309 research outputs found
The Dunhuang chinese sky: a comprehensive study of the oldest known star atlas
This paper presents an analysis of the star atlas included in the medieval
Chinese manuscript (Or.8210/S.3326), discovered in 1907 by the archaeologist
Aurel Stein at the Silk Road town of Dunhuang and now held in the British
Library. Although partially studied by a few Chinese scholars, it has never
been fully displayed and discussed in the Western world. This set of sky maps
(12 hour angle maps in quasi-cylindrical projection and a circumpolar map in
azimuthal projection), displaying the full sky visible from the Northern
hemisphere, is up to now the oldest complete preserved star atlas from any
civilisation. It is also the first known pictorial representation of the
quasi-totality of the Chinese constellations. This paper describes the history
of the physical object - a roll of thin paper drawn with ink. We analyse the
stellar content of each map (1339 stars, 257 asterisms) and the texts
associated with the maps. We establish the precision with which the maps are
drawn (1.5 to 4 degrees for the brightest stars) and examine the type of
projections used. We conclude that precise mathematical methods were used to
produce the atlas. We also discuss the dating of the manuscript and its
possible author and confirm the dates 649-684 (early Tang dynasty) as most
probable based on available evidence. This is at variance with a prior estimate
around +940. Finally we present a brief comparison with later sky maps, both in
China and in Europe.Comment: 19 pages, 5 Tables, 8 Figure
The X-ray emission of magnetic cataclysmic variables in the XMM-Newton era
We review the X-ray spectral properties of magnetic cataclysmic binaries
derived from observations obtained during the last decade with the large X-ray
observatories XMM-Newton, Chandra and Suzaku. We focus on the signatures of the
different accretion modes which are predicted according to the values of the
main physical parameters (magnetic field, local accretion rate and white dwarf
mass). The observed large diversity of spectral behaviors indicates a wide
range of parameter values in both intermediate polars and polars, in line with
a possible evolutionary link between both classes.Comment: To appear in the Proceedings of "The Golden Age of Cataclysmic
Variables (Palermo 2011)", in Mem. Soc. Astron. It. (7 pages, 3 figures
The CNO problem in magnetic cataclysmic variables
Some magnetic CVs like BY Cam are characterized by unusual CNO line ratios
compared to other polars and non-solar abundances have been suggested to
explain this anomaly. We present here a first attempt to constrain the
elemental abundances in these systems by applying a specific ionisation model
combined with a geometrical description of the accretion column where these
lines are thought to be formed. The line luminosities have been computed using
the CLOUDY plasma code for different ionisation spectra and column extension.
We show here selected results and compare to the values observed in "peculiar"
magnetic CVs. The model applied to BY Cam confirms that ionization models with
solar abundances fail to reproduce the observed line intensity ratios. Assuming
the model to be valid, the induced best abundances imply an overabundance of N
(x25), underabundance of C (:8) and nearly solar O (:2), in line with CNO
reprocessing.Comment: 7 pages, 5 figures, To appear in `Magnetic Cataclysmic Variables',
IAU Col. 190, Capetown, Eds.: M. Cropper & S. Vrielman
The eclipsing bursting X-ray binary EXO 0748-676 revisited by XMM-Newton
The bright eclipsing and bursting low-mass X-ray binary EXO 0748-676 has been
observed at several occasions by XMM-Newton during the initial calibration and
performance verification (CAL/PV) phase. We present here the results obtained
from observations with the EPIC cameras. Apart from several type-I X-ray
bursts, the source shows a high degree of variability with the presence of soft
flares. The wide energy coverage and high sensitivity of XMM-Newton allows for
the first time a detailed description of the spectral variability.
The source is found to be the superposition of a central (~2 10^8 cm)
Comptonized emission, most probably a corona surrounding the inner edge of an
accretion disk, associated with a more extended (~3 10^10 cm) thermal halo at a
typical temperature of ~0.6 keV with an indication of non-solar abundances.
Most of the variations of the source can be accounted for by a variable
absorption affecting only the central comptonized component and reaching up to
NH ~1.3 10^23 cm^{-2}. The characteristics of the surrounding halo are found
compatible with an irradiated atmosphere of an accretion disc which intercepts
the central emission due to the system high inclination.Comment: 6 pages, 4 figures, accepted for publication in A&A Letters, XMM
special issu
Search for companions around Sirius
Since the discovery of Sirius-B about 130 yr ago, there have been several
claims of a possible second companion around the brightest star Sirius-A. Such
a companion could, in particular, be responsible of the suspected colour change
of the star, now strongly suggested from two independent historical sources. We
reported here on a new observation of the sky region around Sirius, to search
for such a companion, using a coronographic device.
