695 research outputs found
Discovery of X-ray eclipses from the transient source CXOGC J174540.0-290031 with XMM-Newton
We present the XMM-Newton observations obtained during four revolutions in
Spring and Summer 2004 of CXOGC J174540.0-290031, a moderately bright transient
X-ray source, located at only 2.9" from SgrA*. We report the discovery of sharp
and deep X-ray eclipses, with a period of 27,961+/-5 s and a duration of about
1,100+/-100 s, observed during the two consecutive XMM revolutions from August
31 to September 2. No deep eclipses were present during the two consecutive XMM
revolutions from March 28 to April 1, 2004. The spectra during all four
observations are well described with an absorbed power law continuum. While our
fits on the power law index over the four observations yield values that are
consistent with Gamma=1.6-2.0, there appears to be a significant increase in
the column density during the Summer 2004 observations, i.e. the period during
which the eclipses are detected. The intrinsic luminosity in the 2-10 keV
energy range is almost constant with 1.8-2.3 x 10^34 (d_8kpc)^2 erg/s over the
four observations. In the framework of eclipsing semidetached binary systems,
we show that the eclipse period constrains the mass of the assumed
main-sequence secondary star to less than 1.0 M_odot. Therefore, we deduce that
CXOGC J174540.0-290031 is a low-mass X-ray binary (LMXB). Moreover the eclipse
duration constrains the mass of the compact object to less than about 60
M_odot, which is consistent with a stellar mass black hole or a neutron star.
The absence of deep X-ray eclipses during the Spring 2004 observations could be
explained if the centroid of the X-ray emitting region moves from a position on
the orbital plane to a point above the compact object, possibly coincident with
the base of the jet which was detected in radio at this epoch. [Abstract
truncated].Comment: A&A, accepted for publication (10 pages, 8 figures, 2 Tables
XMM-Newton observations of Sagittarius A East
We present an analysis of a recent XMM-Newton observation of Sgr A East, a
supernova remnant located close to the Galactic Centre. Very high quality X-ray
spectra reveal many emission lines from highly ionized atoms consistent with a
multi-temperature thin thermal plasma in ionization equilibrium. We use a
two-temperature model to fit the spectra and derive temperatures of 1 keV and 4
keV. There is significant concentration of iron towards the centre of the X-ray
source such that the iron abundance varies from ~4 times solar in the core down
to ~0.5 solar in the outer regions, which contrasts with the rather uniform
distribution of other metals such as sulfur, argon and calcium, which have
abundances in the range 1--3. The derived total energy, mass, and the abundance
pattern are consistent with a single supernova event, either of type-Ia or
type-II origin, involving a relatively low-mass progenitor star. A weak 6.4-keV
neutral iron fluorescence line is also detected, the illumination source most
likely being Sgr A East itself. The morphology and spectral characteristics of
Sgr A East show no clear linkage to putative past activity in Sgr A*.Comment: 12 pages, 8 figures, to appear in MNRAS, figures with full resolution
available at http://www.star.le.ac.uk/~mas/research/paper/#Sakano2003mnra
The discovery of a new non-thermal X-ray filament near the Galactic Centre
We report the discovery by XMM-Newton and Chandra of a hard extended X-ray
source (XMM J174540-2904.5) associated with a compact non-thermal radio
filament (the Sgr A-E `wisp'=1LC 359.888-0.086= G359.88-0.07), which is located
within ~4 arcmin of the Galactic Centre. The source position is also coincident
with the peak of the molecular cloud, M -0.13-0.08 (the `20 km/s' cloud). The
X-ray spectrum is non-thermal with an energy index of 1.0 (+1.1 -0.9) and
column density of 38 (+7 -11) x 10^22 H/cm2. The observed 2--10 keV flux of 4 x
10^-13 erg/s/cm2 converts to an unabsorbed X-ray luminosity of 1 x 10^34 erg/s
assuming a distance of 8.0 kpc. The high column density strongly suggests that
this source is located in or behind the Galactic Centre Region. Taking account
of the broad-band spectrum, as well as the source morphology and the positional
coincidence with a molecular cloud, we concluded that both the radio and X-ray
emission are the result of synchrotron radiation. This is the first time a
filamentary structure in the Galactic Centre Region. has been shown,
unequivocally, to have a non-thermal X-ray spectrum.Comment: 5 pages, 4 figures, Accepted for publication in MNRAS, also found in
http://www.star.le.ac.uk/~mas/research/paper/#Sakano2002mnra
Registration of the First Thermonuclear X-ray Burst from AX J1754.2-2754
During the analysis of the INTEGRAL observatory archival data we found a
powerful X-ray burst, registered by JEM-X and IBIS/ISGRI telescopes on April
16, 2005 from a weak and poorly known source AX J1754.2-2754. Analysis of the
