673 research outputs found
Studies and observations of ultraviolet and X-ray sources
The status of research undertaken with the IUE satellite for programs on strong binary X-ray stars and X-ray globular clusters is summarized. Variable Seyfert galaxies and cataclysmic variables were investigated
Do Some X-ray Stars Have White Dwarf Companions?
Some Be stars which are intermittent C-ray sources may have white dwarf companions rather than neutron stars. It is not possible to prove or rule out the existence of Be+WD systems using X-ray or optical data. However, the presence of a white dwarf could be established by the detection of its EUV continuum shortward of the Be star's continuum turnover at 1OOOA. Either the detection or the nondetection of Be+WD systems would have implications for models of Be star variability, models of Be binary system formation and evolution, and models of wind-fed accretion
Do some x-ray stars have white dwarf companions
Some Be stars which are intermittent X-ray sources may have white dwarf companions rather than neutron stars. It is not possible to prove or rule out the existence of Be + WD systems using X-ray or optical data. However, the presence of a white dwarf could be established by the detection of its EUV continuum shortward of the Be star's continuum turnover at 100 A. Either the detection or the nondetection of Be + WD systems would have implications for models of Be star variability, models of Be binary system formation and evolution, and models of wind-fed accretion
Evolution of X-ray astronomy
The evolution of X-ray astronomy up to the launching of the Einstein observatory is presented. The evaluation proceeded through the following major steps: (1) discovery of an extrasolar X-ray source, Sco X-1, orders of magnitude stronger than astronomers believed might exist; (2) identification of a strong X-ray source with the Crab Nebula; (3) identification of Sco X-1 with a faint, peculiar optical object; (4) demonstration that X-ray stars are binary systems, each consisting of a collapsed object accreting matter from an ordinary star; (5) discovery of X-ray bursts; (6) discovery of exceedingly strong X-ray emission from active galaxies, quasars and clusters of galaxies; (7) demonstration that the principal X-ray source is a hot gas filling the space between galaxies
COUP census of X-ray stars in BN-KL and OMC-1S
We present a study of the X-ray sources detected in the vicinity of two
density peaks in the Orion Molecular Cloud 1 (OMC-1) behind the Orion Nebula
Cluster (ONC), as seen in the exceptionally deep (~10 days) exposure of the
Chandra Orion Ultradeep Project (COUP). We focus on a 40"*50" region around the
Becklin-Neugebauer object and Kleinmann-Low nebula (collectively BN-KL) and a
60"*75" region around OMC-1S, a secondary star-forming peak some 90" south of
BN-KL. Forty-three and sixty X-ray sources were detected in BN-KL and OMC-1S,
respectively, of which half and one-third, respectively, were found to be
foreground members of the ONC, while the remaining sources are identified as
obscured X-ray sources with column densities 22<log(NH)/cm^{-2})<24. All but 5
and 18 of these sources have near-infrared stellar counterparts, and 22 of
these appear to be young stellar objects (YSOs). X-ray sources are found close
to four luminous mid-IR sources namely BN, IRc3-i2, IRc2-C, and Source n; their
X-ray variability and spectral properties are typical of coronal activity in
low-mass stars rather than wind emission from massive stars, suggesting that
the X-ray emission may be arising from companions. The X-ray light curve of the
X-ray source close to BN shows a periodicity of ~8.3 days and from an X-ray
image deconvolution of the region around BN, we conclude that either BN itself
or a low mass companion with a projected separation of ~200 AU was detected. On
the other hand, no emission is seen from the bright radio Source I, held by
some to be the main source of luminosity in BN-KL. In OMC-1S, Chandra unveils a
new subcluster of seven YSOs without infrared counterparts (abridged).Comment: 57 pages and 18 Figures. Accepted for publication by ApJS (COUP
special issue). Preprint with higher resolution figures is available at
http://hal.ccsd.cnrs.fr/ccsd-0000464
An exceptionally bright flare from SGR1806-20 and the origins of short-duration gamma-ray bursts
Soft-gamma-ray repeaters (SGRs) are galactic X-ray stars that emit numerous
short-duration (about 0.1 s) bursts of hard X-rays during sporadic active
periods. They are thought to be magnetars: strongly magnetized neutron stars
with emissions powered by the dissipation of magnetic energy. Here we report
the detection of a long (380 s) giant flare from SGR 1806-20, which was much
more luminous than any previous transient event observed in our Galaxy. (In the
first 0.2 s, the flare released as much energy as the Sun radiates in a quarter
of a million years.) Its power can be explained by a catastrophic instability
involving global crust failure and magnetic reconnection on a magnetar, with
possible large-scale untwisting of magnetic field lines outside the star. From
a great distance this event would appear to be a short-duration, hard-spectrum
cosmic gamma-ray burst. At least a significant fraction of the mysterious
short-duration gamma-ray bursts therefore may come from extragalactic
magnetars.Comment: 21 pages, 5 figures. Published in Natur
HRC-I/Chandra X-ray observations towards sigma Orionis
Aims: We investigated the X-ray emission from young stars and brown dwarfs in
the sigma Orionis cluster (tau~3 Ma, d~385 pc) and its relation to mass,
presence of circumstellar discs, and separation to the cluster centre by taking
advantage of the superb spatial resolution of the Chandra X-ray Observatory.
