Leptonic Component Research Articles

THE PAMELA COSMIC RAY SPACE OBSERVATORY

PAMELA is a satellite borne experiment designed to study with great accuracy cosmic rays of galactic, solar, and trapped nature in a wide energy range (protons: 80 MeV–700 GeV, electrons 50 MeV–400 GeV). Main objective is the study of the antimatter component: antiprotons (80 MeV–190 GeV), positrons (50 MeV–270 GeV) and search for antimatter with a precision of the order of 10-8). The experiment, housed on board the Russian Resurs-DK1 satellite, was launched on June, 15 2006 in a 350 × 600 km orbit with an inclination of 70 degrees. The detector is composed of a series of scintillator counters arranged at the extremities of a permanent magnet spectrometer to provide charge, Time-of-Flight and rigidity information. Lepton/hadron identification is performed by a Silicon-Tungsten calorimeter and a Neutron detector placed at the bottom of the device. An Anticounter system is used offline to reject false triggers coming from the satellite. In self-trigger mode the Calorimeter, the neutron detector and a shower tail catcher are capable of an independent measure of the lepton component up to 2 TeV. In this work we present some of its scientific results in its first five years of operation.

The time-dependent one-zone hadronic model

We present a time-dependent approach to the one-zone hadronic model in the case where the photon spectrum is produced by ultrarelativistic protons interacting with soft photons that are produced from protons and low magnetic fields. Assuming that protons are injected at a certain rate in a homogeneous spherical volume containing a magnetic field, the evolution of the system can be described by five coupled kinetic equations, for protons, electrons, photons, neutrons, and neutrinos. Photopair and photopion interactions are modelled using the results of Monte-Carlo simulations and, in particular from the SOPHIA code for the latter. The coupling of energy losses and injection introduces a self-consistency in our approach and allows the study of the comparative relevancy of processes at various conditions, the efficiency of the conversion of proton luminosity to radiation, the resulting neutrino spectra, and the effects of time variability on proton injection, among other topics. We present some characteristic examples of the temporal behaviour of the system and show that this can be very different from the one exhibited by leptonic models. Furthermore, we argue that, contrary to the wide-held belief, there are parameter regimes where the hadronic models can become quite efficient. However, to keep the free parameters at a minimum and facilitate an in-depth study of the system, we have only concentrated on the case where protons are injected; i.e., we did not consider the effects of a co-accelerated leptonic component.

Missing baryons in shells around galaxy clusters

The cluster baryon fraction is estimated from the CMB-scattering leptonic component of the intracluster medium (ICM); however, the observed cluster baryon fraction is less than the cosmic one. Understanding the origin of this discrepancy is necessary for correctly describing the structure of the ICM. We estimate the baryonic mass in the outskirts of galaxy clusters which is difficult to observe because of low electron temperature and density in these regions. The time scale for the electrons and protons to reach equipartition in the outskirts is longer than the cluster age. Since thermal equilibrium is not achieved, a significant fraction of the ICM baryons may be hidden in shells around galaxy clusters. We derive the necessary condition on the cluster mass for the concealment of missing baryons in an outer baryon shell and show that this condition is fulfilled because cluster masses are comparable to the estimated characteristic mass $M=e^4/(m^3_{p} G^2)=1.3x10^{15}$ solar masses. The existence of extreme-ultraviolet emission haloes around galaxy clusters is predicted.

Launch of the space experiment PAMELA

PAMELA is a satellite borne experiment designed to study with great accuracy cosmic rays of galactic, solar, and trapped nature in a wide energy range (protons 80 MeV–700 GeV, electrons 50 MeV–400 GeV). Main objective is the study of the antimatter component: antiprotons (80 MeV–190 GeV), positrons (50 MeV–270 GeV) and search for antimatter with a precision of the order of 10 −8. The experiment, housed on board the Russian Resurs-DK1 satellite, was launched on June 15th, 2006 in a 350 × 600 km orbit with an inclination of 70°. The detector is composed of a series of scintillator counters arranged at the extremities of a permanent magnet spectrometer to provide charge, time-of-flight, and rigidity information. Lepton/hadron identification is performed by a silicon–tungsten calorimeter and a neutron detector placed at the bottom of the device. An anticounter system is used offline to reject false triggers coming from the satellite. In self-trigger mode the calorimeter, the neutron detector, and a shower tail catcher are capable of an independent measure of the lepton component up to 2 TeV. In this work we describe the experiment, its scientific objectives, and the performance in the first months after launch.

