3,926 research outputs found
Monte-Carlo simulations on possible collimation effects of outflows to fan-beamed emission of ultraluminous accreting X-ray pulsars
Pulsating ultraluminous X-ray sources (PULXs) are accreting pulsars with
apparent X-ray luminosity exceeding . We perform
Monte-Carlo simulations to investigate whether high collimation effect (or
strong beaming effect) is dominant in the presence of accretion outflows, for
the fan beam emission of the accretion column of the neutron stars in PULXs. We
show that the three nearby PULXs (RX J0209.67427, Swift J0243.6+6124 and SMC
X-3), namely the three musketeers here, have their main pulsed emission not
strongly collimated even if strong outflows exist. This conclusion can be
extended to the current sample of extragalactic PULXs, if accretion outflows
are commonly produced from them. This means that the observed high luminosity
of PULXs is indeed intrinsic, which can be used to infer the existence of very
strong surface magnetic fields of G, possibly multipole
fields. However, if strong outflows are launched from the accretion disks in
PULXs as a consequence of disk spherization by radiation pressure, regular
dipole magnetic fields of G may be required, comparable to that
of the three musketeers, which have experienced large luminosity changes from
well below their Eddington limit ( for a NS)
to super-Eddington and their maximum luminosity fills the luminosity gap
between Galactic pulsars and extragalactic PULXs.Comment: 26 pages, 8 figures, 1 table. Accepted by Ap
Cu-Ag bi-layer films in dielectric/metal/dielectric transparent electrodes as ITO free electrode in organic photovoltaic devices
Among ITO alternative, dielectric/metal/dielectric multilayer structures are one of the most often studied possible substituent. However, if their optical and electrical properties are systematically investigated it is not the same with regard to their mechanical properties. In the present manuscript we have studied the properties of ZnS/Cu/Ag/ZnS, ZnS/Cu/Ag/MO3 (with M ¼ Mo or W) structures. With a maximum transmission of 90% and a sheet resistance of 5 U/sq the optimum structure exhibits a figure of merit of 82 10-3 Ω-1 when l = 600 nm. Beyond these standard measures we proceeded to the study of the mechanical properties of the multilayer structures. The inner and outer bending tests show that the ZnS/ Cu/Ag/ZnS (or MO3) structures are more flexible than ITO, while their responses to scotch tests show that they exhibit a large adhesion to the substrate, glass or plastic. The scratching adhesion test puts in evidence that the adhesion to the substrate of the Ag layer is smaller than that of ZnS/Cu/Ag/ZnS, which is smaller than that of ITO. On the other hand, this test shows that the ZnS/Cu/Ag/ZnS (no cracks for L = 25 N) is less brittle than ITO (cracks L = 15N). Finally, when used as anode in organic solar cells, the structure ZnS/Cu/Ag/WO3 allows achieving the best efficiency, similar to that obtained with ITO
Relation of cyclotron resonant energy and luminosity in a strongly magnetized neutron star GRO J1008-57 observed by Insight-HXMT
Cyclotron line scattering features are detected in a few tens of X-ray
pulsars (XRPs) and used as direct indicators of a strong magnetic field at the
surface of accreting neutron stars (NSs). In a few cases, cyclotron lines are
known to be variable with accretion luminosity of XRPs. It is accepted that the
observed variations of cyclotron line scattering features are related to
variations of geometry and dynamics of accretion flow above the magnetic poles
of a NS. A positive correlation between the line centroid energy and luminosity
is typical for sub-critical XRPs, where the accretion results in hot spots at
the magnetic poles. The negative correlation was proposed to be a specific
feature of bright super-critical XRPs, where radiation pressure supports
accretion columns above the stellar surface. Cyclotron line in spectra of
Be-transient X-ray pulsar GRO J1008-57 is detected at energies from keV, the highest observed energy of cyclotron line feature in XRPs. We
report the peculiar relation of cyclotron line centroid energies with
luminosity in GRO J1008-57 during the Type II outburst in August 2017 observed
by Insight-HXMT. The cyclotron line energy was detected to be negatively
correlated with the luminosity at 3.2\times 10^{37}\,\ergs, and positively correlated at L\gtrsim 5\times
10^{37}\,\ergs. We speculate that the observed peculiar behavior of a
cyclotron line would be due to variations of accretion channel geometry.Comment: 9 pages. 5 figures, 2 tables, accept for publication in Ap
Reanalysis of the X-ray burst associated FRB 200428 with Insight-HXMT observations
A double-peak X-ray burst from the Galactic magnetar SGR J1935+2154 was
discovered as associated with the two radio pulses of FRB 200428 separated by
28.97+-0.02 ms. Precise measurements of the timing and spectral properties of
the X-ray bursts are helpful for understanding the physical origin of fast
radio bursts (FRBs). In this paper, we have reconstructed some information
about the hard X-ray events, which were lost because the High Energy X-ray
Telescope (HE) onboard the Insight-HXMT mission was saturated by this extremely
bright burst, and used the information to improve the temporal and spectral
analyses of the X-ray burst. The arrival times of the two X-ray peaks by
fitting the new Insight-HXMT/HE lightcurve with multi-Gaussian profiles are
2.77+-0.45 ms and 34.30+-0.56 ms after the first peak of FRB 200428,
respectively, while these two parameters are 2.57+-0.52 ms and 32.5+-1.4 ms if
the fitting profile is a fast rise and exponential decay function. The spectrum
of the two X-ray peaks could be described by a cutoff power-law with cutoff
energy ~60 keV and photon index ~1.4, the latter is softer than that of the
underlying bright and broader X-ray burst when the two X-ray peaks appeared.Comment: 11 pages, 7 figure
Multistage Random Growing Small-World Networks with Power-law degree Distribution
In this paper, a simply rule that generates scale-free networks with very
large clustering coefficient and very small average distance is presented.
These networks are called {\bf Multistage Random Growing Networks}(MRGN) as the
adding process of a new node to the network is composed of two stages. The
analytic results of power-law exponent and clustering coefficient
are obtained, which agree with the simulation results approximately.
In addition, the average distance of the networks increases logarithmical with
the number of the network vertices is proved analytically. Since many real-life
networks are both scale-free and small-world networks, MRGN may perform well in
mimicking reality.Comment: 3 figures, 4 page
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