Akshay Kulkarni - Academia.edu (original) (raw)
Papers by Akshay Kulkarni
Amoebic Liver Abscess (ALA) is a most common extraintestinal manifestation of amoebiasis which is... more Amoebic Liver Abscess (ALA) is a most common extraintestinal manifestation of amoebiasis which is most commonly present with high grade fever with right upper quadrant abdominal pain. Here we present a case of 32 year male patient newly diagnosed as People Living with HIV/AIDS (PLHIV) with ALA. Patient was treated with Metronidazole (500 mg 8 hourly) and therapeutic drainage.
International Journal of Contemporary Medicine, Surgery and Radiology, 2019
AIP Conference Proceedings, 2008
The appearance and time variability of accreting millisecond X-ray pulsars (hereafter AMXPs, e.g.... more The appearance and time variability of accreting millisecond X-ray pulsars (hereafter AMXPs, e.g. Wijnands & van der Klis 1998) depends strongly on the accretion rate, the effective viscosity and the effective magnetic diffusivity of the disk-magnetosphere boundary. The accretion rate is the main parameter which determines the location of the magnetospheric radius of the star for a given stellar magnetic field. We introduce a classification of accreting neutron stars as a function of the accretion rate and show the corresponding stages obtained from our global 3D magnetohydrodynamic (MHD) simulations and from our axisymmetric MHD simulations. We discuss the expected variability features in each stage of accretion, both periodic and quasi-periodic (QPOs). We conclude that the periodicity may be suppressed at both very high and very low accretion rates. In addition the periodicity may disappear when ordered funnel flow accretion is replaced by disordered accretion through the interchange instability.
AIP Conference Proceedings, 2008
We present results of 3D simulations of MHD instabilities at the accretion diskmagnetosphere boun... more We present results of 3D simulations of MHD instabilities at the accretion diskmagnetosphere boundary. The instability is Rayleigh-Taylor, and develops for a fairly broad range of accretion rates and stellar rotation rates and magnetic fields. It produces tall, thin tongues of plasma that penetrate the magnetosphere in the equatorial plane. The shape and number of the tongues changes with time on the inner-disk dynamical timescale. In contrast with funnel flows, which deposit matter mainly in the polar region, the tongues deposit matter much closer to the stellar equator. The instability appears for relatively small misalignment angles, Θ 30 • , between the star's rotation and magnetic axes, and is associated with higher accretion rates. The hot spots and light curves during accretion through instability are generally much more chaotic than during stable accretion. The unstable state of accretion has possible implications for quasi-periodic oscillations and intermittent pulsations from accreting systems.
We investigate the photometric variability of magnetized stars, particularly neutron stars, accre... more We investigate the photometric variability of magnetized stars, particularly neutron stars, accreting through a magnetic Rayleigh-Taylor-type instability at the disk-magnetosphere interface, and compare it with the variability during stable accretion, with the goal of looking for possible quasi-periodic oscillations. The lightcurves during stable accretion show periodicity at the star's frequency and sometimes twice that, due to the presence of two funnel streams that produce antipodal hotspots near the magnetic poles. On the other hand, lightcurves during unstable accretion through tongues penetrating the magnetosphere are more chaotic due to the stochastic behaviour of the tongues, and produce noisier power spectra. However, the power spectra do show some signs of quasi-periodic variability. Most importantly, the rotation frequency of the tongues and the resulting hotspots is close to the inner-disk orbital frequency, except in the most strongly unstable cases. There is theref...
We performed full 3D MHD simulations of disk accretion to a rotating star with a misaligned dipol... more We performed full 3D MHD simulations of disk accretion to a rotating star with a misaligned dipole magnetic field, with the magnetic axis of the star misaligned relative to its rotational axis at an angle Theta . We observed that even at very small misalignment angles matter accretes from the disk to the star through the streams, and forms hot spots at the surface of the star. The shape of the spots reflects the shape of the funnel streams and depends on Theta . At small Theta , they have a shape of a bow, which curves around the magnetic pole. At large Theta , the spots have a shape of an elongated bar, which crosses the magnetic pole. The variability curves were calculated for different misalignment angles and different inclination angles relative to the observer. The spots may rotate faster/slower than the star, thus leading to possible quasi-periodic oscillations. Such oscillations are probable during periods of fast, non-stationary accretion. This research was supported by NASA...
