Synchrotron Process Research Articles

Flux-depending X-ray Spectrum Index of Blazars

Based on a sample of 69 blazars, relationships between flux density and spectral index, and distributions of redshift and spectral index have been investigated for highly frequency peaked BL Lac (HBLs), lowly frequency peaked BL Lac (LBLs) and flat spectrum radio quasars (FSRQs) respectively. Our result shows that (1) an anti-correlation between flux density and spectral index is found for HBLs, there is no clear relation for LBLs or FSRQs, which suggest that the X-ray emission in HBLs are from synchrotron process while that for LBLs and FSRQs from synchrotron self-Compton process, (2) the X-ray spectral index has a consequence for the three subclasses with HBLs showing the softest X-ray spectrum and FSRQs the hardest X-ray spectrum and with LBLs the middle of HBLs and FSRQs.

A Testable Stochastic Acceleration Model for Flares in Sagittarius A*

The near-IR and X-ray flares in Sagittarius A* are believed to be produced by relativistic electrons via synchrotron and synchrotron self-Comptonization, respectively. These electrons are likely energized by turbulent plasma waves through second-order Fermi acceleration that, in combination with the radiative cooling processes, produces a relativistic Maxwellian distribution in the steady state. This model has four principal parameters, namely the magnetic field B, the electron density n and temperature ?cmec2, and the size of the flare region R. In the context of stochastic acceleration, the quantities Rn1/2B and ?cRn should remain nearly constant in time. Therefore, simultaneous spectroscopic observations in the NIR and X-ray bands can readily test the model, which, if proven to be valid, may be used to determine the evolution of the plasma properties during an eruptive event with spectroscopic observations in either band or simultaneous flux density measurements in both bands. The formulae can be applied to other isolated or confined systems, where electrons are accelerated to relativistic energies by plasma wave turbulence and produce most of the emission via synchrotron processes.

Models for Nonthermal Photon Spectra

We describe models of nonthermal photon emission from a homogeneous distribution of relativistic electrons and protons. Contributions from the synchrotron, inverse Compton, nonthermal bremsstrahlung, and neutral-pion decay processes are computed separately using a common parameterization of the underlying distribution of nonthermal particles. The models are intended for use in fitting spectra from multiwavelength observations and are designed to be accurate and efficient. Although our applications have focused on Galactic supernova remnants, the software is modular, making it straightforward to customize for different applications. In particular, the shapes of the particle distribution functions and the shape of the seed photon spectrum used by the inverse Compton model are defined in separate modules and may be customized for specific applications. We assess the accuracy of these models by using a recurrence relation and by comparing them with analytic results and with previous numerical work by other authors.

Radio and X-ray properties of relativistic beaming models for ultraluminous X-ray sources

We calculate the broad-band radio-X-ray spectra predicted by microblazar and microquasar models for ultraluminous X-ray sources (ULXs), exploring the possibility that their dominant power-law component is produced by a relativistic jet, even at near-Eddington mass accretion rates. We do this by first constructing a generalized disc-jet theoretical framework in which some fraction of the total accretion power, P a , is efficiently removed from the accretion disc by a magnetic torque responsible for jet formation. Thus, for different black hole masses, mass accretion rates and magnetic coupling strength, we self-consistently calculate the relative importance of the modified disc spectrum, as well as the overall jet emission due to synchrotron and Compton processes. In general, transferring accretion power to a jet makes the disc fainter and cooler than a standard disc at the same mass accretion rate; this may explain why the soft spectral component appears less prominent than the dominant power-law component in most bright ULXs. We show that the apparent X-ray luminosity and spectrum predicted by the microquasar model are consistent with the observed properties of most ULXs. We predict that the radio synchrotron jet emission is too faint to be detected at the typical threshold of radio surveys to date. This is consistent with the high rate of non-detections over detections in radio counterpart searches. Conversely, we conclude that the observed radio emission found associated with a few ULXs cannot be due to beamed synchrotron emission from a relativistic jet.

The flat synchrotron spectra of partially self-absorbed jets revisited

Flat radio spectra with large brightness temperatures at the core of AGN and X-ray binaries are usually interpreted as the partially self-absorbed bases of jet flows emitting synchrotron radiation. Here we extend previous models of jets propagating at large angles to our line of sight to self-consistently include the effects of energy losses of the relativistic electrons due to the synchrotron process itself and the adiabatic expansion of the jet flow. We also take into account energy gains through self-absorption. Two model classes are presented. The ballistic jet flows, with the jet material travelling along straight trajectories, and adiabatic jets. Despite the energy losses, both scenarios can result in flat emission spectra, however, the adiabatic jets require a specific geometry. No re-acceleration process along the jet is needed for the electrons. We apply the models to observational data of the X-ray binary Cygnus X-1. Both models can be made consistent with the observations. The resulting ballistic jet is extremely narrow with a jet opening angle of only 5. Its energy transport rate is small compared to the time-averaged jet power and therefore suggests the presence of non-radiating protons in the jet flow. The adiabatic jets require a strong departure from energy equipartition between the magnetic field and the relativistic electrons. These models also imply a jet power two orders of magnitude higher than the Eddington limiting luminosity of a 10 solar mass black hole. The models put strong constraints on the physical conditions in the jet flows on scales well below achievable resolution limits.

