We report a study of cloud cover over Indonesia based on meteorological satellite data spanning 15 years (from 1996 to 2010) to aid in the selection of a new astronomical site capable of hosting a multi-wavelength astronomical observatory. High-spatial-resolution meteorological satellite data acquired from Geostationary Meteorological Satellite 5 (GMS 5), Geostationary Operational Environmental Satellite 9 (GOES 9) and Multi-functional Transport Satellite-1R (MTSAT-1R) are used to derive yearly average clear fractions over various regions of Indonesia. This parameter is determined from temperature measurements in the IR3 channel (water vapour, 6.7 µm) for high-altitude clouds (cirrus), and from the IR1 channel (10.7 µm) for lower-altitude clouds. An algorithm is developed to detect the corresponding clouds. The results of this study were used to select the best possible sites in Indonesia, which will be analysed further by performing in situ measurements in the future. The results suggest that regions of East Nusa Tenggara, located in southeastern Indonesia, are the most promising candidates for such an astronomical site. The yearly clear sky fraction of this region may reach better than 70 per cent, with an uncertainty of 10 per cent.
The X-ray emission of non-magnetic cataclysmic variable stars is considered. It is well known that a nearly spherical hot corona is produced around accreting white dwarfs because of strong viscous heating, and of rather inefficient cooling of matter at the boundary layer if the accretion rate is below some critical value. A steady state model of X-ray emitting coronae, into which mass is steadily supplied from the boundary layer, is then constructed and the X-ray spectra are calculated as functions of the mass-supply rate Ṁs. Eclipses of such corona were simulated. It has been found that the thermal energy of the corona is transported downward by conduction to the denser area near to the surface of the white dwarf, where it is emitted as X-ray radiation. Around 5% of the matter supplied by the disk could leave the system as stellar wind. Most of the X-rays are radiated from regions near to the white dwarf surface within this model. Eclipse simulations of this model agree with recent observations of eclipsing system HT Cas.
We present results derived from four stellar occultations by the plutino object (208996) 2003 AZ 84 , detected at January 8, 2011 (single-chord event), February 3, 2012 (multi-chord), December 2, 2013 (single-chord) and November 15, 2014 (multi-chord). Our observations rule out an oblate spheroid solution for 2003 AZ 84 's shape. Instead, assuming hydrostatic equilibrium, we find that a Jacobi triaxial solution with semi axes (470 ± 20) × (383 ± 10) × (245 ± 8) km can better account for all our occultation observations. Combining these dimensions with the rotation period of the body (6.75 h) and the amplitude of its rotation light curve, we derive a density ρ = 0.87 ± 0.01 g cm −3 a geometric albedo p V = 0.097 ± 0.009. A grazing chord observed during the 2014 occultation reveals a topographic feature along 2003 AZ 84 's limb, that can be interpreted as an abrupt chasm of width ∼ 23 km and depth > 8 km or a smooth depression of width ∼ 80 km and depth ∼ 13 km (or an intermediate feature between those two extremes). Subject headings: Kuiper belt objects: individual (208996, 2003 AZ 84 ) -occultations -planets and satellites: surfaces -planets and satellites: fundamental parameters
Type II burst properties of the Rapid Burster have been studied with the X-ray astronomy satellite ASCA. To study spectral evolutions throughout a burst cycle from the onset of a burst to the next burst, we constructed six composite burst-cycles in terms of the burst fluence, and divided each of them into seven phases. We then fitted the 42 spectra with a two-component model consisting of a blackbody (BB) component expected from the boundary layer, and a multi-color-disk (MCD) component expected from the standard disk, systematically. The results show that the luminosity of the BB component is much larger than that of the MCD component during a burst, while the two luminosities become comparable during a persistent phase. They also indicate that the innermost radius of the standard disk tends to recede outwards in the burst decay. These results suggest that although the standard disk extends close to the neutron star surface during a persistent phase, another accretion flow onto the neutron star is needed during a burst phase. We argue that an advection-dominated accretion flow (ADAF) would be a dominant flow onto the neutron star during a burst, and that a limit cycle of the accretion flow between an ADAF and a standard disk can explain the overall properties of the type II bursts.
Active Galactic Nuclei (AGN), such as Seyfert galaxies, quasars, etc., show light variations in all wavelength bands, with various amplitude and in many time scales. The variations usually look erratic, not periodic nor purely random. Many of these objects also show lognormal flux distribution and RMS -flux relation and power law frequency distribution. So far, the lognormal flux distribution of black hole objects is only observational facts without satisfactory explanation about the physical mechanism producing such distribution in the accretion disk. One of the most promising models based on cellular automaton mechanism has been successful in reproducing PSD (Power Spectral Density) of the observed objects but could not reproduce lognormal flux distribution. Such distribution requires the existence of underlying multiplicative process while the existing SOC models are based on additive processes. A modified SOC model based on cellular automaton mechanism for producing lognormal flux distribution is presented in this paper. The idea is that the energy released in the avalanche and diffusion in the accretion disk is not entirely emitted instantaneously as in the original cellular automaton model. Some part of the energy is kept in the disk and thus increase its energy content so that the next avalanche will be in higher energy condition and will release more energy. The later an avalanche occurs, the more amount of energy is emitted to the observers. This can provide multiplicative effects to the flux and produces lognormal flux distribution.
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