Time-dependent photoionization opacities in dense gamma-ray burst environments

The recent detection of a transient absorption feature in the X-ray prompt emission of GRB 990705 showed the importance of such observations in the understanding of gamma-ray bursts and their progenitors. We investigate the time dependence of photoionization edges during the prompt emission of bursts in different environments. We show that their variability can be used to infer the density and geometry of the surrounding medium, giving important clues to unveil the nature of the burst progenitor.     

Crystal-chemical examination of natural diopsides: Some geometrical indications of Si-Ti tetrahedral substitution

Summary The crystal structures of 20 natural diopsides have been refined. For some samples, microprobe analyses show a Si deficiency in the tetrahedron with no A1 present; this suggests that some Ti⁴⁺ may enter the tetrahedral site. Geometrical variations in these samples with respect to the others are discussed in the light of this Si Ti⁴⁺ substitution. The volumes of their M1 sites are larger because of the absence of R³⁺ cations, the octahedron being very regular; in the same way, the volumes of M2 sites of these samples are greater not because of different site occupancies but because of significant charge modifications around the O3 and O1 oxygens, caused by the tetrahedral site. The low distortion of M1 site and the lengthening of (M2-O1) and M2-O3 bond lengths are a clue to the presence of small amounts of Ti at the tetrahedral site.

---

Kristallchemische untersuchung natürlicher diopside: Geometrische hinweise für tetraedrische Si-Ti substitution

Zusammenfassung Die Kristallstrukturen von zwanzig natürlichen Diopsiden sind verfeinert worden. In einigen Proben zeigen Mikrosondenanalysen zu geringe Si-Werte im Tetraeder ohne daß A1 anwesend wäre. Dies weist darauf hin, daß Ti⁴⁺ Tetraederplätze einnehmen könnte. Geometrische Variationen in diesen Proben werden im Lichte der genannten Substitution diskutiert. Die Volumen von M1-Plätzen sind größer wegen der Abwesenheit von R³⁺ Kationen, wobei das Oktaeder sehr gleichmäßig ist. Die Volumina von M2-Plätzen sind noch größer nicht wegen verschiedener Besetzungen dieser Plätze sondern wegen signifikanter Ladungsmodifikationen im Bereich von O3 und O1 Sauerstoffen, die durch die tetraedrische Position bewirkt wird. Die geringe Distortion der M1-Plätze und die Verlängerung der (M2-O1) und (M2-O3) Abstände weisen auf die Anwesenheit von geringen Titangehalten auf Tetraederplätzen hin.

     

Physical investigation of the potentially hazardous Asteroid (144898) 2004 VD17

In this paper we present the observational campaign carried out at ESO NTT and VLT in April and May 2006 to investigate the nature and the structure of the near-Earth object (144898) 2004 VD17. In spite of a great quantity of dynamical information, according to which it will have a close approach with the Earth in the next century, the physical properties of this asteroid are largely unknown. We performed visible and near-infrared photometry and spectroscopy, as well as polarimetric observations. Polarimetric and spectroscopic data allowed us to classify 2004 VD17 as an E-type asteroid. A good agreement was also found with the spectrum of the aubrite meteorite Mayo Belwa. On the basis of the polarimetric albedo (pv=0.45) and of photometric data, we estimated a diameter of about 320 m and a rotational period of about 2 h. The analysis of the results obtained by our complete survey have shown that (144898) 2004 VD17 is a peculiar NEO, since it is close to the breakup limits for fast rotator asteroids, as defined by Pravec and Harris [Pravec, P., Harris, A.W., 2000. Icarus 148, 12-20]. These results suggest that a more robust structure must be expected, as a fractured monolith or a rubble pile in a “strength regime” [Holsapple, K.A., 2002. Speed limits of rubble pile asteroids: Even fast rotators can be rubble piles. In: Workshop on Scientific Requirements for Mitigation of Hazardous Comets and Asteroids, Washington, September, 2002].     

The A.D. 79 eruption as a future explosive scenario in the Vesuvian area: evaluation of associated risk

 Due to the lack of an effective policy of planning and prevention, over the past decades the area around Mt. Vesuvio has undergone a steady increase in population and uncontrolled housing development. Consequently, it has become one of the most hazardous volcanic areas in the world. In order to mitigate the damage that the impact of an explosive event would cause in the area, the Department of Civil Defense has worked out an Emergency Management Plan using the A.D. 1631 subplinian eruption as the most probable short-term event. However, from 25 000 years B.P. to present, the activity of the Somma-Vesuvio volcano has shown a sequence of eight eruptive cycles, which always began with a strong plinian eruption. In this paper we utilize the A.D. 79 eruption as an example of a potential large explosive eruption that might occur again at Vesuvio. A detailed tephrostratigraphic analysis of the eruption products was processed by a multivariate statistical analysis. This analysis proved useful for identifying marker layers in the sequences, thus allowing the recognition of some major phases of synchronous deposition and hence the definition of the chronological and spatial evolution of the eruption. By combining this reconstruction with land-use maps, a scenario is proposed with time intervals in the eruptive sequence similar to those reported in Pliny's letter. Thus, it was calculated that, after 7 h from the start of the eruption, a total area of approximately 300 km² would be covered with the eruption products. In the following 11 h, a total area of approximately 500 km² would be involved. The third and last phase of deposition would not cause significant variation in the total area involved, but it would bring about an increase in the thickness of the pyroclastic deposits in the perivolcanic area. Received: 30 November 1996 / Accepted: 29 May 1997     

