Synchrotron Emission Of Electrons Research Articles

Probing the Efficiency of Electron-Proton Coupling in Relativistic Collisionless Shocks through the Radio Polarimetry of Gamma-Ray Burst Afterglows

The late-time optical/radio afterglows of γ-ray bursts (GRBs) are believed to be synchrotron emission of electrons accelerated in relativistic collisionless shocks propagating in the ambient medium of the sources. However, the fraction f of electrons that are coupled to protons and accelerated remains unclear and a large number of thermal electrons that are not coupled to protons may be left behind. If f < 1, the true explosion energies of GRBs are f−1 times larger than those commonly estimated with f = 1. Thus the value of f gives an important constraint on the nature of the central engine of GRBs and the physics of collisionless shocks. Although early-time radio observations can probe the thermal electrons, they are difficult at present. We show that the Faraday rotation effects of the thermal electrons may suppress the linear polarization of the afterglow at frequencies higher than the absorption frequency in the late time, if the magnetic field is ordered at least in parts, and that f can be constrained through observation of the effects. We find that these effects may be detected with late-time, ≥1 day, polarimetry with ALMA for a burst occurring within 1 Gpc (i.e., z≃ 0.2), if f ∼ 10−1.

GRB 070311: a direct link between the prompt emission and the afterglow

Context. The prompt emission of gamma-ray bursts is mostly thought to be produced in internal shocks of relativistic shells emitted by the progenitor at different times, whereas the late multi-band afterglow is interpreted as the synchrotron emission of electrons swept up by the fireball expanding through the surrounding interstellar medium. The short timescale variability observed in flares superimposed on the X-ray/optical afterglow of several bursts, recently made possible by Swift, has been interpreted as evidence for prolonged activity of the inner engine through internal shocks. Yet, it is not clear whether this applies to all the observed bursts and, in particular, whether the bursts exhibiting single γ-ray pulses with no short timescale variability at late times could also be entirely interpreted as external shocks. Aims. We present prompt γ-ray, early NIR/optical, late optical and X-ray observations of the peculiar GRB 070311 discovered by INTEGRAL, in order to gain clues on the mechanisms responsible for the prompt γ-ray pulse as well as for the early and late multiband afterglow of GRB 070311. Methods. We fitted with empirical functions the gamma-ray and optical light curves and scaled the result to the late time X-rays. Results. The H-band light curve taken by REM shows two pulses peaking 80 and 140 s after the peak of the γ-ray burst and possibly accompanied by a faint γ-ray tail. Remarkably, the late optical and X-ray afterglow underwent a major rebrightening between 3 × 10 4 and 2 × 10 5 s after the burst with an X-ray fluence comparable with that of the prompt emission extrapolated in the same band. Notably, the time profile of the late rebrightening can be described as the combination of a time-rescaled version of the prompt γ-ray pulse and an underlying power law. Conclusions. This result supports a common origin for both prompt and late X-ray/optical afterglow rebrightening of GRB 070311 within the external shock scenario. The main fireball would be responsible for the prompt emission, while a second shell would produce the rebrightening when impacting the leading blastwave in a refreshed shock.

Leptonic Origin of TeV Gamma-Ray Emission from Crab Nebula

We study the nonthermal emission of the Crab nebula in the bands from radio toTeV γ-ray on a simplified time-dependent injectionmodel. In this model, relativistic electrons in the Crab nebulaconsists of two components and their injected spectrum is a brokenpower law with different indices and a break energy. Therelativistic electrons emit nonthermal photons through synchrotronand inverse Compton scattering off soft photon fields inside thenebula. The resulting spectrum calculated with the model is wellconsistent with the observed data ranging from radio to very highenergy γ-rays for the Crab nebula, where the emissionfrom radio to medium γ-rays is from electron's synchrotronemission, whereas the emission above ~100 MeV primarily comesfrom the inverse Compton scattering of the relativistic electrons onsynchrotron photons.

Gamma-ray bursts and collisionless shocks**Invited review presented at the 33rd annual European Physical Society Conference, Rome, 2006.

