67 research outputs found
The emission regions in X-ray binaries: dipping as a diagnostic
X-ray dipping in the black hole binary Cygnus X-1, the Galactic jet source
GRO J1655-40 and in low mass X-ray binaries is discussed. It is shown that
spectral analysis strongly constrains emission models. Measurement of dip
ingress/egress times allows the sizes of extended emission regions to be
determined, notably for the Accretion Disk Corona which is responsible for
Comptonization in X-ray binaries. In LMXB, the radius of the ADC is shown to be
between ~ 10^9 and ~ 5x10^10 cm, an appreciable fraction of the accretion disk
radius. This is inconsistent with Comptonization models requiring a localized
Comptonizing region, for example, in the immediate neighbourhood of the neutron
star. Results from a survey of LMXB using ASCA and BeppoSAX reveal an
approximate equality between the height of the blackbody emission region on the
neutron star and the height of the inner radiatively-supported disk, suggesting
either that there is a direct causal link, such as a radial accretion flow
between the inner disk edge and the star, or an indirect link, as in the case
of accretion flow creep on the surface of the neutron star as suggested by
Inogamov & Sunyaev. Finally, the survey shows that the blackbody cannot
originate on the accretion disk as the required inner radii in many sources are
substantially less than the neutron star radius.Comment: solicited review paper presented at COSPAR 2000 "X-ray and Gamma-ray
Signatures of Black Holes and Weakly Magnetized Neutron Stars"; accepted for
publication in Advances in Space Research (2nd affiliation added
Measurements of accretion disc corona size in LMXB: consequences for Comptonization and LMXB models
We present results of measurements of the radial extent of the accretion disc
corona in low mass X-ray binaries. These results prove conclusively the
extended nature of the ADC, with radial extent varying from 20,000 km in the
faintest sources to 700,000 km in the brightest, a substantial fraction of the
accretion disc radius, typically 15%. This result rules out the Eastern model
for LMXB which is extensively used, in which the Comptonizing region is a small
central region. The ADC size depends strongly on the 1 - 30 keV source
luminosity via a simple relationship r_ADC = L^{0.88 +/- 0.16} (99% confidence)
close to a simple proportionality. We also present limited evidence that the
ADC size agrees with the Compton radius r_C, or maximum radius for hydrostatic
equilibrium. The results are consistent with models in which an extended ADC is
formed by illumination of the disc by the central source. The dependence on
luminosity may reflect the known decrease of coronal temperature as the source
luminosity increases leading to an increase of r_C. The extended nature of the
ADC means that the seed photons for Comptonization must consist of emission
from the disc to the same radial extent as the corona, providing copious
supplies of soft seed photons. We demonstrate the importance of the size of the
ADC to the correct description of Comptonization, and derive the Comptonized
spectrum of a LMXB based on thermal Comptonization of these seed photons and
show that this differs fundamentally from that of the Eastern model which
assumes a cut-off below 1 keV. Finally, we argue that our results are
inconsistent with the assumption often made that the X-ray emission of
accreting Black Holes and Neutron Stars has a common mechanism depending on the
properties of the accretion flow only.Comment: 9 pages, 5 figures, MNRAS in press - final versio
A highly-ionized absorber as a new explanation for the spectral changes during dips from X-ray binaries
Until now, the spectral changes observed from persistent to dipping intervals
in dipping low-mass X-ray binaries were explained by invoking progressive and
partial covering of an extended emission region. Here, we propose a novel and
simpler way to explain these spectral changes, which does not require any
partial covering and hence any extended corona, and further has the advantage
of explaining self-consistently the spectral changes both in the continuum and
the narrow absorption lines that are now revealed by XMM-Newton. In 4U 1323-62,
we detect Fe XXV and Fe XXVI absorption lines and model them for the first time
by including a complete photo-ionized absorber model rather than individual
Gaussian profiles. We demonstrate that the spectral changes both in the
continuum and the lines can be simply modeled by variations in the properties
of the ionized absorber. From persistent to dipping the photo-ionization
parameter decreases while the equivalent hydrogen column density of the ionized
absorber increases. In a recent work (see Diaz Trigo et al. in these
proceedings), we show that our new approach can be successfully applied to all
the other dipping sources that have been observed by XMM-Newton.Comment: 5 pages, 5 figures, to appear in the proceedings of "The X-ray
Universe 2005", San Lorenzo de El Escorial (Spain), 26-30 September 200
Spectral Evolution of the Continuum and Disc Line in Dipping in GRO J1655-40
The discovery is reported of emission features in the X-ray spectrum of GRO
J1655-40 obtained using Rossi-XTE on 1997, Feb 26. The features have been
fitted firstly by two Gaussian lines, which in four spectra have average
energies of 5.85+/-0.08 keV and 7.32+/-0.13 keV, strongly suggestive that these
are the red- and blueshifted wings of an iron disc line from material with
velocity ~0.33 c. The blue wing is apparently less bright than expected for a
disc line subject to Doppler boosting, however, known absorption in the
spectrum of GRO J1655-40 at energies between ~7 and 8 keV can reduce the
apparent brightness of the blue wing. The spectra have also been fitted well
using the full relativistic disc line model of Laor, plus an absorption line.
