163 research outputs found
Swift monitoring observation of the Be/X-ray transient GX 304-1 in quiescence and its current outburst
On 2016 May 17, the Be/X-ray transient GX 30-41 was found (using MAXIand BAT) to exhibit a new outburst although this was not yet expected based on the previous outbursts of the source (Atel #9064)
Quiescent X-ray variability in the neutron star Be/X-ray transient GRO J1750-27
The Be/X-ray transient GRO J1750-27 exhibited a type-II (giant) outburst in
2015. After the source transited to quiescence, we triggered our multi-year
Chandra monitoring programme to study its quiescent behaviour. The programme
was designed to follow the cooling of a potentially heated neutron-star crust
due to accretion of matter during the preceding outburst, similar to what we
potentially have observed before in two other Be/X-ray transients, namely 4U
0115+63 and V 0332+53. However, unlike for these other two systems, we do not
find any strong evidence that the neutron-star crust in GRO J1750-27 was indeed
heated during the accretion phase. We detected the source at a rather low X-ray
luminosity (~10^33 erg/s) during only three of our five observations. When the
source was not detected it had very low-luminosity upper limits (<10^32 erg/s;
depending on assumed spectral model). We interpret these detections and the
variability observed as emission likely due to very low-level accretion onto
the neutron star. We also discuss why the neutron-star crust in GRO J1750-27
might not have been heated while the ones in 4U 0115+63 and V 0332+53 possibly
were.Comment: 13 pages, 6 figures, 5 tables. Accepted for A&
Dramatic spectral transition of X-ray pulsar GX 304-1 in low luminous state
We report on the discovery of a dramatic change in the energy spectrum of the
X-ray pulsar GX 304-1 appearing at low luminosity. Particularly, we found that
the cutoff power-law spectrum typical for accreting pulsars, including GX 304-1
at higher luminosities of erg s,
transformed at lower luminosity of erg s to a
two-component spectrum peaking around 5 and 40 keV. We suggest that the
observed transition corresponds to a change of the dominant mechanism
responsible for the deceleration of the accretion flow. We argue that the
accretion flow energy at low accretion rates is released in the atmosphere of
the neutron star, and the low-energy component in the source spectrum
corresponds to the thermal emission of the optically thick, heated atmospheric
layers. The most plausible explanations for the high-energy component are
either the cyclotron emission reprocessed by the magnetic Compton scattering or
the thermal radiation of deep atmospheric layers partly Comptonized in the
overheated upper layers. Alternative scenarios are also discussed.Comment: 5 pages, 2 figures, accepted by MNRAS Letter
Discovery of accretion-driven pulsations in the prolonged low X-ray luminosity state of the Be/X-ray transient GX 304-1
We present our Swift monitoring campaign of the slowly rotating neutron star
Be/X-ray transient GX 304-1 (spin period of ~275 s) when the source was not in
outburst. We found that between its type-I outbursts the source recurrently
exhibits a slowly decaying low-luminosity state (with luminosities of
10^(34-35) erg/s). This behaviour is very similar to what has been observed for
another slowly rotating system, GRO J1008-57. For that source, this
low-luminosity state has been explained in terms of accretion from a
non-ionised ('cold') accretion disk. Due to the many similarities between both
systems, we suggest that GX 304-1 enters a similar accretion regime between its
outbursts. The outburst activity of GX 304-1 ceased in 2016. Our continued
monitoring campaign shows that the source is in a quasi-stable low-luminosity
state (with luminosities a few factors lower than previously seen) for at least
one year now. Using our NuSTAR observation in this state, we found pulsations
at the spin period, demonstrating that the X-ray emission is due to accretion
of matter onto the neutron star surface. If the accretion geometry during this
quasi-stable state is the same as during the cold-disk state, then matter
indeed reaches the surface (as predicted) during this latter state. We discuss
our results in the context of the cold-disk accretion model.Comment: 5 pages, 2 figures, accepted for A&A Letter
Spinodal decomposition of Fe-Cu nanocrystals: Control of atomic-magnetic-moment and magnetic properties
Experimental results corresponding to the saturation magnetization and coercive field during the decomposition, upon annealing, of bcc and fcc Fe_xCu_(1-x), obtained by mechanical alloying are reported. The overall behavior points out that the decomposition takes place in two steps: (i) at low temperatures a decrease of the saturation magnetic moment as well as an anomalous thermal dependence of coercive field are observed, however, no phase transformation is detected, and (ii) for further annealing temperatures a new phase appears; the magnetization tends to increase and the coercive field abruptly increases. The analysis of the results leads us to conclude that the first step corresponds to a spinodal decomposition. Fluctuations in the local composition give rise to coexistence of adjacent regions with Curie temperature varying continuously in a range of 1000 K across distances of a few nanometers, thus allowing the tailoring of the magnetic nanostructures
Mechanical-alloying and lattice distortions in ball-milled CuFe
A least-square fitting analysis of EXAFS data collected from partially-crystallized Fe_80B_20 thin films (t=15 nm), using data collected from pure phase standards of the crystallization products, was found effective in determining the relative atomic fraction of each crystalline phase present. This fitting scheme provides a means for the quantitative treatment of crystallization and precipitation kinetics in thin films and multilayered structures
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