10,763 research outputs found

    X-rays from Saturn: A study with XMM-Newton and Chandra over the years 2002-05

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    We present the results of the two most recent (2005) XMM-Newton observations of Saturn together with the re-analysis of an earlier (2002) observation from the XMM-Newton archive and of three Chandra observations in 2003 and 2004. While the XMM-Newton telescope resolution does not enable us to resolve spatially the contributions of the planet's disk and rings to the X-ray flux, we can estimate their strengths and their evolution over the years from spectral analysis, and compare them with those observed with Chandra. The spectrum of the X-ray emission is well fitted by an optically thin coronal model with an average temperature of 0.5 keV. The addition of a fluorescent oxygen emission line at ~0.53 keV improves the fits significantly. In accordance with earlier reports, we interpret the coronal component as emission from the planetary disk, produced by the scattering of solar X-rays in Saturn's upper atmosphere, and the line as originating from the Saturnian rings. The strength of the disk X-ray emission is seen to decrease over the period 2002 - 2005, following the decay of solar activity towards the current minimum in the solar cycle. By comparing the relative fluxes of the disk X-ray emission and the oxygen line, we suggest that the line strength does not vary over the years in the same fashion as the disk flux. We consider possible alternatives for the origin of the line. The connection between solar activity and the strength of Saturn's disk X-ray emission is investigated and compared with that of Jupiter. We also discuss the apparent lack of X-ray aurorae on Saturn and conclude that they are likely to lie below the sensitivity threshold of current Earth-bound observatories. A similar comparison for Uranus and Neptune leads to the same disappointing conclusion.Comment: 10 pages, 5 figures; to be published in 'Astronomy and Astrophysics

    Carrier Transport in Magnesium Diboride: Role of Nano-inclusions

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    Anisotropic-gap and two-band effects smear out the superconducting transition (Tc) in literature reported thermal conductivity of MgB2, where large electronic contributions also suppress anomaly-manifestation in their negligible phononic-parts. Present thermal transport results on scarcely explored specimens featuring nano-inclusions exhibit a small but clear Tc-signature, traced to relatively appreciable phononic conduction, and its dominant electronic-scattering. The self-formed MgO as extended defects strongly scatter the charge carriers and minutely the phonons with their longer-mean-free-path near Tc. Conversely, near room temperature, the shorter-dominant-wavelength phonon's transport is hugely affected by these nanoparticles, undergoing ballistic to diffusive crossover and eventually entering the Ioffe-Regel mobility threshold regime.Comment: 14 pages, 4 figures, 28 reference

    The Role of Opacities in Stellar Pulsation

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    We examine the role of opacities in stellar pulsation with reference to Cepheids and RR Lyraes, and examine the effect of augmented opacities on the theoretical pulsation light curves in key temperature ranges. The temperature ranges are provided by recent experimental and theoretical work that have suggested that the iron opacities have been considerably underestimated. For Cepheids, we find that the augmented opacities have noticeable effects in certain period ranges (around log‚Ā°P‚Čą1\log P \approx 1) even though there is a degeneracy with mixing length. We also find significant effects in theoretical models of B-star pulsators.Comment: 6 pages, 3 Figures, Proceeding for the "Workshop on Astrophysical Opacities

    Forbidden oxygen lines at various nucleocentric distances in comets

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    To study the formation of the [OI] lines - i.e., 5577 A (the green line), 6300 A and 6364 A (the two red lines) - in the coma of comets and to determine the parent species of the oxygen atoms using the green to red-doublet emission intensity ratio (G/R ratio) and the lines velocity widths. We acquired at the ESO VLT high-resolution spectroscopic observations of comets C/2002 T7 (LINEAR), 73P-C/Schwassmann-Wachmann 3, 8P/Tuttle, and, 103P/Hartley 2 when they were close to the Earth (< 0.6 au). Using the observed spectra, we determined the intensities and the widths of the three [OI] lines. We have spatially extracted the spectra in order to achieve the best possible resolution of about 1-2", i.e., nucleocentric projected distances of 100 to 400 km depending on the geocentric distance of the comet. We have decontaminated the [OI] green line from C2 lines blends. It is found that the observed G/R ratio on all four comets varies as a function of nucleocentric projected distance. This is mainly due to the collisional quenching of O(1S) and O(1D) by water molecules in the inner coma. The observed green emission line width is about 2.5 km/s and decreases as the distance from the nucleus increases which can be explained by the varying contribution of CO2 to the O(1S) production in the innermost coma. The photodissociation of CO2 molecules seems to produce O(1S) closer to the nucleus while the water molecule forms all the O(1S) and O(1D) atoms beyond 1000 km. Thus we conclude that the main parent species producing O(1S) and O(1D) in the inner coma is not always the same. The observations have been interpreted in the framework of the coupled-chemistry-emission model of Bhardwaj & Raghuram (2012) and the upper limits of CO2 relative abundances are derived from the observed G/R ratios. Measuring the [OI] lines could indeed provide a new way to determine the CO2 relative abundance in comets.Comment: accepted for publication in A&A, the abstract is shortene

    Chandra Observation of an X-ray Flare at Saturn: Evidence for Direct Solar Control on Saturn's Disk X-ray Emissions

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    Saturn was observed by Chandra ACIS-S on 20 and 26-27 January 2004 for one full Saturn rotation (10.7 hr) at each epoch. We report here the first observation of an X-ray flare from Saturn's non-auroral (low-latitude) disk, which is seen in direct response to an M6-class flare emanating from a sunspot that was clearly visible from both Saturn and Earth. Saturn's disk X-ray emissions are found to be variable on time scales of hours to weeks to months, and correlated with solar F10.7 cm flux. Unlike Jupiter, X-rays from Saturn's polar (auroral) region have characteristics similar to those from its disk. This report, combined with earlier studies, establishes that disk X-ray emissions of the giant planets Saturn and Jupiter are directly regulated by processes happening on the Sun. We suggest that these emissions could be monitored to study X-ray flaring from solar active regions when they are on the far side and not visible to Near-Earth space weather satellites.Comment: Total 12 pages including 4 figure
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