Synchrotron Radiation Of Relativistic Electrons Research Articles

Ball lightning and unidentified flying objects (UFOs)

Descriptions of observations of large luminous objects (UFOs) are given. Particularly valuable is information about these objects obtained by pilots who saw them at close range. These objects are capable of flying at airplane speeds and emitting cone-shaped beams of light. Sometimes these rays break off before reaching the ground. When the beam hits a hard surface, it crumbles into “sparkles”. If there is close contact with a UFO, the aircraft's electronic equipment fails. Sometimes these objects “land” on the ground and take off, moving faster than the speed of sound. Objects remain invisible on the radar screen operating in the decimeter range. A hypothesis of the UFO device has been proposed. It is believed that UFO is an ensemble of ball lightning located in a common shell. The reason for the generation of cone beams of light is the synchrotron radiation of relativistic electrons that are part of the nuclei of ball lightning. Objects are capable of moving at supersonic speeds due to the emission of charged micro-clusters of water from their surface.

Resolving acceleration to very high energies along the jet of Centaurus A.

The nearby radio galaxy Centaurus A belongs to a class of active galaxies that are luminous at radio wavelengths. Most show collimated relativistic outflows known as jets, which extend over hundreds of thousands of parsecs for the most powerful sources. Accretion of matter onto the central supermassive black hole is believed to fuel these jets and power their emission1. Synchrotron radiation from relativistic electrons causes the radio emission, and it has been suggested that the X-ray emission from Centaurus A also originates in electron synchrotron processes2-4. Another possible explanation is inverse Compton scattering with cosmic microwave background (CMB) soft photons5-7. Synchrotron radiation needs ultrarelativistic electrons (about 50teraelectronvolts) and, given their short cooling times, requires some continuous re-acceleration mechanism8. Inverse Compton scattering, on the other hand, does not require very energetic electrons, but the jets must stay highly relativistic on large scales (exceeding 1megaparsec). Some recent evidence disfavours inverse Compton-CMB models9-12, although other work seems to be compatible with them13,14. In principle, the detection of extended γ-ray emission, which directly probes the presence of ultrarelativistic electrons, could distinguish between these options. At gigaelectronvolt energies there is also an unusual spectral hardening15,16 in Centaurus A that has not yet been explained. Here we report observations of Centaurus A at teraelectronvolt energies that resolve its large-scale jet. We interpret the data as evidence for the acceleration of ultrarelativistic electrons in the jet, and favour the synchrotron explanation for the X-rays. Given that this jet is not exceptional in terms of power, length or speed, it is possible that ultrarelativistic electrons are commonplace in the large-scale jets of radio-loud active galaxies.

Gamma-Ray Burst Spectrum with a Time-dependent Injection Rate of High-energy Electrons

Abstract Although the physical origin of prompt emission in gamma-ray bursts (GRBs) remains inconclusive, previous studies have considered the synchrotron radiation of relativistic electrons as a promising mechanism. These works usually adopted an invariable injection rate of electrons (Q) which may be discordant with that in a Poynting-flux-dominated jet. In a Poynting-flux-dominated jet (e.g., internal-collision-induced magnetic reconnection and turbulence model), the number of magnetic reconnections occurring simultaneously may grow rapidly with time and result in an increase of Q with time. This paper is dedicated to studying the synchrotron radiation spectrum in this scenario. It is found that the radiation spectrum would obviously get harder if an increasing Q is adopted and a Band-like radiation spectrum can be obtained if the increase of Q is fast enough. The latter is related to the fact that a bump shape rather than a power-law spectrum appears in the low-energy regime of the obtained electron spectrum. This effect can strongly harden the low-energy radiation spectrum. It indicates that an increasing Q can help to alleviate the “fast-cooling problem” of synchrotron radiation for GRBs. Our studies also reveal that a Poynting-flux dominated jet with a large emission radius, a short magnetic reconnection region length, or an injected electron with low minimum energy would prefer to form a Band-like radiation spectrum. We suggest that the Band spectrum found in GRBs may be the synchrotron emission of the electrons with a bump-shape distribution in its low-energy regime.

