Electromagnetic and corpuscular emission from the solar flare of 1991 June 15: Continuous acceleraton of relativistic particles

Data on X-,γ-ray, optical and radio emission from the 1991 June 15 solar flare are considered. We have calculated the spectrum of protons that producesγ-rays during the gradual phase of the flare. The primary proton spectrum can be described as a Bessel-function-type up to 0.8 GeV and a power law with the spectral index ≈3 from 0.8 up to 10 GeV or above. We have also analyzed data on energetic particles near the Earth. Their spectrum differed from that of primary protons producingγ-ray line emission. In the gradual phase of the flare additional pulses of energy release occurred and the time profiles of cm-radio emission andγ-rays in the 0.8–10 MeV energy band and above 50 MeV coincided. A continuous and simultaneous stochastic acceleration of the protons and relativistic electrons at the gradual phase of the flare is considered as a natural explanation of the data.     

The chemical composition of cosmic ray nuclei above 1.3 GeV n−1 and 23 GeV n−1

Measurements made with a balloon-borne counter telescope having an exposure factor of 774 m² sr s are reported. The telescope was flown from Palestine, Tex., during the fall of 1971 for a total of 10 h under an average residual atmospheric depth of 4.4 g cm^–2. The data analysis indicates that the integral flux ratios above 1.3 Gev n^–1 and 23 GeV n^–1 are consistent with energy independence.National Academy of Science-Senior Resident Research Associate     

GeV emission from neutron-rich internal shocks of some long γ-ray bursts

In the neutron-rich internal shocks model for γ-ray bursts (GRBs), the Lorentz factors (LFs) of ion shells are variable, and so are the LFs of accompanying neutron shells. For slow neutron shells with a typical LF of approximate tens, the typical β-decay radius is  ∼10¹⁴–10¹⁵ cm  . As GRBs last long enough  [ T ₉₀ > 14(1 + z ) s]  , one earlier but slower ejected neutron shell will be swept successively by later ejected ion shells in the range  ∼10¹³–10¹⁵ cm  , where slow neutrons have decayed significantly. Part of the thermal energy released in the interaction will be given to the electrons. These accelerated electrons will mainly be cooled by the prompt soft γ-rays and give rise to GeV emission. This kind of GeV emission is particularly important for some very long GRBs and is detectable for the upcoming satellite Gamma-Ray Large Area Space Telescope (GLAST).     

Inverse Compton e± pair cascade model for the γ-ray production in massive binary LSI +61° 303

We apply an inverse Compton   e ^±  pair cascade model for γ-ray production in the massive binary system LSI +61° 303 assuming that electrons are accelerated already inside the inner part of the jet launched by the compact object. γ-ray spectra, affected by the cascade process, and lower energy spectra, from the synchrotron cooling of the highest energy electrons in the jet, are calculated as a function of the phase of this binary system. γ-ray spectra expected in such a model have different shape than those ones produced by electrons in the jet directly to observer. Moreover, the model predicts clear anticorrelation between γ-ray fluxes in the GeV (1–10 GeV) and TeV (>200 GeV) energy ranges with the peak of the TeV emission at the phase ∼0.5 (the peak half-width ranges between the phases ∼0.4–0.9, for the inclination of the binary system equal to 60°, and ∼0.4–0.1 for 30°). The fine features of TeV γ-ray emission (fluxes and spectral shapes) as a function of the phase of the binary system are consistent with recent observations reported by the MAGIC collaboration. Future simultaneous observations in the GeV energies (by the GLAST and AGILE telescopes) and in the TeV energies (by the MAGIC and VERITAS telescopes) should test other predictions of the considered model supporting or disproving the hypothesis of acceleration of electrons already in the inner part of the microquasar jets.     

A local interstellar spectrum for galactic electrons

A heliopause spectrum at 122 AU from the Sun is presented for galactic electrons over an energy range from 1 MeV to 50 GeV that can be considered the lowest possible local interstellar spectrum (LIS). The focus of this work is on the spectral shape of the LIS below ∼1.0 GeV. The study is done by using a comprehensive numerical model for solar modulation in comparison with Voyager 1 observations at ∼112 AU from the Sun and PAMELA data at Earth. Below ∼1.0 GeV, this LIS exhibits a power law with E^{−(1.55 ± 0.05)}, where E is the kinetic energy of these electrons. However, reproducing the PAMELA electron spectrum averaged for 2009, requires a LIS with a different power law of the form E^{−(3.15 ± 0.05)} above ∼5 GeV. Combining the two power laws with a smooth transition from low to high energies yields a LIS over the full energy range that is relevant and applicable to the modulation of cosmic ray electrons in the heliosphere. The break occurs between ∼800 MeV and ∼2 GeV as a characteristic feature of this LIS. The power-law form below ∼1 GeV produces a challenge to the origin of these low energy galactic electrons. On the other hand, the results of this study can be used as a gauge for astrophysical modeling of the local interstellar spectrum for electrons.     

