281 research outputs found
Approximations and exact properties of density functionals from the strong-interaction limit of DFT
BeppoSAX observation of the X-ray binary pulsar Vela X-1
We report on the spectral (pulse averaged) and timing analysis of the ~ 20
ksec observation of the X-ray binary pulsar Vela X-1 performed during the
BeppoSAX Science Verification Phase. The source was observed in two different
intensity states: the low state is probably due to an erratic intensity dip and
shows a decrease of a factor ~ 2 in intensity, and a factor 10 in Nh. We have
not been able to fit the 2-100 keV continuum spectrum with the standard (for an
X--ray pulsar) power law modified by a high energy cutoff because of the
flattening of the spectrum in ~ 10-30 keV. The timing analysis confirms
previous results: the pulse profile changes from a five-peak structure for
energies less than 15 keV, to a simpler two-peak shape at higher energies. The
Fourier analysis shows a very complex harmonic component: up to 23 harmonics
are clearly visible in the power spectrum, with a dominant first harmonic for
low energy data, and a second one as the more prominent for energies greater
than 15 keV. The aperiodic component in the Vela X-1 power spectrum presents a
knee at about 1 Hz. The pulse period, corrected for binary motion, is 283.206
+/- 0.001 sec.Comment: 5 pages, 4 PostScript figure, uses aipproc.sty, to appear in
Proceedings of Fourth Compton Symposiu
CAOS spectroscopy of Am stars Kepler targets
The {\it Kepler} space mission and its {\it K2} extension provide photometric
time series data with unprecedented accuracy. These data challenge our current
understanding of the metallic-lined A stars (Am stars) for what concerns the
onset of pulsations in their atmospheres. It turns out that the predictions of
current diffusion models do not agree with observations. To understand this
discrepancy, it is of crucial importance to obtain ground-based spectroscopic
observations of Am stars in the {\it Kepler} and {\it K2} fields in order to
determine the best estimates of the stellar parameters.
In this paper, we present a detailed analysis of high-resolution
spectroscopic data for seven stars previously classified as Am stars. We
determine the effective temperatures, surface gravities, projected rotational
velocities, microturbulent velocities and chemical abundances of these stars
using spectral synthesis. These spectra were obtained with {\it CAOS}, a new
instrument recently installed at the observing station of the Catania
Astrophysical Observatory on Mt. Etna. Three stars have already been observed
during quarters Q0-Q17, namely: HD\,180347, HD\,181206, and HD\,185658, while
HD\,43509 was already observed during {\it K2} C0 campaign.
We confirm that HD\,43509 and HD\,180347 are Am stars, while HD 52403,
HD\,50766, HD\,58246, HD\,181206 and HD\,185658 are marginal Am stars. By means
of non-LTE analysis, we derived oxygen abundances from O{\sc
I}7771--5{\AA} triplet and we also discussed the results obtained with
both non-LTE and LTE approaches.Comment: accepted in MNRAS main journal 13 pages, 11 figures, 3 tables. arXiv
admin note: text overlap with arXiv:1404.095
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