7,024 research outputs found
Neutral absorber dips in the periodic burster LMXB XB 1323-619 from Suzaku
We present results of an observation with Suzaku of the dipping, periodic
bursting low mass X-ray binary XB 1323-619. Using the energy band 0.8 - 70 keV,
we show that the source spectrum is well-described as the emission of an
extended accretion disk corona, plus a small contribution of blackbody emission
from the neutron star. The dip spectrum is well-fitted by the progressive
covering model in which the extended ADC is progressively overlapped by the
absorbing bulge of low ionization state in the outer accretion disk and that
dipping is basically due to photoelectric absorption in the bulge. An
energy-independent decrease of flux at high energies (20 - 70 keV) is shown to
be consistent with the level of Thomson scattering expected in the bulge. An
absorption feature at 6.67 keV (Fe XXV) is detected in the non-dip spectrum and
other possible weak features. In dipping, absorption lines of medium and highly
ionized states are seen suggestive of absorption in the ADC but there is no
evidence that the lines are stronger than in non-dip. We show that the
luminosity of the source has changed substantially since the Exosat observation
of 1985, increasing in luminosity between 1985 and 2003, then in 2003 - 2007
falling to the initial low value. X-ray bursting has again become periodic,
which it ceased to do in its highest luminosity state, and we find that the
X-ray bursts exhibit both the fast decay and later slow decay characteristic of
the rp burning process. We present arguments against the recent proposal that
the decrease of continuum flux in the dipping LMXB in general can be explained
as absorption in an ionized absorber rather than in the bulge in the outer disk
generally accepted to be the site of absorption.Comment: 12 pages, 6 figures, Astronomy and Astrophysics in pres
Identification of the soft X-ray excess in Cygnus X-1 with disc emission
We present results of a study of the soft X-ray excess in the black hole
candidate Cygnus X-1 made with the {\it Rosat PSPC}, using observations taken
during persistent emission at orbital phases close to 0.5. The soft excess can
be well fitted as a blackbody with temperature = keV. did not vary appreciably with intensity of the
source. By assuming that the distance of the source is its lower limit of 2.5
kpc, a luminosity of the soft excess of erg was obtained. From this, disc temperatures were calculated as a
function of radius, assuming the compact object to be a 10 \rm{M_{\sun}}
black hole, in particular, the temperature at 7 Schwarzschild radii expected to
be highly representative of the total disc emission. This was found to be 0.13
keV, in very good agreement with the spectral fitting result. This good
agreement strongly supports the identification of the soft excess with emission
from the disc around a black hole.Comment: PostScript, 3 figures, accepted for publication Astronomy and
Astrophysics Lette
Chandra HRC Localization of the Low Mass X-ray Binaries X1624-490 and X1702-429: The Infrared Counterparts
We report on the precise localization of the low mass X-ray binaries
X1624-490 and X1702-429 with the Chandra HRC-I. We determine the best positions
to be 16:28:02.825 -49:11:54.61 (J2000) and 17:06:15.314 -43:02:08.69 (J2000)
for X1624-490 and X1702-429, respectively, with the nominal Chandra positional
uncertainty of 0.6". We also obtained deep IR observations of the fields of
these sources in an effort to identify the IR counterparts. A single, faint
(Ks=18.3 +/- 0.1) source is visible inside the Chandra error circle of
X1624-490, and we propose this source as its IR counterpart. For X1702-429, a
Ks=16.5 +/- 0.07 source is visible at the edge of the Chandra error circle. The
brightness of both counterpart candidates is comparable to that of other low
mass X-ray binary IR counterparts when corrected for extinction and distance.Comment: 5 pages, 2 figures, accepted for publication in Ap
BeppoSAX observation of the eclipsing dipping X-ray binary X1658-298
Results of a 2000 August 12-13 BeppoSAX observation of the 7.1 hr eclipsing,
dipping, bursting, transient, low-mass X-ray binary (LMXRB) X1658-298 are
presented. The spectrum outside of eclipses, dips and bursts can be modeled by
the combination of a soft disk-blackbody and a harder Comptonized component
with a small amount (1.3 10E21 atom/cm2) of low-energy absorption. In contrast,
an RXTE observation 18 months earlier during the same outburst, measured an
absorption of 5.0 10E22 atom/cm2. Such a change is consistent with a thinning
of the accretion disk as the outburst progresses. Structured residuals from the
best-fit spectral model are present which are tentatively identified with
Ne-K/Fe-L and Fe-K shell emission. The spectral changes during dips are complex
and may be modeled by a strong (~3 10E23 atom/cm2) increase in absorption of
the Comptonized component only, together with reductions in normalizations of
both spectral components. This behavior is in contrast to the ``complex
continuum'' model for X-ray dip sources, where the softer blackbody component
rapidly suffers strong absorption. It is however, similar to that found during
recent XMM-Newton observations of the eclipsing, dipping, LMXRB EXO0748-676.Comment: 11 pages. Accepted for publication in A&A
The Cessation of Flickering during Dips in Cygnus X-1
We report the discovery of the cessation of flickering in dips in the black
hole candidate Cygnus X-1, detected for the first time in the ASCA observation
of May 9th., 1995. During this observation, particularly deep dipping took
place resulting in strong changes in hardness ratio corresponding to absorption
of the power law spectral component. The deadtime corrected light curve with
high time resolution clearly shows a dramatic decrease in the extent of
flickering in the band 0.7 - 4.0 keV during dipping, but in the band 4.0 - 10.0
keV, there is relatively little change. We show that the rms flickering
amplitude in the band 0.7 - 4.0 keV is proportional to the X-ray intensity in
this band which changes by a factor of almost three. This is direct evidence
that the strong Low State flickering is intrinsic to the power law emission; ie
takes place as part of the emission process. The rms amplitude is proportional
to the intensity in the low energy band, except for a possible deviation from
linearity at the lower intensities. If confirmed, this non-linearity could
imply a process such as electron scattering of radiation which will tend to
smear out the fluctuations, or a process of fluctuation generation which
depends on radial position in the source. Thus timing observations during
absorption dips can give information about the source region and may place
constraints on its size.Comment: 6 pages including 4 figures, accepted for publication in
Astrophysical Journal Letter
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