The energy spectrum of cosmic ray electrons between 10 and 1000 GeV

Measurements made by the Bombay Group on the fluxes of cosmic ray electrons in the energy range 10–1000 GeV have been compared with those of other workers in the same energy domain with a view to understand the present confused situation on the existing observations at these high energies. Such an analysis clearly brings out the current situation in its true perspective and highlights the care and emphasis to be placed on future experimentation in this important field.     

Spectral variability in hard X-rays and the evidence for a 13.5 years period in the bright quasar 3C273

We report the observation of nearest quasar 3C273 made with LASE instrument on November 20th, 1998 as a part of our continuing programme of balloon borne hard X-ray observations in the 20–200 keV band using high sensitivity Large Area Scintillation counter Experiment. Our data clearly show a steep spectrum in the 20–200 keV with power law spectral indexα = 2.26 ± 0.07. This is in complete contrast to the reported data from OSSE and BeppoSAX which suggest the value of 1.3 to 1.6 for the power law index in the X-ray energy band, but is quite consistent with the value derived for the high energy gamma ray data. A single power law fit in the X-ray and gamma ray energy bands points to a common origin of these photons and the absence of spectral break around 1 MeV as suggested in literature. We have reanalyzed the available data to study the temporal variability of the spectrum in the hard X-ray band. Our analysis reveals that 50 keV flux from the source, shows a strong modulation with a period of about 13.5 years. The analysis of the optical light curve of the source also supports the 5000 day period. We discuss the emission mechanism and the possible sites for X-ray photons along with the implications of the long term periodicity with respect to source geometry.     

Extragalactic UHE proton spectrum and prediction for iron-nuclei flux at 10–10 GeV

We investigate the problem of transition from galactic cosmic rays to extragalactic ultra-high energy cosmic rays. Using the model for extragalactic ultra-high energy cosmic rays and observed all-particle cosmic ray spectrum, we calculate the galactic spectrum of iron nuclei in the energy range 10⁸–10⁹ GeV. The flux and spectrum predicted at lower energies agree well with the KASCADE data. The transition from galactic to extragalactic cosmic rays is distinctly seen in spectra of protons and iron nuclei, when they are measured separately. The shape of the predicted iron spectrum agrees with the Hall diffusion.     

Rocket observations of the interaction of auroral electrons with the atmosphere

Electron spectra obtained during the flight of Black Brant VB-31 on August 17, 1970 through a stable aurora to a height of 268 km have been analyzed in detail to obtain the pitch angle distributions from 25 to 155° and the electron energy distributions over an energy range of 18 keV to 20 eV through the region of atmospheric interaction down to 97 km. Backscatter ratios for 140° pitch angle range from 0.065 for 18 keV electrons to 0.22 for 1 keV electrons. Backscatter of lower energy electrons decreases with atmospheric depth below 200 km. The effect of the interactions between auroral electrons and the atmosphere is such as to give a peak in electron flux which moves progressively to higher energies with penetration depth. The secondary electron flux increases monotonically with height up to 200 km. The secondary electron spectrum can be approximated by an energy power over small energy ranges but its form is somewhat dependent on height and on the primary electron spectrum.     

Multiple compton scattering of electrons in a photon field

Consideration is given to the motion of electrons in a photon field of the monoenergetic or power-law spectrum under the conditions when the main mechanism of energy loss is the inverse Compton scattering by field photons. This process changes the primary spectrum of electrons and converts low-energy field photons to high-energy gamma-quanta for which the electron confinement region is assumed to be optically thin. The electron and gamma-ray spectra have been obtained in a wide energy interval including the Klein-Nishina and Thomson regions. A simple qualitative dependence of the solutions found on the field parameters and the primary spectrum of electrons has been established.The electron and gamma-ray spectra have been obtained by numerically solving the kinetic equation dependent on two variables: the energy of electrons and their path (or the time of motion) in a photon field. The results dramatically differ from the solution of the steady-state kinetic equation which depends only on the electron energy and is frequently used in the given problem.     