Gravitational lensing of anisotropic sources

In strong gravitational lensing, the multiple images we see correspond to light rays that leave the source in slightly different directions. If the source emission is anisotropic, the images may differ from conventional lensing predictions (which assume isotropy). To identify scales on which source anisotropy may be important, we study the angle δ between the light rays emerging from the source, for different lensing configurations. If the lens has a power-law profile   M ∝ R ^γ  , the angle δ initially increases with lens redshift and then either diverges (for a steep profile  γ < 1  ), remains constant (for an isothermal profile  γ= 1  ), or vanishes (for a shallow profile  γ > 1  ) as   z _l→ z _s  . The scaling with lens mass is roughly  δ∝ M ^1/(2−γ)  . The results for an Navarro–Frenk–White (NFW) profile are qualitatively similar to those for a shallow power law, with δ peaking at about half the redshift of the source (not half the distance). In practice, beaming could modify the statistics of beamed sources lensed by massive clusters: for an opening angle  θ_jet  , there is a probability as high as   P ∼ 0.02–0.07(θ_jet/0.5°)⁻¹  that one of the lensed images may be missed (for  2 ≲ z _s≲ 6  ). Differential absorption within active galactic nuclei (AGNs) could modify the flux ratios of AGNs lensed by clusters; a sample of AGNs lensed by clusters could provide further constraints on the sizes of absorbing regions. Source anisotropy is not likely to be a significant effect in galaxy-scale strong lensing.     

Eruption and mass-wasting-induced processes during the late holocene destructive phase of Nevado del Ruiz volcano, Colombia

The November 13, 1985, eruption was characteristic of the Arenas eruptive stage of Nevado del Ruiz, the most recent of a series of twelve eruptive stages that have occurred in the past 11,000 years. Eruptive sequences, deposits and processes similar to that of 1985 have characterized the behavior of Nevado del Ruiz during three major prehistorical and historical eruptive stages: the approximately 3300-3100 yr. B.P. Hedionda, the 16th century Azufrado, and the mid-1800's Lagunillas eruptive stages, that partly destroyed the present Ruiz summit.According to the interpretation of the stratigraphic record of prehistorical eruptions and historical accounts, almost every recent magmatic event was small or short-lived. Nevertheless, rockslide-debris avalanches and catastrophic debris flows were triggered in all the eruptions owing to slope failures related to specific tectonic features of Ruiz volcano and/or to significant interactions between pyroclastic debris and the ice cap. Evidence for headward retreat of avalanche scarps during multiple eruptions reinforce the case that large slope failures can occur repeatedly at a large-volume volcano like Ruiz without reconstruction of the edifice. The latest Ruiz eruptions that involved rockslide-debris avalanches resemble in part the Shiveluch 1964 event for which evidence of lateral blast deposits are lacking, but differ in part from this type because non-eruptive and mass-wasting processes also triggered rockslide-debris avalanches.Many factors render the cluster of domes of the Ruiz summit unstable, including: (1) deeply dissected troughs opened toward the north-northeast (Azufrado), east (Lagunillas), and south (Recio) flanks; (2) strongly hydrothermally altered north and east flanks of the summit; (3) currently glaciated or recently deglaciated, high cliffs; (4) steep unstable margins of the ice cap on the north and east.Thus, in light of its past behavior, a small eruption or an earthquake might trigger catastrophic rockslide-debris avalanches. Furthermore, such avalanches as well as glacial outburst-floods and ice avalanches could induce debris flows by mobilizing weathered, water-saturated, and unconsolidated rocks or deposits.     

An efficient algorithm for prioritizing NEA physical observations

The present near‐Earth asteroids (NEA) discovery rate has surpassed 1500 objects per year, thus calling for extensive observation campaigns devoted to physical characterization in order to define successful mitigation strategies in case of possible impactors. A tool is presented which, through a prioritization algorithm, aims to optimize the planning and the execution of NEA physical observations. Two ranking criteria are introduced, Importance and Urgency, accounting for the need of satisfying the two basic observational modes for physical characterization, that is, rapid response and large observing programs, aimed at selecting targets for exploration and mitigation space missions. The resulting tool generates a daily table of observable targets and it can also be run as a stand‐alone tool in order to provide future observing opportunities at a specific date of interest. It has been developed and implemented within the framework of the NEOShield‐2 EU/HORIZON 2020 Project; the output of the prioritization algorithm is publicly available, in tabular format, on the NEOShield‐2 NEO Properties Portal ( http://www.neoshield.net/the-neoshield-project/project-activities/neo-properties-portal ), reachable from the main page of the Project website ( http://www.neoshield.eu ).     

Soft X-ray polarization in thermal magnetar emission

Emission spectra from magnetars in the soft X-ray band likely contain a thermal component emerging directly from the neutron star (NS) surface. However, the lack of observed absorption-like features in quiescent spectra makes it difficult to directly constrain physical properties of the atmosphere. We argue that future X-ray polarization measurements represent a promising technique for directly constraining the magnetar magnetic field strength and geometry. We construct models of the observed polarization signal from a finite surface hotspot, using the latest NS atmosphere models for magnetic fields   B = 4 × 10¹³–5 × 10¹⁴ G  . Our calculations are strongly dependent on the NS magnetic field strength and geometry, and are more weakly dependent on the NS equation of state and atmosphere composition. We discuss how the complementary dependencies of phase-resolved spectroscopy and polarimetry might resolve degeneracies that currently hamper the determination of magnetar physical parameters using thermal models.