Proceedings of the International Astronomical Union, 2007
We discuss a number of topics relevant to disk-magnetosphere interaction and how numerical simula... more We discuss a number of topics relevant to disk-magnetosphere interaction and how numerical simulations illuminate them. The topics include: (1) disk-magnetosphere interaction and the problem of disk-locking; (2) the wind problem; (3) structure of the magnetospheric flow, hot spots at the star's surface, and the inner disk region; (4) modeling of spectra from 3D funnel streams; (5) accretion to a star with a complex magnetic field; (6) accretion through 3D instabilities; (7) magnetospheric gap and survival of protoplanets. Results of both 2D and 3D simulations are discussed.
3D Magnetohydrodynamic simulations show that when matter accretes onto neutron stars, in particul... more 3D Magnetohydrodynamic simulations show that when matter accretes onto neutron stars, in particular if the misalignment angle is small, it does not constantly fall at a fixed spot. Instead, the location at which matter reaches the star moves. These moving hot spots can be produced both during stable accretion, where matter falls near the magnetic poles of the star, and unstable accretion, characterized by the presence of several tongues of matter which fall on the star near the equator, due to Rayleigh-Taylor instabilities. Precise modeling with Monte Carlo simulations shows that those movements could be observed as high frequency Quasi Periodic Oscillations. We performed a number of new simulation runs with a much wider set of parameters, focusing on neutron stars with a small misalignment angle. In most cases we observe oscillations whose frequency is correlated with the mass accretion rateṀ. Moreover, in some cases double QPOs appear, each of them showing the same correlation withṀ.
Proceedings of the International Astronomical Union, 2007
We present results of 3D simulations of MHD instabilities at the accretion disk-magnetosphere bou... more We present results of 3D simulations of MHD instabilities at the accretion disk-magnetosphere boundary. The instability is Rayleigh-Taylor, and develops for a large range of parameter values. It manifests itself in the form of tall, thin tongues of plasma that reach the star by penetrating through the magnetosphere in the equatorial plane. The tongues rotate around the star in the equatorial plane, and their shape and number changes with time on inner-disk dynamical timescales. In contrast with funnel flows, which deposit matter mainly in the polar region, the tongues deposit matter much closer to the stellar equator.
The Astrophysical Journal, 2008
We report on the first global three-dimensional (3D) MHD simulations of disk accretion onto a rot... more We report on the first global three-dimensional (3D) MHD simulations of disk accretion onto a rotating magnetized star through the Rayleigh-Taylor instability. The star has a dipole field misaligned relative to the rotation axis by a small angle Θ. Simulations show that, depending on the accretion rate, a star may be in the stable or unstable regime of accretion. In the unstable regime, matter penetrates deep into the magnetosphere through several elongated "tongues" which deposit matter at random places on the surface of the star, leading to stochastic light-curves. In the stable regime, matter accretes in ordered funnel streams and the light-curves are almost periodic. A star may switch between these two regimes depending on the accretion rate and may thus show alternate episodes of ordered pulsations and stochastic light-curves. In the intermediate regime, both stochastic and ordered pulsations are observed. For Θ > 30 • , the instability is suppressed and stable accretion through funnel streams dominates.
