Observed Photon Spectrum Research Articles

Hadronic versus Leptonic Origin of Gamma-Ray Emission from Supernova Remnants

GeV and TeV emission from the forward shocks of supernova remnants (SNRs) indicates that they are capable particle accelerators, making them promising sources of Galactic cosmic rays (CRs). However, it remains uncertain whether this γ-ray emission arises primarily from the decay of neutral pions produced by very-high-energy hadrons, or from inverse-Compton and/or bremsstrahlung emission from relativistic leptons. By applying a semi-analytic approach to non-linear diffusive shock acceleration, and calculating the particle and photon spectra produced in different environments, we parameterize the relative strength of hadronic and leptonic emission. We show that even if CR acceleration is likely to occur in all SNRs, the observed photon spectra may primarily reflect the environment surrounding the SNR: the emission is expected to look hadronic unless the ambient density is particularly low (with proton number density ≲0.1 cm−3) or the photon background is enhanced with respect to average Galactic values (with radiation energy density u rad ≳ 10 eV cm−3). We introduce a hadronicity parameter to characterize how hadronic or leptonic we expect a source to look based on its environment, which can be used to guide the interpretation of current γ-ray observations and the detection of high-energy neutrinos from SNRs.

The high energy emission of the Crab’s twin pulsar PSR J0540-6919 in the Large Magellanic Cloud

In the present paper, we discuss theoretical interpretation of the X-ray and gamma-ray emission of the extremely bright pulsar PSR J0540-6919, in the Large Magellanic cloud. The source of the high frequency emission is assumed to be the synchrotron radiation, which is generated near the light cylinder during the quasi-linear stage of the cyclotron instability developed in this region. The emitting particles are the particles of the primary beam, with the Lorentz factors of the order of $\gamma _{b}\sim 10^{7}$ that provide generation of the gamma-ray emission with the photon energy up to 10 GeV and the secondary plasma electrons with $\gamma _{t}\sim 10^{5}$ , emitting in the X-ray domain. The theoretical synchrotron radiation spectra obtained in the framework of the model provide natural explanation of the observed photon spectra showing power-law indices 2.2 for gamma-rays and 1.55 for the X-rays. The present model provides natural explanation for pulse-aligned emission peaks at different frequencies and smearing of two-peak pulse profile at higher frequencies that is clearly observed in radio domain.

AGN neutrino flux estimates for a realistic hybrid model

Recent reports of possible correlations between high energy neutrinos observed by IceCube and Active Galactic Nuclei (AGN) activity sparked a burst of publications that attempt to predict the neutrino flux of these sources. However, often rather crude estimates are used to derive the neutrino rate from the observed photon spectra. In this work neutrino fluxes were computed in a wide parameter space. The starting point of the model was a representation of the full spectral energy density (SED) of 3C 279. The time-dependent hybrid model that was used for this study takes into account the full pγ reaction chain as well as proton synchrotron, electron-positron-pair cascades and the full SSC scheme. We compare our results to estimates frequently used in the literature. This allows to identify regions in the parameter space for which such estimates are still valid and those in which they can produce significant errors. Furthermore, if estimates for the Doppler factor, magnetic field, proton and electron densities of a source exist, the expected IceCube detection rate is readily available.

On the Spectral Curvature of VHE Blazar 1ES 1011+496: Effect of Spatial Particle Diffusion

Abstract A detailed multi-epoch study of the broadband spectral behavior of the very high energy (VHE) source 1ES 1011+496 provides us with valuable information regarding the underlying particle distribution. Simultaneous observations of the source at optical/UV/X-ray/γ-ray during three different epochs, as obtained from Swift-UVOT/Swift-XRT/Fermi-LAT, are supplemented with the information available from the VHE telescope array, HAGAR. The long-term flux variability at the Fermi-LAT energies is clearly found to be lognormal. It is seen that the broadband spectral energy distribution of 1ES 1011+496 can be successfully reproduced by synchrotron and synchrotron self Compton emission models. Notably, the observed curvature in the photon spectrum at X-ray energies demands a smooth transition of the underlying particle distribution from a simple power law to a power law with an exponential cutoff, or a smooth broken power law distribution, which may possibly arise when the escape of the particles from the main emission region is energy dependent. Specifically, if the particle escape rate is related to its energy as E 0.5 , then the observed photon spectrum is consistent with the ones observed during the various epochs.

