Search results for "STARS"
showing 10 items of 798 documents
Order in the Chaos: Spin-up and Spin-down during the 2002 Outburst of SAX J1808.4-3658
2006
We present a timing analysis of the 2002 outburst of the accreting millisecond pulsar SAX J1808.4-3658. A study of the phase delays of the entire pulse profile shows a behavior that is surprising and difficult to interpret: superposed to a general trend, a big jump by about 0.2 in phase is visible, starting at day 14 after the beginning of the outburst. An analysis of the pulse profile indicates the presence of a significant first harmonic. Studying the fundamental and the first harmonic separately, we find that the phase delays of the first harmonic are more regular, with no sign of the jump observed in the fundamental. The fitting of the phase delays of the first harmonic with a model whi…
Diagnostics of stellar flares from X-ray observations: from the decay to the rise phase
2007
The diagnostics of stellar flaring coronal loops have been so far largely based on the analysis of the decay phase. We derive new diagnostics from the analysis of the rise and peak phase of stellar flares. We release the assumption of full equilibrium of the flaring loop at the flare peak, according to the frequently observed delay between the temperature and the density maximum. From scaling laws and hydrodynamic simulations we derive diagnostic formulas as a function of observable quantities and times. We obtain a diagnostic toolset related to the rise phase, including the loop length, density and aspect ratio. We discuss the limitations of this approach and find that the assumption of lo…
On the origin of the X-ray emission from Herbig Ae/Be stars
2006
We performed a systematic search for Chandra archival observations of Herbig Ae/Be stars. These stars are fully radiative and not expected to support dynamo action analogous to their convective lower-mass counterparts, the T Tauri stars. Their X-ray emission has remained unexplained. The superior spatial resolution of Chandra with respect to previous X-ray instrumentation has allowed us to examine the possible role of late-type companions in generating the observed X-rays. In the total sample of 17 Herbig Ae/Be stars, 8 are resolved from X-ray emitting faint companions or other unrelated X-ray bright objects within 10". The detection fraction of Herbig Ae/Be stars is 76 %, but lowers to 35 …
XMM-Newton detects a relativistically broadened iron line in the spectrum of the ms X-ray pulsar SAX J1808.4-3658
2008
We report on a 63-ks long XMM-Newton observation of the accreting millisecond pulsar SAX J1808.4-3658 during the latest X-ray outburst which started on September 21st 2008. The pn spectrum shows a highly significant emission line in the energy band where the iron K-alpha line is expected, and which we identify as emission from neutral (or mildly ionized) iron. The line profile appears to be quite broad (more than 1 keV FWHM) and asymmetric; the most probable explanation for this profile is Doppler and relativistic broadening from the inner accretion disc. From a fit with a diskline profile we find an inner radius of the disc of 8.7^(+3.7)_(-2.7) R_g, corresponding to 18.0^(+7.6)_(-5.6) km f…
Caii H and K spectroscopic observations in active binary systems
1990
We have performed high resolution spectroscopic observations of five active binary systems in the Caii H and K lines at La Palma Observatory. We present the absolute fluxes calculated at the surface of the stars and we discuss the spectral characteristics observed, presence of double emissions and phase variations.
Dark Matter Fraction in Lens Galaxies: New Estimates from Microlensing
2014
We present a joint estimate of the stellar/dark matter mass fraction in lens galaxies and the average size of the accretion disk of lensed quasars from microlensing measurements of 27 quasar image pairs seen through 19 lens galaxies. The Bayesian estimate for the fraction of the surface mass density in the form of stars is $\alpha=0.21\pm0.14$ near the Einstein radius of the lenses ($\sim 1 - 2$ effective radii). The estimate for the average accretion disk size is $R_{1/2}=7.9^{+3.8}_{-2.6}\sqrt{M/0.3M_\sun}$ light days. The fraction of mass in stars at these radii is significantly larger than previous estimates from microlensing studies assuming quasars were point-like. The corresponding l…
X-ray optical depth diagnostics of T Tauri accretion shocks
2009
In classical T Tauri stars, X-rays are produced by two plasma components: a hot low-density plasma, with frequent flaring activity, and a high-density lower temperature plasma. The former is coronal plasma related to the stellar magnetic activity. The latter component, never observed in non-accreting stars, could be plasma heated by the shock formed by the accretion process. However its nature is still being debated. Our aim is to probe the soft X-ray emission from the high-density plasma component in classical T Tauri stars to check whether this is plasma heated in the accretion shock or whether it is coronal plasma. High-resolution X-ray spectroscopy allows us to measure individual line f…
Chandra study of the eclipsing M dwarf binary, YY Gem
2012
The eclipsing M dwarf binary system, YY Gem, was observed using Chandra covering 140 ks (2Prot) in total, split into two even exposures separated by 0.76 d (0.94 Prot). The system was extremely active: three energetic flares were observed over the course of these observations. The flaring and non-flaring states of the system are analysed in this paper. The activity level increased between the first and second observations even during the quiescent (non-flaring) phases. An analysis of the dynamics of the X-ray-emitting plasma suggests that both components are significantly active. Contemporaneous Hα spectra also suggest that both components show similar levels of activity. The primary star i…
SOFT X-RAY IRRADIATION OF PURE CARBON MONOXIDE INTERSTELLAR ICE ANALOGUES
2012
There is an increasing evidence for the existence of large organic molecules in the interstellar and circumstellar medium. Very few among such species are readily formed in conventional gas-phase chemistry under typical conditions of interstellar clouds. Attention has therefore focused on interstellar ices as a potential source of these relatively complex species. Laboratory experiments show that irradiation of interstellar ice analogues by fast particles or ultraviolet radiation can induce significant chemical complexity. However, stars are sources of intense X-rays at almost every stage of their formation and evolution. Such radiation may thus provide chemical changes in regions where ult…
Be stars in open clusters. I. uvby $ ^{\bf \beta}$ photometry
1996
We present uvby β photometry for Be stars in eight open clusters and two OB associations. It is shown that Be stars occupy anomalous positions in the photometric diagrams, which can be explained in terms of the circumstellar continuum radiation contribution to the photometric indices. In the - M V plane Be stars appear redder than the non emission B stars, due to the additional reddening caused by the hydrogen free-bound and free-free recombination in the circumstellar envelope. In the c 0 - M V plane the earlier Be stars present lower c 0 values than absorption-line B stars, which is caused by emission in the Balmer discontinuity, while the later Be stars deviate towards higher c 0 values,…