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The X-radiation, occurring in the aurora zone, was first revealed in the summer of 1952 [1,2] rockets above 50 km. This radiation is mainly concentrated between the 62 and 77° geomagnetic latitude, with a maximum near 67.5°. It is generated at fast electron collisions with upper atmosphere particles.
Spectral intensity of high energy solar X rays observed during July 7, 1966 polar event with satellite OGO 3, suggesting nonthermal bremsstrahlung origin
The X-ray source Sco X-1 is known to have a slight linear polarization in visible light. An investigation was conducted to determine whether the origin of the polarization is intrinsic to the source or interstellar. No evidence could be found for unusual polarization properties in Sco X-1 that would point to an intrinsically polarized source, although this cannot be excluded. It is pointed out that the case for interstellar polarization would become very strong if stars in the immediate vicinity are found to be polarized with the same position angle.
The NASA Lunar Polar Orbiter, to be launched by a Delta vehicle in 1980, is designed to perform a global exploration of such moon properties as the gravity field, figure, and surface composition, as well as a lunar resource survey. The paper reviews the mission, spacecraft, and data system concepts in terms of baseline mission sequence, choice of orbits, spacecraft configuration requirements, mission sequence requirements and data system and operations. Scientific experiments planned for the Orbiter mission include geophysical altimetry and gravity experiments, heat flow experiment, magnetic field experiments, measurements of the moon's natural gamma-ray spectrum, X-ray fluorescence of lunar surface materials, sunlight reflectance spectroscopy, and spectro-stereo imaging. Characteristics of instruments designed for use in these investigations are examined. Through the use of advanced sensors and data-system techniques, coupled with well-developed spacecraft systems, the mission promises to return a very large quantity and variety of lunar data at a cost comparable with that of simpler missions in the past.
Observations of the X-ray pulsar Hercules X-1 show a line emission feature at about 60 keV, which has been interpreted as the fundamental electron cyclotron line in a magnetic field of around six trillion gauss. In this interpretation, the line radiation results from transitions between transverse energy levels, which are quantized by the field. The expected line luminosity from the excitation of these levels by protons which are falling into the polar cap of a neutron star are calculated. They are assumed to attain kinetic energies up to around 200 MeV, the gravitational potential energy at the surface. The cross sections for high energy Coulomb encounters between small pitch angle protons and electrons in a strong field are measured and used to calculate the energy loss rate of the infalling protons. This rate, together with the rate of elastic nuclear proton collisions, is then used to calculate the number of line photons an infalling proton can be expected to produce, directly or indirectly. The results are applied to Hercules X-1.
The contribution of pulsars to the gamma-ray flux from the galactic plane is examined using data from the most recent pulsar surveys. It is assumed that pulsar gamma-rays are produced by curvature radiation from relativistic particles above the polar cap and attenuated by pair production in the strong magnetic and electric fields. Assuming that all pulsars produce gamma-rays in this way, their luminosities can be predicted as a function of period and magnetic field strength. Using the distribution of pulsars in the galaxy as determined from data on 328 pulsars detected in three surveys, the local gamma-ray production spectrum, the longitude profile, and the latitude profile of pulsar gamma-ray flux are calculated. The largest sources of uncertainty in the size of the pulsar contribution are the value of the mean interstellar electron density, the turnover in the pulsar radio luminosity function, and the average pulsar magnetic field strength. A present estimate is that pulsars contribute from 15 to 20 % of the total flux of gamma-rays from the galactic plane.
VLA and SMM hard X ray data on the solar flares of November 5, 1980 are analyzed and compared with data from other sources. The VLA provided measurements at 15 GHz at 10 sec intervals, using left and right circular polarizations with a 0.6 arcsec resolution. The hard X ray imaging spectrometer on the SMM obtained data in six bands from 3.5-30 keV, with 8 x 8 arcsec resolution and 1.5 sec separation. The data were examined for a possible nonthermal source for the microwave component of the emissions detected, the origin of 16-30 keV excess fluxes, the relation between the X ray and microwave sources, the magnetic connection between observed loops, and the physical characteristics of the radiating loop. The data were consistent with a model that assumes fast electrons are accelerated to a single power-law energy distribution and freely stream along the magnetic field. The data also agreed with a thick-target model for solar flare X ray emission.
Coordinated high-resolution (1-3 arcsec) observations of two active solar regions (H 421 and H 419) on November 16, 1979, are reported: soft-X-ray filtergrams from a sounding rocket flight, VLA total-intensity and circular-polarization microwave (6-cm) radio maps, KPNO full-disk photospheric magnetograms, and BBSO H-alpha data. The images were converted to 4.8-arcsec/mm-scale transparencies and coaligned on the basis of sunspot positions for comparison. The two active regions are characterized in detail, and intensity, size, and polarization data for the brightest microwave components (BMC) are listed. It is found that 19 of the 32 BMC are farther than 5 arcsec from any sunspot, and that X-ray-emitting structures only rarely correspond to sunspots, or BMC. About one third of the BMC are located at the feet or legs of coronal loops smaller than about 50,000 km. The limitations implied by these obervations for proposed thermal-bremsstrahlung, thermal-gyro-resonance, and nonthermal microwave-emission mechanisms are discussed.