Particle acceleration in collisionless shocks is believed to be responsible for the production of cosmic-rays over a wide range of energies, from a few GeV to > 1020 eV, as well as for the non-thermal emission of radiation from a wide variety of high energy astrophysical sources. A theory of collisionless shocks based on first principles does not, however, exist. Observations of γ-ray burst (GRB) ‘afterglows’ provide a unique opportunity for diagnosing the physics of relativistic collisionless shocks. Most GRBs are believed to be associated with explosions of massive stars. Their ‘afterglows’, delayed low energy emission following the prompt burst of γ-rays, are well accounted for by a model in which afterglow radiation is due to synchrotron emission of electrons accelerated in relativistic collisionless shock waves driven by the explosion into the surrounding plasma. Within the framework of this model, some striking characteristics of collisionless relativistic shocks are implied. These include the generation of downstream magnetic fields with energy density exceeding that of the upstream field by ∼8 orders of magnitude, the survival of this strong field at distances ∼1010 skin-depths downstream of the shock and the acceleration of particles to a power-law energy spectrum, d log n/d logε ≈ −2, possibly extending to 1020 eV. I review in this talk the phenomenological considerations, based on which these characteristics are inferred, and the challenges posed to our current models of particle acceleration and magnetic field generation in collisionless shocks. Some recent theoretical results derived based on the assumption of a self-similar shock structure are briefly discussed.

The Upstream Magnetic Field of Collisionless GRB Shocks

Gamma-ray burst (GRB) afterglow emission is believed to be produced by synchrotron emission of electrons accelerated to high energy by a relativistic collisionless shock propagating into a weakly magnetized plasma. Afterglow observations have been used to constrain the post-shock magnetic field and structure, as well as the accelerated electron energy distribution. Here we show that X-ray afterglow observations on day time scale constrain the pre-shock magnetic field to satisfy B>0.2[n/(1/cc)]^{5/8} mG, where n is the pre-shock density. This suggests that either the shock propagates into a highly magnetized fast, v~10^3 km/s, wind, or that the pre-shock magnetic field is strongly amplified, most likely by the streaming of high energy shock accelerated particles. More stringent constraints may be obtained by afterglow observations at high photon energy at late, >1 d, times.

The Radio and X‐Ray Luminous SN 2003bg and the Circumstellar Density Variations around Radio Supernovae

We report extensive radio and X-ray observations of SN 2003bg, whose spectroscopic evolution shows a transition from a broad-lined Type Ic to a hydrogen-rich Type II, and later to a typical hydrogen-poor Type Ibc. We show that the extraordinarily luminous radio emission is well described by a self-absorption-dominated synchrotron spectrum, while the observed X-ray emission at t ≈ 30 days is adequately fit by inverse Compton scattering of the optical photons off of the synchrotron-emitting electrons. Our radio model implies a subrelativistic ejecta velocity, ≈ 0.24c, at t0 ≈ 10 days after the explosion, which emphasizes that broad optical absorption lines do not imply relativistic ejecta. We find that the total energy of the radio-emitting region evolves as E ≈ 7.3 × 1048(t/t0)0.4 ergs, assuming equipartition of energy between relativistic electrons and magnetic fields (e = B = 0.1). The circumstellar density is well described by a stellar wind profile, with modest (factor of ~2) episodic density enhancements that produce abrupt achromatic flux variations. We estimate an average mass-loss rate of ≈ 3 × 10-4 M☉ yr-1 (assuming a wind velocity of vw = 103 km s-1) for the progenitor, consistent with the observed values for Galactic Wolf-Rayet stars. Comparison with other events reveals that ~50% of radio supernovae show similar short-timescale flux variations, attributable to circumstellar density irregularities. Specifically, the radio light curves of SN 2003bg are strikingly similar to those of the Type IIb SN 2001ig, suggestive of a common progenitor evolution for these two events. Based on the relative intensity of the inferred density enhancements, we conclude that the progenitors of SNe 2003bg and 2001ig experienced quasi-periodic mass-loss episodes just prior to the SN explosion. Finally, this study emphasizes that abrupt radio light-curve variations cannot be used as a reliable proxy for an engine-driven explosion, including off-axis gamma-ray bursts.

