On the analysis of lunar occultations of point-like X-ray sources

In view of the scheduled satellite mission EXOSAT (European X-Ray Observatory Satellite) of ESA (European Space Agency) the lunar occultation technique to determine the position of point-like X-ray sources is investigated. In particular, this paper explores the accuracy of the determination of the starting and ending time of an occultation, both of which are directly related to the size of the resulting position error box. An attempt is made to calculate analytically the distribution function of the estimated starting time, and some corresponding numerical evaluations are given. Finally, it is shown by Monte Carlo simulations that a slightly modifiedX ²-minimum method can be used to analyse the X-ray occultation data.     

Compact sources of suprathermal microwave emission detected in quiescent active regions during lunar occultations

Solar quiescent active regions are known to exhibit radio emission from discrete structures. The knowledge of their dimensions and brightness temperatures is essential for understanding the physics of quiescent, confined plasma regions. Solar eclipses of 10 August, 1980 and 26 January, 1990, observed with high sensitivity and high time resolution at 22 GHz, allowed an unprecedented opportunity to identify Fresnel diffraction effects during lunar occultations of active regions. The results indicate the presence of quiescent discrete sources smaller than one arcsecond in one dimension. Assuming symmetrical sources, their brightness temperatures were larger than 2 × 10⁷ K and 8 × 10⁷ K, for the 1980 and 1990 observations, respectively.In memoriam, 1942–1981.     

Future possibilities for lunar occultation studies of faint X-ray sources

Lunar occultation can be considered of interest for future missions dedicated to X-ray astronomy only if instruments with a large enough collecting area are used. In this case, observations of the numerous, faint X-ray sources occulted by the moon during a typical satellite lifetime of several years, can, in principle, add good source positioning as a free bonus to a high-throughput, medium-imaging mission. In the practical case of the EPIC instrument aboard the XMM ESA satellite, a simple calculation shows the potential usefulness of this technique.     

An analysis of lunar occultations in the years 1955–1980 using the new lunar ephemeris ELP2000

About 60,000 observations of lunar occultations made during 1955–1980 are analysed using recently-developed semi-analytical solution ELP2000-82 for the Moon's position in order to determine the constants in the lunar theory and to investigate the tidal term in the Moon's mean longitude and the motions of the perigee and node of the lunar orbit. The equinox correction and systematic correction to the fundamental star catalogue and the correction to the datum of the lunar-profile in Watts' charts are also investigated. It is confirmed that the occultation observations do not have inconsistent tidal term with the modern observations and the observed mean motions of the perigee and node coincide with the theoretical ones within the error of observations. Some of the values of the constants in the lunar theory and the equinox correction to the fundamental catalogue FK5 obtained in this paper are significantly different from the values obtained using the Brown's theory. The reason of the difference is almost attributed to the deficiencies in the Brown's theory. The obtained correction to the datum of the lunar-profile in Watts' charts is almost consistent with the results by earlier investigators.     

Clusters of black holes as point-like gamma-ray sources

The possibility of identifying some of Galactic gamma-ray sources as clusters of primordial black holes is discussed. The known scenarios of supermassive black hole formation indicate the multiple formation of lower-mass black holes. Our analysis demonstrates that due to Hawking evaporation the cluster of black holes with masses about 10¹⁵ g could be observed as a gamma-ray source. The total mass of typical cluster is ∼10 M_⊙. Detailed calculations have been performed on the basis of specific model of primordial black hole formation.     

A comparison of positions and sizes of sources of centimeter and X-ray bursts

We have made a parallel study of three cm- radio bursts, observed on 9 August, 1973 with the NRAO three-element interferometer at 3.7 and 11.1 cm and the associated X-ray flares observed with the S-054 telescope aboard Skylab. Within the errors of our measurements (± 5) the radio and X-ray events are cospatial. We find good agreement between the size of the X-ray kernel and that of the core of the cm burst, while there is evidence that much of the impulsive radio flux was produced in a larger area.

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Ancient Chinese observations of planetary positions and a table of planetary occultations

Ancient Chinese records of observations of planetary positions are introduced and classified. A table which includes 66 records of planetary occultations is given in this paper. Those ancient records are valuable for research on the secular motions of planets.     

Detection potential to point-like neutrino sources with the NEMO-km3 telescope

The NEMO Collaboration is conducting an R&D activity towards the construction of a Mediterranean km³ neutrino telescope. In this work, we present the results of Monte Carlo simulation studies on the capability of the proposed NEMO telescope to detect and identify point-like sources of high energy muon neutrinos. C. Distefano for the NEMO Collaboration.     