Fast-cooling synchrotron radiation in a decaying magnetic field and γ-ray burst emission mechanism

Synchrotron radiation of relativistic electrons is an important radiation mechanism in many astrophysical sources. In the sources where the synchrotron cooling timescale is shorter than the dynamical timescale, electrons are cooled down below the minimum injection energy. It has been believed that such ‘fast cooling’ electrons have a power-law distribution in energy with an index −2, and their synchrotron radiation has a photon spectral index1−1.5. On the other hand, in a transient expanding astrophysical source, such as a γ-ray burst (GRB), the magnetic field strength in the emission region continuously decreases with radius. Here we study such a system, and find that in a certain parameter regime, the fast-cooling electrons can have a harder energy spectrum. We apply this new physical regime to GRBs, and suggest that the GRB prompt emission spectra whose low-energy photon spectral index has a typical value2,3,4,5−1 could be due to synchrotron radiation in this moderately fast-cooling regime.

X-ray emission from hot accretion flows

AbstractRadiatively inefficient, hot accretion flows are widely considered as a relevant accretion mode in low-luminosity AGNs. We study spectral formation in such flows using a refined model with a fully general relativistic description of both the radiative (leptonic and hadronic) and hydrodynamic processes, as well as with an exact treatment of global Comptonization. We find that the X-ray spectral index–Eddington ratio anticorrelation as well as the cut-off energy measured in the best-studied objects favor accretion flows with rather strong magnetic field and with a weak direct heating of electrons. Furthermore, they require a much stronger source of seed photons than considered in previous studies. The nonthermal synchrotron radiation of relativistic electrons seems to be the most likely process capable of providing a sufficient flux of seed photons. Hadronic processes, which should occur due to basic properties of hot flows, provide an attractive explanation for the origin of such electrons.

Origin of power-law X-ray emission in the steep power-law state of X-ray binaries

We present a new explanation for the origin of the steep power-law (SPL) state of X-ray binaries. The power-law component of X-ray emission is the synchrotron radiation of relativistic electrons in highly magnetized compact spots orbiting near the inner stable circular orbit of a black hole. It has a hard spectrum that extends to above MeV energies, which is determined by the electron acceleration rate. These photons are then down-scattered by the surrounding plasma to form an observed steep spectrum. We discuss the relevance of the model to high-frequency quasi-periodic oscillations and the extremely high luminosity of the SPL state.

ORIGIN OF THE X-RAYS AND POSSIBLE GeV-TeV EMISSION FROM THE WESTERN HOT SPOT OF PICTOR A

Pictor A is a nearby Fanaroff-Riley class II (FR II) radio galaxy with a bright hot spot, the western hot spot. Observation of high polarization in the optical emission of the hot spot indicates that the optical emission could be synchrotron radiation of relativistic electrons in the hot spot. These electrons may be able to produce high energy gamma-ray photons through inverse Compton (IC) scattering. We use single-zone and multi-zone synchrotron + synchrotron-self-Compton (SSC) models to fit the observed spectral energy distribution (SED) from the radio to the X-ray band of the hot spot. Our esults show that in the case of a much weaker magnetic field strength than the equipartition magnetic field, both the single-zone and multi-zone models can fit the SED, but the multi-zone model significantly improves the fit. The two models predict the hot spot as a GeV-TeV source, which might be marginally detectable with Fermi/LAT and HESS. The inverse Compton scattering of cosmic microwave background (IC/CMB) is also considered, but its contribution to GeV-TeV emission is negligible. Note that under the equipartition condition, the SED can also be fit with the multi-zone model, but the predicted flux at 10^22 Hz is too weak to be detectable. The detection of TeV gamma-rays from this FR II radio galaxy, if confirmed, would establish a new subclass of extragalactic source in this energy regime since most of the AGNs detected to date at TeV energies are high-energy-peaked BL Lac objects.

Infrared polarization and variability of blazarst

Correlation between the polarizations in the near-infrared and optical wavebands is studied with simultaneous observational data of blazars. It is found that the degree and angle of polarization in the near-infrared are correlated strongly with those in the optical. And so are the radiation fluxes at the two wavebands. It follows that the near-infrared radiation and polarization of blazars are mainly from synchrotron radiation of relativistic electrons, and come from the same region as the optical waveband. Correlations between the three near-infrared color indices J-H, H-K and J-K are also studied using the 2MASS data. It is found that both J-H and H-K are strongly correlated with J-K, while there is no correlation between J-H and H-K. Lastly, we studied the color index magnitude relationship and found that the near-infrared spectrum gets flatter as the source gets brighter.

