if it is distributed along the height or if it is point-like. In both models of ZSs (the double plasma resonance (DPR) and the whistler model) the source must be distributed along the height, but by contrast to the stationary source in the DPR model, in the whistler model the source should be moving. Moreover, the direction of the space drift of the radio source must correlate with the frequency drift of stripes in the dynamic spectrum. Some models of ZSs require a local source, for example, the models based on the Bernstein modes, or on explosive instability. The selection of the radio emission mechanism for fast broadband pulsations with millisecond duration also depends on the parameters of their radio sources.

Inhibition of type III radio emissions due to the interaction between two electron beams: Observations and simulations

We report the peculiar interaction of two type III bursts observed in the solar wind. As electron beams propagating on the same magnetic field lines cross, a spectacular depletion of the type III radio emission is observed. We combine observations from the WAVES experiment on board the STEREO mission together with kinetic plasma simulations to study the extinction of type III radio emission resulting from the interaction between two electron beams. The remote observations enable to follow the electron beams in the interplanetary medium and show that the level of radiated radio waves is recovered after the beam crossing. The in situ observations of beam‐driven Langmuir waves give evidence for Langmuir decay. The density fluctuations are extracted from in situ observations. The velocity of the beams is independently evaluated from in situ observations of decaying Langmuir waves and remote radio observations. The kinetic simulations show that the level of beam‐driven Langmuir waves is reduced as the two beams cross. We show that the slow beam induced a strong reduction of the quasilinear relaxation of the fast beam, limiting the amplitude of the generated Langmuir waves. Moreover, in the case of two electron beams, the lack of Langmuir wave coherence reduces the efficiency of the Langmuir parametric decay. We thus conclude that the observed depletion of the type III radio emission is independent of the radio emission mechanism, as long as it depends on the Langmuir amplitude and coherence.

THE AUGER ENGINEERING RADIO ARRAY

The Auger Engineering Radio Array currently measures MHz radio emission from extensive air showers induced by high energy cosmic rays with 24 self-triggered radio detector stations. Its unique site, embedded into the baseline detectors and extensions of the Pierre Auger Observatory, allows to study air showers in great detail and to calibrate the radio emission. In its final stage AERA will expand to an area of approximately 20km<sup>2</sup> to explore the feasibility of the radio-detection technique for future cosmic-ray detectors. The concept and hardware design of AERA as well as strategies to enable self-triggered radio detection are presented. Radio emission mechanisms are discussed based on polarization analysis of the first AERA data.

THE AUGER ENGINEERING RADIO ARRAY

The Auger Engineering Radio Array currently measures MHz radio emission from extensive air showers induced by high energy cosmic rays with 24 self-triggered radio detector stations. Its unique site, embedded into the baseline detectors and extensions of the Pierre Auger Observatory, allows to study air showers in great detail and to calibrate the radio emission. In its final stage AERA will expand to an area of approximately 20km 2 to explore the feasibility of the radio-detection technique for future cosmic-ray detectors. The concept and hardware design of AERA as well as strategies to enable self-triggered radio detection are presented. Radio emission mechanisms are discussed based on polarization analysis of the first AERA data.

DO RADIO MAGNETARS PSR J1550–5418 AND J1622–4950 HAVE GIGAHERTZ-PEAKED SPECTRA?

We study the radio spectra of two magnetars PSR J1550-5418 and J1622-4950. We argue that they are good candidates for the pulsars with gigahertz-peaked spectra as their observed flux density decreases at frequencies below 7 GHz. We suggest this behavior is due to the influence of pulsars' environments on the radio waves. Both of the magnetars are associated with supernova remnants, thus are surrounded by a hot, ionized gas, which can be responsible for the free-free absorption of radio-waves. We conclude that the GPS feature of both magnetars and typical pulsars are formed by similar processes in the surrounding media, rather than by different radio-emission mechanisms. Thus, the radio magnetars PSR J1550-5418 and J1622-4950 can be included in the class of the gigahertz-peaked spectra pulsars.