burst profiles and spectrum shows, that it was a type I burst, which result
from thermonuclear explosion on the surface of nutron star. It means that we
can consider AX J1754.2-2754 as an X-ray burster. Certain features of burst
profile at its initial stage witness of a radiation presure driven strong
expansion and a corresponding cooling of the nutron star photosphere. Assuming,
that the luminosity of the source at this phase was close to the Eddington
limit, we estimated the distance to the burst source d=6.6+/-0.3 kpc (for
hidrogen atmosphere of the neutron star) and d=9.2+/-0.4 kpc (for helium
atmosphere).Comment: 12 pages, 6 figure
Unusual X-ray transients in the Galactic Centre
We report the discovery in the Galactic Centre region of two hard X-ray
sources, designated as XMM J174457-2850.3 and XMM J174544-2913.0, which
exhibited flux variations in the 2--10 keV band in excess of a factor of 100 in
observations spanning roughly a year. In both cases the observed hydrogen
column density is consistent with a location near to the Galactic Centre,
implying peak X-ray luminosities of ~5 x 10^34 erg/s. These objects may
represent a new population of transient source with very different properties
to the much more luminous Galactic Centre transients associated with neutron
star and black-hole binary systems. Spectral analysis shows that XMM
J174457-2850.3 has relatively weak iron-line emission set against a very hard
continuum. XMM J174544-2913.0, on the other hand, has an extremely strong
K-line from helium-like iron with an equivalent width of ~2.4keV. The nature of
the latter source is of particular interest. Does it represent an entirely new
class of object or does it correspond to a known class of source in a very
extreme configuration?Comment: 9 pages, 5 figures, to appear in MNRAS, figures with full resolution
are available at
http://www.star.le.ac.uk/~mas/research/paper/#Sakano2004mnra
Eclipse Timings of the Transient Low Mass X-ray Binary EXO0748-676. IV. The Rossi X-Ray Timing Explorer Eclipses
We report our complete database of X-ray eclipse timings of the low mass
X-ray binary EXO0748-676 observed by the Rossi X-Ray Timing Explorer (RXTE)
satellite. As of this writing we have accumulated 443 full X-ray eclipses, 392
of which have been observed with the Proportional Counter Array on RXTE. These
include both observations where an eclipse was specifically targeted and those
eclipses found in the RXTE data archive. Eclipse cycle count has been
maintained since the discovery of the EXO0748-676 system in February 1985. We
describe our observing and analysis techniques for each eclipse and describe
improvements we have made since the last compilation by Wolff et al. (2002).
The principal result of this paper is the database containing the timing
results from a seven-parameter fit to the X-ray light curve for each observed
eclipse along with the associated errors in the fitted parameters. Based on the
standard O-C analysis, EXO0748-676 has undergone four distinct orbital period
epochs since its discovery. In addition, EXO0748-676 shows small-scale events
in the O-C curve that are likely due to short-lived changes in the secondary
star.Comment: Accepted for publication in The Astrophysical Journal Supplement
Series, 5 figures. Analysis revised. Tables 1 & 3 update
Three-dimensional bulk band dispersion in polar BiTeI with giant Rashba-type spin splitting
In layered polar semiconductor BiTeI, giant Rashba-type spin-split band
dispersions show up due to the crystal structure asymmetry and the strong
spin-orbit interaction. Here we investigate the 3-dimensional (3D) bulk band
structures of BiTeI using the bulk-sensitive -dependent soft x-ray angle
resolved photoemission spectroscopy (SX-ARPES). The obtained band structure is
shown to be well reproducible by the first-principles calculations, with huge
spin splittings of meV at the conduction-band-minimum and
valence-band-maximum located in the plane. It provides the first
direct experimental evidence of the 3D Rashba-type spin splitting in a bulk
compound.Comment: 9 pages, 4 figure
Strongly spin-orbit coupled two-dimensional electron gas emerging near the surface of polar semiconductors
We investigate the two-dimensional (2D) highly spin-polarized electron
accumulation layers commonly appearing near the surface of n-type polar
semiconductors BiTeX (X = I, Br, and Cl) by angular-resolved photoemission
spectroscopy. Due to the polarity and the strong spin-orbit interaction built
in the bulk atomic configurations, the quantized conduction-band subbands show
giant Rashba-type spin-splitting. The characteristic 2D confinement effect is
clearly observed also in the valence-bands down to the binding energy of 4 eV.
The X-dependent Rashba spin-orbit coupling is directly estimated from the
observed spin-split subbands, which roughly scales with the inverse of the
band-gap size in BiTeX.Comment: 15 pages 4 figure
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