50 research outputs found
The inner disk radius in the propeller phase and accretion-propeller transition of neutron stars
We have investigated the critical conditions required for a steady propeller
effect for magnetized neutron stars with optically thick, geometrically thin
accretion disks. We have shown through simple analytical calculations that a
steady-state propeller mechanism cannot be sustained at an inner disk radius
where the viscous and magnetic stresses are balanced. The radius calculated by
equating these stresses is usually found to be close to the conventional Alfven
radius for spherical accretion, r_A. Our results show that: (1) a steady
propeller phase can be established with a maximum inner disk radius that is at
least \sim 15 times smaller than r_A depending on the mass-flow rate of the
disk, rotational period and strength of the magnetic dipole field of the star,
(2) the critical accretion rate corresponding to the accretion-propeller
transition is orders of magnitude lower than the rate estimated by equating r_A
to the co-rotation radius. Our results are consistent with the properties of
the transitional millisecond pulsars which show transitions between the
accretion powered X-ray pulsar and the rotational powered radio pulsar states.Comment: 6 pages, accepted for publication in MNRA
Central Compact Objects: some of them could be spinning up?
Among confirmed central compact objects (CCOs), only three sources have
measured period and period derivatives. We have investigated possible
evolutionary paths of these three CCOs in the fallback disc model. The model
can account for the individual X-ray luminosities and rotational properties of
the sources consistently with their estimated supernova ages. For these
sources, reasonable model curves can be obtained with dipole field strengths
a few G on the surface of the star. The model curves
indicate that these CCOs were in the spin-up state in the early phase of
evolution. The spin-down starts, while accretion is going on, at a time yr depending on the current accretion rate, period and the
magnetic dipole moment of the star. This implies that some of the CCOs with
relatively long periods, weak dipole fields and high X-ray luminosities could
be strong candidates to show spin-up behavior if they indeed evolve with
fallback discs.Comment: 5 pages, 3 figures, accepted for publication in MNRA
On the Outbursts of Soft X-ray Transients
We suggest a new scenario to explain the outburst light curves of black hole
soft X-ray transients together with the secondary maximum and the bump seen on
their decay phases. Our explanations are based on the disk instability models
considering the effect of X-ray irradiation. The scenario is consistent with
the observed X-ray delays by a few days with respect to the optical rise for
both the main outburst and the later maxima. We test our scenario by
numerically solving the disk diffusion equation. The obtained model curve fits
well to the observed X-ray outburst photon flux curve of the black hole soft
X-ray transient GS/GRS 1124-68, a typical representative of the four BH SXTs
with very similar light curves.Comment: 11 pages, 8 figures, accepted for publication in A&
Long-term evolution of anomalous X-ray pulsars and soft gamma repeaters
We have investigated the long-term evolution of individual anomalous X-ray
pulsars (AXPs) and soft gamma repeaters (SGRs) with relatively well constrained
X-ray luminosity and rotational properties. In the frame of the fallback disc
model, we have obtained the ranges of disc mass and dipole field strength that
can produce the observed source properties. We have compared our results with
those obtained earlier for dim isolated neutron stars (XDINs). Our results show
that (1) the X-ray luminosity, period and period derivative of the individual
AXP/SGR sources can be produced self-consistently in the fallback disc model
with very similar basic disc parameters to those used earlier in the same model
to explain the long-term evolution of XDINs, (2) except two sources, AXP/SGRs
are evolving in the accretion phase; these two exceptional sources, like XDINs,
completed their accretion phase in the past and are now evolving in the final
propeller phase and still slowing down with the disc torques, (3) the dipole
