R. Ramaty

Universities Space Research Association, Houston, Texas, United States

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Publications (290)882.92 Total impact

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    R. Ramaty, R. E. Lingenfelter
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    ABSTRACT: Assuming that cosmic-ray deuterons and helium-3 nuclei are of secondary origin, we show that a unique determination of both the cosmic-ray path-length and the residual interplanetary field modulation at solar minimum may be made from a comparison of the calculated and measured intensities of these two nuclei. This determination does not depend on any assumptions regarding either the source spectra or the unmodulated proton to alpha particle ratio of the primary cosmic rays. The production of deuterium and helium-3 by cosmic-ray interactions in the galaxy is calculated considering energy-dependent cross sections, interaction kinematics, and demodulated cosmic-ray spectra. The resulting flux at the earth is obtained by taking into account leakage from the galaxy, ionization losses, nuclear breakup, and modulation. From a comparison of these calculations with the measured deuterium and helium-3 intensities at the earth, we conclude that within the experimental uncertainties all the data can be understood in terms of an energy-independent cosmic-ray path-length of 4 ± 1 g/cm2 and a residual interplanetary field modulation at solar minimum of the form exp(–η/Pβ) with η = 0.4 ± 0.1 BV, where P and β are the rigidity and velocity.
    Canadian Journal of Physics 02/2011; 46(10). · 0.90 Impact Factor
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    ABSTRACT: The Transient Gamma-Ray Spectrometer (TGRS) experiment is a high-resolution germanium detector launched on the WIND satellite on 1994 November 1. 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 1995 January through 1995 October. These results were obtained from the TGRS occultation mode, in which a lead absorber occults the Galactic center region for of each spacecraft rotation, thus chopping the 511 keV signal. The occulted region is a band in the sky of width 16° that passes through the Galactic center. We detect the narrow annihilation line from the Galactic center with flux = (1.64 ± 0.09) × 10-3 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° cannot be ruled out. No evidence for temporal variability on timescales longer than 1 month was found.
    The Astrophysical Journal 01/2009; 463(2):L75. · 6.73 Impact Factor
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    ABSTRACT: We used SMM gamma-ray data from 19 solar flares to study ambient elemental abundances in the solar atmosphere. We found that the abundance ratios of low FIP (first ionization potential) to high FIP elements are enhanced relative to photospheric abundances, but that the variability of these ratios from flare to flare is limited to a narrower range than that inferred from EUV and X-ray observations. The mean of the gamma-ray derived Mg/O (a low FIP to high FIP element abundance ratio) is coronal and the individual values are always higher than the photospheric Mg/O. The value of Ne/O (~0.25) is higher than the coronal value of 0.15 obtained from solar energetic particle data, but not inconsistent with some EUV and X-ray determinations. To avoid Ne/O higher than 0.3 a steep accelerated particle energy spectrum extending down to about 1 MeV per nucleon is needed. This implies that a large fraction of the available flare energy is contained in accelerated ions.
    The Astrophysical Journal 01/2009; 455(2):L193. · 6.73 Impact Factor
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    ABSTRACT: From the measured fluences of αα and narrow gamma-ray lines we derive He abundances (He/O) in the gamma-ray production region of solar flares, most likely subcoronal regions located above the photosphere. The αα line is a spectral feature due to de-excitations in 7Li and 7Be produced by the interactions of α-particles with He. We find that the derived values of He/O are consistent with the photospheric He/O only if the accelerated particle α/p 0.5. More conventional values (α/p 0.1) imply a higher He abundance in the subcoronal regions than in the photosphere. Indications for the existence of such He-enriched regions in the solar atmosphere are obtained from solar wind observations. We show how future high-resolution observations with Ge detectors could distinguish between the two possibilities of high α/p or high He/O (and hence high He/H) values by resolving an already observed spectral feature at ~1.02 MeV. This feature is dominated by lines produced exclusively in α-particle interactions with 56Fe and 3He interactions with 16O. In addition to the determination of α/p and the ambient He abundance, this will also determine the accelerated 3He/4He and its time development as the acceleration progresses. The relative intensities of the αα and narrow de-excitation lines confirm our previous finding that in the gamma-ray production region the abundance of Ne exceeds its measured coronal and generally accepted photospheric value. Thus, the subcoronal abundances of the two elements with the highest first ionization potential, He and Ne, are higher than their corresponding coronal abundances.