The influence of jet geometry on light curves and spectra of GRB afterglows

We have performed detailed calculations of spectra and light curves of GRB afterglows assuming that the observed GRBs can have a jet geometry. The calculations are based on an expanding relativistic shock GRB afterglow model where the afterglow is the result of synchrotron radiation of relativistic electrons with power-law energy distribution at the front of external shock being decelerated in a circumstellar medium. To determine the intensity on the radiation surface we numerically solve the full time-, angle- and frequency-dependent special relativistic transfer equation in the comoving frame using the method of long characteristics.

SECONDARY ELECTRONS IN CLUSTERS OF GALAXIES AND GALAXIES

We investigate the role of secondary electrons in galaxy clusters and in ultra-luminous infrared galaxies (ULIGs). The radio emission in galaxy clusters and ULIGs is believed to be produced by the synchrotron radiation of relativistic electrons. Nonetheless, the sources of these relativistic electrons are still unclear. Relativistic secondary electrons can be produced from the hadronic interactions of cosmic-ray nuclei with the intra-cluster media (ICM) of galaxy clusters and the dense molecular clouds of ULIGs. We estimate the contribution of the secondary electrons in galaxy clusters and ULIGs by comparing observational results with theoretical calculations for the radio emission in these sources. We find that the radio halos of galaxy clusters can not be produced from the secondary electrons; on the other hand, at least for some ULIGs, the radio emission can be dominated by the synchrotron emission of the secondary electrons.

The multifrequency variability of Mrk 421

We present a model of the multifrequency variability of the blazar Mrk 421. The model explains correlated variability observed from Very High Energy (VHE) gamma rays to radio frequencies. We assume that the dominant part of the stationary emission from the radio frequencies to the X-rays is generated by the synchrotron radiation of relativistic electrons ejected from the central engine. The particles move from the center of the source with relativistic velocities and form an inhomogeneous jet. We perform detailed calculations of the radiation transfer and calculate evolution of the electron energy spectrum along the jet. We explain the observed variability by the evolving synchrotron and Inverse-Compton (IC) radiation of a compact component (a blob) which travels along the jet. Two scenarios have been considered as mechanisms to generate VHE flares. The first scenario assumes that the high energy electrons, necessary for generation of the VHE flares, are injected into the jet, directly from the central engine or from an acceleration zone (e.g., a shock wave). The second scheme assumes that the high energy electrons are generated in situ by acceleration, for example by diffusive shock waves or a localized turbulence inside the jet. The particles evolve along the jet. They are cooled by the radiative processes and by the adiabatic expansion which compete with the acceleration process and the injection of high energy electrons. We present new observations we obtained in the radio domain for Mrk 421. The radio data gathered in February–April 2001 show a well defined radio outburst which corresponds to an X-ray outburst observed by RXTE-ASM and a gamma-ray flare detected by HEGRA in the TeV range. The best of our knowledge, this is the first direct observational evidence for a flare observed simultaneously in the radio range and at very high energies. Our scenario with acceleration of electrons in the middle part of the jet describes well the temporal evolution of such multispectral flare.

Modelling of the non-thermal flares in the Galactic microquasar GRS 1915+105

The microquasars recently discovered in our Galaxy offer a unique opportunity for a deep insight into the physical processes in relativistic jets observed in different source populations. We study the temporal and spectral evolution of the radio flares detected from the relativistic ejecta in the microquasar GRS 1915+105, and propose a model that suggests that these flares are caused by synchrotron radiation of relativistic electrons suffering radiative, adiabatic and energy-dependent escape losses in fast-expanding plasmoids (radio clouds). Analytical solutions to the kinetic equation for relativistic electrons in the expanding magnetized clouds are found, and the synchrotron radiation of these electrons is calculated. Detailed comparison of the calculated radio fluxes with the ones detected from the prominent flare of GRS 1915+105 during 1994 March/April provides conclusive information on the basic parameters in the ejecta, such as the absolute values and temporal evolution of the magnetic field, the speed of expansion, the rates of continuous injection of relativistic electrons into and their energy-dependent escape from the clouds, etc. The data from radio monitoring of the pair of resolved ejecta enable unambiguous determination of the parameters of the bulk motion of the counter-ejecta and the degree of asymmetry between them, and also contain important information on the prime energy source for accelerated electrons. These data allow us, in principle, to distinguish between the scenarios of bow-shock powered and relativistic magnetized wind powered plasmoids. Assuming that the electrons in the ejecta can be accelerated up to very high energies, we also calculate the fluxes of the synchrotron and inverse Compton components of the radiation that could be expected during the flares in the broad band from radio frequencies to very high-energy γ-rays.

