NGC 4151 and MCG 8-11-11: Two X-ray seyfert galaxies with strong soft γ-ray emission

During a balloon flight of the MISO telescope on the 30th September 1979, the Seyfert galaxies NGC 4151 and MGC 8-11-11 were studied in the hard X-ray range (E_X > 20 keV) and low-energy -ray range up to 19 MeV. An emission at the 4.5 level above 20 keV (4 above 260 keV) was detected in the direction of NGC 4151. -ray emission at the 3.9 level above 90 keV was also observed from the direction of MCG 8-11-11. The emission photon spectrum shows a high-energy cutoff at about 3 MeV. A large amount of the observed low-energy -ray diffuse background could be produced by a few percent of the X-ray emitting Seyfert galaxies having a -ray luminosity comparable to that observed from the regions of NGC 4151 or MCG 8-11-11.     

Absorption of high energy gamma-rays by low energy intergalactic photons

Following our previously proposed technique, we have used the recent -ray observations of Mkr421 to place theoretically significant constraints on the magnitude of the intergalactic infrared radiation field (IIRF). Our 2 upper limits are consistent with normal IR production by stars and dust in galaxies. They rule out exotic mechanisms proposed to produce a larger IIRF. Although they are still subject to revision and are unconfirmed, the data on the spectrum of Mkr421 hint at a possible absorption cutoff which could be produced by an IIRF of the magnitude expected from stellar emission and reprocessing in galaxies. Using models for the low energy intergalactic photon spectrum from microwave to ultraviolet energies, we calculate the opacity of intergalactic space to -rays as a function of energy and redshift. These calculations indicate that the GeV -ray burst recently observed by the CGRO EGRET detector originates at a redshift less than 1.5.     

Theoretical studies of the flux and energy spectrum of gamma radiation from the Sun

The purpose of this work is to study the various -ray-production mechanisms in solar flares and to calculate the flux, the spectrum, and the decay curves of radiation. Using the continuity equation and taking into account the energy losses for solar-flare-accelerated particles, we obtain the time-dependent particle distribution and thus the time behavior of the resulting rays. The important processes for producing rays in solar flares are found to be nonthermal electron bremsstrahlung, decay of neutral mesons, positron annihilation, neutron capture, and decay of excited nuclei. The results are applied to several known solar flares. For a large flare such as the class 3⁺ on February 23, 1956, continuous rays with energies up to 100 MeV from electron bremsstrahlung and neutral meson decays are observable at the orbit of the Earth by existing -ray detectors. Line rays from positron annihilation (0.51 MeV), neutron capture (2.23 MeV), and deexcitation of excited nuclei O¹⁶ (6.14 and 7.12 MeV) and C¹² (4.43 MeV) are particularly strong and well above the continuous -ray background due to electron bremsstrahlung. These lines can be detected at the Earth.NASA-NRC Resident Research Associate.     

Gamma-ray astrophysics

-ray astronomy is the study of the most energetic photons originating in our Galaxy and beyond, and therefore, provides the most direct means of studying the largest transfers of energy occurring in astrophysical processes. The first certain detection of celestial-rays came from a satellite experiment flown on OSO-III (Kraushaaret al., 1972); more recently two second generation spark chamber-ray telescopes, flown on the SAS-2 (Fichtelet al., 1975) and COS-B (Bennettet al., 1974) satellites, are now obtaining more detailed results on the high energy celestial radiation causing-ray astronomy to move from the discovery phase to the exploratory phase. The most striking feature of the celestial sphere when viewed in the frequency range of-rays is the emission from the galactic plane, which is particularly intense in the galactic longitudinal region from 300° to 50°. The longitudinal and latitudinal distributions are generally correlated with galactic structural features and when studied in detail suggest a non-uniform distribution of cosmic rays in the galaxy. Several point-ray sources have now been observed, including four radio pulsars. This last result is particularly striking since only one radio pulsar has been seen at either optical or X-ray frequencies. Nuclear-ray lines have been seen from the Sun during a large solar flare and future satellite experiments are planned to search for-ray lines from supernovae and their remnants. A general apparently diffuse flux of-rays has also been seen whose energy spectrum has interesting implications; however, in view of the possible contribution of point sources and the observation of galactic features such as Gould's belt, its interpretation must await-ray experiments with finer spatial and energy resolution, as well as greater sensitivity. Instruments with much greater sensitivity and improved energy and angular resolution are now available and will greatly enhance our understanding of high energy processes in astrophysics, especially in view of the high penetrating power of-rays, which for example permit them to reach the solar system from the far side of the galaxy essentially unattenuated.     