The air shower maximum probed by Cherenkov effects from radio emission

Radio detection of cosmic-ray-induced air showers has come to a flight the last decade. Along with the experimental efforts, several theoretical models were developed. The main radio-emission mechanisms are established to be the geomagnetic emission due to deflection of electrons and positrons in Earth’s magnetic field and the charge-excess emission due to a net electron excess in the air shower front. It was only recently shown that Cherenkov effects play an important role in the radio emission from air showers. In this article we show the importance of these effects to extract quantitatively the position of the shower maximum from the radio signal, which is a sensitive measure for the mass of the initial cosmic ray. We also show that the relative magnitude of the charge-excess and geomagnetic emission changes considerably at small observer distances where Cherenkov effects apply.

High-frequency cutoff and change of radio emission mechanism in pulsars

Pulsars are fast rotating neutron stars with a strong magnetic field, that emit over a wide frequency range. In spite of all efforts during the 40 years after the discovery of pulsars, the mechanism of their radio emission so far remains unknown. We propose a new approach to solving this problem for a subset of pulsars with a high-frequency cutoff of the spectrum from the Pushchino catalogue (the “Pushchino” sample). We provide a theoretical explanation of the observed dependence of the high-frequency cutoff on the pulsar period, and we predict the dependence of the cutoff position from the magnetic field. This explanation is based on a new mechanism for electron radio emission in pulsars. Namely, radiation occurs in the inner (polar) gap, when electrons are accelerated in the electric field that is increasing from zero level at the star surface. In this case the acceleration of electrons passes through a maximum and goes to zero when the electron velocity approaches the speed of light. All the radiated power is located within the radio frequency band. The averaging of radiation intensity over the polar cap, with some natural assumptions of the coherence of the radiation, leads to the observed spectra. It also leads to an acceptable estimate of the power of radio emission.

Radio Measurements of Air Showers with LOPES

LOPES is a digital antenna array for the radio measurement of cosmic-ray air showers at energies around 1017 eV. It is triggered by the KASCADE-Grande air-shower array at the Karlsruhe Institute of Technology (KIT), Germany. Because of an absolute amplitude calibration and a sophisticated data analysis, LOPES can test models for the radio emission to an up-to-now unachieved level, thus improving our understanding of the radio emission mechanisms. Recent REAS simulations of the air-shower radio emission come closer to the measurements than any previously tested simulations. We have determined the radio-reconstruction precision of interesting air-shower parameters by comparing LOPES reconstructions to both REAS simulations and KASCADE-Grande measurements, and present our latest results for the angular resolution, the energy and the Xmax reconstruction based on the radio measurement of about 500 air showers. Although the precision of LOPES is limited by the high level of anthropogenic noise at KIT, it opens a promising perspective for next-generation radio arrays in regions with a lower ambient noise level.

Anomalous Microwave Emission from Star Forming Regions

The evidence for microwave emission from spinning dust grains has been strengthened considerably by its detection in a number of discrete astrophysical objects associated with star formation. These detections, in combination with statistical constraints on its presence on large angular scales in the diffuse ISM, have provided strong observational confirmation of an emission mechanism still referred to as anomalous. This emission has a peaked spectrum with a maximum in the microwave band; the present review discusses the continuum radio emission mechanisms which can contribute to this region of the electromagnetic spectrum, collects published results on the prevalence of anomalous microwave emission in a variety of star formation regions, presents the overall conclusions that may be drawn from the detections so far, and discusses the prospects for future research on the anomalous microwave emission attributed to spinning dust within star forming regions.

MULTI-EPOCH OBSERVATIONS OF THE RED WING EXCESS IN THE SPECTRUM OF 3C 279

It has been previously determined that there is a highly significant correlation between the spectral index from 10 GHz to 1350 $\AA$ and the amount of excess luminosity in the red wing of quasar CIV $\lambda$1549 broad emission lines (BELs). Ostensibly, the prominence of the red excess is associated with the radio jet emission mechanism and is most pronounced for lines of sight close to the jet axis. Studying the scant significant differences in the UV spectra of radio loud and radio quiet quasars might provide vital clues to the origin of the unknown process that creates powerful relativistic jets that appear in only about ten percent of quasars. In this study, the phenomenon is explored with multi-epoch observations of the MgII $\lambda$2798 broad line in 3C 279 which has one of the largest known redwing excesses in a quasar spectrum. The amount of excess that is detected appears to be independent of all directly observed optical continuum, radio or submm properties (fluxes or polarizations). The only trend that occurs in this sparse data is: the stronger the BEL, the larger the fraction of flux that resides in the redwing. It is concluded that more monitoring is needed and spectropolarimetry with a large telescope is essential during low states to understand more.