The spectral flattening of the low-energy component in gamma-ray bursts

Observations from BATSE show that the spectra of gamma-ray bursts are very wide, which cannot be described by a simple law. The distribution of the peak energy Ep [defined as the maximum of the Eγ2N(Eγ) spectrum], and of the spectral indices below and above Ep, is wide. In particular, the slope below Ep lies mainly between 0.5 and 1, which is inconsistent with the 4/3 predicted by the simple synchrotron radiation spectrum. Here we discuss the emission model for gamma-ray bursts based on the shock model proposed by Mészáros & Rees. We find that the synchrotron radiation of relativistic electrons accelerated by a strong shock, together with the finite lifetime of synchrotron radiation and the non-uniform magnetic field distribution, can explain the gamma-ray burst spectra, and the spectral index below the peak energy Ep can be flattened by the non-uniform magnetic field distribution; this flattening is insensitive to the specific form of the magnetic field variance. Furthermore, the peak energy Ep ∝ Γ4 is sensitively dependent on the bulk Lorentz factor.

The Radio Spectrum of Sgr A∗

We discuss the radio spectrum of Sgr A∗ in the frequency range between ≈ 1 GHz and ≈ 1 000 GHz, show that it can be explained by optically thin synchrotron radiation of relativistic electrons, and point toward a possible correlation between the spectrum of Sgr A∗ and larger-scale (≲ 50 pc) radio emission from the Galactic Center region.

Nonthermal radio emission from hot star winds

Nonthermal radio emission has been observed from some of the most luminous hot star winds. It is understood to be synchrotron radiation of the relativistic electrons in the winds. To understand how the electrons are accelerated to such high energies and to correctly explain the observed radio flux and spectra require an exhaustive investigation of all the relevant physical processes involved and possibly point to a complex wind structure. In this paper we discuss the logical path toward a comprehensive model of the nonthermal radio emission from hot star winds. Based on the available observational data and fundamental theoretical considerations, we found that the only physically viable and self-consistent scenario is: the nonthermal radio emission is synchrotron radiation of relativistic electrons (left right harpoon) the electrons are accelerated by shocks via the first-order Fermi mechanism (left right harpoon) the acceleration has to be in situ in the radio emitting region (left right harpoon) the shocks formed at the base of the winds have to propagate to beyond the radio photosphere.

A possible mechanism of jet formation in the nuclei of radio galaxies

A hypothesis is being put forward that the formation of jets in the nuclei of radio galaxies is due to a high-speed energy excretion (explosion) in the accretion disk around a massive black hole. The explosion can be induced, for example, by a fall of the star into the black hole. For the accretion disk featuring an exponential high-density distribution, an asymmetrical explosion can be obtained: the shock front moves in the direction of decreasing the density accelerately and achieves the relativistic velocity swiftly, carrying away the most fraction of the explosion energy. Radio emission of the jet involves synchrotron radiation of relativistic electrons which are accelerated by such shock wave in the magnetic field driven up by the shock front.

Efficiency of Acceleration of Relativistic Particles in the Crab Nebula

The energy density of cosmic rays comparable to that of star light suggests the surprisingly high efficiency of converting the thermal energy into kinetic energy of relativistic particles. Although the storage of cosmic rays in the galactic magnetic field lowers the over-all efficiency, there must be some objects in which the acceleration is very efficient, since most of the stars, like the sun, are known to be rather poor producers of cosmic rays. An attempt is made to show on a semiquantitative basis that the high efficiency is really seen in the Crab nebula, the remnant of a supernova in 1054. Finally, the arguments are based on hypotheses, which seem to be plausible, that (a) the electromagnetic radiations are caused by the synchrotron radiation of relativistic electrons and (b) the so called rapid process of forming transuranic elements that took place in the explosion of the supernova in 1054.