Possible proton synchrotron origin of X-ray and gamma-ray emission in large-scale jet of 3C 273

Abstract The large-scale jet of quasar 3C 273 has been observed in radio to gamma-ray frequencies. Earlier the X-ray emission from knot A of this jet has been explained with inverse Compton scattering of the cosmic microwave background radiations by the shock accelerated relativistic electrons in the jet. More recently it has been shown that this mechanism overproduces the gamma-ray flux at GeV energy and violates the observational results from Fermi LAT. We have considered the synchrotron emission from a broken power-law spectrum of accelerated protons in the jet to explain the observed X-ray to gamma-ray flux from knot A. The two scenarios discussed in our work are (i) magnetic field is high, synchrotron energy loss time of the protons is shorter than their escape time from the knot region and the age of the jet and (ii) their escape time is shorter than their synchrotron energy loss time and the age of the jet. These scenarios can explain the observed photon spectrum well for moderate values of Doppler factor. The required jet luminosity is high ∼1046 erg s−1 in the first scenario and moderate ∼1045 erg s−1 in the second, which makes the second scenario more favourable.

BAYESIAN CONFIDENCE LIMITS OF ELECTRON SPECTRA OBTAINED THROUGH REGULARIZED INVERSION OF SOLAR HARD X-RAY SPECTRA

Many astrophysical observations are characterized by a single, non-repeatable measurement of a source brightness or intensity, from which we are to construct estimates for the true intensity and its uncertainty. For example, the hard X-ray count spectrum from transient events such as solar flares can be observed only once, and from this single spectrum one must determine the best estimate of the underlying source spectrum I({epsilon}), and hence the form of the responsible electron spectrum F(E). Including statistical uncertainties on the measured count spectrum yields a 'confidence strip' that delineates the boundaries of electron spectra that are consistent with the observed photon spectrum. In this short article, we point out that the expectation values of the source brightness and its variance in a given photon energy bin are in general not (as has been assumed in prior works) equal to n, the number of counts observed in that energy bin. Rather, they depend both on n and on prior knowledge of the overall photon spectrum. Using Bayesian statistics, we provide an explicit procedure and formulas for determining the 'confidence strip' (Bayesian credible region) for F(E), thus providing rigorous bounds on the intensity and shape of the accelerated electron spectrum.

Neutrinos from Cosmic Accelerators including Magnetic Field and Flavor Effects

We review the particle physics ingredients affecting the normalization, shape, and flavor composition of astrophysical neutrinos fluxes, such as different production modes, magnetic field effects on the secondaries (muons, pions, and kaons), and flavor mixing, where we focus onpγinteractions. We also discuss the interplay with neutrino propagation and detection, including the possibility to detect flavor and its application in particle physics, and the use of the Glashow resonance to discriminatepγfromppinteractions in the source. We illustrate the implications on fluxes and flavor composition with two different models: (1) the target photon spectrum is dominated by synchrotron emission of coaccelerated electrons and (2) the target photon spectrum follows the observed photon spectrum of gamma-ray bursts. In the latter case, the multimessenger extrapolation from the gamma-ray fluence to the expected neutrino flux is highlighted.

DERIVATION OF THE ELECTRON DISTRIBUTION IN SUPERNOVA REMNANT RX J1713.7–3946 VIA A SPECTRAL INVERSION METHOD

We show that the radio, X-ray and gamma-ray spectrum of the supernova remnant RX J1713.7-3946 can be accounted for with the simplest emission model, where all of these emissions are attributed to a population of relativistic electrons interacting with the cosmic microwave background radiation, IR interstellar photons, and a background magnetic field. With a spectral inversion method (Johns & Lin 1992), the parent electron distribution and its uncertainties are derived from the observed photon spectrum. These results are independent on the model of particle acceleration and strongly support the leptonic scenario for the TeV emission.

Lepto-hadronic origin of γ-rays from the G54.1+0.3 pulsar wind nebula

Abstract G54.1+0.3 is a Crab-like pulsar wind nebula (PWN) with the highest γ-ray to X-ray luminosity ratio among all the nebulae driven by young rotation-powered pulsars. We model the spectral evolution of the PWN and find it difficult to match the observed multiband data with leptons alone using reasonable model parameters. In a lepton–hadron hybrid model, however, TeV photons come mainly from π0 decay in proton–proton interaction and the observed photon spectrum can be well reproduced. The newly discovered infrared loop and molecular cloud that lie in or closely around the PWN can work as the target for the bombardment of the PWN protons.