Evidence for accelerated electrons upstream from Jupiter's bow shock: Ulysses results

Several transient increases of electrons with energies in the range 40–100 keV have been detected upstream and immediately downstream from the Jovian bow shock (and only in these regions), by instruments on the Ulysses spacecraft during February 1992. The energy spectra of these electrons differ markedly from the energy spectrum of the trapped magnetospheric electrons measured by the same instrument. Two populations of the upstream electrons were identified. Type I electrons appear at times when the direction of the interplanetary magnetic field at the spacecraft could have been tangent to the Jovian bow shock surface thus paralleling, for the first time at another planetary bow shock, the rather well understood situation at Earth's bow shock. Type II electrons have the same energy spectrum as Type I electrons, but are not so clearly associated with the tangent field-line condition. They occur at high southerly latitudes only while the Type I electrons are detected both on the inbound and outbound passages. Type II electrons have never been reported at the Earth's bow shock or any other planetary bow shock. Under the assumption that the field line that goes through Ulysses connects to the bow shock in a straight line, two possible explanations for the Type II electrons may be: (1) very large distortions of the bow shock surface, perhaps caused by deformations of the magnetopause, may permit the tangent condition; and (2) upstream electrons are preferentially, but not necessarily, accelerated when the IMF is tangent to the bow-shock surface.     

Study of electron energy and angular distributions and calculations of X-ray,EUV line flux and rise times

Evolution of energy and angular distributions of electrons has been studied by combining small-angle analytical treatment with large-angle Monte Carlo calculations as a function of column density for initially monoenergetic and monodirectional electrons. The incident electron energies considered are 20, 30 and 60 keV at 0°, 30° and 60° angles of incidence. Using these distributions, time evolution of extreme ultraviolet (EUV) spectrum has been studied. The slopes of the curves calculated compare well with the experimentally observed curve.     

Evidence for thin-target X-ray emission in a small solar flare on 26 February 1972

We compare solar X-ray observations from the UCSD experiment aboard OSO-7 with high resolution energetic electron observations from the UCAL experiment on IMP-6 for a small solar flare on 26 February 1972. A proportional counter and NaI scintillator covered the X-ray energy range 5–300 keV, while a semiconductor detector telescope covered electrons from 18 to 400 keV. A series of four non-thermal X-ray spikes were observed from 1805 to 1814 UT with average spectrum dJ/d (hv) (hv)^–4.0 over the 14–64 keV range. The energetic electrons were observed at 1 AU beginning 1840 UT with a spectrum dJ/dE E ^–3.1. If the electrons which produce the X-ray emission and those observed at 1 AU are assumed to originate in a common source, then these observations are consistent with thin target X-ray production at the Sun and inconsistent with thick target production. Under a model consistent with the observed soft X-ray emission, we obtain quantitative estimates of the total energy, total number, escape efficiency, and energy lost in collisions for the energetic electrons.     

Energy correction based on fluorescence attenuation of DAMPE

The major scientific goals of the DArk Matter Particle Explorer(DAMPE) are to study cosmicray electrons(including positrons) and gamma rays from 5 GeV to 10 TeV and nuclei from Z = 1 to 26 up to 100 TeV. The deposited energy measured by the Bismuth Germanate Oxide(BGO) calorimeter of DAMPE is affected by fluorescence attenuation in BGO crystals that are 600 mm long. In this work, an in-orbit attenuation calibration method is reported, and energy correction of the sensitive detector unit of the BGO calorimeter is also presented.     

The flattening of the X-ray spectrum of sco X-1 at energies beyond 40 keV

The measurements of the hard X-ray spectrum of Sco X-1 in the energy interval 20–150 keV in three balloon flights from Hyderabad, India are reported. These results show conclusively that the spectrum of Sco X-1 is very flat in the energy interval 40–150 keV and the measured fluxes beyond 60 keV are several orders of magnitude higher than those expected on the basis of an extrapolation of bremsstrahlung spectrum from a thin hot plasma at a temperature corresponding tokT=5 keV, which is applicable for Sco X-1 for energies <40 keV. The results are compared with those of other investigators of hard X-rays from Sco X-1, and the implication of the results is briefly discussed.     

Particle acceleration in supernova-remnant shocks

It has been known for over 50 years that the radio emission from shell supernova remnants (SNRs) indicates the presence of electrons with energies in the GeV range emitting synchrotron radiation. The discovery of nonthermal X-ray emission from supernova remnants is now 30 years old, and its interpretation as the extension of the radio synchrotron spectrum requires electrons with energies of up to 100 TeV. SNRs are now detected at GeV and TeV photon energies as well. Strong suggestions of the presence of energetic ions exist, but conclusive evidence remains elusive. Several arguments suggest that magnetic fields in SNRs are amplified by orders of magnitude from their values in the ambient interstellar medium. Supernova remnants are thus an excellent laboratory in which to study processes taking place in very high Mach-number shocks. I review the observations of high-energy emission from SNRs, and the theoretical framework in which those observations are interpreted.     