A Multiwavelength Study of the Pulsar PSR B1929+10 and Its X‐Ray Trail

We report on the emission properties of PSR B1929+10 and its putative trail from a multiwavelength study performed using optical, X-ray, and radio data. XMM-Newton observations confirm the existence of the diffuse emission with a trail morphology lying in a direction opposite to the transverse motion of the pulsar. The trail spectrum is nonthermal and produced by electron-synchrotron emission in the shock between the pulsar wind and the surrounding medium. Radio data from the Effelsberg 11 cm radio continuum survey show an elongated feature that roughly coincides with the X-ray trail. Three not fully resolved radio sources seen in the NVSS survey data at 1.4 GHz match with part of the elongated radio feature seen at 11 cm. The emission properties observed from PSR B1929+10 are in excellent agreement with a nonthermal, and thus magnetospheric-radiation-dominated, emission scenario. The pulsar's X-ray spectrum is best described by a single power-law model with a photon index of 2.72. A flux contribution from the thermal emission of heated polar caps of at most ~7% is inferred from a best-fitting composite Planckian and power-law spectral model. A pure thermal emission spectrum consisting of two Planckian spectra is regarded as unlikely. A broken power-law spectral model with Ebreak = 0.83 keV and the photon indexes α1 = 1.12 and α2 = 2.48 can describe the optical and X-ray data entirely in terms of a nonthermal magnetospheric origin. The X-ray pulse profile observed in the 0.2-10 keV band is found to be markedly different from the broad sinusoidal pulse profile seen in the low statistic Rontgensatellit (ROSAT) data. Fitting Gaussians to the X-ray light curve indicates the possible existence of three pulse components. A small narrow pulse, characterized by energies greater than 1 keV, is found to lead the radio main pulse by ~20°. The fraction of pulsed photons in the 0.2-10 keV band is 32% ± 4%. For the subbands 0.2-1.0 and 1.0-2.1 keV the pulsed fraction is 24% ± 5% and 44% ± 6%, respectively, indicating a mild energy dependence at a ~2 σ level. Simulations in the framework of an outer gap emission model are able to reproduce the observed X-ray pulse profile and its phase shift relative to the radio pulse.

74 MHz Discrete HiiAbsorption Regions Toward the Inner Galaxy

At low radio frequencies (ν ≲ 100 MHz), classical H II regions may become optically thick (optical depth τ ≥ 1) and can be observed as discrete absorption regions against the Galactic nonthermal background emission created by Galactic cosmic-ray electrons spiraling around magnetic fields. However, the historically poor angular resolution (>30') of previous low-frequency surveys has limited such observations to the largest and nearest H II regions. The significantly enhanced resolution and surface brightness sensitivity of the 74 MHz system on the Very Large Array now allow for the detection of absorption regions on scale sizes of just a few arcminutes that can be readily identified with many more H II regions previously cataloged in emission at higher frequencies. These absorption measurements directly constrain the brightness temperature of the cosmic-ray synchrotron emission emanating from behind the H II regions based on reasonable physical assumptions. Many such observations could be used to map out the three-dimensional cosmic-ray emissivity in the Galaxy without resorting to a priori assumptions about Galactic structure. This measurement is unique to low-frequency radio astronomy. In this work we present 74 MHz observations in the region 26° > l > -15°, -5° < b < 5°; report the detection of 92 absorption features associated with known H II regions; and derive the brightness temperature of the Galactic cosmic-ray electron synchrotron emission emanating from the column behind these regions. For the 42 H II regions with known distances, the average emissivity of the column behind the H II region is derived. The 74 MHz emissivity values range between 0.3 and 1.0 K pc-1 for a model assuming uniform distribution of emissivity. Methods for using this type of data to model the three-dimensional distribution of cosmic-ray emissivity and the possibility of using this method to break the H II region kinematic distance degeneracy are discussed.

