113 research outputs found
Spectral evolution of bright NS LMXBs with INTEGRAL: an application of the thermal plus bulk Comptonization model
The aim of this work is to investigate in a physical and quantitative way the
spectral evolution of bright Neutron Star Low-Mass X-ray Binaries (NS LMXBs),
with special regard to the transient hard X-ray tails. We analyzed INTEGRAL
data for five sources (GX 5-1, GX 349+2, GX 13+1, GX 3+1, GX 9+1) and built
broad-band X-ray spectra from JEM-X1 and IBIS/ISGRI data. For each source,
X-ray spectra from different states were fitted with the recently proposed
model compTB. The spectra have been fit with a two-compTB model. In all cases
the first compTB describes the dominant part of the spectrum that we interpret
as thermal Comptonization of soft seed photons (< 1 keV), likely from the
accretion disk, by a 3-5 keV corona. In all cases, this component does not
evolve much in terms of Comptonization efficiency, with the system converging
to thermal equilibrium for increasing accretion rate. The second compTB varies
more dramatically spanning from bulk plus thermal Comptonization of blackbody
seed photons to the blackbody emission alone. These seed photons (R < 12 km,
kT_s > 1 keV), likely from the neutron star and the innermost part of the
system, the Transition Layer, are Comptonized by matter in a converging flow.
The presence and nature of this second compTB component (be it a pure blackbody
or Comptonized) are related to the inner local accretion rate which can
influence the transient behaviour of the hard tail: high values of accretion
rates correspond to an efficient Bulk Comptonization process (bulk parameter
delta > 0) while even higher values of accretion rates suppress the
Comptonization, resulting in simple blackbody emission (delta=0).Comment: 12 pages, 10 figures, accepted for publication in A&
First results on the HMXRB Pulsar SAXJ2103.5+4545 with INTEGRAL
We report on the preliminary timing and spectral analysis of the High Mass
X-ray Binary Pulsar SAXJ2103.5+4545 as observed with INTEGRAL during the
Galactic Pla n Scan of the Core Program. The source shows a hard spectrum,
being detected up to 100 keV. The timing analysis performed on IBIS/ISGRI data
shows evidence for a spin-up wi th respect to previous observations, performed
in 1997 and 1999 with BeppoSAX an d RossiXTE.Comment: esapub.cls, 4 pages, Proceedings of the V INTEGRAL Workshop, Munich
16-20 February 2004. ESA SP-5524. Revised version according to referee
suggestions, added reference
First simultaneous multi-wavelength observations of the black hole candidate IGR J17091-3624: ATCA, INTEGRAL, Swift, and RXTE views of the 2011 outburst
We present the results of the first four (quasi-)simultaneous radio (ATCA),
X-ray (Swift, RXTE), and Gamma-ray (INTEGRAL) observations of the black hole
candidate IGR J17091-3624, performed in February and March 2011. The X-ray
analysis shows that the source was in the hard state, and then it transited to
a soft intermediate state. We study the correlated radio/X-ray behaviour of
this source for the first time. The radio counterpart to IGR J17091-3624 was
detected during all four observations with the ATCA. In the hard state, the
radio spectrum is typical of optically thick synchrotron emission from a
self-absorbed compact jet. In the soft intermediate state, the detection of
optically thin synchrotron emission is probably due to a discrete ejection
event associated with the state transition. The position of IGR J17091-3624 in
the radio versus X-ray luminosity diagram (aka fundamental plane) is compatible
with that of the other black hole sources for distances greater than 11 kpc.
IGR J17091-3624 also appears as a new member of the few sources that show a
strong quenching of radio emission after the state transition. Using the
estimated luminosity at the spectral transition from the hard state, and for a
typical mass of 10 M_sun, we estimate a distance to the source between ~11 and
~17 kpc, compatible with the radio behaviour of the source.Comment: 6 pages, 4 figures, 1 online table, accepted for publication in A&
Two years of INTEGRAL monitoring of GRS 1915+105 Part 1: multiwavelength coverage with INTEGRAL, RXTE, and the Ryle radio Telescope
(Abridged) We report the results of monitoring observations of the Galactic
microquasar GRS 1915+105 performed simultaneously with INTEGRAL and RXTE Ryle .