Simulation studies of MACE-I: Trigger rates and energy thresholds

The MACE (Major Atmospheric Cherenkov Experiment) is an upcoming Very High Energy (VHE) γ-ray telescope, based on imaging atmospheric Cherenkov technique, being installed at Hanle, a high altitude astronomical site in Ladakh, India. Here we present Monte Carlo simulation studies of trigger rates and threshold energies of MACE in the zenith angle range of 0°–60° for on-axis γ-ray coming from point source and various cosmic ray species. We have simulated the telescope’s response to γ-rays, proton, electron and alpha initiated atmospheric Extensive Air Showers (EAS) in the broad energy range of 5 GeV to 20 TeV. For γ-rays we consider power law and log parabolic spectra while other particles are simulated with their respective cosmic ray spectrum. Trigger rates and threshold energies are estimated for the trigger configuration of 4 Close Cluster Nearest Neighbour(CCNN) pixels as implemented in MACE hardware, in combination with single channel discriminator threshold ranging from 6–10 photo electrons (pe). We find that MACE can achieve the γ-ray trigger energy threshold of ∼ 17 GeV (4 CCNN, 9 pe) at 0° zenith angle for power law spectrum. The total trigger rate at 0° zenith is expected to be ∼650 Hz, with protons contributing ∼ 80% to it. For the zenith range of 0°-40° we find that the telescope can achieve γ-ray trigger threshold energies of ∼22 GeV at 20° zenith angle and ∼40 GeV at 40° zenith angle. Integral rates are also almost constant for this zenith angle range. At zenith angle of 60°, trigger energy threshold increases to ∼173 GeV and total integral rate falls down to ∼305 Hz.     

A calculation of auroral hiss with improved models for geoplasma and magnetic field

Intensities of auroral hiss generated by the Cerenkov radiation process by electrons in the lower magnetosphere are calculated with respect to a realistic model of the Earth's magnetosphere. In this calculation, the magnetic field is expressed by the “Mead-Fairfield Model” (1975), and a static model of the iono-magnetospheric plasma distribution is constructed with data accumulated by recent satellites (Alouette-I, -II, ISIS-I, OGO-4, -6 and Explorer 22). The energy range of hiss producing electrons and the frequency range of the calculated VLF are 100–200 keV, and 2–200 kHz, respectively. Intensities with a maximum around 20 kHz, of the order of 10^?14 W/m²/Hz¹ at the ground seem to be ascribable to the incoherent Cerenkov emission from soft electrons with a differential energy spectrum E^?2 having an intensity of the order of 10⁸cm^?2/sec/sr/eV at 100 eV. It is shown that the frequency of the maximum hiss spectral density at geomagnetic latitudes 80° on the day-side and 70° on the night-side is around 20 kHz for the soft spectrum (～E^?2) electrons, which shifts toward lower frequency (～10 kHz) for a hard spectrum (～E^?1·2) electrons. The maximum hiss intensity produced by soft electrons is more than one order higher than that of hard electron produced hiss. The higher rate of hiss occurrence in the daytime side, particularly in the soft electron precipitation zone in the morning sector, and the lesser occurrence of auroral hiss in night-time sectors must be, therefore, due to the local time dependence of the energy spectra of precipiating electrons rather than the difference in the geomagnetic field and in the geoplasma distributions.     

Modification of gamma-ray spectrum in a photon field

The changes in the high-energy gamma-ray spectra due to electron-photon cascading in the photon field (e.g., microwave background radiation) are investigated.The propagation of gamma-rays with an initial power-law spectrum through an isotropic monoenergetic photon field is considered. Two processes of the cascade particle-photon field interaction are taken into account, namely, the photoproduction of electron-positron pairs and the inverse Compton effect. The effect of the synchrotron energy loss by cascade electrons in a magnetic field on the cascade gamma-quantum spectrum is studied. The results may be used to estimate the absorption and the variations in the shape of the spectrum of radiation from compact gamma-ray sources in the metagalaxy.The cascade characteristics have been obtained by numerically solving the cascade equations by the moment method.     

Spectral development of a solar X-ray burst observed on OSO-7

The UCSD solar X-ray instrument on the OSO-7 satellite observes X-ray bursts in the 2–300 keV range with 10.24 s time resolution. Spectra obtained from the proportional counter and scintillation counter are analyzed for the event of November 16, 1971, at 0519 UT in terms of thermal (exponential spectrum) and non-thermal (power law) components. The energy content of the approximately 20 × 10⁶K thermal plasma increased with the 60 s duration hard X-ray burst which entirely preceded the 5 keV soft X-ray maximum. If the hard X-rays arise by thick target bremsstrahlung, the nonthermal electrons above 10 keV have sufficient energy to heat the thermally emitting plasma. In the thin target case the collisional energy transfer from non-thermal electrons suffices if the power law electron spectrum is extrapolated below 10 keV, or if the ambient plasma density exceeds 4 × 10¹⁰ cm^–3.Formerly at UCSD.