The Astrophysical Journal, 2005
We model the variability profiles of millisecond-period X-ray pulsars. We performed three-dimensi... more We model the variability profiles of millisecond-period X-ray pulsars. We performed three-dimensional magnetohydrodynamic simulations of disk accretion to millisecond-period neutron stars with a misaligned magnetic dipole moment, using the pseudo-Newtonian Paczyński-Wiita potential to model general relativistic effects. We found that the shapes of the resulting funnel streams of accreting matter and the hot spots on the surface of the star are quite similar to those for more slowly rotating stars obtained from earlier simulations using the Newtonian potential. The funnel streams and hot spots rotate approximately with the same angular velocity as the star. The spots are bowshaped (bar-shaped) for small (large) misalignment angles. We found that the matter falling on the star has a higher Mach number when we use the Paczyński-Wiita potential than in the Newtonian case. Having obtained the surface distribution of the emitted flux, we calculated the variability curves of the star, taking into account general relativistic, Doppler, and light-travel time effects. We found that general relativistic effects decrease the pulse fraction (flatten the light curve), while Doppler and light-travel time effects increase it and distort the light curve. We also found that the light curves from our hot spots are reproduced reasonably well by spots with a Gaussian flux distribution centered at the magnetic poles. We also calculated the observed image of the star in a few cases and saw that for certain orientations, both the antipodal hot spots are simultaneously visible, as noted by earlier authors.
The Astrophysical Journal, 2007
Thermonuclear burning on the surface of a neutron star causes the expansion of a thin outer layer... more Thermonuclear burning on the surface of a neutron star causes the expansion of a thin outer layer of the star, ∆R(t). The layer rotates slower than the star due to angular momentum conservation. The shear between the star and the layer acts to twist the star's dipole magnetic field giving at first a trailing spiral field. The twist of the field acts in turn to 'torque up' the layer increasing its specific angular momentum. As the layer cools and contracts, its excess specific angular momentum causes it to rotate faster than the star which gives a leading spiral magnetic field. The process repeats, giving rise to torsional oscillations. We derive equations for the angular velocity and magnetic field of the layer taking into account the diffusivity and viscosity which are probably due to turbulence. The magnetic field causes a nonuniformity of the star's photosphere (at the top of the heated layer), and this gives rise to the observed X-ray oscillations. The fact that the layer periodically rotates faster than the star means that the X-ray oscillation frequency may "overshoot" the star's rotation frequency. Comparison of the theory is made with observations of Chakrabarty et al. (2003) of an X-ray burst of SAX J1804.4-3658.
Monthly Notices of the Royal Astronomical Society, 2013
We present an analytical formula for the position and shape of the spots on the surface of accret... more We present an analytical formula for the position and shape of the spots on the surface of accreting magnetized stars in cases where a star has a dipole magnetic field tilted at a small misalignment angle 30 • about the rotational axis, and the magnetosphere is 2.5-5 times the radius of the star. We observed that the azimuthal position of the spots varies significantly when the position of the inner disc varies. In contrast, the polar position of the spots varies only slightly because of the compression of the magnetosphere. The azimuthal width of the spots strongly varies with : spots have the shape of an arc at larger misalignment angles, and resemble a ring at very small misalignment angles. The polar width of the spots varies only slightly with changes in parameters. The motion of the spots in the azimuthal direction can provide phase-shifts in accreting millisecond pulsars, and the 'drift' of the period in Classical T Tauri stars. The position and shape of the spots are determined by three parameters: misalignment angle ; normalized corotation radius, r c /R * , and normalized magnetospheric radius, r m /R * , where R * is the stellar radius. We also use our data to check the formula for the Alfvén radius, where the main dependences are r m ∼ (μ 2 /Ṁ) 2/7 , where μ is the magnetic moment of the star, andṀ is the accretion rate. We found that the dependence is more gradual, r m ∼ (μ 2 /Ṁ) 1/5 , which can be explained by the compression of the magnetosphere by the disc matter and by the non-dipole shape of the magnetic field lines of the external magnetosphere.
Monthly Notices of the Royal Astronomical Society, 2009
We investigate the photometric variability of magnetized stars, particularly neutron stars, accre... more We investigate the photometric variability of magnetized stars, particularly neutron stars, accreting through a magnetic Rayleigh-Taylor-type instability at the diskmagnetosphere interface, and compare it with the variability during stable accretion, with the goal of looking for possible quasi-periodic oscillations. The lightcurves during stable accretion show periodicity at the star's frequency and sometimes twice that, due to the presence of two funnel streams that produce antipodal hotspots near the magnetic poles. On the other hand, lightcurves during unstable accretion through tongues penetrating the magnetosphere are more chaotic due to the stochastic behaviour of the tongues, and produce noisier power spectra. However, the power spectra do show some signs of quasi-periodic variability. Most importantly, the rotation frequency of the tongues and the resulting hotspots is close to the inner-disk orbital frequency, except in the most strongly unstable cases. There is therefore a high probability of observing QPOs at that frequency in longer simulations. In addition, the lightcurves in the unstable regime show periodicity at the star's rotation frequency in many of the cases investigated here, again except in the most strongly unstable cases which lack funnel flows and the resulting antipodal hotspots. The noisier power spectra result in the fractional rms amplitudes of the Fourier peaks being smaller. We also study in detail the effect of the misalignment angle between the rotation and magnetic axes of the star on the variability, and find that at misalignment angles 25 • , the star's period always appears in the lightcurves.