Prompt emission of high-energy photons from gamma ray bursts

Within the internal shock scenario, we consider different mechanisms of high-energy ( > 1 MeV) photon production inside a gamma ray burst (GRB ) fireball and derive the expected high-energy photon spectra from individual GRBs during the prompt phase. The photon spectra of leptonic and hadronic origins are compared within different sets of parameter regimes. Our results suggest that the high-energy emission is dominated by the leptonic component if the fraction of shock energy carried by electrons is not very small (e.g. ∈ e > 10 -3 ). For very small values of ∈ e , the hadronic emission component could be comparable to or even exceed the leptonic component in the GeV-TeV regime. However, in this case a much larger energy budget of the fireball is required to account for the same level of the observed sub-MeV spectrum. The fireballs are therefore extremely inefficient in radiation. For a canonical fireball bulk Lorentz factor (e.g. r = 400), emissions above ∼ 10 GeV are attenuated by two-photon pair production processes. For a fireball with an even higher Lorentz factor, the cut-off energy is higher, and emissions of 10 TeV-PeV due to 7r°-decay can also escape from the internal shocks. The flux level is, however, too low to be detected by current TeV detectors, and these photons also suffer attenuation by external soft photons. The GLAST Large Area Telescope can detect prompt emission of bright long GRBs above 100 MeV. For short GRBs, the prompt emission can be only barely detected for nearby bright ones with relatively 'long' durations (e.g. ∼1 s). With the observed high-energy spectrum alone, it appears that there is no clean picture to test the leptonic versus hadronic origin of the gamma rays. Such an issue may be, however, addressed by collecting both prompt and afterglow data. A moderate-to-high radiative efficiency would suggest a leptonic origin of high-energy photons, while a GRB with an extremely low radiative efficiency but an extended high-energy emission component would be consistent with (but not a proof for) the hadronic origin.

Launch and commissioning of the PAMELA experiment on board the Resurs-DK1 satellite

PAMELA is a satellite borne experiment designed to study with great accuracy cosmic rays of galactic, solar, and trapped nature in a wide energy range (protons: 80 MeV–700 GeV, electrons 50 MeV–400 GeV). Main objective is the study of the antimatter component: antiprotons (80 MeV–190 GeV), positrons (50 MeV–270 GeV) and search for antimatter (with a precision of the order of 10 −8). The experiment, housed on board the Russian Resurs-DK1 satellite, was launched on June, 15th 2006 in a 350 × 600 km orbit with an inclination of 70°. The detector consists of a permanent magnet spectrometer core to provide rigidity and charge sign information, a Time-of-Flight system for velocity and charge information, a silicon–tungsten calorimeter and a neutron detector for lepton/hadron identification. An anticounter system is used off-line to reject false triggers coming from the satellite. In self-trigger mode the calorimeter, the neutron detector and a shower tail catcher are capable of an independent measure of the lepton ( e + + e −) component up to 2 TeV. In this work we focus on the first months of operations of the experiment during the commissioning phase.

Gamma-ray bursts prompt emission spectrum: an analysis of a photosphere model

A thermal radiative component is likely to accompany the first stages of the prompt emission of gamma-ray bursts (GRBs) and X-ray flashes. We analyse the effect of such a component on the observable spectrum, assuming that the observable effects are due to a dissipation process occurring below or near the thermal photosphere. For comparable energy densities in the thermal and leptonic components, the dominant emission mechanism is Compton scattering. This leads to a nearly flat energy spectrum (nuFnu proportional, 0) above the thermal peak at approximately 10-100 keV and below 10-100 MeV, for a wide range of optical depths 0.03 less, similar tau less, similar 100, regardless of the details of the dissipation mechanism or the strength of the magnetic field. For higher values of the optical depth, a Wien peak is formed at 100 keV to 1 MeV. In particular, these results are applicable to the internal shock model of GRBs, as well as to slow dissipation models, e.g. as might be expected from reconnection, if the dissipation occurs at a sub-photospheric radii. We conclude that dissipation near the thermal photosphere can naturally explain (i) clustering of the peak energy at sub-MeV energies at early times, (ii) steep slopes observed at low energies, and (iii) a flat spectrum above 10 keV at late times. Our model thus provides an alternative scenario to the optically thin synchrotron-synchrotron self-Compton model.

