502 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
AX J1749.1-2733 and AX J1749.2-2725 - the close pair of X-ray pulsars behind the Galactic Center: an optical identification
Two faint X-ray pulsars, AX J1749.2-2725 and AX J1749.1-2733, located in the
direction to the Galactic Center, were studied in detail using data of
INTEGRAL, XMM-Newton and Chandra observatories in X-rays, the SOFI/NTT
instrument in infrared and the RTT150 telescope in optics. X-ray positions of
both sources were determined with the uncertainty better than ~1 arcsec, that
allowed us to identify their infrared counterparts. From the subsequent
analysis of infrared and optical data we conclude that counterparts of both
pulsars are likely massive stars of B0-B3 classes located behind the Galactic
Center at distances of 12-20 kpc, depending on the type, probably in further
parts of galactic spiral arms. In addition, we investigated the extinction law
towards the galactic bulge and found that it is significantly different from
standard one.Comment: 6 pages, 7 figures, will be published in MNRA
ASCA X-ray source catalogue in the Galactic Center region
The ASCA satellite made 107 pointing observations on a 5 x 5 deg^2 region
around the center of our Milky Way Galaxy (the Galactic Center) from 1993 to
1999. In the X-ray images of the 0.7--3 keV or 3--10 keV bands, we found 52
point sources and a dozen diffuse sources. All the point sources are uniformly
fitted with an absorbed power-law model. For selected bright sources, Sgr A*,
AX J1745.6-2901, A 1742-294, SLX 1744-300, GRO J1744-28, SLX 1737-282, GRS
1734-292, AX J1749.2-2725, KS 1741-293, GRS 1741.9-2853, and an unusual flare
source XTE J1739-302, we present further detailed spectral and timing analyses,
and discuss their nature.
The dozen extended X-ray sources comprise radio supernova remnants, giant
molecular clouds, and some new discoveries. Most show emission lines from
either highly ionized atoms or low-ionized irons.
The X-ray spectra were fitted with either a thin thermal or power-law model.
This paper summarizes the results and provides the ASCA X-ray source catalogue
in the Galactic Center region.Comment: 33 pages, 8 figures, Accepted for publication in ApJS, also found in
http://www.star.le.ac.uk/~mas/research/paper/#Sakano2001apj
Bursting behavior of the Galactic Center faint X-ray transient GRS 1741.9-2853
The neutron star low-mass X-ray binary GRS 1741.9-2853 is a known type-I
burster of the Galactic Center. It is transient, faint, and located in a very
crowded region, only 10 arcmin from the supermassive black hole Sgr A*.
Therefore, its bursting behavior has been poorly studied so far. In particular,
its persistent emission has rarely been detected between consecutive bursts,
due to lack of sensitivity or confusion. This is what made GRS 1741.9-2853 one
of the nine "burst-only sources" identified by BeppoSAX a few years ago. The
physical properties of GRS 1741.9-2853 bursts are yet of great interest since
we know very little about the nuclear regimes at stake in low accretion rate
bursters. We examine here for the first time several bursts in relation with
the persistent emission of the source, using INTEGRAL, XMM-Newton, and Swift
observations. We investigate the source flux variability and bursting behavior
during its 2005 and 2007 long outbursts. The persistent luminosity of GRS
1741.9-2853 varied between ~1.7 and 10.5 10^36 erg s^-1, i.e. 0.9-5.3% of the
Eddington luminosity. The shape of the spectrum as described by an absorbed
power-law remained with a photon index Gamma ~ 2 and a column density $N_{\rm
H} ~ 12 10^22 cm^-2 throughout the outbursts. We discovered 11 type-I bursts
with INTEGRAL, and inspected 4 additional bursts: 2 recorded by XMM-Newton and
2 by Swift. From the brigthest burst, we derive an upper limit on the source
distance of ~7 kpc. The observed bursts characteristics and source accretion
rate suggest pure helium explosions igniting at column depths y_{ign} ~ 0.8-4.8
10^8 g cm^-1, for typical energy releases of ~1.2-7.4 10^39 erg.Comment: 11 pages, 7 figures, accepted for publication in A&
A New 626 s Periodic X-ray Source in the Direction of the Galactic Center
Here we report the detection of a 626 s periodic modulation from the X-ray
source 2XMM J174016.0-290337 located in the direction of the Galactic center.
We present temporal and spectral analyses of archival XMM-Newton data and
photometry of archived near-infrared data in order to investigate the nature of
this source. We find that the X-ray light curve shows a strong modulation at
626 +/- 2 s with a confidence level > 99.9% and a pulsed fraction of 54%.
Spectral fitting demonstrates that the spectrum is consistent with an absorbed
power law. No significant spectral variability was observed over the 626 s
period. We have investigated the possibility that the 626 s period is orbital
in nature (either that of an ultra-compact X-ray binary or an AM CVn) or
related to the spin of a compact object (either an accretion powered pulsar or
an intermediate polar). The X-ray properties of the source and the photometry
of the candidate near-infrared counterparts are consistent with an accreting
neutron star X-ray binary on the near-side of the Galactic bulge, where the 626
s period is most likely indicative of the pulsar spin period. However, we
cannot rule out an ultra-compact X-ray binary or an intermediate polar with the
data at hand. In the former case, if the 626 s modulation is the orbital period
of an X-ray binary, it would be the shortest period system known. In the latter
case, the modulation would be the spin period of a magnetic white dwarf.