The Spectral Properties of Shocked Two-Component Accretion Flows in the Presence of Synchrotron Emission

Two-component advective flows have Keplerian accretion disks on the equatorial plane that is surrounded by sub-Keplerian transonic flows. In this Letter, we study the spectral properties of these flows when the shocks are present. The shock acceleration produces nonthermal electrons in the postshock region that in turn produce power-law synchrotron radiation. The soft photons generated by the bremsstrahlung and synchrotron processes in the sub-Keplerian flow, as well as the multicolor blackbody emission from the Keplerian disk, are Comptonized by the thermal and nonthermal electrons. By varying Keplerian and sub-Keplerian rates, we are able to reproduce the observed soft and hard states as far as the X-ray region is concerned and low γ-ray intensity and high γ-ray intensity states as far as the soft γ-ray region is concerned. We also find two pivotal points where the spectra intersect, as is observed in, e.g., Cyg X-1.

A unified picture for gamma-ray burst prompt and X-ray afterglow emissions

Abstract Data from the Swift satellite have enabled us for the first time to provide a complete picture of the gamma-ray (γ-ray) burst emission mechanism and its relationship with the early afterglow emissions. We show that γ-ray photons for two bursts, 050126 and 050219A, for which we have carried out detailed analysis were produced as a result of the synchrotron self-Compton process in the material ejected in the explosion when it was heated to a mildly relativistic temperature at a distance from the centre of explosion of order the deceleration radius. Both of these bursts exhibit rapidly declining early X-ray afterglow light curves; this emission is from the same source that produced the γ-ray burst. The technique that we exploit to determine this is very general and makes no assumption about any particular model for γ-ray generation except that the basic radiation mechanism is some combination of synchrotron and inverse Compton processes in a relativistic outflow. For GRB 050219A we can rule out the possibility that energy from the explosion is carried outward by magnetic fields, and that the dissipation of this field produced the γ-ray burst.

Log-parabolic spectra and particle acceleration in blazars

Curved broad-band spectral distributions of non-thermal sources like blazars are described well by a log-parabolic (LP) law where the second degree term measures the curvature. LP energy spectra can be obtained for relativistic electrons by means of a statistical acceleration mechanism whose probability of acceleration depends on energy. In this paper we compute the spectra radiated by an electron population via synchrotron (S) and Synchro-Self Compton(SSC) processes to derive the relations between the LP parameters. These spectra were obtained by means of an accurate numerical code. We found that the ratio between the curvature parameters of the S spectrum to that of the electrons is equal to about 0.2 instead of 0.25, the value foreseen in the delta approximation. Inverse Compton spectra are also intrinsically curved and can be approximated by a log-parabola only in limited ranges. The curvature parameter, estimated around the SED peak, may vary from a lower value than that of the S spectrum up to that of emitting electrons depending on whether the scattering is in the Thomson or in the Klein-Nishina regime. We applied this analysis to computing the SSC emission from the BL Lac object Mkn 501 during the large flare of April 1997. We fit simultaneous BeppoSAX and CAT data and reproduced intensities and spectral curvatures of both components with good accuracy. The large curvature observed in the TeV range was found to be mainly intrinsic, and therefore did not require a large pair production absorption against the extragalactic background. We regard this finding as an indication that the Universe is more transparent at these energies than previously assumed by several models found in the literature. This conclusion is supported by recent detection of two relatively high redshift blazars with H.E.S.S.