Possible episodic gamma-ray sources

A model for production of episodic -ray event at interaction of a moving gas target with, a beam of relativistic particles is proposed. The typical duration of -ray emission is limited by the flight time of the target across the beam as well as by the time of destruction and/or expulsion of the target by luminous beam. The time-dependent radiation spectra of the expanding and moving gas cloud irradiated by the beam are calculated for the galactic binary systems Her X-1 and AE Aquarii which are reported as episodic -ray emitters at very high energies. Some predictions and observational tests for the model are discussed.On leave from Yerevan Physics Institute, Armenia     

TeV emission from close binaries

It is commonly accepted that candidates for very high energy -ray sources are neutron stars, binary systems, black holes etc. Close binary systems containing a normal hot star and a neutron star (or a black hole) form an important class of very high energy -ray sources. Such systems are variable in any region of the electromagnetic spectrum and they enable us to study various stages of stellar evolution, accretion processes, mechanisms of particle acceleration, etc. Phenomena connected with this class of very high energy -ray sources are discussed. Particular emphasis has been placed on the TeV energy region.     

Determination of the characteristics of the gamma-ray telescope Gamma-1

The Gamma-1 telescope has been developed through a collaboration of scientists in the USSR and France in order to conduct -ray astronomical observations within the energy range from 50 to 5000 MeV. The major characteristics of the telescope were established by Monte-Carlo simulations and calibrations made with the aid of electron and tagged -ray beams produced by an accelerator, and these have been found to be as follows: the effective area for photons coming along the instrument's axis varies from about 50 cm² at E = 50 MeV to approximately 230 cm² at E 300 MeV; the angular resolution (half opening of the cone embracing 68% events) is equal to 2.7° at E = 100 MeV, and 1.8° at E = 300 MeV; the energy resolution (FWHM) varies from 70% to 35% as the energy of the detected photons increases from 100 to 550 MeV; the telescope's field-of-view at the half-sensitivity level is 300–450 square degrees depending upon the spectrum of the detected radiation, and the event selection logic. Proceeding from the thus obtained characteristics it is demonstrated that a point source producing a photon flux J (E 100 MeV) = 3 × 10^-7 cm^-2 s^-1, can be detected with a 5 significance by observing it during 10⁶ s at the level of the Cygnus background, and a source having intensity J (E 100 MeV) = 10^-6 cm^-2 s^-1 can be detected to within a mean square positional accuracy of about 15.     

Signatures of Energetic Protons and Electrons in the Galaxy

Models of the cosmic-ray, -ray and synchrotron properties of the Galaxy allow conclusions to be drawn about the cosmic-ray injection spectrum and propagation parameters. While the simplest models fail to reproduce the data, reasonable extensions can explain a range of observational facts. Explanations for the diffuse -ray GeV excess found by EGRET are considered; inverse-Compton emission resulting from a hard electron injection spectrum appears most promising. Meanwhile the -ray emission at MeV energies is unlikely to originate entirely from cosmic-ray electrons, and a point source component is required in addition.     

Gamma-rays from Active Galactic Nuclei: Emission processes and particle acceleration

It is argued that the high-energy X-ray and -ray emission from flaring blazars is beamed radiation from the relativistic jet supporting the relativistic beaming hypothesis and the unified scenario for AGNs. Most probably the high-energy emission results from inverse Compton scattering by relativistic electrons and positrons in the jet of radiation originating external to the jet plus pair annihilation radiation from the jet. Future positive TeV detections of EGRET AGN sources will be decisive to identify the prominent target photon radiation field. Direct -ray production by energetic hadrons is not important for the flaring phase in -ray blazars, but the acceleration of energetic hadrons during the quiescent phase of AGNs is decisive as the source of secondary electrons and positrons through photo-pair and photo-pion production. Injection of ultrahigh energy secondary electrons and positrons into a stochastic quasilinear acceleration scheme during the quiescent AGN phase leads to cooling electron-positron distribution functions with a strong cut-off at low but relativistic energy that under certain local conditions may trigger a plasma instability that gives rise to an explosive event and the flaring -ray phase.     

