arXiv subjects

R. Ramaty

Publications and source records attributed to R. Ramaty.

23 records · Page 2Linked to original sources

arXiv1996

LiBeB: High and low energy cosmic ray production and comparison with neutrino induced nucleosynthesis in SNII

We present new calculations of LiBeB production by accelerated particles with various compositions and energy spectra ranging from low energies to relativistic energies, and various ambient medium metallicities ($Z/Z_\odot$). The observed, essentially constant, Be/Fe ratio as a function of $Z/Z_\odot$ can be best understood if the metallicity of the accelerated particles (high energy or low energy) does not vary much with $Z/Z_\odot$. This could be achieved if the particles are accelerated directly from the ejecta of Type II supernovae (SNII) and not from the interstellar medium. Using the observed Be/Fe and the fact that most of the Fe at low $Z/Z_\odot$ is from SNII, we derive the energy content in accelerated particles per SNII (a few times 10$^{50}$ erg). We show that additional $^{11}$B production by neutrinos is consistent with the available data, allowing a neutrino yield from SNII less than or equal to the nominal published yields. We further show that the observed $^6$Li at low $Z/Z_\odot$ suggests that the accelerated particles responsible for the LiBeB at such $Z/Z_\odot$ are confined to low energies and have a relatively high He/O abundance.

astro-ph↗

arXiv1996

TGRS Observation of the Galactic Center Annihilation Line

The TGRS (Transient Gamma-Ray Spectrometer) experiment is a high-resolution germanium detector launched on the WIND satellite on Nov. 1, 1994. Although primarily intended to study gamma-ray bursts and solar flares, TGRS also has the capability of studying slower transients (e.g. x-ray novae) and certain steady sources. We present here results on the narrow 511 keV annihilation line from the general direction of the Galactic Center accumulated over the period Jan. 1995 through Oct. 1995. These results were obtained from the TGRS occultation mode, in which a lead absorber occults the Galactic Center region for 1/4 of each spacecraft rotation, thus chopping the 511 keV signal. The occulted region is a band in the sky of width 16 degrees that passes through the Galactic Center. We detect the narrow annihilation line from the galactic center with flux = $(1.64\pm0.09)\times10^{-3} {photons} {cm}^{-2} {s}^{-1}$. The data are consistent with a single point source at the galactic center, but a distributed source of extent up to ~30 degrees cannot be ruled out. No evidence for temporal variability on time scales longer than 1 month was found.

astro-ph↗

arXiv1995

Astrophysical Gamma Ray Emission Lines

We review the wide range of astrophysical observations of gamma ray emission lines and we discuss their implications. We consider line emission from solar flares, the Orion molecular cloud complex, supernovae 1987A and 1991T, the supernova remnants Cas A and Vela, the interstellar medium, the Galactic center region and several Galactic black hole candidates. The observations have important, and often unique, implications on particle acceleration, star formation, processes of nucleosynthesis, Galactic evolution and compact object physics.

astro-ph↗

arXiv1994

Thermal Comptonization in Mildly Relativistic Pair Plasmas

We use a Monte Carlo simulation to calculate the spectra of mildly relativistic thermal plasmas in pair balance. We use the exact integral expression for the electron-positron thermal annihilation spectrum, and provide accurate expressions for the Gaunt factors of electron-ion, electron-electron, and electron-positron thermal bremsstrahlung in the transrelativistic temperature regime. The particles are assumed to be uniformly distributed throughout a sphere, and the pair opacity is self-consistently calculated from the energy and angular distribution of scattered photons. The resultant photon spectra are compared with the nonrelativistic diffusion treatment of Sunyaev and Titarchuk, the bridging formulas of Zdziarski, and the relativistic corrections proposed by Titarchuk. We calculate allowed pair-balanced states of thermal plasmas with no pair escape which include bremsstrahlung and internal soft photons. The results are presented in the spectral index/compactness plane, and can be directly compared with observations of spectra from AGNs and Galactic black hole candidates. By comparing with X-ray spectral indices of Seyfert AGNs and compactnesses inferred from X-ray variability data, we find that the allowed solutions for pair equilibrium plasma imply that the temperatures of Seyfert galaxies are $\lesssim 300$ keV. This prediction can be tested with more sensitive gamma-ray observations of Seyfert galaxies. We find that if the X-ray variability time scale gives an accurate measure of the compactness, pair-dominated solutions are inconsistent with the data.

astro-ph↗

arXiv1994

Compton Scattering in Jets: A Mechanism for $\sim$ 0.4 and $\lsim$ 0.2 Mev Line Production

We show that gamma ray line emission at $\sim$0.4 MeV and $\lsim 0.2$ MeV can be produced by Compton scattering of beamed radiation in the jets of Galactic black hole candidates. This mechanism has the novel feature of not invoking the presence of $e^+$--$e^-$ pairs. To produce the two lines we employ a symmetric double sided jet with bulk flow velocity of about 0.5 c and incident beam radiation with a hard energy spectrum. We show that the two lines can be seen at viewing angle cosines relative to the jet ranging from 0.2 to 0.6. This comprises 40\% of the total solid angle. In addition, the line radiation is approximately 10\% polarized. Depending on the bulk flow and viewing angle the model can produce lines at other energies as well. In particular a broad feature near 1 MeV can be seen by viewing the jet close to its axis. Our model can also accommodate single line spectra if the beamed gamma ray emission or the jets themselves are asymmetric.

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