16 research outputs found
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
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
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
Discovery of narrow X-ray absorption features from the dipping low-mass X-ray binary X 1624-490 with XMM-Newton
We report the discovery of narrow X-ray absorption features from the dipping
low-mass X-ray binary X 1624-490 during an XMM-Newton observation in 2001
February. The features are identified with the K alpha absorption lines of Fe
xxv and Fe xxvi and have energies of 6.72 +/- 0.03 keV and 7.00 +/- 0.02 keV
and equivalent widths (EWs) of -7.5 +1.7 -6.3 eV and -16.6 +1.9 -5.9 eV,
respectively. The EWs show no obvious dependence on orbital phase, except
during a dip, and correspond to a column of greater than 10^17.3 Fe atom /cm2.
In addition, faint absorption features tentatively identified with Ni xxvii K
alpha and Fe xxvi K beta may be present. A broad emission feature at 6.58 +0.07
-0.04 keV with an EW of 78 +19 -6 eV is also evident. This is probably the 6.4
keV feature reported by earlier missions since fitting a single Gaussian to the
entire Fe-K region gives an energy of 6.39 +0.03 -0.04 keV. A deep absorption
feature is present during the dip with an energy consistent with Fe xxv K
alpha. This is the second dipping LMXRB source from which narrow Fe absorption
features have been observed. Until recently the only X-ray binaries known to
exhibit narrow X-ray absorption lines were two superluminal jet sources and it
had been suggested that these features are related to the jet formation
mechanism. It now appears likely that ionized absorption features may be common
characteristics of accreting systems with accretion disks.Comment: 6 pages. To appear in A&
Spectral changes during dipping in low-mass X-ray binaries due to highly-ionized absorbers
X-ray observations have revealed that many microquasars and low-mass X-ray
binaries (LMXBs) exhibit narrow absorption features identified with resonant
absorption from Fe XXV and Fe XXVI and other abundant ions. In many well
studied systems there is evidence for blue-shifts, indicating outflowing
plasmas. We succesfully model the changes in both the X-ray continuum and the
Fe absorption features during dips from all the bright dipping LMXBs observed
by XMM-Newton (EXO 0748-676, XB 1254-690, X 1624-490, MXB 1659-298, 4U 1746-371
and XB 1916-053) as resulting primarily from an increase in column density and
a decrease in the ionization state of a highly-ionized absorber in a similar
way as was done for 4U 1323-62. This implies that the complex spectral changes
in the X-ray continua observed from the dip sources as a class can be most
simply explained primarily by changes in the highly ionized absorbers present
in these systems. There is no need to invoke unusual abundances or partial
covering of extended emission regions. Outside of the dips, the absorption line
properties do not vary strongly with orbital phase. This implies that the
ionized plasma has a cylindrical geometry with a maximum column density close
to the plane of the accretion disk. Since dipping sources are simply normal
LMXBs viewed from close to the orbital plane this implies that ionized plasmas
are a common feature of LMXBs.Comment: 19 pages, 11 figures, accepted for publication by Astronomy and
Astrophysic
A study of the spectral evolution during dipping in XB 1323-619 with Rossi-XTE and BeppoSAX
We report results from analysis of the observations of the dipping low mass
X-ray binary XB 1323-619 made with BeppoSAX} and Rossi-XTE. The dust-scattered
halo contributes significantly in this source, and the observation made with
BeppoSAX on 1997 August was used to provide MECS radial intensity profiles at
several energies. From these, the halo fractions were obtained and thus an
optical depth to dust scattering of 1.8 +/- 0.4 derived. In the Rossi-XTE
observation of April 25-28, 1997, seven X-rays dips were observed together with
7 bursts repeating approximately periodically. Non-dip and dip PCA spectra can
be well-described by assuming the emission consists of point-like blackbody
emission identified with the neutron star, plus Comptonized emission from an
extended ADC. The blackbody temperature is 1.79 +/- 0.21 keV and the cut-off
power law photon index 1.61 +/- 0.04. Spectral evolution in dipping is well
described by progressive covering of the extended Comptonizing region by
absorber plus more rapid removal of the point-like blackbody. The effects of
dust scattering and of the X-ray pulsar 1SAX J1324.4-6200 also in the field of
view are included in the fitting. We detect an iron line at ~6.4 keV and its
probable origin in the ADC is discussed.Comment: 10 pages, 5 ps figures; Astron. and Astrophysics in pres
An explanation of the Z-track sources
We present an explanation of the Z-track phenomenon based on spectral fitting
of RXTE observations of GX340+0 using the emission model previously shown to
describe the dipping LMXB. In our Z-track model, the soft apex is a quiescent
state of the source with lowest luminosity. Moving away from this point by
ascending the normal branch the strongly increasing luminosity of the Accretion
Disc Corona (ADC) Comptonized emission L_ADC provides substantial evidence for
a large increase of mass accretion rate Mdot. There are major changes in the
neutron star blackbody emission, kT increasing to high values, the blackbody
radius R_BB decreasing, these changes continuing monotonically on both normal
and horizontal branches. The blackbody flux increases by a factor of ten to
three times the Eddington flux so that the physics of the horizontal branch is
dominated by the high radiation pressure of the neutron star, which we propose
disrupts the inner disc, and an increase of column density is detected. We
further propose that the very strong radiation pressure is responsible for the
launching of the jets detected in radio on the horizontal branch. On the
flaring branch, we find that L_ADC is constant, suggesting no change in Mdot so
that flaring must consist of unstable nuclear burning. At the soft apex, the
mass accretion rate per unit area on the neutron star m_dot is minimum for the
horizontal and normal branches and about equal to the theoretical upper limit
for unstable burning. Thus it is possible that unstable burning begins as soon
as the source arrives at this position, the onset being consistent with theory.
