Browsing by Author "Wetro, Legesse (PhD)"

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    2d Geometry of Quadrapole Magnetic field Lines from Neutron Star and Associated Radiation Pressure
    (Addis Ababa University, 2011-06) Girma, Melkameshet; Wetro, Legesse (PhD)
    In this thesis we have derived the vector potential to determine the analytic expressions for the magnetic eld of dipole and quadrupole components of neutron star, we develop the eld line equations and the magnetic eld line geometry of both dipole and quadrupole terms. The magnetic eld varies with time and as well as the induced electric eld. As a result the neutron star can generate electromagnetic radiation. This electromagnetic radiation has pressure. We have derived the radiation pressure at any distance r out side of the surface of the neutron star
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    Braking index of Isolated Pulsars According to the Relativistic Plasma Diffusion theory for Pulsar fields
    (Addis Ababa University, 2005-06) Tadesse, Tilaye; Wetro, Legesse (PhD)
    The braking index of a pulsar is expected to be 3 if it slows down under ordinary magnetic dipole radiation . However, measured braking indices of young as well as old pulsars are experimentally determined to be different from the above value. Sometimes these get to large positive and negative numbers. This is something that is not well understood. We will describe pulsar braking index variations based on Relativistic plasma diffusion theory which is currently developed by us. Using this theory we will calculate the braking indices of four-young well known pulsars and two old pulsars. It is also important to establish whether the observed(measured ) braking index riots is time dependent . We will also derive braking indices of three pulsars as a function of time and draw the graphs to show the time evolution of braking index
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    The Contribbution of Coulombic Pressure to the Stability of thin Keplerian Accretion Discs Around a Neutron Star with Axisymmetric Magnetic Dipole
    (Addis Ababa University, 2013-06) Jemal, Muktar; Wetro, Legesse (PhD)
    Based on the contribution of intraction between various charged components of the system the stability analysis of an axisymmetric geometrically thin and optically thick accretion disk around a magnetized neutron star is presented.The disk we consider is modi ed by including the e ect of coulombic pressure on the total pressure.We consider three region of the disks outer, middle and inner region. The outer and middle region of the disk is dominated by gas pressure and the inner region is dominated by radiation pressure. The opacity in the middle and inner region is mainly due to electron scattering whereas that in the outer region is mainly due to free-free emission. Starting from the vertically integrated non-relativistic hydrodynamics equations we set up the basic equations which govern the structure of the disc and for the stability analysis of the disc model we have kept the time dependencies in the equations. Even if we include coulombic pressure We nd that the gas pressure dominated region is thermally stable.The graph of stability parameter as afunction of the disk radius shows that the inner region of the disc is viscously unstable
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    The Contribution of Coulombic Pressure to the Stability of thin Keplerian Accretion Discs around a Neutron Star with Axisymmetric Magnetic Dipole
    (Addis Ababa University, 2011-06) Hagos, Gereziher; Wetro, Legesse (PhD)
    We determine the stability of an axisymmetric geometrically thin and optically thick accretion disc around a magnetized neutron star in the region where gas pressure is dominant. Analysis is made with the inclusion of pressure from coulombic sources. The opacity in the middle region is mainly due to electron scattering whereas that in the outer region is mainly due to free-free emission. Starting from the vertically integrated non-relativistic hydrodynamics equations we set up the basic equations which govern the structure of the disc and for the stability analysis of the disc model we have kept the time dependencies in the equations. Although we include the e ect of coulombic pressure it is thermally stable as if there were gas pressure only and our graphical solution given at the end of this manuscript shows that the disc is viscously unstable
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    Contribution of Gravitational and Magnetic Dipole Radiations to the Ellipticity of Pulsars
    (Addis Ababa University, 2005-06) Kassa, Netsanet; Wetro, Legesse (PhD)
    Accurate and reliable calculation of the ellipticity parameter is very important in order to theoretically study the gravitational waves emitted by pulsars which is a hot research area in the field astrophysics. Ellipticity is a result of lack in symmetry of spherical bodies. In the case of pulsars rotation and magnetic fields are the causes of deformation. According to relativistic plasma diffusion model for neutron star magnetic fields, the surface magnetic field of neutron stars depends on their rotation frequency. Hence it can be deduced that the parameter ellipticity depends on the rotational frequency. But gravitational and magnetic dipole radiations affect the rotational frequency of pulsars which in turn will also affect the parameter ellipticity. In this work we will estimate the contributions of gravitational and dipole radiations to the parameter ellipticity
