163 research outputs found
Evidence of Compton cooling during an X-ray flare supports a neutron star nature of the compact object in 4U1700-37
Based on new Chandra X-ray telescope data, we present empirical evidence of
plasma Compton cooling during a flare in the non pulsating massive X-ray binary
4U1700-37. This behaviour might be explained by quasispherical accretion onto a
slowly rotating magnetised neutron star. In quiescence, the neutron star in
4U1700-37 is surrounded by a hot radiatively cooling shell. Its presence is
supported by the detection of mHz quasi periodic oscillations likely produced
by its convection cells. The high plasma temperature and the relatively low
X-ray luminosity observed during the quiescence, point to a small emitting area
about 1 km, compatible with a hot spot on a NS surface. The sudden transition
from a radiative to a significantly more efficient Compton cooling regime
triggers an episode of enhanced accretion resulting in a flare. During the
flare, the plasma temperature drops quickly. The predicted luminosity for such
transitions, Lx = 3 x 10^35 erg s-1, is very close to the luminosity of
4U1700-37 during quiescence. The transition may be caused by the accretion of a
clump in the stellar wind of the donor star. Thus, a magnetised NS nature of
the compact object is strongly favoured.Comment: Accepted for publication in MNRA
A Chandra survey of fluorescence Fe lines in X-ray Binaries at high resolution
In this paper we present a comprehensive survey of 41 X-ray binaries (10
HMXBs and 31 LMXBs) with Chandra, with specific emphasis on the Fe K region and
the narrow Fe Kalpha line, at the highest resolution possible. We find that: a)
The Fe Kalpha line is always centered at 1.9387 +- 0.0016 Angstroms, compatible
with Fe I up to Fe X; we detect no shifts to higher ionization states nor any
difference between HMXBs and LMXBs. b) The line is very narrow, with FWHM < 5
mAngstroms, which means that the reprocessing material is not rotating at high
speeds. c) Fe Kalpha fluorescence is present in all the HMXB in the survey
while such emissions are very rare (~ 10% ) among LMXBs. d) The lack of Fe line
emission is always accompanied by the lack of any detectable K edge. e) We
obtain the empirical curve of growth of the equivalent width of the Fe Kalpha
line versus the density column of the reprocessing material, i.e. EW_{Kalpha}
vs N_{H}, and show that it is consistent with a reprocessing region spherically
distributed around the compact object. f) We show that fluorescence in X-ray
binaries follows the X-ray Baldwin effect. We interpret this finding as
evidence of decreasing neutral Fe abundance with increasing X-ray illumination
and use it to explain some spectral states of Cyg X-1 and as a possible cause
of the lack of narrow Fe line emission in LMXBs. g) Finally, we study anomalous
morphologies. We present the first evidence of a Compton shoulder in the HMXB
X1908+075. Also the Fe Kalpha lines of 4U1700-37 and LMC X-4 present asymmetric
wings suggesting the presence of highly structured stellar winds in these
systems.Comment: 12 pages, 11 figures, accepted for publication in Ap
The hard X-ray emission of X Per
We present an analysis of the spectral properties of the peculiar X-ray
pulsar X Per based on INTEGRAL observations. We show that the source exhibits
an unusually hard spectrum and is confidently detected by ISGRI up to more than
100 keV. We find that two distinct components may be identified in the
broadband 4-200 keV spectrum of the source. We interpret these components as
the result of thermal and bulk Comptonization in the vicinity of the neutron
star and describe them with several semi-phenomenological models. The
previously reported absorption feature at ~30 keV is not required in the
proposed scenario and therefore its physical interpretation must be taken with
caution. We also investigated the timing properties of the source in the
framework of existing torque theory, concluding that the observed phenomenology
can be consistently explained if the magnetic field of the neutron star is
~10^14 G.Comment: Published as a letter in A&A; 4 pages, 2 figure
El auge de la ciudad de Concepción en Penco, las variables geohistóricas del siglo XVII
Fundada en 1550 por el conquistador Pedro de Valdivia, en el sitio que hasta entonces ocupaba el asentamiento aborigen de “Carapenco”, la antigua ciudad de Concepción del Nuevo Extremo tuvo un proceso de desarrollo accidentado y difícil, determinado por los vaivenes de la guerra de Arauco y catástrofes telúricas que, desde su establecimiento, afectaron la consolidación de aquel emblemático enclave colonial
The X-ray emission of the gamma Cassiopeiae stars
Long considered as the "odd man out" among X-ray emitting Be stars, \gamma
Cas (B0.5e IV) is now recognized as the prototype of a class of stars that emit
hard thermal X-rays. Our classification differs from the historical use of the
term "gamma Cas stars" defined from optical properties alone. The luminosity
output of this class contributes significantly to the hard X-ray production in
massive stars in the Galaxy. The gamma Cas stars have light curves showing
variability on a few broadly-defined timescales and spectra indicative of an