High‐Energy Photon Emission in the Early Afterglow of GRBs

We consider the emission within the fireball model framework of very high energy, ∼ 1 GeV to > 1 TeV photons, on a minute time scale, during the onset of fireball deceleration due to interaction with surrounding medium. Our time dependent numerical model includes exact treatment of electron synchrotron emission, inverse-Compton scattering, pair production, and evolution of electromagnetic cascades (initiated by pair production or photo-production of pions). We find that (i) The 1 GeV– 10 GeV flux is not sensitive to model parameters and is ∼ 10 7 ergcm 2 sec 1 for z = 1 bursts, well within the detection capabilities of GLAST; (ii) The sub-TeV flux depends on the surrounding medium density and on the fraction of thermal energy carried by the magnetic field, ǫB: It ranges from ∼ 10 7 ergcm 2 sec 1 in the case of typical ISM density and ǫB . 10 4 to 10 10 ergcm 2 sec 1 in the case of a source surrounded by a wind and ǫB ∼ 10 0.5 ; (iii) The sub-TeV flux is detectable by high energy γ-ray experiments such as HESS, MAGIC, MILAGRO, and VERITAS; (iv) Combined ∼ 1keV, ∼ 1GeV and sub-TeV observations will allow to determine both ǫB and the ambient medium density; (v) The spectra depend only weakly on the spectral index of the energy distribution of the accelerated electrons. Pion production energy loss of high energy protons may contribute significantly in the wind case to the luminosity of high energy photons. However, it is difficult to distinguish in this case between the electron and proton contributions since the spectral shape is determined primarily by the energy dependence of the pair production optical depth. Subject headings: gamma rays: bursts — gamma rays: theory — radiation mechanisms: nonthermal

Faraday Rotation Measure Gradients from a Helical Magnetic Field in 3C 273

Using high-frequency (12-22 GHz) VLBA observations, we confirm the existence of a Faraday rotation measure gradient of ~500 rad m-2 mas-1 transverse to the jet axis in the quasar 3C 273. The gradient is seen in two epochs spaced roughly 6 months apart. This stable transverse rotation-measure gradient is expected if a helical magnetic field wraps around the jet. The overall order to the magnetic field in the inner projected 40 pc is consistent with a helical field. However, we find an unexpected increase in fractional polarization along the edges of the source, contrary to expectations. This high fractional polarization rules out internal Faraday rotation but is not readily explained by a helical field. After correcting for the rotation measure, the intrinsic magnetic field direction in the jet of 3C 273 changes from parallel to nearly perpendicular to the projected jet motion at two locations. If a helical magnetic field causes the observed rotation measure gradient, then the synchrotron-emitting electrons must be separate from the helical field region. The presence or absence of transverse rotation-measure gradients in other sources is also discussed.

Detection of the Sgr A* activity at 3.8 and 4.8 $\mathsf{\mu}$m with NACO

L'-band (lambda=3.8 microns) and M'-band (lambda=4.8 microns) observations of the Galactic Center region, performed in 2003 at VLT (ESO) with the adaptive optics imager NACO, have lead to the detection of an infrared counterpart of the radio source Sgr A* at both wavelengths. The measured fluxes confirm that the Sgr A* infrared spectrum is dominated by the synchrotron emission of nonthermal electrons. The infrared counterpart exhibits no significant short term variability but demonstrates flux variations on daily and yearly scales. The observed emission arises away from the position of the dynamical center of the S2 orbit and would then not originate from the closest regions of the black hole.

Detection of the Sgr A activity at 3.8 and 4.8 µm with NACO

e-mail:rainer@ph1.uni-koeln.de Received ; accepted Abstract. L'-band (λ=3.8� m) and M'-band (λ=4.8� m) observations of the Galactic Center region, performed in 2003 at VLT (ESO) with the adaptive optics imager NACO, have lead to the detection of an infrared counterpart of the radio source Sgr A* at both wavelengths. The measured fluxes confirm that the Sgr A * infrared spectrum is dominated by the synchrotron emission of nonthermal electrons. The infrared counterpart exhibits no significant short term variability but demonstrates flux variations on daily and yearly scales. The observed emission arises away from the position of the dynamical center of the S2 orbit and would then not originate from the closest regions of the black hole.

XMM-Newton and Geminga

A deep XMM-Newton/EPIC observation of the field of the Geminga pulsar unveiled the presence of two elongated parallel X-ray tails trailing the neutron star. They are aligned with the object's supersonic motion, extend for ∼ 2′, and have a nonthermal spectrum produced by electron-synchrotron emission in the bow shock between the pulsar wind and the surrounding medium. Such a first ever X-ray detection of a pulsar bow shock allows us to gauge the pulsar electron injection energy and the shock magnetic field while constraining the angle of Geminga's motion and the local matter density.