field strength (at the poles) of XDINs are in the G range,
while AXP/SGRs have relatively strong dipole fields between G, and (4) the source properties can be obtained with large ranges of
disc masses which do not allow a clear test of correlation between disc masses
and the magnetic dipole fields for the whole AXP/SGRs and XDIN population.Comment: 10 pages, 6 figures, Accepted for publication in MNRA
X-ray outbursts of AXPs and SGRs
We show that the X‐ray enhancement light curves of transient AXP∕SGRs can be reproduced by the active fallback disk model. We solve the diffusion equation for the relaxation of a disk that has been pushed back by a soft gamma‐ray burst. Our preliminary results indicate that a critical temperature around 1500 K leads to a thermal‐viscous instability in the fallback disks of all AXP∕SGRs. The effect of the instability on the light curves are different for transient and persistent sources due to different pre‐burst disk conditions in these systems
Fallback disks, magnetars and other neutron stars
The presence of matter with angular momentum, in the form of a fallback disk around a young isolated neutron star will determine its evolution. This leads to an understanding of many properties of different classes of young neutron stars, in particular a natural explanation for the period clustering of AXPs, SGRs and XDINs. The spindown or spinup properties of a neutron star are determined by the dipole component of the magnetic field. The natural
possibility that magnetars and other neutron stars may have different strengths of the dipole and higher multipole components of the magnetic field is now actually required by observations on the spindown rates of some magnetars. This talk gives a broad overview and some applications of the fallback disk model to particular neutron stars. Salient points are: (i) A fallback disk has already been observed around the AXP 4U 0142+61 some years ago. (ii) The low observed
spindown rate of the SGR 0418+5729 provides direct evidence that the dipole component of the field is in the 1012G range. All properties of the SGR 0418+5729 at its present age can be explained by spindown under torques from a fallback disk. (iii) The anomalous braking index of PSR J1734-3333 can also be explained by the fallback disk model which gives the luminosity, period, period derivative and the period second derivative at the present age. (iv) These and all applications to a variety of other sources employ the same disk physics and evolution, differing only in the initial conditions of the disk
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&
Hayvan Sağlığında Son Gelişmeler
Bilindiği üzere 14 Haziran 2012 tarihinde yayınlanan EC 505 /2012 sayılı Direktif doğrultusunda ulusal mevzuatımızda yapılan değişiklikler 24 Mayıs 2013 tarihinde Resmi Gazetede yayınlanarak yürürlüğe girmiştir.
Yeni yönetmelik ile önerilen Fitoterapötik ürünler 17 Aralık 2011 tarihinde yayınlanarak yürürlüğe giren “Tıbbi olmayan veteriner sağlık ürünleri “ yönetmeliğine göre, Bakanlıkça ruhsatlandırılmaktadır. Adı geçen Yönetmelik çerçevesinde Temmuz 2013 tarihi itibariyle atlar, pet hayvanları ve gevişenler de kullanılmak üzere toplam 11 ürün ruhsatlandırılmış, olup, bu ürünler reçeteye tabi değildir.
Homeopatik ürünlerin dâhil olduğu Veteriner tıbbi ürünler hakkındaki yönetmelik 24.12.2011 tarihinde yayınlanarak yürürlüğe girmiş olup, reçeteye tabidir.
Bakanlıkça Temmuz 2013 tarihi itibariyle toplam 17 homeopatik ürün ruhsatlandırılmıştır
On the Enhanced X-ray Emission from SGR 1900+14 after the August 27 Giant Flare
We show that the giant flares of soft-gamma ray repeaters (E~10^44 ergs) can
push the inner regions of a fall-back disk out to larger radii by radiation
pressure, while matter remains bound to the system for plausible parameters.
The subsequent relaxation of this pushed-back matter can account for the
observed enhanced X-ray emission after the August 27 giant flare of SGR
1900+14. Based on the results of our models, we estimate that the ratio of the
fluences of the enhanced X-ray emissions to that of the preceding bursts
remains constant for a particular SGR with similar preburst inner disk
conditions, which is consistent with the four different burst observations of
SGR 1900+14.Comment: 13 pages, 3 figures, accepted for publication in ApJ