optically thin plasma consisting of one or more hot thermal components. By now
9--13 Galactic \approx B0-1.5e main sequence stars are judged to be members or
candidate members of the \gamma Cas class. Conservative criteria for this
designation are for a \approxB0-1.5e III-V star to have an X-ray luminosity of
10^{32}--10^{33} ergs s^{-1}, a hot thermal spectrum containing the short
wavelength Ly \alpha FeXXV and FeXXVI lines and the fluorescence FeK feature
all in emission. If thermality cannot be demonstrated, for example from either
the presence of these Ly \alpha lines or curvature of the hard continuum; these
are the gamma Cas candidates. We discuss the history of the discovery of the
complicated characteristics of the variability in the optical, UV, and X-ray
domains, leading to suggestions for the physical cause of the production of
hard X-rays. These include scenarios in which matter from the Be star accretes
onto a degenerate secondary star and interactions between magnetic fields on
the Be star and its decretion disk. The greatest aid to the choice of the
causal mechanism is the temporal correlations of X-ray light curves and spectra
with diagnostics in the optical and UV wavebands. We show why the magnetic
star-disk interaction scenario is the most tenable explanation for the creation
of hard X-rays on these stars.Comment: Review paper for "X-ray Emissions from Hot Stars and their Winds"
compendium to be published by Advances in Space Research in mid-2016. Paper
is comprised of 66 pages, 15 figure
Probing stellar winds and accretion physics in high-mass X-ray binaries and ultra-luminous X-ray sources with LOFT
This is a White Paper in support of the mission concept of the Large
Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We
discuss the potential of LOFT for the study of high-mass X-ray binaries and
ultra-luminous X-ray sources. For a summary, we refer to the paper.Comment: White Paper in Support of the Mission Concept of the Large
Observatory for X-ray Timing. (v2 few typos corrected
4U 1909+07: a well-hidden pearl
We present the first detailed spectral and timing analysis of the High Mass
X-ray Binary (HMXB) 4U 1909+07 with INTEGRAL and RXTE. 4U 1909+07 is detected
in the ISGRI 20-40 keV energy band with an average countrate of 2.6 cps. The
pulse period of ~604 sec is not stable, but changing erratically on timescales
of years. The pulse profile is strongly energy dependent: it shows a double
peaked structure at low energies, the secondary pulse decreases rapidly with
increasing energy and above 20 keV only the primary pulse is visible. This
evolution is consistent between PCA, HEXTE, and ISGRI. The phase averaged
spectrum can be well described by the sum of a photoabsorbed power law with a
cutoff at high energies and a blackbody component. To investigate the pulse
profile, we performed phase resolved spectral analysis. We find that the
changing spectrum can be best described with a variation of the folding energy.
We rule out a correlation between the black body component and the continuum
variation and discuss possible accretion geometries.Comment: 9 pages, 11 figures, accepted for publication in A&A Sect.
XMM-Newton observations of IGRJ18410-0535: The ingestion of a clump by a supergiant fast X-ray transient
IGRJ18410-0535 is a supergiant fast X-ray transients. This subclass of
supergiant X-ray binaries typically undergoes few- hour-long outbursts reaching
luminosities of 10^(36)-10^(37) erg/s, the occurrence of which has been
ascribed to the combined effect of the intense magnetic field and rotation of
the compact object hosted in them and/or the presence of dense structures
("clumps") in the wind of their supergiant companion. IGR J18410-0535 was
observed for 45 ks by XMM-Newton as part of a program designed to study the
quiescent emission of supergiant fast X-ray transients and clarify the origin
of their peculiar X-ray variability. We carried out an in-depth spectral and
timing analysis of these XMM-Newton data. IGR J18410-0535 underwent a bright
X-ray flare that started about 5 ks after the beginning of the observation and
lasted for \sim15 ks. Thanks to the capabilities of the instruments on-board
XMM-Newton, the whole event could be followed in great detail. The results of
our analysis provide strong convincing evidence that the flare was produced by
the accretion of matter from a massive clump onto the compact object hosted in
this system. By assuming that the clump is spherical and moves at the same
velocity as the homogeneous stellar wind, we estimate a mass and radius of Mcl
\simeq1.4\times10^(22) g and Rcl \simeq8\times10^(11) cm. These are in
qualitative agreement with values expected from theoretical calculations. We
found no evidence of pulsations at \sim4.7 s after investigating coherent
modulations in the range 3.5 ms-100 s. A reanalysis of the archival ASCA and
Swift data of IGR J18410-0535, for which these pulsations were previously
detected, revealed that they were likely to be due to a statistical fluctuation
and an instrumental effect, respectively.Comment: Accepted for publication on A&A. V2: Inserted correct version of
Fig.1
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