Cosmic-ray acceleration and transport,and diffuse galactic γ-ray emission

Cosmic-ray acceleration and transport is considered from the point of view of application to diffuse galactic -ray sources. As an introduction we review several source models, in particular supernovae exploding inside or near large interstellar clouds. The complex problem of cosmic ray transport in random electromagnetic fields is reduced to three cases which should be sufficient for practical purposes. As far as diffusive acceleration is concerned, apart from reviewing the basic physical principles, we point out the relation between shock acceleration and 2nd order Fermi acceleration, and the relative importance of the two processes around interstellar shock waves. For -ray source models the interaction of cosmic rays with dense clouds assumes great importance. Past discussions had been confined to static interactions of clouds with the ambient medium in the sense that no large scale mass motions in the ambient interstellar medium were considered. The well-known result then is that down to some tens of MeV or less, cosmic-ray nucleons should freely penetrate molecular clouds of typical masses and sizes. The self-exclusion of very low energy nucleons however may affect electron transport with consequences for the Bremsstrahlung -luminosity of such clouds.In this paper we consider also the dynamical interaction of dense clouds with a surrounding hot interstellar medium. Through cloud evaporation and accretion there exist mass flows in the cloud surroundings. We argue that in the case of (small) cloud evaporation the galactic cosmic rays will be essentially excluded from the clouds. The dynamic effects of cosmic rays on the flow should be minor in this case. For the opposite case of gas accretion onto (large) clouds, cosmic-ray effects on the flow will in general be large, limiting the cosmic-ray compression inside the cloud to dynamic pressure equilibrium. This should have a number of interesting and new consequences for -ray astronomy. A first, qualitative discussion is given in the last section.Proceedings of the XVIII General Assembly of the IAU: Galactic Astrophysics and Gamma-Ray Astronomy, held at Patras, Greece, 19 August 1982.     

Clouds and Diffuse Baryonic Dark Matter

The nature of the dark matter in the halo of our galaxy is still largely unknown. The microlensing events found so far towards the Large Magellanic Cloud suggest that at most about 20% of the halo dark matter is in form of MACHOs (Massive Astrophysical Compact Halo Objects). The dark matter could also, at least partially, be made of cold molecular clouds (mainly H₂). We proposed a model for baryonic dark matter, according to which dark clusters of brown dwarfs and cold self-gravitating H₂ clouds populate the outer galactic halo. A signature would be a diffuse -ray emission from the galactic halo. Basically, cosmic-ray protons in the galactic halo scatter on the clouds clumped into dark clusters, giving rise to a -ray flux. An analysis of EGRET data has led to the discovery of a statistically significant diffuse -ray emission from the galactic halo, which turns out to be in remarkably good agreement with our prediction.     

The effect of rotation on the hydrodynamics of stellar collapse

Summary Using values of d, _min, and _max that Van Riper (1978) has found most promising for a hydrodynamic envelope ejection, we have shown that even a small amount of rotation in the initial core can stop its collapse before nuclear densities are reached. We expected _i > 0.02 to produce significant deviations from a spherically symmetric collapse, but have found that _i as much as ten times smaller than this will not allow the core to reach densities as high as in the spherical collapse. In no case, however, does the core flatten very much, nor does the value of become very large. Low final 's preclude the formation of an axisymmetric torus. They also indicate that deformation of an iron core into a triaxial configuration or fragmentation of the core during its collapse is an extremely unlikely event. (Note: Classically, must exceed 0.27 before a dynamic instability to non-axisymmetric perturbations is encountered.)The small degree of flattening of the core also suggests that the reduced moment of inertia I of the core will always be relatively small in magnitude and hence that the third time derivative of I, which is proportional to the energy emitted in gravity wave radiation, will not be very significant. Numerically calculated estimates of I- during some of these model evolutions supports this suspicion. If the _min and used here are found to be realistic values after the detailed physics of the core collapse is well understood, it is clear that gravitational radiation from a core collapse will be difficult to measure.Finally, we should point out that it is the relatively large values of Y_min (near 4/3) combined with values of d near unity that (a) prevented the core from flattening significantly in these models and (b) prevented the core from reaching high configurations. If realistic values of either one (or both) of these parameters are found to be much smaller in more complete models of the core collapse, then the core will have to become flatter (and denser) before pressure gradients will support it along the rotation axis. All of the conclusions drawn here would be modified accordingly under those circumstances. It should also be noted that in general relativistic models, the critical for spherical collapse is somewhat larger than 4/3 (Van Riper, 1979). Therefore, we predict that when fully general relativistic core collapses are performed including rotation, a given choice of _min and _i will produce a slightly flatter and slightly denser core than the corresponding model that has been presented here.     