This gives a restframe energy between 6.4 and 6.8 keV indicating that the line
is from highly ionized iron K_alpha. The Laor model also shows that the line
originates at radii extending from ~10 Schwarzschild radii (r_S) outwards. The
line is direct evidence for the black hole nature of the compact object. The
continuum is well described by dominant disc blackbody emission plus
Comptonized emission. During dipping, spectral evolution is well modelled by
allowing progressive covering of the disc blackbody and simple absorption of
the Comptonized emission showing that the thermal emission is more extended.
Acceptable fits are only obtained by including the disc line in the covering
term, indicating that it originates in the same inner disc region as the
thermal continuum. Dip ingress times and durations are used to provide the
radius of the disc blackbody emitter as 170-370 r_S, and the radius of the
absorber.Comment: paper presented at COSPAR 2000 "X-ray and Gamma-ray Signatures of
Black Holes and Weakly Magnetized Neutron Stars"; accepted for publication in
Advances in Space Researc
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
Results of a LMXB survey: variation in the height of the neutron star blackbody emission region
We present results of a survey of the spectra of Low Mass X-ray Binaries
using ASCA. It is shown that all sources in the survey are well-fitted by the
same two-component emission model that we have previously shown is able to
describe both the non-dip and dip spectra of the dipping class of LMXB. This
model consists of point-like blackbody emission from the neutron star plus
Comptonized emission from a disk-like accretion disk corona of radius typically
50,000 km. Additional data from results published elsewhere by us from BeppoSAX
and ASCA are added to the survey. The large variation in blackbody luminosity
of survey sources is shown to be due primarily to major changes in blackbody
emitting area. Fitting a multi-temperature disk blackbody plus Comptonization
model to the survey spectra requires values of inner disk radius substantially
less than the neutron star radius in many cases, making disk origin of the
blackbody highly unlikely. Assuming that the emission is from an equatorial
strip on the neutron star, it is shown that the half-height of the strip h
agrees well with the half-height H of the radiatively-supported inner accretion
disk, this agreement spanning three orders of magnitude in each parameter.
Possible mechanisms for the agreement are discussed, including radial accretion
flow between inner disk and star, and accretion flow ``creep' on the surface of
the neutron star.Comment: 10 pages, 7 ps figures; accepted for publication in A&A Main Journa
Physical changes during Z-track movement in Sco X-1 on the flaring branch
We present results of a detailed study of X-ray flaring in the Z-track source
Sco X-1 in a highly super-Eddington state made using high quality Rossi-XTE
data from the PCA and HEXTE instruments. The emission model successfully used
to explain the dipping LMXB, and other classes of LMXB in recent years, was
applied to study the physical evolution along the Z-track which remains a major
problem. This model consists of blackbody emission from the neutron star plus
Comptonized emission from an extended accretion disk corona. As found in
earlier work, major changes take place in the neutron star blackbody emission
with kT increasing in flaring, and the blackbody radius R_BB increasing
substantially to a maximum value of 9.4 +/- 0.6 km, consistent with the radius
of the neutron star, after which R_BB decreases. Thus this result is a
measurement of neutron star radius. The behaviour of Sco X-1 in flaring is
compared with our previous results for the strong flaring that takes place in
the bright dipping, flaring LMXB X 1624-490. Remarkably, during movement along
the Normal Branch towards the apex with the Flaring Branch, the luminosities of
both spectral components decrease, suggesting the possibility that Mdot may
decrease on the Normal Branch, contrary to the widely-held view that Mdot
increases monotonically along the Z-track. During flaring, we detect for the
first time an increase of the Comptonization cut-off energy which may suggest
heating of the ADC plasma by the neutron star flare. The energy of a broad
Gaussian line at 6.4 keV does not change, but the intensity of the line
increases in flaring suggesting either an increase in ADC size in flaring or
the effects of irradiation by the neutron star.Comment: 12 pages including 8 figures, Astronomy and Astrophysics in press;
reference list correcte
The Ephemeris and Dipping Spectral Behavior of X1624-490
We present striking results from Rossi X-ray Timing Explorer (RXTE)
observations of the 21-hr low mass X-ray binary X1624-490, showing five complex
dips in unprecedented detail. For the first time, dipping is detected up to 15
keV. Prominent flares are also observed in the light curves, limited to
energies above 8 keV. Spectra selected by intensity during dip episodes can be
well fit with a two-component model consisting of a point-like blackbody from
the neutron star and progressive covering of an extended Comptonized region,
presumably an accretion disk corona (ADC), corrected for photons scattered into
and out of the X-ray beam by a interstellar dust halo. We find that the outer
regions of the absorber are highly ionized and that electron scattering is
totally responsible for the X-ray attenuation during shallow dipping. The
timescales of dip ingress and egress indicate that the envelope of material
absorbing the ADC has smaller angular size than the ADC itself, and that the
ADC is likely limited to a height-to-radius ratio of 10%, rather than being
spherical in extent. In addition, we have analyzed 4.5 yrs of RXTE All Sky
Monitor (ASM) coverage to derive the first accurate orbital ephemeris for
X1624-490, with phase zero (the time of dip centers) well-described by the
relation 2450088.63918(69) + N*0.869907(12) (JD).Comment: 12 pages, 5 figure
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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