However, we find no evidence for absorption dips over the 626 s period, a low
temperature black body spectral component, or Fe Kalpha emission lines. These
features are commonly observed in intermediate polars, making 2XMM
J174016.0-290337 a rather unusual member of this class if confirmed. We instead
suggest that 2XMM J174016.0-290337 could be a new addition to the emerging
class of symbiotic X-ray binaries.Comment: 11 pages, 10 figures, submitted to A&A on 18th January 2010, accepted
for publication 20th August 201
OH (1720 MHz) Masers and Mixed-Morphology Supernova Remnants
Radio surveys of supernova remnants (SNRs) in the Galaxy have uncovered 19
SNRs accompanied by OH maser emission at 1720 MHz. This unusual class of maser
sources is suggested to be produced behind a shock front from the expansion of
a supernova remnant running into a molecular cloud. An important ingredient of
this model is that X-ray emission from the remnant enhances the production of
OH molecule. The role of X-ray emission from maser emitting (ME) SNRs is
investigated by comparing the X-ray induced ionization rate with theory. One
aspect of this model is verified: there is a strong association between maser
emitting and mixed-morphology (MM) or thermal composite SNRs --center-filled
thermal X-ray emission surrounded by shell-like radio morphology. We also
present ROSAT and ASCA observations of two maser emitting SNRs: G21.8--0.6 (Kes
69) and G357.7--0.1 (Tornado).Comment: 13 pages, 2 tables, 1 figure, ApJ (in press
New fast X-ray transient IGR J18462-0223 discovered by the INTEGRAL observatory
Details of the discovery of a new X-ray source, IGR J18462-0223, on October
12, 2007, during a short (several hours), intense (~ 35 mCrab at the peak)
outburst of hard radiation by the IBIS/ISGRI gamma-ray telescope onboard the
INTEGRAL observatory are given. The detection of another earlier outburst from
this source occurred on April 28, 2006, in the archival data of the telescope
is reported. We present the results of the source's localization and our
spectral/timing analysis of the observational data. The source may turn out to
be yet another representative of the continuously growing population of fast
X-ray transients, which are the focus of attention because of the
identification of their optical counterparts with early-type supergiants
Optical Identification of the ASCA Large Sky Survey
We present results of optical identification of the X-ray sources detected in
the ASCA Large Sky Survey. Optical spectroscopic observations were done for 34
X-ray sources which were detected with the SIS in the 2-7 keV band above 3.5
sigma. The sources are identified with 30 AGNs, 2 clusters of galaxies, and 1
galactic star. Only 1 source is still unidentified. The flux limit of the
sample corresponds to 1 x 10^{-13} erg s^{-1} cm^{-2} in the 2-10 keV band.
Based on the sample, the paper discusses optical and X-ray spectral
properties of the AGNs, contribution of the sources to the Cosmic X-ray
Background, and redshift and luminosity distributions of the AGNs. An
interesting result is that the redshift distribution of the AGNs suggests a
deficiency of high-redshift (0.5 10^{44}
erg s^{-1}) absorbed narrow-line AGNs (so called type 2 QSOs).Comment: Accepted for publication in ApJ. 57 pages with 13 figures, 9 JPG
plates, 5 additional PS tables. Original EPS plates (gzipped format
~1Mbyte/plate) and TeX tables are available from
ftp://ftp.kusastro.kyoto-u.ac.jp/pub/akiyama/0001289
Further studies of 1E 1740.7-2942 with ASCA
We report the ASCA results of the Great Annihilator 1E 1740.7-2942 obtained
with five pointing observations in a time span of 3.5 years. The X-ray spectrum
for each period is well fitted with a single power-law absorbed by a high
column of gas. The X-ray flux changes by a factor of 2 from period to period,
but the other spectral parameters show no significant change. The photon index
is flat with \Gamma = 0.9--1.3. The column densities of hydrogen N_H is
1.0 x 10^{23} H cm^{-2} and that of iron N_{Fe} is 10^{19} Fe cm^{-2}.
These large column densities indicate that 1E 1740.7-2942 is near at the
Galactic Center. The column density ratio leads the iron abundance to be 2
times larger than the other elements in a unit of the solar ratio. The
equivalent width of the K\alpha-line from a neutral iron is less than 15 eV in
90% confidence. This indicates that the iron column density within several
parsecs from 1E 1740.7-2942 is less than 5 x 10^{17} Fe cm^{-2}. In addition,
the derived hydrogen column density is about 1/6 of that of giant molecular
clouds in the line of sight. All these facts support that 1E 1740.7-2942 is not
in a molecular cloud, but possibly in front of it; the X-rays are not powered
by accretion from a molecular cloud, but from a companion star like ordinary
X-ray binaries.Comment: To appear in ApJ July 20, 1999 issue, Vol. 520 #1, 23 pages LaTeX
files, uses aasms4.sty and psfig.sty, also available at
http://www-cr.scphys.kyoto-u.ac.jp/member/sakano/work/paper/index-e.htm
Analysis of Shot Noise at Finite Temperatures in Fractional Quantum Hall Edge States
We investigate shot noise at {\it finite temperatures} induced by the
quasi-particle tunneling between fractional quantum Hall (FQH) edge states. The
resulting Fano factor has the peak structure at a certain bias voltage. Such a
structure indicates that quasi-particles are weakly {\it glued} due to thermal
fluctuation. We show that the effect makes it possible to probe the difference
of statistics between FQH states where quasi-particles have the
same unit charge.Finally we propose a way to indirectly obtain statistical
angle in hierarchical FQH states.Comment: 5 pages, 3 figure
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