494 research outputs found
Spectral formation in a radiative shock: application to anomalous X-ray pulsars and soft gamma-ray repeaters
In the fallback disk model for the persistent emission of Anomalous X-ray
pulsars (AXPs) and soft gamma-ray repeaters (SGRs), the hard X-ray emission
arises from bulk- and thermal Comptonization of bremsstrahlung photons, which
are generated in the accretion column. The relatively low X-ray luminosity of
these sources implies a moderate transverse optical depth to electron
scattering, with photons executing a small number of shock crossings before
escaping sideways. We explore the range of spectral shapes that can be obtained
with this model and characterize the most important parameter dependencies. We
use a Monte Carlo code to study the crisscrossing of photons in a radiative
shock in an accretion column and compute the resulting spectrum. As expected,
high-energy power-law X-ray spectra are produced in radiative shocks with
photon-number spectral index larger than or about 0.5. We find that the
required transverse optical depth is between 1 and 7. Such spectra are observed
in low-luminosity X-ray pulsars. We demonstrate here with a simple model that
Compton upscattering in the radiative shock in the accretion column can produce
hard X-ray spectra similar to those seen in the persistent and transient
emission of AXPs and SGRs. In particular, one can obtain a high-energy
power-law spectrum, with photon-number spectral index ~ 1 and a cutoff at 100 -
200 keV, with a transverse Thomson optical depth of ~ 5, which is shown to be
typical in AXPs/SGRs.Comment: Accepted for publication in A&
On the evolution of the radio pulsar PSR J1734−3333
Recent measurements showed that the period derivative of the ‘hig
h-B’ radio pulsar PSR J1734−3333 is increasing with time. For neutron stars evolving with fallback disks, this rotational behavior is expected in certain phases of the long-term evolution. Using the same model as employed earlier to explain the evolution of anomalous X-ray pulsars and soft gamma-ray repeaters, we show that the period,the first and second period derivatives and the X-ray luminosity of this source can simultaneously acquire the observed values for a neutron star evolving with a fallback disk. We find that the required strength of the dipole field that can produce the source properties is in the range of 10^12 − 10^13 G on the pole of the neutron star. When the model source
reaches the current state properties of PSR J1734−3333, accretion onto the star has not started yet, allowing the source to operate as a regular radio pulsar. Our results imply that PSR J1734−3333 is at an age of ∼3×10^4 −2×10^5years. Such sources will have properties like the X-ray dim isolated neutron stars or transient AXPs at a later epoch of weak accretion from the diminished fallback disk
The Energy Spectrum of Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters
Assuming that AXPs and SGRs accrete matter from a fallback disk, we attempt
to explain both the soft and the hard X-ray emission as the result of the
accretion process. We also attempt to explain their radio emission or the lack
of it. We test the hypothesis that the power-law, hard X-ray spectra are
produced in the accretion flow mainly by bulk-motion Comptonization of soft
photons emitted at the neutron star surface. Fallback disk models invoke
surface dipole magnetic fields of G, which is what we
assume here. Unlike normal X-ray pulsars, for which the accretion rate is
highly super-Eddington, the accretion rate is approximately Eddington in AXPs
and SGRs and thus the bulk-motion Comptonization operates efficiently. As an
illustrative example we reproduce both the hard and the soft X-ray spectra of
AXP 4U 0142+61 well using the XSPEC package compTB. Our model seems to explain
both the hard and the soft X-ray spectra of AXPs and SGRs, as well as their
radio emission or the lack of it, in a natural way. It might also explain the
short bursts observed in these sources. On the other hand, it cannot explain
the giant X-ray outbursts observed in SGRs, which may result from the
conversion of magnetic energy in local multipole fields.Comment: 6 pages, 2 figures, minor corrections, accepted for publication in
A&
Electrodeposition of Nickel Nanowires and Nanotubes Using Various Templates
Nickel nanotubes and nanowires are grown by galvanostatic electrodeposition in the pores of 1000, 100, and 15 nm polycarbonate as well as in anodised alumina membranes at a current density of 10 mA cm-2. The effects of pore size, porosity, electrodeposition time, effective current density, and pore aspect ratio are investigated. Nickel nanotube structures are obtained with 1000 nm pore size polycarbonate membrane without any prior treatment method. At the early stages of electrodeposition hollow nickel nanotubes are produced and nanotubes turn into nanowires at longer depositon times. As effective current density accounting for the membrane porosity decreases, the axial growth direction is favoured yielding nanowires rather than nanotubes. However, for smaller pore size polycarbonate membranes, nanowires are obtained even though effective current densities were higher. We believe that when the pore diameter is below a critical size, nanowires grow regardless of current density since narrow pores promote layer by layer growth of nanorods due to smaller surface area of the pore bottom compared to pore walls. Pore size has a dominant effect over effective current density in determining the structure of the fibres produced for small pores. Nickel nanowires are also obtained in the small pores of anodised alumina, which has higher aspect ratios. High aspect ratio membranes favour the fabrication of nanowires regardless of current density
Electrodeposition of Nickel Nanowires and Nanotubes Using Various Templates