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    Contribution of Magnetic Stress Energy to Supernova Bounce
    (Addis Ababa University, 2010-06) Teweldebirhan, Kinfe; Wetro, Legesse (PhD)
    In this paper, we have analyzed the contribution of the magnetic stress energy to the supernova bounce. A calculation of Maxwell stress tensor is proposed when leads to the expression of the magnetic force density. By making the link between the the magnetic force density and the Maxwell stress tensor we derived the magnetic pressure, required to ensure core stability or support the star from gravitational collapse, we calculated the associated eld stranght at the surface of the compact object (the neutron star- NS) to be B & 1018 G and showed that this eld is exert a pressure Pmag 1035 g cms2 which is able to cause supernova bounce of the infalling stellar material
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    The Contribution of Radiation Pressure to the Stability of a Standard thin Keplerian Accretion Disk around a Neutron Star with Axisymmetric Magnetic Field Dipole
    (Addis Ababa University, 2011-06) G/Mariam, Haftamu; Wetro, Legesse (PhD)
    In this thesis, we have studied the dynamic properties of an accretion disk formed from the in ow of plasma from a blotted out companion star. The disk extends from an inner radius of 106m ( RA Alfeven radius) to 100RA (see gure 1.4). We have divided the disk into three regions: an outer region dominated by gas pressure and free-free opacity, a middle region dominated by gas pressure and electron scattering, and an inner region dominated by radiation pressure and electron scattering. We have also derived the radiation pressures and gas pressure in the inner region of the disk as a function of "r" using radial dependence of the central temperature and the density. The latter was obtained using the basic equations for thin accretion in non-relativistic case . Analyses of the instability of the disk is made between RA and 10RA (RA < r < 10RA) based on the instability condition ( o > 3=5). This is occurred at high temperature, at which the opacity is dominated by electron scattering and radiation pressure is strong
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    The Dynamics of Accretion Disks around Compact Stars with Complex Magnetic Fields
    (Addis Ababa University, 2014-04) Kare, Anno; Wetro, Legesse (PhD)
    Strongly magnetized stellar objects have magnetospheres characterized by such activities that define the geometry of the inner edge of the disk as well as control the inflow of matter to the NS surface itself. Of all possible components of the surface magnetic field of the central object (neutron star-NS), we have only considered the quadrupole term to investigate what ever is going on nearer to the surface of the NS. There is a highly important difference of the accretion flow in a quadrupolar and dipolar magnetic fields. The dipolar magnetic field will in the end always present a barrier to the accretion flow since the field lines are perpendicular to the plane of the disk, but the quadrupolar magnetic field will in the simplest case lie in the plane of the disk, and thus it will rather channel the accretion flow all the way down to the stellar equator. This work involves a mathematical treatment of an accretion disk around a magnetized star. In order to define the disk structure magnetohydrodynamic (MHD) equations are solved in cylindrical coordinates. For the detailed results an ordinary differential equation (ODE) derived from the angular momentum equation is numerically solved. So, both Keplerian and non-Keplerian cases of thin accretion disk are solved. Further, introductory work on slim disk is included as a part of this work. The results of our analysis indicate the existence of two different regions: a super-Keplerian innermost region and a broader sub-Keplerian outer region. The effects of stellar and toroidal magnetic fields on the variations of viscosity, temperature and density have also been studied. We have identified the nature of the inner portion of an accretion disk. The velocity of the transition varies from corotating magnetospheric boundary to super-Keplerian for low density inner most portion of accretion disk, that extends from 0:5RM to the peak and then to sub-Keplerian. Our results are applicable to accreting astrophysical systems such viii ix as neutron stars (NSs) and white dwarfs (WDs). It can also explain observational results not yet fully backed with theories
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    Gamma Ray Production in an Accreting Neutron Stars
    (Addis Ababa University, 2007-07) Reshid, Remudin; Wetro, Legesse (PhD)
    Inverse compton (IC) scattering of lower frequency photons by relativistic electrons was used as a method for the production of high-energy gamma-rays in an accreting neutron stars. A spectrum function is derived.Based on which we obtained two new results. First there is a maximum scattered photon energy for a given resonant scattering which depends on both the incident electron energy and the magnetic field, but is independent of the incident photon energy. It is also found that the common upper limit of the scattered photon energy is the highest scattered photon energy for any given incident photon and electron energy. Thus this may have physical implications for Gamma-ray production
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    Inverse Beta-Decay of Polarized Neutron in Curved Spacetime in the Presence of Strong Magnetic Field
    (Addis Ababa University, 2014-06) Tibesso, Tahir; Wetro, Legesse (PhD)
