Search results for "stars [X-rays]"
showing 10 items of 31 documents
Modeling accretion shocks on CTTSs and their X-ray emission
2009
Recent high spectral resolution X-ray observations of some CTTSs show the presence of high density plasma (ne=10^{11}-10^{13} cm^{-3}) at temperature T=2-3 MK. This plasma is likely heated up by an accretion shock on the star surface. We investigate this issue by an accurate modelling of the impact of an accretion stream onto the stellar chromosphere. Specifically, we present a large set of 1D hydrodynamical simulations aimed at investigating the physical properties of the system as a function of the density, and the velocity of the accretion stream and of the abundances of the heavy elements. We also synthesize the plasma X-ray emission from the simulations results, in order to link the ob…
High Resolution X-ray Spectroscopy of Pre-Main-Sequence Stars: TWA 5 and PZ Tel
2004
We report on the analysis of high resolution X-ray spectra of two pre-main-sequence stars: TWA 5 (observed with XMM-Newton) and PZ Telescopii (observed with Chandra/HETGS). TWA 5 is a classical T Tauri star in the TW Hydrae association while PZ Tel is a rapidly rotating weak-lined T Tauri star in the beta-Pictoris moving group. For both stars we have reconstructed the emission measure distribution and derived the coronal abundances to check for possible patterns of the abundances related to the first ionization potential of the various elements. We have also derived estimates of the plasma density from the analysis of the He-like triplets. We compare the characteristics of our targets with …
A possible solution of the puzzling variation of the orbital period of MXB 1659-298
2017
MXB 1659-298 is a transient neutron star Low-Mass X-ray binary system that shows eclipses with a periodicity of 7.1 hr. The source went to outburst in August 2015 after 14 years of quiescence. We investigate the orbital properties of this source with a baseline of 40 years obtained combining the eight eclipse arrival times present in literature with 51 eclipse arrival times collected during the last two outbursts. A quadratic ephemeris does not fit the delays associated with the eclipse arrival times and the addition of a sinusoidal term with a period of $2.31 \pm 0.02$ yr is required. We infer a binary orbital period of $P=7.1161099(3)$ hr and an orbital period derivative of $\dot{P}=-8.5(…
Modeling the X-ray emission from jets observed with Chandra
2009
The Second APOKASC Catalog: The Empirical Approach
2018
We present a catalog of stellar properties for a large sample of 6676 evolved stars with APOGEE spectroscopic parameters and \textit{Kepler} asteroseismic data analyzed using five independent techniques. Our data includes evolutionary state, surface gravity, mean density, mass, radius, age, and the spectroscopic and asteroseismic measurements used to derive them. We employ a new empirical approach for combining asteroseismic measurements from different methods, calibrating the inferred stellar parameters, and estimating uncertainties. With high statistical significance, we find that asteroseismic parameters inferred from the different pipelines have systematic offsets that are not removed b…
Mass accretion to young stars triggered by flaring activity in circumstellar discs
2011
Young low-mass stars are characterized by ejection of collimated outflows and by circumstellar discs which they interact with through accretion of mass. The accretion builds up the star to its final mass and is also believed to power the mass outflows, which may in turn remove the excess angular momentum from the star-disc system. However, although the process of mass accretion is a critical aspect of star formation, some of its mechanisms are still to be fully understood. A point not considered to date and relevant for the accretion process is the evidence of very energetic and frequent flaring events in these stars. Flares may easily perturb the stability of the discs, thus influencing th…
Meistara Ekharta mācība D. T. Sudzuki transcendentālfilozofijas interpretācijā
2021
Maģistra darbs “Meistara Ekharta mācība D. T. Sudzuki transcendentālfilozofijas interpretācijā” ir veltīts tam, lai pētītu 20.gadsimta japāņu domātāja Daisecu Teitaro Sudzuki (Daisetz Teitaro Suzuki) darbos pausto Meistara Ekharta – 13.gadsmta vācu filozofa, teologa un mistiķa – domas salīdzinājumu ar budisma mācību. D. T. Sudzuki ir domātājs, kurš izvirza pieņēmumu, ka Meistars Ekharts savos darbos pauž uzskatus, kas ir ļoti tuvi vai pat vienādi ar budisma filozofiju, kaut arī 13.gadsimtā Eiropā budisms nebija zināms.
Observatory science with eXTP
2019
Disponible preprint en: arXiv:1812.04023v1 [astro-ph.HE] [v1] Mon, 10 Dec 2018 19:00:52 UTC (4,376 KB)
Broad-band Spectral Evolution of Scorpius X-1 along its Color-Color Diagram
2007
We analyze a large collection of RXTE archive data from April 1997 to August 2003 of the bright X-ray source Scorpius X-1 in order to study the broadband spectral evolution of the source for different values of the inferred mass accretion rate by studying energy spectra from selected regions in the Z-track of its Color-Color Diagram. A two-component model, consisting of a soft thermal component interpreted as thermal emission from an accretion disk and a thermal Comptonization component, is unable to fit the whole 3--200 keV energy spectrum at low accretion rates. Strong residuals in the highest energy band of the spectrum require the addition of a third component that can be fitted with a …
23 GHz VLBI observations of SN 2008ax (Research Note)
2009
We report on phase-referenced 23 GHz Very-Long-Baseline-Interferometry (VLBI) observations of the type IIb supernova SN 2008ax, made with the Very Long Baseline Array (VLBA) on 2 April 2008 (33 days after explosion). These observations resulted in a marginal detection of the supernova. The total flux density recovered from our VLBI image is 0.8 ± 0.3 mJy (one standard deviation). As it appears, the structure may be interpreted as either a core-jet or a double source. However, the supernova structure could be somewhat confused with a possible close by noise peak. In such a case, the recovered flux density would decrease to 0.48 ± 0.12 mJy, compatible with the flux densities measured with the…