High‐Energy Gamma Rays from FR I Jets

Thanks to Hubble and Chandra telescopes, some of the large scale jets in extragalactic radio sources are now being observed at optical and X-ray frequencies. For the FR I objects the synchrotron nature of this emission is surely established, although a lot of uncertainties--connected for example with the particle acceleration processes involved--remain. In this paper we study production of high energy {gamma}-rays in FR I kiloparsec-scale jets by inverse-Compton emission of the synchrotron-emitting electrons. We consider different origin of seed photons contributing to the inverse-Compton scattering, including nuclear jet radiation as well as ambient, stellar and circumstellar emission of the host galaxies. We discuss how future detections or non-detections of the evaluated {gamma}-ray fluxes can provide constraints on the unknown large scale jet parameters, i.e. the magnetic field intensity and the jet Doppler factor. For the nearby sources Centaurus A and M 87, we find measurable fluxes of TeV photons resulting from synchrotron self-Compton process and from comptonization of the galactic photon fields, respectively. In the case of Centaurus A, we also find a relatively strong emission component due to comptonization of the nuclear blazar photons, which could be easily observed by GLAST at energy {approx} 10 GeV, providing importantmore » test for the unification of FR I sources with BL Lac objects.« less

Geminga's tails: a pulsar bow shock probing the interstellar medium.

We report the X-ray Multimirror Mission-Newton European Photon Imaging Camera observation of two elongated parallel x-ray tails trailing the pulsar Geminga. They are aligned with the object's supersonic motion, extend for approximately 2', and have a nonthermal spectrum produced by electron-synchrotron emission in the bow shock between the pulsar wind and the surrounding medium. Electron lifetime against synchrotron cooling matches the source transit time over the x-ray features' length. Such an x-ray detection of a pulsar bow shock (with no Halpha emission) allows us to gauge the pulsar electron injection energy and the shock magnetic field while constraining the angle of Geminga's motion and the local matter density.

Relativistic parsec–scale jets: II. Synchrotron emission

We calculate the optically thin synchrotron emission of fast electrons and positrons in a spiral stationary magnetic field and a radial electric field of a rotating relativistic strongly magnetized force–free jet consisting of electron-positron pair plasma. The magnetic field has a helical structure with a uniform axial component and a toroidal component that is maximal inside the jet and decreasing to zero towards the boundary of the jet. Doppler boosting and swing of the polarization angle of synchrotron emission due to the relativistic motion of the emitting volume are calculated. The distribution of the plasma velocity in the jet is consistent with the electromagnetic field structure. Two spatial distributions of fast particles are considered: uniform, and concentrated in the vicinity of the Alfvén resonance surface. The latter distribution corresponds to the regular acceleration by an electromagnetic wave in the vicinity of its Alfvén resonance surface inside the jet. The polarization properties of the radiation have been obtained and compared with the existing VLBI polarization measurements of parsec–scale jets in BL Lac sources and quasars. Our results give a natural explanation of the observed bimodality in the alignment between the electric field vector of the polarized radiation and the projection of the jet axis on the plane of the sky. We interpret the motion of bright knots as a phase velocity of standing spiral eigenmodes of electromagnetic perturbations in a cylindrical jet. The degree of polarization and the velocity of the observed proper motion of bright knots depend upon the angular rotational velocity of the jet. The observed polarizations and velocities of knots indicate that the magnetic field lines are bent in the direction opposite to the direction of the jet rotation.

A [ITAL]Chandra[/ITAL] Detection of Diffuse Hard X-Ray Emission Associated with the Lobes of the Radio Galaxy 3C 452

An 80 ks Chandra ACIS observation of the radio galaxy 3C 452 is reported. A diffuse X-ray emission associated with the lobes has been detected with high statistical significance, together with the X-ray nucleus of the host galaxy. The 0.5-5 keV ACIS spectrum of the diffuse emission is described by a two-component model, consisting of a soft thermal plasma emission from the host galaxy halo and a hard nonthermal power-law component. The hard component is ascribed to the inverse Comptonization of cosmic microwave background photons by the synchrotron-emitting electrons in the lobes, because its spectral energy index, 0.68 ± 0.28, is consistent with the radio synchrotron index, 0.78. These results reveal a significant electron dominance in the lobes. The electrons are inferred to have a relatively uniform distribution, while the magnetic field is compressed toward the lobe periphery.