Nickel nanotubes and nanowires are grown by galvanostatic electrodeposition in the pores of 1000, 100, and 15 nm polycarbonate as well as in anodised alumina membranes at a current density of 10 mA cm-2. The effects of pore size, porosity, electrodeposition time, effective current density, and pore aspect ratio are investigated. Nickel nanotube structures are obtained with 1000 nm pore size polycarbonate membrane without any prior treatment method. At the early stages of electrodeposition hollow nickel nanotubes are produced and nanotubes turn into nanowires at longer depositon times. As effective current density accounting for the membrane porosity decreases, the axial growth direction is favoured yielding nanowires rather than nanotubes. However, for smaller pore size polycarbonate membranes, nanowires are obtained even though effective current densities were higher. We believe that when the pore diameter is below a critical size, nanowires grow regardless of current density since narrow pores promote layer by layer growth of nanorods due to smaller surface area of the pore bottom compared to pore walls. Pore size has a dominant effect over effective current density in determining the structure of the fibres produced for small pores. Nickel nanowires are also obtained in the small pores of anodised alumina, which has higher aspect ratios. High aspect ratio membranes favour the fabrication of nanowires regardless of current density
On the X-Ray Light Curve, Pulsed-Radio Emission, and Spin Frequency Evolution of the Transient Anomalous X-Ray Pulsar Xte J1810--197 During its X-Ray Outburst
We show that: (i) the long-term X-ray outburst light curve of the transient
AXP XTE J1810-197 can be accounted for by a fallback disk that is evolving
towards quiescence through a disk instability after having been heated by a
soft gamma-ray burst, (ii) the spin-frequency evolution of this source in the
same period can also be explained by the disk torque acting on the
magnetosphere of the neutron star, (iii) most significantly, recently observed
pulsed-radio emission from this source coincides with the epoch of minimum
X-ray luminosity. This is natural in terms of a fallback disk model, as the
accretion power becomes so low that it is not sufficient to suppress the beamed
radio emission from XTE J1810-197.Comment: 13 pages, 2 Figures, accepted for publication in Ap
X-ray enhancement and long-term evolution of swift J1822.3-1606
We investigate the X-ray enhancement and the long-term evolution of the recently discovered second "low-B magnetar" Swift J1822.3-1606 in the frame of the fallback disk model. During a soft gamma burst episode, the inner disk matter is pushed back to larger radii, forming a density gradient at the inner disk. Subsequent relaxation of the inner disk could account for the observed X-ray enhancement light curve of Swift J1822.3-1606. We obtain model fits to the X-ray data with basic disk parameters similar to those employed to explain the X-ray outburst light curves of other anomalous X-ray pulsars and soft gamma repeaters. The long period (8.4 s) of the neutron star can be reached by the effect of the disk torques in the long-term accretion phase ((1-3) x 10(5) yr). The currently ongoing X-ray enhancement could be due to a transient accretion epoch, or the source could still be in the accretion phase in quiescence. Considering these different possibilities, we determine the model curves that could represent the long-term rotational and the X-ray luminosity evolution of Swift J1822.3-1606, which constrain the strength of the magnetic dipole field to the range of (1-2) x 10(12) G on the surface of the neutron star
Effect of Synthesis Time and Treatment on Porosity of Mesoporous Silica Materials
Nitrogen adsorption at 77 K on mesoporous silica materials (MPS) with varying synthesis time and treatment conditions was investigated. Scanning electron microscope (SEM) and X-ray diffraction (XRD) were also used to characterize the mesoporous materials. This study was performed at 6, 24 and 72-h synthesis times. It is shown that 6-h is not enough for complete formation of the MPS material and at least 24-h is necessary. The pore structure starts decaying for the 72-h synthesis time. The three-after-synthesis treatment conditions used were 1) washed, 2) washed and calcined and 3) directly calcined after synthesis. Ethanol/HCl mixtures were used for washing and calcinations were performed at 550°C. Among these samples, directly washed sample yields the lowest adsorption capacity while washed and calcined sample yields the highest adsorption capacity. Hence, it is concluded that washing stabilizes the structure before high temperature treatment
Effect of Synthesis Time and Treatment on Porosity of Mesoporous Silica Materials
Nitrogen adsorption at 77 K on mesoporous silica materials (MPS) with varying synthesis time and treatment conditions was investigated. Scanning electron microscope (SEM) and X-ray diffraction (XRD) were also used to characterize the mesoporous materials. This study was performed at 6, 24 and 72-h synthesis times. It is shown that 6-h is not enough for complete formation of the MPS material and at least 24-h is necessary. The pore structure starts decaying for the 72-h synthesis time. The three-after-synthesis treatment conditions used were 1) washed, 2) washed and calcined and 3) directly calcined after synthesis. Ethanol/HCl mixtures were used for washing and calcinations were performed at 550°C. Among these samples, directly washed sample yields the lowest adsorption capacity while washed and calcined sample yields the highest adsorption capacity. Hence, it is concluded that washing stabilizes the structure before high temperature treatment
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