Gamma Rays From Molecular Clouds

High energy -rays from individual giant molecular clouds contain unique information about the hidden sites of acceleration of galactic cosmic rays, and provide a feasible method for study of propagation of cosmic rays in the galactic disk on scales 100 pc. I discuss the spectral features of ⁰-decay -radiation from clouds/targets located in proximity of relatively young proton accelerators, and speculate that such `accelerator+target systems in our Galaxy can be responsible for a subset of unidentified EGRET sources. Also, I argue that the recent observations of high energy -rays from the Orion complex contain evidence that the level of the `sea of galactic cosmic rays may differ significantly from the flux and the spectrum of local (directly detected) particles.     

Dissipation of magnetic fields in active galactic nuclei

A model for the emission processes causing rapid variability (less than one day) in active galactic nuclei is developed. Relativistic electron beams escape from reconnection sheets in coronae of accretion disks and excite plasma turbulence with a typical frequency , which depends on the electron number densityn (see also the contribution by R. van Oss). The finite lengths of different beams emerging from different reconnection sheets allows that the waves arecoherently scattered to frequencies ²_pe. For Lorentz factors 10³ and densities typical for disk coronaen10⁶ cm ^–3 (derived from iron line observations) one easily reaches the optical, frequency range. The time scale of the variability is then caused by the relaxation of the electron beams. Likewise, this model explains the very rapid variability in the X-ray (less than 10 minutes) by changing the parameters slightly. According to this scenario the higher the variable frequency is, the closer to the central black hole it should originate.     

A high-resolution Ge spectrometer for Gamma-Ray burst astronomy

The Transient Gamma-Ray Spectrometer (TGRS) to be flown aboard the WIND spacecraft is primarily designed to perform high resolution spectroscopy of transient -ray events, such as cosmic -ray bursts and solar flares over the energy range 25 keV to 8.2 MeV with an expected spectroscopic resolution of 3 keV at 1 MeV. The detector itself consists of a 215 cm³ high purityn-type Ge crystal kept at cryogenic temperatures by a passive radiative cooler. The geometric field of view defined by the cooler is 1.8 steradian. To avoid continuous triggers by soft solar events, a thin BeCu Sun-shield around the sides of the cooler has been provided. A passive Mo/Pb occulter, which modulates signals from within ±5° of the ecliptic plane at the spacecraft spin frequency, is used to identify and study solar flares, as well as emission from the galactic plane and center. Thus, in addition to transient event measurements, the instrument will allow the search for possible diffuse background lines and monitor the 511 keV positron annihilation radiation from the galactic center. In order to handle the typically large burst count rates, which can be in excess of 100 kHz, burst data are stored directly in an onboard 2.75 Mbit burst memory with an absolute timing accuracy of ±1.5 ms after ground processing. The memory is capable of storing the entire spectral data set of all but the largest bursts. WIND is scheduled to be launched on a Delta II launch vehicle from Cape Canaveral on November 1, 1994. After injection into a phasing orbit, the spacecraft will execute a double lunar swing-by before being moved into a controlled halo orbit about theL1 Lagrangian point (250R _e towards the Sun). This will provide a 5 light-second light travel time with which to triangulate gamma-ray burst sources with Earth-orbiting systems, such as those on-board the Gamma-Ray Observatory (GRO). The response of instrument to transient -ray events such as GRB's and solar flares will be presented as well as the expected response to steady state point sources and galactic center line emission.     

Primordial Helium and ΔY/ΔZ from H II Regions and from Fine Structure in the Main Sequence Based on Hipparcos Parallaxes

The primordial helium abundance YP is important for cosmology and the ratio Y/Z of the changes relative to primordial abundances constrains models of stellar evolution. While the most accurate estimates of YP come from emission lines in extragalactic H II regions, they involve an extrapolation to zero metallicity which itself is closely tied up with the slope Y/Z. Recently certain systematic effects have come to light in this exercise which make it useful to have an independent estimate of Y/Z from fine structure in the main sequence of nearby stars. We derive such an estimate from Hipparcos data for stars with Z Z and find values between 2 and 3, which are consistent with stellar models, but still have a large uncertainty.     

Gamma-ray line production in astronomical objects

This review focuses on the conditions for -ray line production in the most interesting astronomical objects, in light of the planned experiments: Gamma-1, GRO, Sigma, GRASP, and others. Among these objects are the Sun, the galactic center region, molecular and dust clouds, accreting and exploding stars.     