TeV GAMMA-RAY EMISSION FROM VELA X: LEPTONIC OR HADRONIC?

We study nonthermal photon emission properties from Vela X in both a leptonic and a hadronic-leptonic model. In the leptonic model, it is difficult to reproduce the observed photon spectrum from X-ray to TeV bands using reasonable model parameters; the model results can compare with either the observed data at the TeV band (the total energy of electrons required to match the observations is W e ≈ 6.1 × 1045 erg) or those at the X-ray band (W e ≈ 1.3 × 1045 erg). In the hadronic-leptonic model, however, the observed photon spectrum can be reproduced in the hadronic-leptonic model and TeV photons are mainly from π0 decay in proton-proton interaction. The energy required for iron nuclei released in the pulsar wind is W Fe ≈ 2.1 × 1048 erg and the energy of iron nuclei transferred to positrons is erg in this case.

Understanding the electroluminescence emitted by single molecules in scanning tunneling microscopy experiments

We explore theoretically the electroluminescence of single molecules. We adopt a local-electrode framework that is appropriate for scanning tunneling microscopy (STM) experiments where electroluminescence originates from individual molecules of moderate size on complex substrates: Couplings between the STM tip and molecule and between the molecule and multiple substrate sites are treated on the same footing as local electrodes contacting the molecule. Electron flow is modeled with the Lippmann-Schwinger Green's function scattering technique. The evolution of the electronic energy levels of the molecule under bias is modeled assuming the total charge of the molecule to be invariant, consistent with Coulomb blockade considerations, but the electronic occupations of the molecular highest occupied molecular orbital and lowest unoccupied molecular orbital levels vary with changing bias. The photon-emission rate is calculated using Fermi's golden rule. We apply this theoretical approach to the STM/Zn-etioporphyrin/${\text{Al}}_{2}{\text{O}}_{3}/\text{NiAl}(110)$ system and simulate various configurations of coupling strength between the molecule and substrate. We compare our results to the experimental observations of Qiu et al. [Science 299, 542 (2003)] for this system and find that our model provides a comprehensive explanation of a multitude of previously unexplained observations. These include the different types of current-voltage characteristics (CVCs) that are observed experimentally, the observed association of electroluminescence with some CVCs and not others, and key properties of the observed photon spectra. Theoretical predictions are presented for further single-molecule electroluminescence experiments.

Solar flare hard X-ray spectra possibly inconsistent with the collisional thick target model

Recent progress in solar hard X-ray (HXR) observations with RHESSI data and methods for spectral inversion allow us to study model-independent mean electron flux spectra in solar flares. We report several hard X-ray events observed by RHESSI in which the photon spectra I( ϵ) are such that the inferred source mean electron spectra are not consistent with the standard model of collisional transport in solar flares. The observed photon spectra are so flat locally that the recovered mean electron flux spectra show a dip around 17–31 keV. While we note that alternative explanations, unrelated to electron transport, have not been ruled out, we focus on the physical implications of this tentative result for the collisional thick target model.

The Effect of the Compton Backscattering Component on the Photon Spectra of Three Yohkoh /Hard X-Ray Telescope Flares

A study of the photon spectra of three Yohkoh/hard X-ray telescope (HXT) events at the nonthermal peak times is presented. It is found that the photon spectra are very hard (gamma similar to 2.6-3.1) and have a single power law approximately between the HXT M1 and H channels (23-93 keV). However, the photon spectra become very flattened between the HXT L and M1 channels (13-33 keV); i.e., the M1/L channel ratio is excessively large. Consequently, the observed photon spectra cannot be produced simply by a single-power-law electron spectrum with a low-energy cutoff/flattening. We understand that this property of the photon spectra may result from the superposition of a strong Compton backscattering component in the observed hard X-ray spectra. This Letter points out that the contribution of the Compton backscattering may actually be significant for the received hard X-ray emissions in the deka-keV energy range, which has been indicated by previous researchers through theoretical calculations.

