Richard E. Lingenfelter

Nuclear gamma-rays from energetic particle interactions

Gamma ray line emission from nuclear deexcitation following energetic particle reactions is evaluated. The compiled nuclear data and the calculated gamma ray spectra and intensities can be used for the study of astrophysical sites which contain large fluxes of energetic protons and nuclei. A detailed evaluation of gamma ray line production in the interstellar medium is made.

The lunar neutron flux revisited

Abstract Cosmic ray-produced neutron equilibrium spectra are calculated for a variety of lunar surface compositions. From these spectra production rates of 80Kr, 82Kr, 114Cd, 131Xe, 150Sm, 152Sm, 152Gd, 156Gd, 158Gd and 187Re due to neutron capture on 79Br, 81Br, 113Cd, 130Ba, 149Sm, 151Eu, 155Gd, 157Gd and 186W are determined and compared with measurements.

EVIDENCE OF X-RAY SYNCHROTRON EMISSION FROM ELECTRONS ACCELERATED TO 40 TeV IN THE SUPERNOVA REMNANT CASSIOPEIA A

We present the 2‐ 60 keV spectrum of the supernova remnant Cassiopeia A measured using the Proportional Counter Array and the High Energy X-Ray Timing Experiment on the Rossi X-Ray Timing Explorer satellite. In addition to the previously reported strong emission-line features produced by thermal plasmas, the broadband spectrum has a high-energy “tail” that extends to energies at least as high as 120 keV. This tail may be described by a broken power law that has photon indices of G1 5 1.820.6

Solar gamma rays

The theory of gamma-ray production in solar flares is treated in detail. Both lines and continuum are produced. The strongest line predicted at 2.225 MeV with a width of less than 100 eV and detected at 2.24±0.02 MeV, is due to neutron capture by protons in the photosphere. Its intensity is dependent on the photospheric 3He abundance. The neutrons are produced in nuclear reactions of flare accelerated particles which also produce positrons and prompt nuclear deexcitation lines. The strongest prompt lines are at 4.43 MeV from 12C and at ∼6.2 from 16O and 15N. These lines result from both direct excitation and spallation. The widths of individual prompt lines are determined by nuclear kinematics. The width of the 4.43 MeV line is ∼100 keV and that of the 6.2 MeV feature is ∼300 keV. Both these lines have been observed from a solar flare. Other potentially observable lines are predicted at 0.845 and 1.24 MeV from 56Fe, at 1.63 MeV principally from 14N and 20Ne, at 1.78 MeV from 28Si, at ∼5.3 MeV from 15O and 15N, and at 7.12 MeV from 16O. The widths of the iron lines are only a few keV, while those of the other lines are about 100 keV. The only other observed line is at 0.511 MeV from positron annihilation. The width of this line is determined by the temperature, and its temporal variation depends on the density of the ambient medium in the annihilation region. Positrons can also annihilate from the 3S state of positronium to produce a 3-photon continuum below 0.511 MeV. In addition, the lines of 7Li and 7Be at 0.478 keV and 0.431 keV, which have kinematical widths of ∼30 keV, blend into a strong feature just below the 0.511 MeV line.From the comparison of the observed and calculated intensities of the line at 4.4 MeV to that of the 2.2 MeV line it is possible to obtain information on the spectrum of accelerated nuclei in flares. Moreover, from the absolute intensities of these lines the total number of accelerated nuclei at the Sun and their heating of the flare region can be estimated. We find that about 1033 protons of energies greater than 30 MeV were produced in the 1972, August 4 flare.The gamma-ray continuum, produced by electron bremsstrahlung, allows the determination of the spectrum and number of accelerated electrons in the MeV region. From the comparison of the line and continuum intensities we find a proton-to-electron ratio of about 10 to 102 at the same energy for the 1972, August 4 flare. For the same flare the protons above 2.5 MeV which are responsible for the gamma-ray emission produce a few percent of the heat generated by the electrons which make the hard X-rays above 20 keV.

Light Elements and Cosmic Rays in the Early Galaxy

Observations of Be and B in low-metallicity halo stars formed during the first 109 yr of Galactic evolution show that cosmic-ray acceleration must have taken place in the early Galaxy. The observed abundances of these elements relative to Fe, which, in the early Galaxy, is almost exclusively produced in Type II supernovae, strongly suggest that the cosmic-ray acceleration is also related to such supernovae with the particles being accelerated out of freshly nucleosynthesized matter before it mixes into the ambient, essentially nonmetallic interstellar medium. The observed abundances require that about 3 × 1049 to 2 × 1050 ergs per Type II supernova be imparted to these metallic cosmic rays, depending on whether or not H and He are accelerated along with the metals. The current data, however, are not sufficient to decide whether these cosmic rays are predominantly low energy or high energy. But, in any case, arguments of energetics imply a hard-energy spectrum extending up in energy to at least 50 MeV nucleon-1. This rules out Be and B production by supernova ejecta without further acceleration. In addition to production by cosmic rays, there must also be significant 11B production by neutrinos. This argument is driven by the observed 11B/10B ratio in meteorites that is very difficult to reproduce by cosmic-ray interactions. Observations of 6Li and Li in the early Galaxy provide information on the acceleration of nonmetallic cosmic rays out of the interstellar medium.