Methods: We used public HRC-I/Chandra data from a 97.6 ks pointing towards the
cluster centre and complemented them with X-ray data from IPC/Einstein,
HRI/ROSAT, EPIC/XMM-Newton, and ACIS-S/Chandra together with optical and
infrared photometry and spectroscopy from the literature and public catalogues.
On our HRC-I/Chandra data, we measured count rates, estimated X-ray fluxes, and
searched for short-term variability. We also looked for long-term variability
by comparing with previous X-ray observations. Results: Among the 107 detected
X-ray sources, there were 70 cluster stars with known signposts of youth, two
young brown dwarfs, 12 cluster member candidates, four field dwarfs, and two
galaxies with optical-infrared counterpart. The remaining sources had
extragalactic nature. Based on a robust Poisson-chi^2 analysis, nine cluster
stars displayed flares or rotational modulation during the HRC-I observations,
while other eight stars and one brown dwarf showed long-term X-ray flux
variations. We constructed a cluster X-ray luminosity function from O9.5 (~18
Msol) to M6.5 (~0.06 Msol). We found: a tendency of early-type stars in
multiple systems or with spectroscopic peculiarities to display X-ray emission,
that the two detected brown dwarfs and the least-massive star are among the
sigma Orionis objects with the highest L_X/L_J ratios, and that a large
fraction of known classical T Tauri stars in the cluster are absent in this and
other X-ray surveys. We concluded that dozens X-ray sigma Orionis stars and
brown dwarfs are still to be detected [abridged].Comment: A&A, in pres
The Elephant Trunk Nebula and the Trumpler 37 cluster: Contribution of triggered star formation to the total population of an HII region
Rich young stellar clusters produce HII regions whose expansion into the
nearby molecular cloud is thought to trigger the formation of new stars.
However, the importance of this mode of star formation is uncertain. This
investigation seeks to quantify triggered star formation (TSF) in IC 1396A
(a.k.a., the Elephant Trunk Nebula), a bright rimmed cloud (BRC) on the
periphery of the nearby giant HII region IC 1396 produced by the Trumpler 37
cluster. X-ray selection of young stars from Chandra X-ray Observatory data is
combined with existing optical and infrared surveys to give a more complete
census of the TSF population. Over 250 young stars in and around IC 1396A are
identified; this doubles the previously known population. A spatio-temporal
gradient of stars from the IC 1396A cloud toward the primary ionizing star HD
206267 is found. We argue that the TSF mechanism in IC 1396A is the
radiation-driven implosion process persisting over several million years.
Analysis of the X-ray luminosity and initial mass functions indicates that >140
stars down to 0.1 Msun were formed by TSF. Considering other BRCs in the IC
1396 HII region, we estimate the TSF contribution for the entire HII region
exceeds 14-25% today, and may be higher over the lifetime of the HII region.
Such triggering on the periphery of HII regions may be a significant mode of
star formation in the Galaxy.Comment: Accepted for publication in MNRAS; 28 pages, 18 figure
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