Nonuniform Target Ionization and Fitting Thick Target Electron Injection Spectra to RHESSI Data

Past analyses of flare hard X-ray (HXR) spectra have largely ignored the effect of nonuniform ionization along the electron paths in the thick-target model, though it is very significant for well-resolved spectra. The inverse problem (photon spectrum to electron injection spectrum F 0(E 0)) is disturbingly non-unique. However, we show that it is relatively simple to allow for the effect in forward fitting of parametric models of F 0(E 0) and provide an expression to evaluate it for the usual single power-law form of F 0(E 0). The expression involves the column depth N * of the transition region in the flare loop as one of the parameters so data fitting can enable derivation of N * (and its evaporative evolution) as part of the fitting procedure. The fit to RHESSI data on four flares for a single power law F 0(E 0) is much improved when ionization structure is included compared to when the usual fully ionized approximation is used. This removes the need, in these events at least, to invoke broken power laws, or other forms, of the acceleration spectrum F 0(E 0) to explain the observed photon spectrum

Inversion of Thick Target Bremsstrahlung Spectra from Nonuniformly Ionised Plasmas

The effects of non-uniform plasma target ionisation on the spectrum of thick-target HXR bremsstrahlung from a non-thermal electron beam are analysed. In particular the effect of the target ionisation structure on beam collisional energy losses, and hence on inversion of an observed photon spectrum to yield the electron injection spectrum, is considered and results compared with those obtained under the usual assumption of a fully ionised target.

Does the inverse Compton mechanism alone explain the GeV/TeV gamma-ray emission from the Crab Nebula?

Abstract The origin of the broad-band photon spectrum of the Crab Nebula is discussed. The synchrotron radiation calculated for the spatial distribution of the nebular magnetic field derived from the MHD flow model is in a good agreement with the observed photon spectrum from radio wavelengths to γ-rays, up to E γ ≤ 1 GeV. The inverse Compton scattering of the accelerated electrons on the synchrotron, “dust” IR, and 2.7 K microwave background radiation fields is the most effective γ-ray production mechanism at photon energies above 100 GeV. We argue that the bremsstrahlung radiation of relativistic electrons produced in the Crab filaments could play a principal role in the formation of the spectrum of the Crab Nebula in the intermediate energy domain between 1 and 100 GeV.

Preequilibrium emission of hard photons in proton-nucleus reactions.

The preequilibrium hybrid model is extended to account for emission of hard photons using the quasideuteron radiative concept and detailed balance. We analyze proton-nucleus reactions, find good accord with the observed photon spectra at 72 MeV incident proton energy, and predict two times higher spectra than the observed ones at 140 MeV.

Bremsstrahlung spectra from thick-target electron beams with noncollisional energy losses

A generalization of the Brown's (1971) formulation of the relationship between bremsstrahlung spectrum and mean electron spectrum (electrons in the range 10-100 keV) is presented with an objective of demonstrating the information content of bremsstrahlung spectra from a thick target. It is shown that the observed photon spectrum can be inverted to yield an integral functional of the electron spectrum and the effective energy loss rate. Furthermore, if observational or theoretical electron injection spectrum is known, an effective 'phenomenological' energy loss function can be obtained.

Inversion of synchrotron spectra

The problem of synchrotron radiation spectra is treated from the viewpoint of deconvolving the spectrum of ultrarelativistic source electrons from the observed photon spectrum. It is shown that for homogeneous sources the problem amounts to inversion of a Meijer transform with a modified Bessel finction kernel. A precise analytic inversion is only possible in the complex plane but Meijer transform tables are available for a wide range of functions. More convenient inversion formulae prove possible by use of a Laplace transform approximation or by analysing the spectra in terms of their integral moments. The filtering property of the transform is also established showing that the contribution to the synchrotron spectrum of high frequency components in the electron spectrum declines exponentially with their frequency. Thus, as with other Laplace-like transforms, only a few terms in an electron spectrum expansion can be deconvolved for any plausible noise level in the synchrotron spectrum.

The spectroscopy of ion-induced photon emission and its application to quantitative surface analysis

Experimentally observed photon spectra produced by ion bombardment of transition elements Sc, Ti, V, Cr, Fe, Ni, and Cu and Zn with Ar + ions are very similar to arc spectra of these same elements, with respect to relative intensities of lines observed. Standard spectrochemical analysis techniques, based on the assumption of local thermodynamic equilibrium in the arc plasma, have been applied to the spectra of the pure elements, to determine the effective arc temperature under relatively clean conditions and in an oxygen environment. These techniques have then been applied to the analysis of a stainless steel sample, again under relatively clean and oxygen environments. In order to explain differences in the Fe Cr ratios measured, the behaviour of the intensities of the Fe and Cr emission has been investigated as a function of the oxygen background pressure. The results indicate that while analysis is possible the results depend on the oxygen background pressure.