Search results for "astronomia"
showing 10 items of 673 documents
A re-analysis of the NuSTAR and XMM-Newton broad-band spectrum of Serpens X-1
2017
Context. High-resolution X-ray spectra of neutron star low-mass X-ray binaries (LMXBs) in the energy range 6.4-6.97 keV are often characterized by the presence of K alpha transition features of iron at different ionization stages. Since these lines are thought to originate by reflection of the primary Comptonization spectrum over the accretion disk, the study of these features allows us to investigate the structure of the accretion flow close to the central source. Thus, the study of these features gives us important physical information on the system parameters and geometry. Ser X-1 is a well studied LMXB that clearly shows a broad iron line. Several attempts to fit this feature as a smear…
Monte Carlo Markov Chain DEM reconstruction of isothermal plasmas
2012
In this paper, we carry out tests on the Monte Carlo Markov Chain (MCMC) technique with the aim of determining: 1) its ability to retrieve isothermal plasmas from a set of spectral line intensities, with and without random noise; 2) to what extent can it discriminate between an isothermal solution and a narrow multithermal distribution; and 3) how well it can detect multiple isothermal components along the line of sight. We also test the effects of 4) atomic data uncertainties on the results, and 5) the number of ions whose lines are available for the DEM reconstruction. We find that the MCMC technique is unable to retrieve isothermal plasmas to better than Delta log T = 0.05. Also, the DEM…
Evidence of Widespread Hot Plasma in a Nonflaring Coronal Active Region from Hinode/X-Ray Telescope
2009
Nanoflares, short and intense heat pulses within spatially unresolved magnetic strands, are now considered a leading candidate to solve the coronal heating problem. However, the frequent occurrence of nanoflares requires that flare-hot plasma be present in the corona at all times. Its detection has proved elusive until now, in part because the intensities are predicted to be very faint. Here, we report on the analysis of an active region observed with five filters by Hinode/X-Ray Telescope (XRT) in 2006 November. We have used the filter ratio method to derive maps of temperature and emission measure (EM) both in soft and hard ratios. These maps are approximate in that the plasma is assumed …
MHD modeling of coronal loops: the transition region throat
2014
The expansion of coronal loops in the transition region may considerably influence the diagnostics of the plasma emission measure. The cross sectional area of the loops is expected to depend on the temperature and pressure, and might be sensitive to the heating rate. The approach here is to study the area response to slow changes in the coronal heating rate, and check the current interpretation in terms of steady heating models. We study the area response with a time-dependent 2D MHD loop model, including the description of the expanding magnetic field, coronal heating and losses by thermal conduction and radiation from optically thin plasma. We run a simulation for a loop 50 Mm long and qu…
Lobster eye optics for nano-satellite x-ray monitor
2011
The Lobster eye design for a grazing incidence X-ray optics provides wide field of view of the order of many degrees, for this reason it would be a convenient approach for the construction of space X-ray monitors. In this paper, we compare previously reported measurements of prototype lobster eye X-ray optics called P-25 with computer simulations and discuss differences between the theoretical end experimentally obtained results. Usability of this prototype lobster eye and manufacturing technology for the nano-satellite mission is assessed. The specific scientific goals are proposed.
XMM-Newton survey of two upper Scorpius regions
2006
We study X-ray emission from young stars by analyzing deep XMM-Newton observations of two regions of the Upper Scorpius association, having an age of 5 Myr. Based on near infrared and optical photometry we identify 22 Upper Scorpius photometric members among the 224 detected X-ray sources. We derive coronal properties of Upper Scorpius stars by performing X-ray spectral and timing analysis. The study of four strong and isolated stellar flares allows us to derive the length of the flaring loops. Among the 22 Upper Scorpius stars, 13 are identified as Upper Scorpius photometric members for the first time. The sample includes 7 weak-line T Tauri stars and 1 classical T Tauri star, while the na…
Emission-Line Intensity Ratios in F[CLC]e[/CLC] [CSC]xvii[/CSC] Observed with a Microcalorimeter on an Electron Beam Ion Trap
2000
We report new observations of emission line intensity ratios of Fe XVII under controlled experimental conditions, using the National Institute of Standards and Technology electron beam ion trap (EBIT) with a microcalorimeter detector. We compare our observations with collisional-radiative models using atomic data computed in distorted wave and R-matrix approximations, which follow the transfer of the polarization of level populations through radiative cascades. Our results for the intensity ratio of the 2p6 1S0-2p53d 1P1 15.014 A line to the 2p6 1S0-2p53d 3D1 15.265 A line are 2.94 ± 0.18 and 2.50 ± 0.13 at beam energies of 900 and 1250 eV, respectively. These results are not consistent wit…
A model to interpret pulse phase shifts in AMXPs: SAX J1808.4-3658 as a proof of concept
2011
Abstract: Observational evidences of erratic 1(st) harmonic pulse phase shifts in accreting millisecond X-ray pulsars pulse phase evolution was reported by several authors. This effect always go together with much more stable 2(nd) harmonics pulse phase delays. Different possible explanations of these phase shifts have been given in literature. But all these interpretations do not explain why the 2(nd) harmonic are more stable than the 1(st) harmonic. The explanation of such a behaviour is of fundamental importance in order to gain an insight on the NS rotational behaviour and to remove the still present interpretative ambiguity on the results of timing analysis. We propose a simple toy-mod…
Mass Accretion Impacts in Classical T Tauri Stars: A Multi-disciplinary Approach
2019
Accretion of matter is a process that plays a central role in the physics of young stellar objects. The analysis of the structure by which matter settles on the star can unveil key information about the process of star formation by providing details on mass accretion rates, stellar magnetic field configurations, possible effects of accretion on the stellar coronal activity, etc. Here we review some of the achievements obtained by our group by exploiting a multi-disciplinary approach based on the analysis of multi-dimensional magnetohydrodynamic simulations, multi-wavelength observations, and laboratory experiments of accretion impacts occurring onto the surface of classical T Tauri stars (C…
Bright hot impacts by erupted fragments falling back on the Sun: a template for stellar accretion.
2013
Impacts of falling fragments observed after the eruption of a filament in a solar flare on 7 June 2011 are similar to those inferred for accretion flows on young stellar objects. As imaged in the ultraviolet (UV)-extreme UV range by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory, many impacts of dark, dense matter display uncommonly intense, compact brightenings. High-resolution hydrodynamic simulations show that such bright spots, with plasma temperatures increasing from ~10(4) to ~10(6) kelvin, occur when high-density plasma (>>10(10) particles per cubic centimeter) hits the solar surface at several hundred kilometers per second, producing high-energy emission as …