By comparison of the new stellar field with a similar image obtained by us
13 yr ago and using the Sirius proper motion, we are able to eliminate
the most obvious companion candidates down to a magnitude m17 in a
field from 30 arcsec to 2.5 arcmin of the central star. None of the visible
stars appears consistent in magnitude and colours with what expected from
current theoretical models and observations of low-mass stars.
From the study of the same field, it is also shown that the Sirius companion,
consistently reported by observers during the years 1920-1930, is most probably
an unrelated m12 background star, now 1 arcmin away but
located precisely on the Sirius proper motion trajectory. The closest apparent
conjunction with Sirius was realized in 1937 with a minimum angular distance of
6.9 arcsec, of the same order than the Sirius A-B binary separation.
The reported observations do not eliminate the possibility of a second
companion but now confined the search to the more central 30 arcsec region
around Sirius. In particular, the existence of a long period companion cannot
definitively be ruled out since the arbitrary orientation of the orbit can
yield an observed projected position on sky inside this more central region.Comment: 6 pages, 4 figures, published in Astronomy and Astrophysic
The ephemeris, orbital decay, and masses of 10 eclipsing HMXBs
We take advantage of more than 10 years of monitoring of the eclipsing HMXB
systems LMC X-4, Cen X-3, 4U 1700-377, 4U 1538-522, SMC X-1, IGR J18027-2016,
Vela X-1, IGR J17252-3616, XTE J1855-026, and OAO 1657-415 with the ASM
on-board RXTE and ISGRI on-board INTEGRAL to update their ephemeris. These
results are used to refine previous measurements of the orbital period decay of
all sources (where available) and provide the first accurate values of the
apsidal advance in Vela X-1 and 4U 1538-522. Updated values for the masses of
the neutron stars hosted in the ten HMXBs are also provided, as well as the
long-term lightcurves folded on the sources best determined orbital parameters.
These lightcurves reveal complex eclipse ingresses and egresses, that are
understood mostly as being due to the presence of accretion wakes. The results
reported in this paper constitute a database to be used for population and
evolutionary studies of HMXBs, as well as theoretical modelling of long-term
accretion in wind-fed X-ray binaries.Comment: Accepted for publication on A&
Numerical Simulations of Naturally Tilted, Retrogradely Precessing, Nodal Superhumping Accretion Disks
Accretion disks around black hole, neutron star, and white dwarf systems are
thought to sometimes tilt, retrogradely precess, and produce hump-shaped
modulations in light curves that have a period shorter than the orbital period.
Although artificially rotating numerically simulated accretion disks out of the
orbital plane and around the line of nodes generates these short-period
superhumps and retrograde precession of the disk, no numerical code to date has
been shown to produce a disk tilt naturally. In this work, we report the first
naturally tilted disk in non-magnetic Cataclysmic Variables (CVs) using 3D
Smoothed Particle Hydrodynamics (SPH). Our simulations show that after many
hundreds of orbital periods, the disk has tilted on its own and this disk tilt
is without the aid of radiation sources or magnetic fields. As the system
orbits, the accretion stream strikes the bright spot (which is on the rim of
the tilted disk) and flows over and under the disk on different flow paths.
These different flow paths suggest the lift force as a source to disk tilt. Our
results confirm the disk shape, disk structure, and negative superhump period
and support the source to disk tilt, source to retrograde precession, and
location associated with X-ray and He II emission from the disk as suggested in
previous works. Our results identify the fundamental negative superhump
frequency as indicator of disk tilt around the line of nodes
The peculiar source XSS J12270-4859: a LMXB detected by FERMI ?
The X-ray source XSS J12270-4859 has been first suggested to be a magnetic
cataclysmic variable of Intermediate Polar type on the basis of its optical
spectrum and a possible 860 s X-ray periodicity. However further X-ray
observations by the Suzaku and XMM-Newton satellites did not confirm this
periodicity but show a very peculiar variability, including moderate repetitive
flares and numerous absorption dips. These characteristics together with a
suspected 4.3 h orbital period would suggest a possible link with the so-
called "dipping sources", a sub-class of Low-Mass X-ray Binaries (LMXB). Based
on the released FERMI catalogues, the source was also found coincident with a
very high energy (0.1-300 GeV) VHE source 2FGL J1227.7-4853. The good
positional coincidence, together with the lack of any other bright X-ray
sources in the field, makes this identification highly probable. However, none
of the other standard LMXBs have been so far detected by FERMI. Most galactic
VHE sources are associated with rotation-powered pulsars. We present here new
results obtained from a 30 ksec high-time resolution XMM observations in
January 2011 that confirm the flaring-dipping behaviour and provide upper
limits on fast X-ray pulsations. We discuss the possible association of the
source with either a microquasar or an accreting rotation powered pulsar.Comment: To appear in the Proceedings of "The Golden Age of Cataclysmic
Variables (Palermo 2011)", in Mem. Soc. Astron. It. (4 pages, 2 figures
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