We present the results of the whole \integral campaign, report the sources that
are detected and their fluxes and identify the classes of variability in which
GRS 1915+105 is found. The accretion ejection connections are studied in a
model independent manner through the source light curves, hardness ratio, and
color color diagrams. During a period of steady ``hard'' X-ray state (the
so-called class chi) we observe a steady radio flux. We then turn to 3
particular observations during which we observe several types of soft X-ray
dips and spikes cycles, followed by radio flares. During these observations GRS
1915+105 is in the so-called nu, lambda, and beta classes of variability. The
observation of ejections during class lambda are the first ever reported. We
generalize the fact that a (non-major) discrete ejection always occurs, in GRS
1915+105, as a response to an X-ray sequence composed of a spectrally hard
X-ray dip terminated by an X-ray spike marking the disappearance of the hard
X-ray emission above 18 keV. We also identify the trigger of the ejection as
this X-ray spike. A possible correlation between the amplitude of the radio
flare and the duration of the X-ray dip is found in our data. In this case the
X-ray dips prior to ejections could be seen as the time during which the source
accumulates energy and material that is ejected later.Comment: 17 pages, 14 figures. Accepted for publication in ApJ, scheduled for
the March 20, 2008, vol676 issue. Table 3 has been degrade
A large spin-up rate measured with INTEGRAL in the High Mass X-ray Binary Pulsar SAXJ2103.5+4545
The High Mass X-ray Binary Pulsar SAXJ2103.5+4545 has been observed with
INTEGRAL several times during the last outburst in 2002-2004. We report a
comprehensive study of all INTEGRAL observations, allowing a study of the pulse
period evolution during the recent outburst. We measured a very rapid spin-up
episode, lasting 130days, which decreased the pulse period by 1.8s. The spin-up
rate, pdot=-1.5e-7 s/s, is the largest ever measured for SAXJ2103.5+4545, and
it is among the fastest for an accreting pulsar. The pulse profile shows
evidence for temporal variability, apparently not related to the source flux or
to the orbital phase. The X-ray spectrum is hard and there is significant
emission up to 150keV. A new derivation of the orbital period, based on RXTE
data, is also reported.Comment: 8 pages, 7 figures, accepted for publication in A&
Scientific Performance of the ISDC Quick Look Analysis
The INTEGRAL Science Data Centre (ISDC) routinely monitors the Near Real Time
data (NRT) from the INTEGRAL satellite. A first scientific analysis is made in
order to check for the detection of new, transient or highly variable sources
in the data. Of primary importance for this work is the Interactive Quick Look
Analysis (IQLA), which produces JEM-X and ISGRI images and monitors them for
interesting astrophysical eventsComment: 4 pages, 3 figures. Proceedings of 5th INTEGRAL Workshop: The
INTEGRAL Universe, Munich, 16-20 February 2004. Accepted for publication in
European Space Agency Special Publication 552. See paper for institute
affiliation
Wind accretion: Theory and observations
A review of wind accretion in high-mass X-ray binaries is presented. We focus on different regimes of quasi-spherical accretion onto the neutron star (NS): the supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically towards the NS magnetosphere, and subsonic (settling) accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. These two regimes of accretion are separated by an X-ray luminosity of about 4 × 1036 erg s-1. In the subsonic case, which sets in at lower luminosities, a hot quasi-spherical shell must form around the magnetosphere, and the actual accretion rate onto NS is determined by the ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instability. In turn, two regimes of subsonic accretion are possible, depending on plasma cooling mechanism (Compton or radiative) near the magnetopshere. The transition from the high-luminosity with Compton cooling to the lowluminosity (Lx ≲ 3 × 1035 erg s-1) with radiative cooling can be responsible for the onset of the off states repeatedly