The results of observations of Sco X-1 from three separate scanning epochs during 1977 and 1978 by the UCSD/MIT instruments aboard HEAO 1 in the 15-200 keV range are reported. Intensity and spectral variations have been studied on three time scales: 33-minute scans, daily averages, and weekly averages. No relationship was found between intensity and temperature over the very narrow range of temperatures observed, but comparison of the variability of the intensity, emission measure, and temperature implies at least two different emission mechanisms are at work. It is found that the spectral data are best fitted by either a thin-thermal-bremsstrahlung model of about 5 keV or a Comptonized thermal model of temperature about 3 keV. At higher energies (50-200 keV), no flux is found in excess of the thermal (as claimed by previous observers), and a 2-sigma upper limit from scanning data of 10 to the -5th photons/sq cm sec keV is set. This is an order of magnitude lower than reported detections. Using the distance derived from optical and UV measurements, X-ray luminosities from Sco X-1 incompatible with predictions for spherical accretion onto a white dwarf and emission volumes inconsistent with polar caps of white dwarfs are found.
The COS-B X-ray data in the direction of the rho Oph dark cloud show an extended structure; at the same time, the region of highest intensity has a spatial distribution compatible with a localized source; 2CG353+16 which is designated Oph gamma. The possibility of an excess gamma ray flux over what is expected on the basis of the interaction of average density cosmic rays with an estimated cloud mass of 2 to 4 000 M is still open, pending an extended CO survey matching the gamma ray data. Estimates for this excess factor are in the range 2 to 4. While the cloud mass may be underestimated, it should be noted that an excess of the same order appears to be present in the nearby Oph-Sag area, well surveyed in CO with the Columbia dish. Possible reasons for a gamma ray excess, in view of two recent observational developments: an Einstein X-ray survey and a VLA radio survey, both covering the approx 2 deg diameter Oph gamma error box. Current interpretations link the gamma ray excess to the cloud gas, in which some active agent is present: stellar winds, or interaction with the North Polar Spur.
The creation of high energy pairs and photons in the conversion of gravitational to thermal energy is a process common to most accretion models for active galactic nuclei. These are two observational methods designed to explore this process: direct observations of the hot photons, through hard X-ray and gamma-ray data, and indirect observations of the energetic pairs, through their polarized, nonthermal low frequency radiation. However, interpretation of these observations in terms of the conditions in the inner accretion flow requires understanding of the various processes which modify the pair and photon distributions within the hot, dense core. These processes include opacity effects within the pair/photon plasma, Compton losses on external photons, further acceleration of the pairs and further radiation by the pairs, and the dynamic interaction of the pair/photon plasma with the surrounding gas. Current observational and theoretical work is reviewed and new directions are considered in a search for constraints on or tests of accretion models of active nuclei.
The X-ray oscillations observed in the eclipsing spectroscopic binary V471 Tau by Exosat (Jensen, 1985) and attributed to rotational modulation of an X-ray-opaque pole due to He and metals accreted onto the DA2 white dwarf component (Jensen, 1986) are investigated theoretically. The observational data are summarized in a table and compared with the results of numerical simulations of the accretion of wind/flare material onto the white dwarf. The rotational modulation model is found to reproduce the observed line features only if the magnetic field is strong (about 10 kG), the accretion rate is about 10 to the -15th solar mass/yr, and the polar cap area is much larger than the theoretical lower limit of 5.7 x 10 to the 14th sq cm. It is recommended that nonradial g-mode oscillations of the white dwarf be investigated as an alternative explanation.
Nitric and hydrochloric acids have been postulated to condense in the winter polar stratosphere to become an important component of polar stratospheric clouds. One implication is that the removal of NO(y) from the gas phase by this mechanism allows high Cl(x) concentrations to react with O3, because the formation of ClNO3 is inhibited. Contributions of NO3 and Cl to the stratospheric aerosol were determined during the 1987 Airborne Antarctic Ozone Experiment by testing for the presence of nitrates and chlorides in the condensed phase. Aerosol particles were collected on four 500 micron diameter gold wires, each pretreated differently to give results that were specific to certain physical and chemical aerosol properties. One wire was carbon-coated for concentration and size analyses by scanning electron microscopy; X-ray energy dispersive analyses permitted the detection of S and Cl in individual particles. Three more wires were coated with Nitron, barium chloride and silver nitrate, respectively, to detect nitrate, sulfate and chloride in aerosol particles. All three ions, viz., sulfates, nitrates and chlorides were detected in the Antarctic stratospheric aerosol. In terms of number concentrations, the aerosol was dominated by sulfates, followed by chlorides and nitrates. An inverse linear regression can be established between nitrate concentrations and ozone mixing ratio, and between temperature and nitrates.