X-Ray Synchrotron Spectral Hardenings from Compton and Synchrotron Losses in Extended [ITAL]Chandra[/ITAL] Jets

Chandra observations of knots and hot spots in spatially resolved X-ray jets of radio galaxies show that the X-ray fluxes often lie above an extrapolation from the radio-to-optical continuum fluxes. We show that combined synchrotron and Compton losses on a single-power-law electron injection function can produce a hardening in the electron spectrum at electron Lorentz factors of ? ? 2 ? 108/[?(1 + z)] owing to Klein-Nishina energy losses on the cosmic microwave background radiation. Here ? is the bulk Lorentz factor of the outflow and z is the source redshift. This produces a flattening in the spectrum at frequencies of 8 ? 1016?B?G/[?2(1 + z)3] Hz, where B?G is the magnetic field in the comoving plasma frame in units of microgauss and ? is the Doppler factor. A single population of synchrotron-emitting electrons may therefore produce the radio-to-X-ray continuum in some radio galaxy knots, such as those in 3C 273.

High‐Energy Spectral Components in Gamma‐Ray Burst Afterglows

We investigate two high-energy radiation mechanisms, the proton-synchrotron and the electron inverse Compton emission, and explore their possible signatures in the broadband spectra and in the keV-GeV light curves of gamma-ray burst afterglows. We develop a simple analytical approach, also allowing for the effects of photon-photon pair production, and explore the conditions under which one or the other of these components dominates. We identify three parameter-space regions in which different spectral components dominate: (1) a region where the proton-synchrotron and other hadron-related emission components dominate, which is small; (2) a region in which the electron inverse Compton component dominates, which is substantial; and (3) a third substantial region in which electron-synchrotron emission dominates. We discuss the prospects and astrophysical implications of directly detecting the inverse Compton and the proton high-energy components in various bands, in particular, in the GeV band, with future missions such as the Gamma-Ray Large Area Space Telescope (GLAST) and in the X-ray band with Chandra. We find that regime II parameter space is the most favorable regime for high-energy emission. The inverse Compton component is detectable by GLAST within hours for bursts at typical cosmological distances and by Chandra within days if the ambient density is high.

Simulating Electron Transport and Synchrotron Emission in Radio Galaxies: Shock Acceleration and Synchrotron Aging in Three‐Dimensional Flows

We introduce a simple and economical but effective method for including relativistic electron transport in multidimensional simulations of radio galaxies. The method is designed to follow explicitly diffusive acceleration at shocks, and, in smooth flows, second-order Fermi acceleration, plus adiabatic and synchrotron losses for electrons in the energy range responsible for radio emission in these objects. We are able to follow both the spatial and the energy (momentum) distributions of the electrons, so that direct synchrotron emission properties can be modeled in time-dependent simulated flows of this type for the first time. That feature is essential if simulations are to bridge successfully the fundamental physical gap between flow dynamics and observed emissions. As an initial step toward that goal, we present results from some axisymmetric MHD simulations of Mach 20 light jet flows. These explicitly explore the effects of shock acceleration, as well as adiabatic expansion and synchrotron in smooth flows. The simulations demonstrate the importance of the fact that even for steady inflows jet terminal shocks are not simple, steady plane structures. Most importantly, this should play a very major role in determining the properties of synchrotron emission within the terminal hot spot and in the lobes generated by the jet back flow. In fact, the outflows are inherently complex, because of the basic driven character of a jet flow. Consequently, the nonthermal electron population emerging from the jet may encounter a wide range of shock types and strengths, as well as of magnetic field environments. We may expect to find a complex range in synchrotron spectral and brightness patterns associated with terminal hot spots and lobes. These include the possibility of steep spectral gradients (of either sign) within hot spots, the potential in lobes for islands of flat-spectrum electrons within steeper spectral regions (or the reverse), and spectral gradients coming from the dynamical history of a given flow element rather than from synchrotron of the embedded electrons. Finally, synchrotron aging in the lobes tends to proceed more slowly than one would estimate from regions of high emissivity. This is a consequence of the fact that those regions are ordinarily places where the magnetic fields are the strongest, so that the instantaneous rates of energy loss are atypical of the full history of the electron population. This feature supports earlier suggestions that nonuniform field structures may help to explain why dynamical ages of FR II sources often seem to be greater than the apparent age of the electrons radiating in the lobes, as measured in terms of spectral steepening, or the absence thereof.