DETECTION OF RADIO EMISSION FROM THE HYPERACTIVE L DWARF 2MASS J13153094–2649513AB

We report the detection of radio emission from the unusually active L5e + T7 binary 2MASS J13153094-2649513AB made with the Australian Telescope Compact Array. Observations at 5.5 GHz reveal an unresolved source with a continuum flux of 370+/-50 microJy, corresponding to a radio luminosity of L_rad = nuL_nu = (9+/-3)x10^23 erg/s and log10(L_rad/L_bol) = -5.44+/-0.22. No detection is made at 9.0 GHz to a 5 sigma limit of 290 microJy, consistent with a power law spectrum S_nu ~ nu^-a with a > 0.5. The emission is quiescent, with no evidence of variability or bursts over 3 hr of observation, and no measurable polarization (V/I < 34%). 2MASS J1315-2649AB is one of the most radio-luminous ultracool dwarfs detected in quiescent emission to date, comparable in strength to other cool sources detected in outburst. Its detection indicates no decline in radio flux through the mid-L dwarfs. It is unique among L dwarfs in having strong and persistent Halpha and radio emission, indicating the coexistence of a cool, neutral photosphere (low electron density) and a highly active chromosphere (high electron density and active heating). These traits, coupled with the system's mature age and substellar secondary, makes 2MASS J1315-2649AB an important test for proposed radio emission mechanisms in ultracool dwarfs.

Microwave and radio emission of dusty star-forming galaxies: implication for the cosmic radio background

We use the most up-to-date cosmological evolution models of star-forming (SF) galaxies and radio sources to compute the extragalactic number counts and the cosmic background from 408MHz to 12THz. The model of SF galaxies reproduces the constraints obtained by Spitzer, Herschel, and ground-based submm/mm experiments: number counts, redshift distribution of galaxies, cosmic background intensity and anisotropies. The template SEDs of this model are extrapolated to the radio adding synchrotron, free-free, and spinning dust emissions. To fix the synchrotron intensity, we use the IR/radio flux ratio, q70, and a spectral index beta=-3. For a constant q70, our model added to the AGN contribution provides a good fit to the number counts from 12THz to 408MHz and to the CIB. Spinning dust accounts for up to 20% of the cosmic microwave background produced by SF galaxies, but for less than 10% of the total background when AGN are included. The SF galaxies account for 77.5% of the number counts at 1.4GHz for a flux of 1e-4Jy. However, the model does not explain the CRB measured with the ARCADE2 experiment. Considering the case when q70 decreases strongly with redshift, this still does not explain the ARCADE2 results. It also yields to an overestimate of the low-flux number counts in the radio. Thus, we rule out a steep variation of q70 with redshift at least for z<3.5. Adding a population of faint SF galaxies at high redshift (Lir<1e11Lsun and 4<z<6), which would reproduce the ARCADE2 results, leads to predictions of the CIB much higher than what is observed, ruling out this as the explanation for the ARCADE2 results. Considering our findings and previous studies, we conclude that if the radio emission measured by ARCADE2 is astrophysical in origin, it has to originate in the Galaxy or in a new kind of radio sources (with no mid- to far-IR counterparts) or emission mechanism still to be discovered.

Radio Emission From Low Mass Young Stellar Objects

AbstractCompact radio emission provides a reliable method for the detection of low luminosity young stellar objects (YSOs), and is particularly useful for detecting the earliest stages of protostellar evolution where the source itself may still be heavily embedded in its natal dust envelope. For such Class 0 and Class I objects the dominant radio emission mechanism is expected to be free-free, however unlike massive YSOs the way in which this radio emission is produced remains a subject of debate. As larger samples of radio YSOs become available the relationship between the radio luminosity of the Class 0/I population and their wider global properties is now being clarified. Furthermore, the broader scientific applications of such samples are also becoming increasingly apparent. These improved constraints on the nature of the radio emission from YSOs are now contributing to our understanding of not only the evolutionary physics of protostars themselves but also their wider impact on their surroundings. He...