A broadband leptonic model for gamma-ray emitting microquasars

Observational and theoretical studies point to microquasars (MQs) as possible counterparts of a significant fraction of the unidentified gamma-ray sources detected so far. At present, a proper scenario to explain the emission beyond soft X-rays from these objects is not known, nor what the precise connection is between the radio and the high-energy radiation. We develop a new model where the MQ jet is dynamically dominated by cold protons and radiatively dominated by relativistic leptons. The matter content and power of the jet are both related with the accretion process. The magnetic field is assumed to be close to equipartition, although it is attached to and dominated by the jet matter. For the relativistic particles in the jet, their maximum energy depends on both the acceleration efficiency and the energy losses. The model takes into account the interaction of the relativistic jet particles with the magnetic field and all the photon and matter fields. Such interaction produces significant amounts of radiation from radio to very high energies through synchrotron, relativistic Bremsstrahlung, and inverse Compton (IC) processes. Variability of the emission produced by changes in the accretion process (e.g. via orbital eccentricity) is also expected. The effects of the gamma-ray absorption by the external photon fields on the gamma-ray spectrum have been taken into account, revealing clear spectral features that might be observed. This model is consistent to the accretion scenario, energy conservation laws, and current observational knowledge, and can provide deeper physical information of the source when tested against multiwavelength data.

A focal plane detector design for a wide-band Laue-lens telescope

The energy range above 60 keV is important for the study of many open problems in high energy astrophysics such as the role of Inverse Compton with respect to synchrotron or thermal processes in GRBs, non thermal mechanisms in SNR, the study of the high energy cut-offs in AGN spectra, and the detection of nuclear and annihilation lines. Recently the development of high energy Laue lenses with broad energy bandpasses from 60 to 600 keV have been proposed for a Hard X ray focusing Telescope (HAXTEL) in order to study the X-ray continuum of celestial sources. The required focal plane detector should have high detection efficiency over the entire operative range, a spatial resolution of about 1 mm, an energy resolution of a few keV at 500 keV and a sensitivity to linear polarization. We describe a possible configuration of the focal plane detector based on several CdTe/CZT pixelated layers stacked together to achieve the required detection efficiency at high energy. Each layer can operate both as a separate position sensitive detector and polarimeter or work with other layers to increase the overall photopeak efficiency. Each layer has a hexagonal shape in order to minimize the detector surface required to cover the lens field of view. The pixels would have the same geometry so as to provide the best coupling with the lens point spread function and to increase the symmetry for polarimetric studies.

Interpretation of the Radio/X‐Ray Knots of AGN Jets within the Internal Shock Model Framework

The dynamics of relativistically moving blobs ejected out of a central AGN are considered. It is assumed that the collision between two blobs is completely inelastic, such that the bulk kinetic energy lost in the collision is used to energize electrons to relativistic energies via acceleration in internal shocks that are formed by the collision. These high-energy electrons, which are produced on a timescale corresponding to the collision timescale, cool by radiative losses due to synchrotron and inverse Compton processes. The model is applied to the radio/X-ray knots of several AGNs. For three of these sources we have analyzed long (>40 ks) Chandra observations and report on constraints on the X-ray spectral indices. In the framework of this model the AGNs are inferred to sporadically eject relativistic blobs on timescales ranging from 1011 to 1012 s for different sources. It is shown that the collision timescales can be longer than the age of the knot, and hence nonthermal electrons are continuously being injected into the system. This continuous injection, in contrast to an instantaneous one-time injection, gives rise to a characteristic spectral break rather than a high-energy cutoff in the spectrum.

Orbital X‐Ray Variability of the Microquasar LS 5039

The properties of the orbit and the donor star in the high mass X-ray binary microquasar LS 5039 indicate that accretion processes should mainly occur via a radiatively driven wind. In such a scenario, significant X-ray variability would be expected due to the eccentricity of the orbit. The source has been observed at X-rays by several missions, although with a poor coverage that prevents to reach any conclusion about orbital variability. Therefore, we conducted RossiXTE observations of the microquasar system LS 5039 covering a full orbital period of 4 days. Individual observations are well fitted with an absorbed power-law plus a Gaussian at 6.7 keV, to account for iron line emission that is probably a diffuse background feature. In addition, we have taken into account that the continuum is also affected by significant diffuse background contamination. Our results show moderate power-law flux variations on timescales of days, as well as the presence of miniflares on shorter timescales. The new orbital ephemeris of the system recently obtained by Casares et al. have allowed us to show, for the first time, that an increase of emission is seen close to the periastron passage, as expected in an accretion scenario. Moreover, the detected orbital variability is a factor of ~4 smaller than the one expected by using a simple wind accretion model, and we suggest that an accretion disk around the compact object could be responsible for this discrepancy. On the other hand, significant changes in the photon index are also observed clearly anti-correlated with the flux variations. We interpret the overall X-ray spectral characteristics of LS 5039 in the context of X-ray radiation produced by inverse Compton and/or synchrotron processes in the jet of this microquasar.

