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71.
72.
M. Vietri C. Perola L. Piro L. Stella 《Monthly notices of the Royal Astronomical Society》1999,308(4):L29-L33
Recently, BeppoSAX and ASCA have observed an unusual resurgence of soft X-ray emission during the afterglows of GRB 970508 and 970828, together with marginal evidence for the existence of Fe lines in both objects. We consider the implications of the existence of a torus of iron-rich material surrounding the sites of gamma-ray bursts, as would be expected in the supra-nova model; in particular, we show that the fireball will quickly hit this torus, and bring it to a temperature of ≈3×107 K. Bremsstrahlung emission from the heated-up torus will cause a resurgence of the soft X-ray emission with all expected characteristics (flux level, duration and spectral hardening with time) identical to those observed during the re-burst. Also, thermal emission from the torus will account for the observed iron line flux. These events are also observable, for instance by new missions such as SWIFT , when beaming away from our line of sight makes us miss the main burst, as fast (soft) X-ray transients, with durations of ≈103 s and fluences of ≈10−7 –10−4 erg cm−2 . This model provides evidence in favour of the supra-nova model for gamma-ray bursts. 相似文献
73.
Andrea Tiengo Paolo Esposito Sandro Mereghetti Lara Sidoli Diego Götz Marco Feroci Roberto Turolla Silvia Zane Gian Luca Israel Luigi Stella Peter Woods 《Astrophysics and Space Science》2007,308(1-4):33-37
We present a systematic analysis of all the BeppoSAX data of SGR1900+14. The observations spanning five years show that the source was brighter than usual on two occasions: ~20 days after the August 1998 giant flare and during the 105?s long X-ray afterglow following the April 2001 intermediate flare. In the latter case, we explore the possibility of describing the observed short term spectral evolution only with a change of the temperature of the blackbody component. In the only BeppoSAX observation performed before the giant flare, the spectrum of the SGR1900+14 persistent emission was significantly harder and detected also above 10 keV with the PDS instrument. In the last BeppoSAX observation (April 2002) the flux was at least a factor 1.2 below the historical level, suggesting that the source was entering a quiescent period. 相似文献
74.
Human pressure on mountain resources poses certain threats to their natural and man-made environment. Although there are many parameters involved, one of the driving forces behind this situation is the emphasis given so far only to the market value of mountain goods and services, ignoring the true social and environmental costs of using mountain resources. Towards this direction, the use of non-market valuation techniques could prove to be beneficial. Bearing in mind the above remarks, the paper presents an application of the Contingent Valuation Method, which aims at estimating the monetary value of a mountain settlement’s vernacular architecture, namely Metsovo. For this purpose, the survey focuses on Metsovo visitors’ willingness to pay a single annual voluntary contribution to an institution that would be founded in order to undertake all necessary actions required to preserve the town’s traditional character. The results are very promising and indicate that vernacular architecture holds a significant economic value that could justify the implementation of appropriate policies towards the protection of vernacular settlements in mountain areas. 相似文献
75.
G. L. Israel L. Angelini S. Campana P. Giommi L. Stella & N. E. White 《Monthly notices of the Royal Astronomical Society》1998,298(2):502-506
During a systematic search for periodic signals in a sample of ROSAT PSPC (0.1–2.4 keV) light curves, we have discovered ∼12-min large-amplitude X-ray pulsations in 1WGA J1958.2+3232, an X-ray source which lies close to the Galactic plane. The energy spectrum is well fitted by a power law with a photon index of 0.8, corresponding to an X-ray flux level of ∼ 10−12 erg cm−2 s−1 . The source is probably a long-period, low-luminosity X-ray pulsar, similar to X Per, or an intermediate polar. 相似文献
76.
A qualitative landslide susceptibility zonation map (scale 1:100,000) of a sector of the Río Mendoza Valley was prepared by overlapping thematic maps of conditioning factors. The main parameters affecting the distribution of landslides in the area were ranked according to slope instability. Geomorphological surveys and field observations enabled identification of 300 historical or prehistoric landslides. A landslide inventory was developed classifying the processes involved in slope instability, and analysing the degree of activity. The two most influential factors in decreasing slope stability were lithology and slope angle. 相似文献
77.