Chronology and sources of lunar impact bombardment

The Moon has suffered intense impact bombardment ending at 3.9 Gyr ago, and this bombardment probably affected all of the inner Solar System. Basin magnetization signatures and lunar crater size-distributions indicate that the last episode of bombardment at about 3.85 Gyr ago was less extensive than previously thought. We explore the contribution of the primordial Mars-crosser population to early lunar bombardment. We find that Mars-crosser population initially decays with a 80-Myr half-life, with the long tail of survivors clustering on temporarily non-Mars-crossing orbits between 1.8 and 2 AU. These survivors decay with half-life of about 600 Myr and are progenitors of the extant Hungaria asteroid group in the same region. We estimate the primordial Mars-crosser population contained about 0.01–0.02 Earth masses. Such initial population is consistent with no lunar basins forming after 3.8 Gya and the amount of mass in the Hungaria group. As they survive longer and in greater numbers than other primordial populations, Mars-crossers are the best candidate for forming the majority of lunar craters and basins, including most of the Nectarian system. However, this remnant population cannot produce Imbrium and Orientale basins, which formed too late and are too large to be part of a smooth bombardment. We propose that the Imbrian basins and craters formed in a discrete event, consistent with the basin magnetization signatures and crater size-distributions. This late “impactor shower” would be triggered by a collisional disruption of a Vesta-sized body from this primordial Mars-crossing population (Wetherill, G.W. [1975]. Proc. Lunar Sci. Conf. 6, 1539–1561) that was still comparable to the present-day asteroid belt a 3.9 Gya. This tidal disruption lead to a short-lived spike in bombardment by non-chondritic impactors with a non-asteroidal size–frequency distribution, in agreement with available evidence. This body (“Wetherill’s object”) also uniquely matches the constraints for the parent body of mesosiderite meteorites. We propose that the present-day sources of mesosiderites are multi-km-sized asteroids residing in the Hungaria group, that have been implanted there soon after the original disruption of their parent 3.9 Gyr ago.     

X-ray observations of ultraluminous X-ray sources

Ultraluminous X-ray sources (ULXs) are amongst the most intriguing of X-ray source classes. Their extreme luminosities—greater than 10³⁹ erg s^?1 in the 0.3–10 keV band alone—suggest either the presence of black holes larger than those regularly encountered in our own Galaxy (the Galactic centre excepted), or sources apparently radiating well above the Eddington limit. We review the insights afforded us by studies of their X-ray emission, focussing on what this reveals about the underlying compact object. In particular, we discuss recent deep observations of ULXs by the XMM-Newton observatory, and how the unprecedented data quality provided by this mission is starting to discriminate between the different physical models for these extraordinary X-ray emitters.     

The results of an accurate analysis of EAO charts of the Moon marginal zone constructed on the basis of lunar occultations

A computer‐readable version of the Moon's charts which refer to the center of its mass and the principal axes of the inertia were constructed on the basis of large‐scale star calibrated lunar photographies. The accuracy of six charts have been examined by reduction of observations of lunar occultations on the basis of the theory LE200.     

X-ray digital-imaging petrography: Technique development for lunar mare soils

Abstract Remote sensing studies are the primary means of solar system exploration. In particular, spectral reflectance measurements involve determinations of the nature and compositions of other worlds through analysis of absorption features characteristic of the surface chemistry and mineralogy. These studies are particularly applicable to “airless” solar system bodies (e.g., the Moon), because atmospheres, such as on Earth, tend to interfere with the reflectance spectrum. The precision of the spectral measurements is greatly increased by calibration with actual lunar soils. In the past, these calibrations were done using particle-counting data collected for the study of soil formation processes, soil classification, and provenance determination. These particle counting data, while valuable in those areas of study, neither identify the true volume percentages of soil particles, nor give the true modal values for the various phases (i.e., minerals and glasses) which make up the soil grains. These data are paramount for accurate spectral reflectance calibrations. Therefore, in this paper, a new technique is presented that involves x-ray digital-imaging of lunar soils using an energy dispersive spectrometer (EDS) on an electron microprobe. In contrast to particle counting with an optical microscope, the digital-imaging method allows precise volume percentages of soil grains to be determined, including absolute modal abundances of the various phases locked within the particles as well as their chemistry. In order to validate this method for characterization of lunar soils, the technique was applied to four Apollo 17 soils that were previously described by Heiken and McKay (1974) via particle counts with an optical microscope, and similar results were obtained. In addition to verifying the x-ray digital-imaging technique, the obtained data were applied in order to better understand the lunar-soil formational process, specifically the variation of particle types with maturity.     