The Observable Effects of a Photospheric Component on GRB and XRF Prompt Emission Spectrum

A thermal radiative component is likely to accompany the first stages of the prompt emission of gamma-ray bursts (GRBs) and X-ray flashes (XRFs). We analyze the effect of such a component on the observable spectrum, assuming that the observable effects are due to a dissipation process occurring below or near the thermal photosphere. We consider both the internal shock model and a slow heating model as possible dissipation mechanisms. For comparable energy densities in the thermal and leptonic components, the dominant emission mechanism is Compton scattering. This leads to a nearly flat energy spectrum (νFν ∝ ν0) above the thermal peak at ≈10-100 keV and below 10-100 MeV, for a wide range of optical depths 0.03 τγe 100, regardless of the details of the dissipation mechanism or the strength of the magnetic field. At lower energies steep slopes are expected, while above 100 MeV the spectrum depends on the details of the dissipation process. For higher values of the optical depth, a Wien peak is formed at 100 keV-1 MeV, and no higher energy component exists. For any value of τγe, the number of pairs produced does not exceed the baryon-related electrons by a factor of larger than a few. We conclude that dissipation near the thermal photosphere can naturally explain both the steep slopes observed at low energies and a flat spectrum above 10 keV, thus providing an alternative scenario to the optically thin synchrotron-SSC model.

Spectral and temporal signatures of ultrarelativistic protons in compact sources

We present calculations of the spectral and temporal radiative signatures expected from ultrarelativistic protons in compact sources. The coupling between the protons and the leptonic component is assumed to occur via Bethe-Heitler pair production. This process is treated by modeling the results of Monte-Carlo simulations and incorporating them in a time-dependent kinetic equation, that we subsequently solve numerically. Thus, the present work is, in many respects, an extension of the leptonic `one-zone' models to include hadrons. Several examples of astrophysical importance are presented, such as the signature resulting from the cooling of relativistic protons on an external black-body field and that of their cooling in the presence of radiation from injected electrons. We also investigate and refine the threshold conditions for the 'Pair Production/Synchrotron' feedback loop which operates when relativistic protons cool efficiently on the synchrotron radiation of the internally produced Bethe-Heitler pairs. We demonstrate that an additional component of injected electrons lowers the threshold for this instability.

Afterglow Emission from Pair‐loaded Blast Waves in Gamma‐Ray Bursts

The MeV radiation front of gamma-ray bursts creates copious e± pairs as it propagates through an ambient medium. The created pairs enrich the leptonic component of the medium by a large factor at distances R < Rload ~ 1016 cm from the burst center. The following blast wave sweeps up the pair-rich medium and then emits the observed afterglow radiation. We find that the afterglow has a memory of e± loading outside Rload. The e± remain in the swept-up material and slowly cool down by emitting synchrotron radiation. They are likely to dominate the blast wave emission in IR, optical, and UV bands during the first minutes of the observed afterglow. The e± afterglow is described by a simple formula, which is derived analytically and checked by numerical integration of synchrotron emission over the blast material; a suitable Lagrangian formalism is developed for such calculations. The main signature of e± radiation is its flat (white) spectrum in a broad range of frequencies from IR to UV and possibly soft X-rays. This radiation can be detected by the Swift satellite, which would enable new observational tests for the explosion physics.

Gauge freedom of Dirac theory in complexified spacetime algebra

A Dirac theory over complexified spacetime algebra leads unavoidably to a U(1) ⊗ U(1) ⊗ SU(3) global gauge freedom. We argue the SU(3) freedom corresponds to colour. The U(1) ⊗ U(1) gauge freedom mixes to give two photon fields responsible for the charge assignments of the standard model. In addition to the usual up/down spin and particle/anti-particle decomposition of the Dirac field, this 16-dimensional field decomposes into a colourless leptonic component and a three-coloured quarkonic component.

Nonthermal Emission from a Supernova Remnant in a Molecular Cloud

In evolved supernova remnants (SNRs) interacting with molecular clouds, such as IC 443, W44, and 3C391, a highly inhomogeneous structure consisting of a forward shock of moderate Mach number, a cooling layer, a dense radiative shell and an interior region filled with hot tenuous plasma is expected. We present a kinetic model of nonthermal electron injection, acceleration and propagation in that environment and find that these SNRs are efficient electron accelerators and sources of hard X- and gamma-ray emission. The energy spectrum of the nonthermal electrons is shaped by the joint action of first and second order Fermi acceleration in a turbulent plasma with substantial Coulomb losses. Bremsstrahlung, synchrotron, and inverse Compton radiation of the nonthermal electrons produce multiwavelength photon spectra in quantitative agreement with the radio and the hard emission observed by ASCA and EGRET from IC 443. We distinguish interclump shock wave emission from molecular clump shock wave emission accounting for a complex structure of molecular cloud. Spatially resolved X- and gamma- ray spectra from the supernova remnants IC 443, W44, and 3C391 as might be observed with BeppoSAX, Chandra XRO, XMM, INTEGRAL and GLAST would distinguish the contribution of the energetic lepton component to the gamma-rays observed by EGRET.