High-resolution images of the sun in the soft-X-ray/XUV range have been obtained with normal-incidence multilayer coated optics (MCOs) during a sounding-rocket flight on October 23, 1987 from White Sands. MCOs were used in two Cassegrain telescopes, as off-axis primary mirrors, and as tertiary mirrors operating with a Wolter-Schwarzschild (WS) grazing-incidence mirror. The inherent energy-selective property of MCOs allowed distinct groups of emission lines to be isolated in the solar corona and the transition region. Images were recorded at 173 and 256 A with the Cassegrain telescopes, at 256 A with a Herschelian telescope, and at 44, 173, and 256 A with the WS hybrid telescope. In addition, soft-X-ray images in the 8-18-A bandpass were obtained at the prime focus of the WS optics. The images show many features of the solar corona and transition region, including magnetically confined loops of hot solar plasma, coronal plumes, polar coronal holes, supergranulation, and features associated with overlying cool prominences.
A standard data format for the archival and transport of x-ray events generated by ray trace models is described. Upon review and acceptance by the Advanced X-ray Astrophysics Facility (AXAF) Software Systems Working Group (SSWG), this standard shall become the official AXAF data format for ray trace events. The Flexible Image Transport System (FITS) is well suited for the purposes of the standard and was selected to be the basis of the standard. FITS is both flexible and efficient and is also widely used within the astronomical community for storage and transfer of data. In addition, software to read and write FITS format files are widely available. In selecting quantities to be included within the ray trace standard, the AXAF Mission Support team, Science Instruments team, and the other contractor teams were surveyed. From the results of this survey, the following requirements were established: (1) for the scientific needs, each photon should have associated with it: position, direction, energy, and statistical weight; the standard must also accommodate path length (relative phase), and polarization. (2) a unique photon identifier is necessary for bookkeeping purposes; (3) a log of individuals, organizations, and software packages that have modified the data must be maintained in order to create an audit trail; (4) a mechanism for extensions to the basic kernel should be provided; and (5) the ray trace standard should integrate with future AXAF data product standards.
Simultaneous soft X-ray, microwave, and photospheric magnetic field observations were taken during the Coronal Magnetic Structures Observing Campaign (CoMStOC '87). The plasma electron temperature and emission measures determined from the X-ray data are used to predict the free-free emission expected at 20 and 6 cm. Comparing these predictions with the microwave observations, it is found that the predicted 20 cm brightness temperatures are higher than the observed, requiring cool absorbing material between the hot X-ray plasma and the observer. The model that is most consistent with all the observations and minimizes the required coronal fields indicates that this 20 cm emission is either free-free or a combination of free-free and fourth harmonic cyclotron emanating from the X-ray plasma with an electron temperature of approximately 3.1 x 10(exp 6) K and an emission measure of approximately 1.3 x 10(exp 29)/cm(exp 5). The observed 20 cm polarization requires a field strength of greater than or equal to 150 G. In addition, the 6 cm emission is free-free, emanating from cooler plasma with an electron temperature of approximately 1.5 x 10(exp 6) K and an emission measure of approximately 3-6 x 10(exp 29)/cm(exp 5). This model is consistent with the rather unusual combination of high 20 cm and low 6 cm polarization as well as the low extrapolated coronal fields.
Model calculation of inhomogenuous magnetized neutron star atmospheres are used to analyze the phase-dependent spectra of 4U 1538-52 and Vela X-1 taking into account general relativistic effects. A chi (exp 2) fitting procedure is used on the Ginga data for these objects to determine the geometry and the magnetic structure of the polar caps, as well as their size and location on the surface of the stars. Other parameters determined by the fit are the masses and radii of neutron stars, the accretion rates, and the rotation and magnetic inclination angles including an azimuthal offset to reproduce the asymmetry of the pulse shapes. We find that general relativistic effects play a significant role in determining the derived cap sizes, the fluxes, and the observational appearance of the pulses. There is evidence that the polar caps are unequal and nonantipodal, suggesting either an off-center or bent magnetic axis, or a strong nondipole component. The magnetic field structure is approximated by a two-component model. The rotation axes are inferred to be moderately close to the mean magnetic axes and seen at large inclinations with respect to the line of sight.
The pulsation of Geminga has been detected to date only at high energies (E greater than 0.1 keV). Since x-ray exposures are short and Geminga is at best only marginally detected in gamma-rays at E less than 30 MeV, the primary means of timing Geminga is with high-energy gamma-rays. The EGRET observations of Geminga now span 4 years. These data are analyzed to determine the 1995 ephemeris for Geminga which is provided here. We continue to count every revolution of Geminga during the GRO mission with a rotational phase resolution which improves with additional exposure. Proper motion is now apparent in gamma-ray timing, consistent with the optical measurement of Bignami et al. With improved statistics, two additional peaks are tentatively detected in the 'minor bridge' region. More exposure is required to confirm them. If found to be real, they are difficult to understand with polar cap models, but are expected for the outer gap model, and provide sorely needed constraints.