    The Astrophysical Journal 01/2009; 489(1):L99. · 6.73 Impact Factor
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    ABSTRACT: We investigated in detail the shape of the broad nuclear gamma-ray line emission resulting from accelerated C, N, and O interacting with ambient H and He in an attempt to fit the recent COMPTEL observations of Orion. The fact that the observed gamma-ray emission shows a strong enhancement in the 3-7 MeV region with significant structure within this energy band is the main motivation for assuming that the emission is due to nuclear de-excitations in C, N, and O. We found that the strong emission peak observed at ~4 MeV, with a red wing extending down to almost 3 MeV, can be understood as broad line emission produced by accelerated C, N, and O with a relatively hard spectrum but with the blue wing of the 12C line suppressed by the anisotropy of the interactions. The fact that the observed spectrum does not show narrow emission peaks at 4.44 and 6.13 MeV indicates that the bulk of the observed emission is produced by accelerated C, N, and O, rather than by accelerated protons and α-particles. The anisotropic interaction scenario removes the constraint placed by the line widths on the hardness of the accelerated ion spectrum, a useful feature because harder ion energy spectra are energetically more efficient than softer spectra. We have incorporated in our calculations the anisotropic angular distribution of the gamma-ray line emission in the rest frame of the heavy ions leading to line-splitting effects. The only feature in the COMPTEL data that would indicate the presence of splitting is a structure in the 5-7.5 MeV region. However, for calculations that account for the ~4 MeV peak, the characteristic maxima due to line splitting in this region are apparent only in models employing multiple sources.
    The Astrophysical Journal 01/2009; 484(1):286. · 6.73 Impact Factor
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    ABSTRACT: The Transient Gamma-Ray Spectrometer (TGRS) on the Wind satellite is designed to perform spectroscopy of bright gamma-ray bursts in the ~20-8000 keV energy range, having a resolution 5-30 times better than that of earlier generation detectors. GRB 950822 was the brightest burst observed by TGRS in its first year of operation, with a peak flux (1024 ms) of ~78.3 photons cm-2 s-1 and a fluence (50-300 keV) of ~1.4 × 10-4 ergs cm-2; therefore, the GRB data in this paper represent the first high-resolution spectroscopy ever performed on a burst of this brightness. The continuum spectrum of GRB 950822 has the classical gamma-ray burst shape and exhibits typical evolution from hard to soft over the course of the burst. We found no evidence of line features or any other spectral fine structure with significance greater than 3.5 σ in our data.
    The Astrophysical Journal 01/2009; 491(2):697. · 6.73 Impact Factor
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    ABSTRACT: We have obtained spectra of the Galactic center at energies of 400-600 keV from high-resolution data acquired by the Transient Gamma-Ray Spectrometer Ge detector on board the WIND mission during 1995-1997. The data were obtained using an on-board occulter and are relatively free from systematics and backgrounds. Analysis of the spectra reveals a well-resolved electron-positron annihilation line at 511 keV and the associated continuum due to annihilation via positronium formation. Measurements of the line width and the continuum-to-line ratio allow some constraints to be placed on the interstellar sites where annihilation occurs.