The Edison infrared space observatory

For five years, theEdison program has had the goal of developing new designs for infrared space observatories which will break the cost curve by permitting more capable missions at lower cost. Most notably, this has produced a series of models for purely radiative and radiative/mechanical (hybrid) cooling which do not use cryogens and optical designs which are not constrained by the coolant tanks. Purely radiatively-cooled models achieve equilibrium temperatures as low as about 20 K at a distance of 1 AU from the sun. More advancedEdison designs include mechanical cooling systems attached to the telescope assembly which lower the optical system temperature to 5 K or less. Via these designs, near-cryogenic temperatures appear achievable without the limitations of cryogenic cooling. OneEdison model has been proposed to the European Space Agency as the next generation infrared space observatory and is presently under consideration as a candidate ESA Cornerstone mission. The basic design is also the starting point for elements of future infrared space interferometers.     

Effects of Particle Beams in the Solar Atmosphere

This work addresses the observational and physical effects of particle beams in the solar atmosphere. Mainly electron beams are considered, but also some effects of proton and neutral beams are mentioned. Briefly describing acceleration mechanisms of superthermal particles, the main attention is devoted to effects influencing the particle beam propagation. The collisional energy losses and pitch-angle scattering, return current effects, mirroring in the converging magnetic field, and the scattering in the Alfvén and whistler wave turbulence in specific situations are considered. The role of quasi-linear relaxation is discussed. Examples of observations showing effects of particle beams in the solar atmosphere are presented throughout the paper. Separate chapters are devoted to processes connected with particle beam bombardment of dense layers of the solar atmosphere: hard X-ray and -ray flare emissions, evaporation process, asymmetry of optical chromospheric lines, and impact linear H line polarization. The beam induced energy release processes are also included. The presented effects of particle beams are summarized in the conclusions and future prospects are suggested.     

Diffuse cosmic X- and gamma radiation: The isotropic component

SummaryA. Spectral features The ability of the various theories to explain the three main spectral features at 1/4 keV, 60 keV and 1 MeV is summarized in Tables II and III.Clearly, confirmation of the reality of these features, especially the soft X-ray and -ray excesses, is one of the key elements in enabling us to decide between the competing theoretical interpretations.B. Energy requirements None of the proposed interpretations are easily explained in terms of the available energy in cosmic rays (except perhaps the Seyfert galaxy proposal, and this runs into difficulties). It seems that one either has to regard normal galaxies at the present epoch as prolific sources of cosmic rays ( 10⁶⁰ erg/galaxy in protons), as is required by the Brecher-Morrison model, or to argue that at early stages in their evolution far more energy is available than at present. One ends up with much the same energy requirement in this approach.One could conceivably identify such an early phase with the radio galaxy or QSO phenomena: in any event, cosmological evolution plays a major role. Cosmology does ease the energy requirements, but only for the inefficient mechanisms, such as nonthermal bremsstrahlung or ° -production.It seems that one still needs the metagalactic cosmic ray flux to be 10^-2 of the galactic flux in the diffuse inverse Compton models, and 10^-2–10^-4 in the nonthermal bremsstrahlung models.Faced with problems of energetics, one is tempted to turn to the most energetic objects in the Universe, namely Seyfert nuclei and QSO's, to provide the basic energy source, whether directly or indirectly, for the diffuse X-ray background. A direct connection could be more readily investigated when X-ray observations are available of more extra-galactic sources.C. Angular variations Another approach, complementary to that of looking for remote discrete sources, is to seek angular fluctuations, or limits on such fluctuations in the diffuse X-ray background.The best results presently available are those from the X-ray experiment on board OSO 3. Schwartz (1970) reports a limit of I/Ifour percent on small-scale (10°) fluctuations over 10–100 keV over about one-quarter of the sky. If one assumes a astrophysics, namely the origin of cosmic rays, is intimately linked to the origin of the X-ray background.It may well be that no single mechanism suffices to account for the entire spectrum of isotropic X- and -radiation. Nature is sufficiently perverse for there to be a reasonable probability that several different processes are contributing, and considerable ingenuity will be required to ascertain which mechanism, if any, is assigned the dominant role in a given spectral region.This review is based on an invited paper presented at the joint meeting of the A. A. S. Division of High Energy Astrophysics, and the A. P. S. Division of Cosmic Physics, Washington, D. C., 28 April–1 May, 1970