On the possibility of investigating the galactic center region with the integral observatory by the method of lunar and terrestrial occultations

The possibility of investigating the sky region near the Galactic center with instruments of the INTEGRAL orbital astrophysical gamma-ray observatory by the method of its occultation by the Earth and the Moon is considered. Existing engineering constraints on the observing conditions, such as the admissible orientation of the INTEGRAL satellite relative to the direction to the Sun and the performance of measurements only outside the Earth??s radiation belts, are taken into account. Long time intervals during which the lunar occultation center passes at angular distances of less than 2° from the Galactic center have been found. Such events occur under the adopted constraints two or three times per year without any correction of the INTEGRAL satellite orbit. The orbit can be corrected to reduce the angular distance between the Moon and the Galactic center in occultation events. The required velocity impulses do not exceed several meters per second. The possibility of the Galactic center being occulted by the Earth has been analyzed. In this case, to perform measurements, the admissible (in radiation exposure) height of the working segment of the orbit should be reduced to 25 000 km, which can be problematic. At the same time, part of the Galaxy??s equatorial region is shadowed by the Earth for a time long enough to carry out the corresponding experiments.     

The eccentric-orbit binary model for the transient X-ray sources

Eccentric-orbit binary models for transient X-ray sources are investigated. In these models, a compact star is in an eccentric orbit around a more massive star. As the compact star accretes mass from the stellar wind of the massive star, the accretion rate becomes time-dependent. The accretion rate is determined by Bondi's accretion radius, which depends on both the relative velocity of the stellar wind to the compact star and the sound velocity through the stellar wind. With reasonable sets of the eccentricity, the semi-major axis, the stellar wind velocity and the sound velocity, we obtain the variations of the light curves compatible with observations for the transient X-ray sources. It is likely that many transient X-ray sources are explainable by eccentric-orbit binary models.     

A new method for determining the sensitivity of X-ray imaging observations and the X-ray number counts

We present a new method for determining the sensitivity of X-ray imaging observations, which correctly accounts for the observational biases that affect the probability of detecting a source of a given X-ray flux, without the need to perform a large number of time-consuming simulations. We use this new technique to estimate the X-ray source counts in different spectral bands (0.5–2, 0.5–10, 2–10 and 5–10 keV) by combining deep pencil-beam and shallow wide-area Chandra observations. The sample has a total of 6295 unique sources over an area of  11.8 deg²  and is the largest used to date to determine the X-ray number counts. We determine, for the first time, the break flux in the 5–10 keV band, in the case of a double power-law source count distribution. We also find an upturn in the 0.5–2 keV counts at fluxes below about  6 × 10⁻¹⁷ erg s⁻¹ cm⁻²  . We show that this can be explained by the emergence of normal star-forming galaxies which dominate the X-ray population at faint fluxes. The fraction of the diffuse X-ray background resolved into point sources at different spectral bands is also estimated. It is argued that a single population of Compton thick active galactic nuclei (AGN) cannot be responsible for the entire unresolved X-ray background in the energy range 2–10 keV.     

Search for point-like sources of cosmic rays with energies above 10 eV in the HiRes-I monocular data set

We report the results of a search for point-like deviations from isotropy in the arrival directions of ultra-high energy cosmic rays in the northern hemisphere. In the monocular data set collected by the High-Resolution Fly’s Eye, consisting of 1525 events with energy exceeding 10^18.5 eV, we find no evidence for point-like excesses. We place a 90% c.l. upper limit of 0.8 hadronic cosmic rays/km² yr on the flux from such sources for the northern hemisphere and place tighter limits as a function of position in the sky.     

Search for point-like sources of cosmic rays with energies above 1018.5 eV in the HiRes-I monocular data set

We report the results of a search for point-like deviations from isotropy in the arrival directions of ultra-high energy cosmic rays in the northern hemisphere. In the monocular data set collected by the High-Resolution Fly’s Eye, consisting of 1525 events with energy exceeding 10^18.5 eV, we find no evidence for point-like excesses. We place a 90% c.l. upper limit of 0.8 hadronic cosmic rays/km² yr on the flux from such sources for the northern hemisphere and place tighter limits as a function of position in the sky.     

X-ray sources and the diffuse background

Possible numbers of X-ray sources at various flux levels are considered and their contribution to the background evaluated. Our conclusions are: The mean spectrum of Quasi-Stellar Objects and the nuclei of Seyfert galaxies between optical and X-ray wavelengths is characterized by a spectral index well in excess of 1.0. These objects may make an important contribution to the soft X-ray background. Observations of Q.S.O. may allow a decisive test for the cosmological interpretation of the redshifts of the Q.S.O. With regard to galactic sources we find that few undetected sources of the Sgr-Sco type exist, that these sources are located in the immediate neighborhood of the galactic center and that they account for at least 80% of the total galactic source luminosity. However, many intrinsically much fainter galactic sources may exist.