    We calculate the cross section of the inverse beta decay process, e+n ! p+e􀀀 in curved spacetime in the presence of strong magnetic eld and.Using exact solutions of the Dirac equation in a curved space-time and Spinors of the Dirac Equation and its solution In Robertson-Walker Spacetime, we nd the cross section for arbitrary polarization of the initial neutrons.The magnetic eld might provide a net polarization of the neutrons, which we take into account
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    Inverse Compton Scattering in Curved Spacetime in the Presence of Strong Magnetic Field
    (Addis Ababa University, 2014-09) Teka, Demisew; Wetro, Legesse (PhD)
    We calculate the scattering cross section for inverse compton scattering in curved spacetime in the presence of strong magnetic eld. An approximate solution of Dirac equation in a spatially at RW spacetime has been established. And its solution in Friedman universe is also discussed. The scattering cross section we found contains the expanision parameter of the expanding universe and this makes main di erence from the special relativistic results. We investigate the role of the expanission parameter to the scattering cross section of the inverse compton scattering
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    Inverse Compton Scattering in strong Magnetic Field: a Possible Mechanism for Hard x-Ray production from Accreting Neutron stars
    (Addis Ababa University, 2007-07) Mekonnen, Getachew; Wetro, Legesse (PhD)
    We began by considering Dirac equation to derive the inverse compton scattering cross section in strong magnetic field and this cross section has simple form for the scattering of a low frequency photon with relativistic electron. We also derived the spectrum function from this cross section,which has a direct application on the hardening of thermal photons(soft x-rays)through inverse compton scattering by relativistic electron beams on the surface of strongly magnetized neutron stars to get the highest scattered photon energy of magnitude: !f = 4 2!i for the incident photon angle i = and scattered photon angle f = 0 which can give hard x-ray spectrum in the energy range of 12KeV-50MeV by considering soft X-ray as incident photon and to get this spectrum the electron energy for pulsars(rotating Neutron stars)is considered as well. And we showed the application of Inverse compton scattering in X-ray Astronomy during Roche Lobe overflow and formation of Accretion disc in the compact object in our case(Strongly magnetized Neutron stars)
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    Landau Number to the Dirac Solution in Curved Space Time
    (Addis Ababa University, 2013-06) Tilahun, Negessa; Wetro, Legesse (PhD)
    In this paper we study the Landau number arising within the relativistic dynamics of a neutral particle which possesses a permanent magnetic dipole moment interacting with an external electric eld in the curved spacetime background. The eigenfunction and eigenvalues of the Hamiltonian are obtained. We show that the presence of the torsion eld breaks the in nite degeneracy of the Landau levels arising in this system
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    Lifetime Estimate of a Neutron Star due to Gravitational Radiation Reaction
    (Addis Ababa University, 2009-07) Shiferaw, Tadele; Wetro, Legesse (PhD)
    A rotating neutron star/pulsar can emit gravitational radiation.The third time derivative of quadrupole moment of of an isolated systems must be non-zero in order for it to emit gravitational radiation. We follow the emission of gravitational radiation using quadrupole approximation. The energy and angular momentum of a rotating neutron star can slowly decrease as it get older. We estimate the average lifetime of a gravitational wave damping for a neutron star wobble.Our calculation is based on the well known idea that energy loss within an isolated wobbling neutron star causes the axis of inertia of the star to align with its angular momentum vector. We model the neutron star as a rigid body with quadrupled deformation of its moment of inertia tensor.We find out the numerical lifetime estimate of a neutron star with two different dervations of the result: one is by adding the Burke-Thorne radiation reaction potential to the newtonian equation of motion and the other is based on energy and angular momentum balance Key words: neutron star, quadrupole moment, gravitational radiation, angular momentum, lifetime of neutron star
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    Magnetic Dipole-Quadrupole Interaction field of Neutron Star
    (Addis Ababa University, 2013-06) Muleta, Gemechu; Wetro, Legesse (PhD)
    Pulsurs are fast spinning ( 104 Hz) and highly magnetized (1013􀀀14G) neutron stars(NSs), compact objects (R 10 km; 1013􀀀14g=cm3) resulting from supernova explosions that normally occur at the ends of the lives of massive stars.Measurements of electromagnetic emission related to curvature radiation from known pulsurs such as Crab or Vela pulsur strongly indicate that the mag- netic eld structure near the surface of these objects sigini cantly di er from the customarily expected pure star centered dipole structure.Scientists now believe that possible non-dipolar components in the magnetic eld structure will resolve this long time issue.The structure of magnetic eld lines also de nes the type and geometry of accretion discs around neutron star in binaries.In this thesis we have determined the geometry of the combined dipole and quadrupole magnetic eld lines. In idealized situations the eld lines near these compact objects are closed; but they may be open at large distances due to interactions with magnetic elds or stress energy from other sources such as the accretion discs, for example.The method we used is simulating eld line with matlab.The result shows the eld lines are open at a given distance from the surface of the star (at rmax) and it has only one solution of r( ) for a speci c angle in one quadrant.There are three neutral lines for r( ) and the eld line is more bulged than the pure dipole and quadrupole