    The Astrophysical Journal 01/2009; 501(1):L55. · 6.73 Impact Factor
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    ABSTRACT: We show that the Be abundances in old halo stars formed in the early Galaxy strongly suggest that the refractory cosmic-ray metals, in both the early Galaxy and the present epoch, are accelerated from fresh supernova ejecta rather than from the interstellar medium, as proposed in most current models. To resolve this problem, we suggest that the high-velocity grains formed in supernova ejecta are the injection source for these cosmic rays, including C and O. We show how this cosmic-ray injection source can refute previous arguments against cosmic-ray acceleration from fresh supernova ejecta: the enrichment of the highly refractory elements relative to the volatiles and the similarity of the refractory cosmic-ray source and solar elemental and isotopic abundance ratios. We also show that the cosmic-ray source O abundance may be the direct consequence of the condensation of O as refractory oxides and that the problematic cosmic-ray C-to-O ratio may be understood if a large fraction of the C in the ejecta is in graphite grains. We further show that if the refractory metals are accelerated from fresh ejecta, the cosmic-ray source 54Fe/56Fe should be higher than the corresponding solar value. Such a difference is apparent in some recent cosmic-ray measurements, but forthcoming Advanced Composition Explorer results are needed to confirm this finding.
    The Astrophysical Journal 01/2009; 500(2):L153. · 6.73 Impact Factor
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    ABSTRACT: A series of narrow gamma-ray lines at 0.339, 0.429, 0.478, 0.937, 1.00, 1.04, 1.05, and 1.08 result almost exclusively from the bombardment of ambient He, O, and Fe by accelerated α-particles and 3He nuclei. Study of these lines, combined with the 56Fe line at 0.847 MeV and the 16O line at 6.129 MeV, allows the determination of the abundances of α-particles and 3He accelerated in solar flares and of the ambient He in the gamma-ray production region in the solar atmosphere. Using the Solar Maximum Mission/GRS and Compton Gamma Ray Observatory/OSSE data for 20 flares, we find that with significance not exceeding about 2.5 σ there are flares that exhibit α-particle abundance enhancements (α/p>0.1), show evidence for the presence of accelerated 3He, and indicate ambient He abundance enhancements. In some cases the accelerated 3He/4He~1 , and for essentially all of the flares 3He/4He could be 0.1, consistent with our earlier conclusion that in both impulsive and gradual flares the particles that interact and produce gamma rays are always accelerated by the same mechanism that operates in impulsive flares, namely, stochastic acceleration through gyroresonant wave-particle interactions. The ambient He abundance enhancements suggest that there are chromospheric regions where He/O exceeds its photospheric value, a result that could have important implications for solar atmospheric dynamics.
    The Astrophysical Journal 01/2009; 518(2):918. · 6.73 Impact Factor
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    ABSTRACT: 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.
    The Astrophysical Journal 12/2008; 534(2):747. · 6.73 Impact Factor
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    ABSTRACT: By introducing a hitherto ignored 6Li producing process, due to accelerated 3He reactions with 4He, we show that accelerated particle interactions in solar flares produce much more 6Li than 7Li. By normalizing our calculations to gamma-ray data, we demonstrate that the 6Li produced in solar flares, combined with photospheric 7Li, can account for the recently determined solar wind lithium isotopic ratio, obtained from measurements in lunar soil, provided that the bulk of the flare-produced lithium is evacuated by the solar wind. Further research in this area could provide unique information on a variety of problems, including solar atmospheric transport and mixing, solar convection and the lithium depletion issue, and solar wind and solar particle acceleration.
    The Astrophysical Journal 12/2008; 534(2):L207. · 6.73 Impact Factor
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    ABSTRACT: We have developed new neutron production kinematics and thoroughly updated the neutron production cross sections, and we have included ion pitch-angle scattering and magnetic mirroring in our Monte Carlo simulation programs, to make new calculations of anisotropic neutron emission produced in the solar flare magnetic loop models. The anisotropy in these models arises from the combined effects of converging magnetic field lines and a rapidly increasing ambient density in the portion of the loop below the chromosphere-corona transition. We have carried out new calculations of the depth, time, angle, and energy dependences of the neutron production, the angle distributions and energy spectra of the escaping neutrons, and the energy spectrum of the surviving neutrons at the distance 1 AU from the Sun. These new calculations will now allow much more reliable and detailed analyses of the various solar flare neutron spectral observations.