Neutrino synchrotron radiation in electro-weak interaction

The neutrino synchrotron process has been considered within the framework of the electro-weak theory. An electron moving in a magnetic field with spiral path changes the Landau level to release the energy in terms of neutrino antineutrino pair. The calculation of scattering cross-section for the neutrino synchrotron radiation has been carried out. We also calculate the energy loss rate and compare our result with Landstreet’s result, graphically. Finally, the neutrino luminosity (expressed in the unit of solar luminosity) at different central temperatures of white dwarfs and neutron stars have been calculated to study the significance of this process in the later stages of the stellar evolution.

The Two‐sided Parsec‐Scale Structure of the Low‐Luminosity Active Galactic Nucleus in NGC 4278

We present new Very Long Baseline Interferometry observations of the LINER galaxy NGC 4278. The observations were taken with the Very Long Baseline Array (VLBA) and a single antenna of the Very Large Array (VLA) at 5 GHz and 8.4 GHz and have a linear resolution of <0.1 pc. Our radio data reveal a two sided structure, with symmetric S-shaped jets emerging from a flat spectrum core. We fit the jet brightness with gaussian components, which we identify from a previous observation taken five years before. By comparing the positions of the components in the two epochs, we measure motions between 0.45 +/- 0.14 and 3.76 +/- 0.65 mas, corresponding to apparent velocities < 0.2c, and to ages in the range 8.3 - 65.8 years. Assuming that the radio morphology is intrinsically symmetric and its appearance is governed by Doppler beaming effects, we find that NGC4278 has mildly relativistic jets (beta ~ 0.75), closely aligned to the line-of-sight (2 degrees < theta < 4 degrees). Alternatively, the source could be oriented at a larger angle and asymmetries could be related to the jet interaction with the surrounding medium. We also present new simultaneous VLA observations between 1.4 and 43 GHz, and a 5 GHz light curve between 1972 and 2003. The radio spectrum can be fit by a relatively steep power-law (alpha = 0.54). We find significant variability at 5 GHz. All these arguments indicate that the radiation from NGC 4278 is emitted via the synchrotron process by relativistic particles accelerated by a supermassive black hole. Despite a much lower power, this is the same process that takes place in ordinary radio loud AGNs.

Diffuse inverse Compton and synchrotron emission from dark matter annihilations in galactic satellites

Annihilating dark matter particles produce roughly as much power in electrons and positrons as in gamma ray photons. The charged particles lose essentially all of their energy to inverse Compton and synchrotron processes in the galactic environment. We discuss the diffuse signature of dark matter annihilations in satellites of the Milky Way (which may be optically dark with few or no stars), providing a tail of emission trailing the satellite in its orbit. Inverse Compton processes provide x rays and gamma rays, and synchrotron emission at radio wavelengths might be seen. We discuss the possibility of detecting these signals with current and future observations, in particular EGRET and GLAST for the gamma rays.

Constraining X‐Ray Binary Jet Models via Reflection

Although thermal disk emission is suppressed or absent in the hard state of X-ray binaries, the presence of a cold, thin disk can be inferred from signatures of reprocessing in the ~2-50 keV band. The strength of this signature is dependent on the source spectrum and flux impinging on the disk surface and is thus very sensitive to the system geometry. The general weakness of this feature in the hard state has been attributed to either a truncation of the thin disk, large ionization, or beaming of the corona region away from the disk with β ~ 0.3. This latter velocity is comparable to jet nozzle velocities, so we explore whether a jet can account for the observed reflection fractions. It has been suggested that jets may contribute to the high-energy spectra of X-ray binaries, via either synchrotron from around (100-1000)rg along the jet axis or from inverse Compton (synchrotron self-Compton and/or external Compton) from near the base. Here we calculate the reflection fraction from jet models wherein either synchrotron or Compton processes dominate the emission. Using as a guide a data set for GX 339-4, for which the reflection fraction previously has been estimated as ~10%, we study the results for a jet model. We find that the synchrotron case gives less than 2% reflection, while a model with predominantly synchrotron self-Compton in the base gives ~10%-18%. This shows for the first time that an X-ray binary jet is capable of significant reflection fractions and that extreme values of the reflection may be used as a way of discerning the dominant contributions to the X-ray spectrum.