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    Neutrino Emission and Thermal History of Neutron Stars
    (Addis Ababa University, 2010-07) Lemi, Tufa; Wetro, Legesse (PhD)
    In this work we have examined the formation of neutron stars (NSs) in the core of a supernova explosion during the death of massive stars, neutrino emission mechanisms in the interior of NSs under di erent assumptions on their internal structure, and the thermal history of NSs by determining neutrino luminosities and the temperature as a function of time. The emphasis is made on new results such as modi ed Urca process in a NS core with spherical nuclear structures since both momentum and energy are conserved in these reactions. Neutrino emission from NS cores containing exotic phases of matter (pion or kaon condensates,quarks) is also outlined. The e ects of neutrino emission on thermal histories of NSs , particularly cooling isolated stars, are examined
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    Neutrino Transport in Strongly Magnetized Proto-Neutron Stars and the Origin of fast Spinning Pulsar Kicks
    (Addis Ababa University, 2013-06) Teklehaimanot, Tesfaye; Wetro, Legesse (PhD)
    The Fermi LAT collaboration has recently reported the discovery of the pul- sations of the -ray pulsar J1823-3021A with a luminosity which is the highest observed to date for any millisecond pulsar (MSP). This large luminosity implies a large spin down rate _P and therefore a large magnetic eld which seems to be incompatible with the observed short rotation period P . In proto-neutron stars with strong magnetic elds, the cross section for e ( e) absorption on neutrons (protons) depends on the local magnetic eld strength resulting from the quantiza- tion of energy levels for the e􀀀 (e+) produced in the nal state. If the neutron star possesses an asymmetric magnetic eld topology in the sense that the magnitude of magnetic eld in the north pole is di erent from that in the south pole, then asymmetric neutrino emission may be generated. We calculate the absorption cross sections of e and e in strong magnetic elds as a function of the neutrino energy. These cross sections exhibit oscillatory behaviors that occur because new Landau levels for the e􀀀 (e+) become accessible as the neutrino energy increases. By evaluating the appropriately averaged neutrino opacities, we demonstrate that the change in the local neutrino ux caused by the modi ed opacities is rather small. To generate appreciable kick velocity ( 300 km s􀀀1) to the newly formed neutron star, the di erence between the eld strengths at the two opposite poles of the star must be at least 1016 G. We also consider the magnetic eld e ect on the spectral neutrino energy uxes. The oscillatory features in the absorption opacities give rise to modulations in the emergent spectra of e and e
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    Non-Dipolar Components of Neutron Star Magnetic Fields
    (Addis Ababa University, 2007-07) Kifle, Mekuanint; Wetro, Legesse (PhD)
    Due to the existence of both observational and theoretical evidences for the presence of non-dipolar NS magnetic fields, it has become more and more clear that neutron stars posses magnetic field structures, which are much more complicated than the simple assumption of a dipolar neutron star (NS) magnetic field. This thesis intends to show the presence of non-dipolar component of the NS magnetic fields as derived from a recent model by kebede[5]. We will derive both the dipolar and non dipolar components of the field separately. We approach the problem by applying Post-Newtonian approximation, to find the angle dependent charge distribution on the surface of the neutron star, which results in the non dipolar components of the NS magnetic field
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    Non-Dipolar Components of Pulsar Magnetic Fields
    (Addis Ababa University, 2010-07) Sado, Feyisso; Wetro, Legesse (PhD)
    The recent observations indicate that the presence of non-dipolar pulsars magnetic elds. Post-Newtonian approximation(PN) used in solving Einstein's eld equation in general relativity was made to derive the much more complicated non-dipolar pulsar magnetic field. In this work, we analyze the presence of non-dipolar components of the pulsar magnetic field, as derived from a recent model by Kebede[5]. We will derive both the dipolar and non dipolar components of the field. We consider the quadrupole and octopole fields, which result in the non dipolar components of the pulsar's magnetic field
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    Pulsar Alignment due to Radiation Reaction Torque
    (Addis Ababa University, 2010-07) Mohammed, Endris; Wetro, Legesse (PhD)
    We calculate the time of alignment of the magnetic dipole moment with the axis of rotation. For alignment we take into acount the electromagnetic torque acting on the magnetic dipole moment, the frictional torque between the crust and the core and gravitational effects. The anomalous electromagnetic torque, usually neglected in the a rigid star model, here plays a crucial role for the alignment of the magnetic dipole