A Relativistic Bremsstrahlung/Inverse Compton Origin for 2EG J1857+0118 Associated with Supernova Remnant W44

We show that relativistic bremsstrahlung and inverse Compton scattering of radio-emitting electrons can easily account for the observed γ-ray spectrum of 2EG J1857+0118 if the field strength in the shell is below ~30 μG. This source is located at the eastern border of the composite SNR W44, where the expanding radio shell is interacting with a dense molecular cloud. The nondetection of this remnant above 250 GeV implies a cutoff or steepening in the electron spectrum above ~100 GeV. The E-1.66 spectrum of this radio/γ-ray-emitting electron component is too flat to have its origin in standard first-order Fermi acceleration, but electron injection into the shell by the pulsar PSR B1853+01 over the 2 × 104 yr lifetime may explain why the Crab-like radio spectrum (Sν ∝ ν-0.33) is about the hardest of all shell-type remnants. The injected energy would be sufficient to account for the required energy of 6 × 1049 ergs if the initial spin-down power of PSR B1853+01 was about 10 times larger than the present spin-down power of the Crab pulsar. A steeper Fermi electron component may be present, but the observational data are not constraining enough to provide a meaningful limit on the presence of an additional ~E-2 shell-type electron component. The predicted γ-ray contribution from high-energy proton-gas interactions is about 20% of the observed EGRET flux above 100 MeV, which confirms our conclusion that the γ-ray emission from W44 is dominated by a leptonic component.

Electron Acceleration and Efficiency in Nonthermal Gamma-Ray Sources

In energetic nonthermal sources such as gamma-ray bursts, AGN or galactic jet sources, etc., one expects both relativistic and transrelativistic shocks acompanied by violent motions of moderately relativistic plasma. We present general considerations indicating that these sites are electron and positron accelerators leading to a modified power law spectrum. The electron (or $e^\pm$) energy index is very hard, $\propto \gamma^{-1}$ or flatter up to a comoving frame break energy $\gamma_\ast$, and becomes steeper above that. In the example of gamma-ray bursts the Lorentz factor reaches $\gamma_\ast\sim 10^3$ for $e^{\pm}$ accelerated by the internal shock ensemble on subhydrodynamical time scales. For pairs accelerated on hydrodynamical timescales in the external shocks similarly hard spectra are obtained, and the break Lorentz factor can be as high as $\gamma_\star \siml 10^5$. Radiation from the nonthermal electrons produces photon spectra with shape and characteristic energies in qualitative agreement with observed generic gamma-ray burst and blazar spectra. The scenario described here provides a plausible way to solve one of the crucial problems of nonthermal high energy sources, namely the efficient transfer of energy from the proton flow to an apropriate nonthermal lepton component.

AcoplanarDi-leptons andMixed events on the basis of two supergravity model predictions

Purpose of this note is to propose a search for typical signatures produced by charginos, neutralinos and sleptons on the basis of two supergravity model predictions, at the highest e+ e− energy which will be available with LEP-II. The typical signatures are of two main classes: i) «pure leptonic» states; ii) «mixed lepton-jets» states. The «pure leptonic» states consist of acoplanardi-leptons with «leading» and «non-leading» leptonic components plusmulti-di-lepton states. All with at least two missing momenta. The «mixed lepton-jets» states contain at least two jets and one or three leptons plus missing momenta. All these final states represent clear signatures for SUSY. Cross-sections and branching ratios are computed using two «local» supersymmetry models:i.e. the minimalSU(5) supergravity and theSU(5)×U(1) supergravity. The corresponding numbers of «pure leptonic» and of «mixed lepton-jets» final states are computed. We also discuss the upgrading needed in an experimental set-up at LEP-II especially suited for the detection of the supersymmetry signals expected on the basis of the two supergravity models quoted. These models are at present the most reliable in terms of physically sound assumptions.

Simplistic Weak Interaction

A phenomenological weak interaction is proposed on the principle of requiring a minimal addition of new elementary fermions. It also has the feature that decay of all charmed particles must show a leptonic component. Various experimental tests are suggested.