A Survey of Extended Radio Jets withChandraand theHubble Space Telescope

We present the results from an X-ray and optical survey of a sample of 17 radio jets in active galactic nuclei performed with Chandra and the Hubble Space Telescope. The sample was selected from the radio and is unbiased toward detection at shorter wavelengths, but preferentially it includes beamed sources. We find that X-ray emission is common on kiloparsec scales, with over half (10/17) the radio jets exhibiting at least one X-ray knot on the Chandra images. A similar detection rate is found for the optical emission, although not all X-ray knots have optical counterparts and vice versa. The distributions of the radio-to-X-ray and radio-to-optical spectral indices, αrx and αro, for the detected jets are similar to the limits for the nondetections, suggesting that all bright radio jets have X-ray counterparts that will be visible in longer observations. Comparing the radio and X-ray morphologies shows that the majority of the X-ray jets have structures that closely map the radio. Analysis of the spectral energy distributions of the jet knots suggests that the knots in which the X-ray and radio morphologies track each other produce X-rays by inverse Compton (IC) scattering of the cosmic microwave background (CMB). The remaining knots produce X-rays by the synchrotron process. Spectral changes are detected along the jets, with the ratio of the X-ray-to-radio and optical-to-radio flux densities decreasing from the inner to the outer regions. This suggests an additional contribution to the X-ray flux in the jet's inner part, either from synchrotron radiation or IC scattering of the stellar light. Alternatively, in a pure IC/CMB scenario, the plasma decelerates as it flows from the inner to the outer regions. Finally, the X-ray spectral indices for the brightest knots are flat (photon index Γx ~ 1.5), indicating that the bulk of the luminosity of the jets is emitted at GeV energies and raising the interesting possibility of future detections with GLAST.

The Most Likely Sources of High‐Energy Cosmic‐Ray Electrons in Supernova Remnants

Evidences of non-thermal X-ray emission and TeV gamma-rays from the supernova remnants (SNRs) has strengthened the hypothesis that primary Galactic cosmic-ray electrons are accelerated in SNRs. High energy electrons lose energy via synchrotron and inverse Compton processes during propagation in the Galaxy. Due to these radiative losses, TeV electrons liberated from SNRs at distances larger than ~1 kpc, or times older than ~10^5 yr, cannot reach the solar system. We investigated the cosmic-ray electron spectrum observed in the solar system using an analytical method, and considered several candidate sources among nearby SNRs which may contribute to the high energy electron flux. Especially, we discuss the effects for the release time from SNRs after the explosion, as well as the deviation of a source spectrum from a simple power-law. From this calculation, we found that some nearby sources such as the Vela, Cygnus Loop, or Monogem could leave unique signatures in the form of identifiable structure in the energy spectrum of TeV electrons and show anisotropies towards the sources, depending on when the electrons are liberated from the remnant. This suggests that, in addition to providing information on the mechanisms of acceleration and propagation of cosmic-rays, specific cosmic-ray sources can be identified through the precise electron observation in the TeV region.

X-ray variability and emission process of the radio jet in M87

We monitored the M87 jet with the ACIS-S detector on Chandra with 5 observations between 2002 January and 2002 July. Our goal was to determine the presence and degree of variability in morphology, intensity, and spectral parameters. We find strong variability of the core and HST-1, the knot lying 0.8″ from the core. These observations were designed to constrain the X-ray emission process: whereas synchrotron emission would necessitate the presence of extremely high energy electrons with a halflife of a few years or less, inverse Compton emission from a relativistic jet would arise from low energy electrons with very long halflives. Currently, all indications point to a synchrotron process for the X-ray emission from the M87 jet. We give key parameters for a ‘modest beaming’ synchrotron model.

Nonthermal Particles and Radiation Produced by Cluster Merger Shocks

We have developed a numerical model for the temporal evolution of particle and photon spectra resulting from nonthermal processes at the shock fronts formed in merging clusters of galaxies. Fermi acceleration is approximated by injecting power-law distributions of particles during a merger event, subject to constraints on maximum particle energies. We consider synchrotron, bremsstrahlung, Compton, and Coulomb processes for the electrons, nuclear, photomeson, and Coulomb processes for the protons, and knock-on electron production during the merging process. The broadband radio through gamma-ray emission radiated by nonthermal protons and primary and secondary electrons is calculated both during and after the merger event. Using ROSAT observations to establish typical parameters for the matter density profile of clusters of galaxies, we find that typical merger shocks are weak and accelerate particles with relatively soft spectra. We consider the prospects for detecting nonthermal radio and gamma-ray emission from clusters of galaxies and implications for the origin of ultra-high energy cosmic rays and the diffuse gamma-ray background. Our results suggest that only a few of the isotropically-distributed unidentified EGRET sources are due to shocks formed in cluster mergers, and that only a minor contribution to the diffuse extragalactic gamma-ray background can originate from cluster merger shocks. Cluster merger shocks can accelerate protons to $\lesssim 10^{19}$ eV for the standard parameters considered here. We predict that {\it GLAST} will detect several cluster mergers and, depending on the mean magnetic fields in the intracluster medium, the Low Frequency Array could detect anywhere from several to several hundred.