observed in several low-luminosity slowly accreting pulsars, such as Vela X-1, GX 301-2, and 4U 1907+09. The triggering of the transitionmay be due to a switch in the X-ray beam pattern in response to a change in the optical depth in the accretion column with changing luminosity. We also show that in the settling accretion theory, bright X-ray flares (~1038-1040 erg) observed in supergiant fast X-ray transients (SFXT) can be produced by sporadic capture of magnetized stellar wind plasma. At sufficiently low accretion rates, magnetic reconnection can enhance the magnetospheric plasma entry rate, resulting in copious production of X-ray photons, strong Compton cooling and ultimately in unstable accretion of the entire shell. A bright flare develops on the free-fall time scale in the shell, and the typical energy released in an SFXT bright flare corresponds to the mass of the shell
The Galactic LMXB Population and the Galactic Centre Region
Seventeen years of hard X-ray observations with the instruments of the
INTEGRAL observatory, with a focus on the Milky Way and in particular on the
Galactic Centre region, have provided a unique database for exploration of the
Galactic population of low-mass X-ray binaries (LMXBs). Our understanding of
the diverse energetic phenomena associated with accretion of matter onto
neutron stars and black holes has greatly improved. We review the large variety
of INTEGRAL based results related to LMXBs. In particular, we discuss the
spatial distribution of LMXBs over the Galaxy and their X-ray luminosity
function as well as various physical phenomena associated with Atoll and Z
sources, bursters, symbiotic X-ray binaries, ultracompact X-ray binaries and
persistent black hole LMXBs. We also present an up-to-date catalogue of
confirmed LMXBs detected by INTEGRAL, which comprises 166 objects. Last but not
least, the long-term monitoring of the Galactic Centre with INTEGRAL has shed
light on the activity of Sgr A* in the recent past, confirming previous
indications that our supermassive black hole experienced a major accretion
episode just ~100 years ago. This exciting topic is covered in this review too.Comment: 60 pages, 26 figures, 2 tables, accepted for publication in New
Astronomy Review
Two Years of INTEGRAL monitoring of GRS 1915+105 Part 2: X-Ray Spectro-Temporal Analysis
(abridged) This is the second paper presenting the results of two years of
monitoring of GRS 1915+105 with \integral and \rxte and the Ryle Telescope. We
present the X-ray spectral and temporal analysis of four observations which
showed strong radio to X-ray correlations. During one observation GRS 1915+105
was in a steady state, while during the three others it showed cycles of X-ray
dips and spikes (followed by radio flares). We present the time-resolved
spectroscopy of these cyclesand show that in all cases the hard X-ray component
(the Comptonized emission from a coronal medium) is suppressed in coincidence
with a soft X-ray spike that ends the cycle. We interpret these results as
evidence that the soft X-ray spike is the trigger of the ejection, and that the
ejected medium is the coronal material. In the steady state observation, the
X-ray spectrum is indicative of the hard-intermediate state, with the presence
of a relatively strong emission at 15 GHz. The X-ray spectra are the sum of a
Comptonized component and an extra power law extending to energies >200 keV
without any evidence for a cut-off. We observe a possible correlation of the
radio flux with that of the power law component, which may indicate that we see
direct emission from the jet at hard X-ray energies. We study the energy
dependence of a ~4 Hz QPO during the hard-intermediate state observation. The
QPO-``spectrum'' is well modeled by a power law with a cut-off at an energy
about 11 keV that clearly differs from the relative contribution of the
Comptonized component to the overall flux. This may rule out models of global
oscillations of the Compton corona.Comment: 13 pages, 8 figures (3 color), accepted for publication in ApJ,
scheduled for the March 20, 2008, vol676 issue. Paper 1 is the next astro-ph
numbe
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