The large increase in R_BB in flaring is reminiscent of radius expansion in
X-ray bursts. Finally, in our model, Mdot does not increase monotonically along
the Z-track as often previously thought.Comment: 14 pages, 8 figures, accepted for publication in Astronomy and
Astrophysic
A comparison of neutron star blackbody luminosities in LMXB with the theory of accretion flow spreading on the stellar surface
We present a comparison of the results of the ASCA survey of LMXB with the
Inogamov and Sunyaev theory of accretion flow spreading on the surface of
neutron stars. The ASCA survey of LMXB of Church and Balucinska-Church (2001)
revealed a systematic variation of the luminosity of blackbody emission from
the neutron star spanning 3 decades in total X-ray luminosity suggesting that
the level of blackbody emission is controlled by the physics of the inner
disk/stellar interface, which we can hope to understand. Two types of
explanation exist: firstly that there is radial flow between the inner disk and
star at all vertical positions above the orbital plane so that the height of
the disk directly determines the area of star emitting. Secondly, the height of
the emitting region on the star is not directly related to the disk properties
but depends on the mass accretion rate as suggested by Inogamov and Sunyaev
(1999) in their theory of accretion flow spreading on the stellar surface. We
find that the survey results for the emitting area agree with this theory at
the lowest luminosities. However, for higher luminosities, the blackbody
emission is stronger than predicted by spreading theory suggesting that the
emitting area is controlled by radial flow between disk and star.Comment: 8 pages, 6 ps figures; Astron. and Astrophysics in pres
Neutron star blackbody contraction during flaring in X1624-490
We present results of an investigation of the physical changes taking place
in the emission regions of the LMXB X1624-490 during strong flaring in RXTE
observations. Based on the detailed light curve, we propose that the flaring
consists of a superposition of X-ray bursts. It is shown that major changes
take place in the blackbody emission component, the temperature kT_BB
increasing to ~2.2 keV in flaring. Remarkably, the blackbody area decreases by
a factor of ~5 in flaring. During flare evolution, the blackbody luminosity
remains approximately constant, constituting a previously unknown Eddington
limiting effect which we propose is due to radiation pressure of the blackbody
as kT_BB increases affecting the inner disk or accretion flow resulting in a
decreased emitting area on the star. We argue that the large decrease in area
cannot be explained in terms of modification of the blackbody spectrum by
electron scattering in the atmosphere of the neutron star. The height of the
emitting region on the non-flaring neutron star is shown to agree with the
height of the inner radiatively-supported disk as found for sources in the ASCA
survey of LMXB of Church & Balucinska-Church (2001). The decrease in height
during flaring is discussed in terms of possible models, including radial
accretion flow onto the stellar surface and the theory of accretion flow
spreading on the neutron star surface of Inogamov & Sunyaev (1999). We
demonstrate that the intensity of the broad iron line at 6.4 keV is strongly
correlated with the luminosity of the blackbody emission from the neutron star,
and discuss the probable origin of this line in the ADC. Finally, possible
reasons for non-detection of a reflection component in this source, and LMXB in
general, are discussed.Comment: 14 pages including 11 figures, Astronomy and Astrophysics in pres
Long term variability of Cygnus X-1. IV, Spectral evolution 1999–2004
Continuing the observational campaign initiated by our group, we present the long term spectral evolution of the Galactic black hole candidate Cygnus X-1 in the X-rays and at 15 GHz. We present ∼200 pointed observations taken between early 1999 and late 2004 with the Rossi X-ray
Timing Explorer and the Ryle radio telescope. The X-ray spectra are remarkably well described by a simple broken power law spectrum with an exponential cutoff. Physically motivated Comptonization models, e.g., by Titarchuk (1994, ApJ, 434, 570, compTT) and by Coppi (1999, in High Energy Processes in Accreting Black Holes, ed. J. Poutanen, & R. Svensson (San Francisco: ASP), ASP Conf. Ser., 161, 375, eqpair), can reproduce this simplicity; however, the success of the phenomenological broken power law models cautions against “overparameterizing” the more physical models. Broken power law models reveal a significant linear correlation between the photon index of the lower energy
power law and the hardening of the power law at ∼10 keV. This phenomenological soft/hard power law correlation is partly attributable to correlations of broad band continuum components, rather than being dominated by the weak hardness/reflection fraction correlation present in the Comptonization model. Specifically, the Comptonization models show that the bolometric flux of a soft excess (e.g., disk component) is strongly correlated with the compactness ratio of the Comptonizing medium, with L disk
∝( h / s ) −0.19 . Over the course of our campaign, Cyg X-1 transited several times into the soft state, and exhibited a large number of “failed state transitions”. The fraction of the time spent in such low radio emission/soft X-ray spectral states has increased from ∼10% in 1996–2000 to ∼34% since early 2000. We find that radio flares
typically occur during state transitions and failed state transitions (at h / s ∼ 3), and that there is a strong correlation between the 10–50 keV X-ray flux and the radio luminosity of the source. We demonstrate that rather than there being distinctly separated states, in contrast to the timing properties the spectrum of Cyg X-1 shows variations between extremes of properties, with clear cut examples of spectra at every intermediate point in the observed spectral correlations