Cosmic-Ray Acceleration from Supernova Ejecta in Superbubbles

We suggest that the cosmic rays are accelerated primarily out of the supernova ejecta-enriched matter in the interiors of superbubbles. These hot, low-density superbubbles, which reach dimensions of several hundred parsecs, are generated by the winds and ejecta of supernova explosions of massive stars formed in giant molecular cloud OB associations that last for tens of megayears. Since these bubbles expand with shell velocities that are much faster than the dispersion velocities of the O and B star progenitors of the supernovae that power the bubbles, the bulk of the supernovae occur in their cores. The expanding remnants of each of these supernovae fill only less than 1% of this core before they have slowed to sonic velocities. Thus, the bulk of these supernovae remnants, together with their metal-rich grain and gas ejecta and their cosmic-ray-accelerating shocks, are well confined within the cores of superbubbles. These cores can thus provide a source of cosmic-ray matter of essentially constant metallicity throughout the age of the Galaxy, which is required to account for the constancy of cosmic-ray-produced Be relative to supernova-produced Fe observed in halo stars formed in the early Galaxy. The interactions of the grains and gas in metal-rich superbubbles, with recurrent supernova shocks every ~3×105 yr, also reconcile the requirement of a supernova ejecta source of cosmic rays with the recent observations that require a greater than 105 yr delay between nucleosynthesis and acceleration for the cosmic-ray metals. Supernova-enriched bubble metallicity may also explain the X-ray emission from the interiors of superbubbles in the Large Magellanic Cloud.

Nature versus Nurture: The Origin of Soft Gamma-Ray Repeaters and Anomalous X-Ray Pulsars

Soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are young and radio-quiet X-ray pulsars that have been rapidly spun-down to slow spin periods clustered in the range 5-12 s. Most of these unusual pulsars also appear to be associated with supernova shell remnants (SNRs) with typical ages less than 30 kyr. By examining the sizes of these remnants versus their ages, we demonstrate that the interstellar media that surrounded the SGR and AXP progenitors and their SNRs were unusually dense compared to the environments around most young radio pulsars and SNRs. We explore the implications of this evidence on magnetar and propeller-based models for the rapid spin-down of SGRs and AXPs. We find that evidence of dense environments is not consistent with the magnetar model unless a causal link can be shown between the development of magnetars and the external interstellar medium. Propeller-driven spin-down by fossil accretion disks for SGRs and AXPs appears to be consistent with dense environments since the environment can facilitate the formation of such a disk. This may occur in two ways: (1) formation of a pushback disk from the innermost ejecta pushed back by prompt reverse shocks from supernova remnant interactions with massive progenitor wind material stalled in dense surrounding gas or (2) acquisition of disks by a high-velocity neutron stars, which may be able to capture sufficient amounts of comoving outflowing ejecta slowed by the prompt reverse shocks in dense environments.

Solar flare neutron production and the angular dependence of the capture gamma-ray emission

The depth dependence of the production of neutrons and capture gamma-ray line emission are calculated by Monte Carlo simulation of the nuclear processes taking place when flare-accelerated ions interact with the solar atmosphere. The calculations also give the heliocentric-angular dependence of the 2.223 MeV neutron capture line emission as a function of accelerated-ion energy spectrum and angular distribution. These results are compared with observations to determine the energy spectrum shape and total ion number for various flares.

OB Associations, Supernova-generated Superbubbles, and the Source of Cosmic Rays

We have considered the effects of both the spatial and temporal clustering of OB stars and their subsequent core-collapse supernovae on their generation of superbubbles and their resultant role as the primary source of cosmic rays. Employing a wide range of astronomical and astrophysical observations, we determine quantitatively the fraction of Galactic core-collapse supernovae that occur in superbubbles. We show that the fraction of core-collapse supernovae occurring in superbubbles is high, ranging from ~80% (solely temporal correlations) to ~90% (only spatial correlations). In addition, we find that the singleton end of our stellar cluster distribution is sufficient to reproduce the observed relative number of OB field stars. Core-collapse supernovae (Types II and Ib/c) constitute 85% of Galactic supernovae; only a small fraction of the remaining class of supernovae, Type Ia, occur in superbubbles. Thus, ~75% of all Galactic supernovae are expected to occur within superbubbles. The occurrence of the great majority of Galactic supernovae in superbubbles has major implications for cosmic-ray acceleration. Acceleration of cosmic-ray nuclei heavier than He in enriched (ZSB ? 3 Z?) superbubble interiors can consistently explain the anomalous cosmic-ray 22Ne/20Ne ratio, the cosmic-ray actinde/Pt group and UPuCm/Th ratios, and the constant LiBeB/(C+O) ratio observed in very old, metal-poor stars. Finally, although only ~75% of supernovae occur in superbubbles, ~88% of the cosmic-ray heavy particles are accelerated there because of the factor of ~3 enhanced superbubble core metallicity.

Light-Element Evolution and Cosmic-Ray Energetics

Using cosmic-ray energetics as a discriminator, we investigate the viability of evolutionary models for the light elements Li, Be, and B (LiBeB). We find that models in which the cosmic rays are accelerated mainly out of the average interstellar medium which is increasingly metal-poor at early times significantly underpredict the measured Be abundance of the early Galaxy, the possible increase in [O/Fe] with decreasing [Fe/H] indicated by some recent data notwithstanding. On the other hand, if the cosmic-ray metals are accelerated primarily out of supernova ejecta-enriched superbubbles, such that the cosmic-ray source composition as a function of [Fe/H] remains similar to that of the current epoch, the measured Be abundances are consistent with a cosmic-ray acceleration efficiency that is in very good agreement with the current epoch data. This model requires the incorporation of neutrino-produced 11B. We show that, even though the production histories of the cosmic-ray-produced B and Be and the neutrino-produced 11B are different, B/Be can remain essentially constant as a function of [Fe/H]. We also find that neither the above cosmic-ray origin models nor a model employing low-energy cosmic rays originating from the supernovae of only very massive progenitors can account for the 6Li data at values of [Fe/H] below -2.