    The Astrophysical Journal Supplement Series 12/2008; 140(2):563. · 16.24 Impact Factor
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    ABSTRACT: A search for spectral lines in gamma-ray bursts detected with the Transient Gamma-Ray Spectrometer has been unable to confirm their existence. The spectrometer, aboard the Wind spacecraft, has detected gamma-ray bursts and other transients since 1995. We have performed a systematic search for narrow spectral lines in gamma-ray bursts detected with this instrument that augments and extends the results of a similar search of bursts detected with BATSE. This search procedure tests for statistically significant lines at all possible times and durations during a burst using the method of maximum likelihood and C-statistic. Simulations demonstrated the effectiveness of this procedure, particularly on Poisson distributed data, for distinguishing real features from statistical fluctuations. The most promising line candidates were consistent with chance fluctuations, given the large number of spectra searched.
    The Astrophysical Journal 12/2008; 543(1):77. · 6.73 Impact Factor
  • R. RAMATY, R. E. LINGENFELTER
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    ABSTRACT: Recent observations suggest that galactic line emission at 511 keV results from the superposition of contributions from a variable, compact source and an interstellar distribution of positrons resulting from the decay of radionuclei produced by thermonuclear burning supernovae. The compact point source could have turned on as recently as 1977 and has not been seen since 1979. Photon-photon pair production in the vicinity of a relatively small black hole (<103 Msolar) could be the source of the annihilating positrons in the point source. It is not known whether this compact object lies exactly at the Galactic Center.
    Annals of the New York Academy of Sciences 12/2006; 655(1):319 - 325. · 4.38 Impact Factor
  • Annals of the New York Academy of Sciences 12/2006; 759(1):392 - 396. · 4.38 Impact Factor
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    ABSTRACT: The observations and interpretations of cosmic (nonsolar) gamma ray lines are discussed. The most prominent of these lines is the e(+)e(-) annihilation line which was observed from the Galactic Center and from several gamma ray transients. At the Galactic Center the e(+)e(-) pairs are probably produced by an accreting massive black hole (solar mass of approximately one million) and annihilate within the central light year to produce a line at almost exactly 0.511 MeV. In gamma ray transients the annihilation line is redshifted by factors consistent with neutron star surface redshifts. Other observed transient gamma ray lines appear to be due to cyclotron absorption in the strong magnetic fields of neutron stars, and nuclear deexcitations and neutron capture, which could also occur on or around these objects.
    Annals of the New York Academy of Sciences 12/2006; 375(1):338 - 346. · 4.38 Impact Factor
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    ABSTRACT: RHESSI is the sixth in the NASA line of Small Explorer (SMEX) missions and the first managed in the Principal Investigator mode, where the PI is responsible for all aspects of the mission except the launch vehicle. RHESSI is designed to investigate particle acceleration and energy release in solar flares, through imaging and spectroscopy of hard X-ray/gamma- ray continua emitted by energetic electrons, and of gamma-ray lines produced by energetic ions. The single instrument consists of an imager, made up of nine bi-grid rotating modulation collimators (RMCs), in front of a spectrometer with nine cryogenically-cooled germanium detectors (GeDs), one behind each RMC. It provides the first high-resolution hard X- ray imaging spectroscopy, the first high-resolution gamma-ray line spectroscopy, and the first imaging above 100 keV including the first imaging of gamma-ray lines. The spatial resolution is as fine as similar to 2.3 are see with a full-Sun (greater than or similar to1degrees) field of view, and the spectral resolution is similar to 1 - 10 keV FWHM over the energy range from soft X-rays (3 keV) to gamma-rays (17 MeV). An automated shutter system allows a wide dynamic range ( gt 10(7)) of flare intensities to be handled without instrument saturation. Data for every photon is stored in a solid-state memory and telemetered to the ground, thus allowing for versatile data analysis keyed to specific science objectives. The spin-stabilized (similar to 15 rpm) spacecraft is Sun- pointing to within similar to 0.2degrees and operates autonomously. RHESSI was launched on 5 February 2002, into a nearly circular, 38degrees inclination, 600-km altitude orbit and began observations a week later. The mission is operated from Berkeley using a dedicated 11-m antenna for telemetry reception and command uplinks. All data and analysis software are made freely and immediately available to the scientific community.
    Solar Physics 01/2002; · 3.26 Impact Factor
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    ABSTRACT: We obtain limits on gamma-ray line emission at 478 keV from both known and undiscovered novae. The instrument has good spectral resolution and a complete view of the southern ecliptic hemisphere. Although most of this field of view of unmodulated and acquires only background spectra, an occulter, taking advantage of the 3 s rotation period of the Wind satellite, modulates signals from the Galactic center (GC). The backgrounds are low and extremely stable, allowing us to extract cosmic signals from the background spectrum. The instrumental background contribution to the 478 keV line is described by a simple model; no departures from it (i.e. nova signals) were detected during 1995-1997. In a separate analysis the quasi-steady emission at 478 keV from integrated novae towards the GC is measured in the occulted data set. .
    10/2001;
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    ABSTRACT: We discuss the new information that the light elements, particularly Be, have brought to cosmic-ray studies, specifically to the issue of the origin of the seed material of the cosmic rays. The primary nature of the Be evolution strongly suggests that supernova ejecta are the sources of this material. We discuss the superbubble models that emerged as the most likely site for the acceleration of supernova ejecta, and we review the arguments that support the model in which the present epoch cosmic rays have the same origin as those that produce the light elements throughout the evolutionary history of the Galaxy. These arguments include the facts that the bulk of the Galactic supernovae are confined within the interiors of superbubbles, where their ejecta could dominate the metallicity, and that high velocity grains, which condense out of the cooling and expanding ejecta, serve as the injection source for shock acceleration, via sputtering of grain material and scattering of volatile gas atoms. We also review the evolutionary calculations that show that a secondary origin for the evolution of Be as a function of the O abundance is energetically untenable, and unnecessary if cosmic-ray transport is properly taken into account.
    Space Science Reviews 09/2001; 99(1):51-60. · 5.52 Impact Factor
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    ABSTRACT: The "Wind" spacecraft carrying the Transient Gamma Spectrometer (TGRS) moves in an extremely elliptical orbit that largely avoids Earth's trapped radiation belts and albedo gamma-radiation. The TGRS therefore enjoys a relatively low level of background that is also extremely stable. We show how this stability enables modeling of the time variability of background lines, which in turn enables a novel technique of background subtraction to be used in the detection of transient astrophysical lines. We apply a simple version of this method to the line at 478 keV that is expected to arise from nucleosynthesis of 7Be in nearby novae. This search covers the entire southern ecliptic hemisphere during 1995-1997, including five known individual events, and possible undiscovered individual events. The TGRS design also uses "Wind"'s 3 s rotation period to modulate signals from the Galactic Center (GC). We use this feature of the instrument to search for a quasi-constant level of 478 keV emission from the accumulation of 7Be from several novae that are expected to occur in the direction of the GC during that isotope's 53 d half-life. We derive upper limits on the transient (single-nova) emission which improve on previous limits by about an order of magnitude, and limits on the steady (many-nova) emission which represent a factor 2 improvement. Only weak limits can be placed on the key parameters in the nucleosynthesis and ejection of 7Be, however.
    09/2001;

Publication Stats

1k Citations
882.92 Total Impact Points

Institutions

  • 2001
    • Universities Space Research Association
      Houston, Texas, United States
  • 1983–2001
    • Tel Aviv University
      • Department of Physics and Astronomy
      Tel Aviv, Tel Aviv, Israel
  • 1993–2000
    • University of Maryland, College Park
      • Department of Astronomy
      Maryland, United States
  • 1985–1997
    • NASA
      Washington, West Virginia, United States
  • 1992
    • ISTS
      Lutetia Parisorum, Île-de-France, France
    • Georgian National Academy of Sciences
      Tbilsi, T'bilisi, Georgia
  • 1987
    • Stony Brook University
      Stony Brook, New York, United States
  • 1984
    • Loyola University Maryland
      Baltimore, Maryland, United States
  • 1981
    • National University (California)
      San Diego, California, United States