Monthly Notices of the Royal Astronomical Society, 2011
Recent spectropolarimetric observations of the classical T Tauri star BP Tau and analysis of its ... more Recent spectropolarimetric observations of the classical T Tauri star BP Tau and analysis of its surface magnetic field have shown that the magnetic field can be approximated as a superposition of slightly tilted dipole and octupole moments with respective strengths of the polar magnetic fields of 1.2 kG and 1.6 kG (Donati et al. 2008, hereafter D08). We adopt the measured properties of BP Tau and model the disc accretion onto the star by performing global three-dimensional magnetohydrodynamic simulations. We observed in simulations that the disc is disrupted by the dipole component and matter flows towards the star in two funnel streams which form two accretion spots below the dipole magnetic poles. The octupolar component becomes dynamically important very close to the star and it redirects the matter flow to higher latitudes and changes the distribution and shape of the accretion spots. The spots are meridionally elongated and are located at higher latitudes, compared with the pure dipole case, where crescent-shaped, latitudinally elongated spots form at lower latitudes. The position and shape of the spots are in good agreement with observations. The disk-magnetosphere interaction leads to the inflation of the field lines and to the formation of magnetic towers above and below the disk. The magnetic field of BP Tau is close to the potential inside the magnetospheric surface, where magnetic stress dominates over the matter stress. However, it strongly deviates from the potential at larger distances from the star. A series of simulation runs were performed at different accretion rates. In one of them, the disk is truncated at r ≈ (6 − 7)R which is close to the corotation radius, R cor ≈ 7.5R. However, the accretion rate, 1.4 × 10 −9 M yr −1 , is lower than that obtained from most of the observations. In a sample model with a higher accretion rate 8.5 × 10 −9 M yr −1 , the disk is truncated at r ≈ 3.6R , but such a state can not be a typical state for the slowly rotating BP Tau if it is in the rotational equilibrium. However, torque acting on the star is also small: it is about an order of magnitude lower than that which is required for the rotational equilibrium. We suggest that a star could lose most of its angular momentum at earlier stages of its evolution.
Monthly Notices of the Royal Astronomical Society, 2010
We present recent results of 3D magnetohydrodynamic simulations of neutron stars with small misal... more We present recent results of 3D magnetohydrodynamic simulations of neutron stars with small misalignment angles, as regards the features in light curves produced by regular movements of the hot spots during accretion on to the star. In particular, we show that the variation of position of the hot spot created by the infalling matter, as observed in 3D simulations, can produce high-frequency quasi-periodic oscillations (QPOs) with frequencies associated with the inner zone of the disc. Previously reported simulations show that the usual assumption of a fixed hot spot near the polar region is valid only for misalignment angles relatively large. Otherwise, two phenomena challenge the assumption: one is the presence of Rayleigh-Taylor instabilities at the disc-magnetospheric boundary, which produce tongues of accreting matter that can reach the star almost anywhere between the equator and the polar region; the other one is the motion of the hot spot around the magnetic pole during stable accretion. In this paper, we start by showing that both phenomena are capable of producing short-term oscillations in the light curves. We then use Monte Carlo techniques to produce model light curves based on the features of the movements observed, and we show that the main features of kHz QPOs can be reproduced. Finally, we show the behaviour of the frequencies of the moving spots as the mass accretion rate changes, and propose a mechanism for the production of double QPO peaks.
Monthly Notices of the Royal Astronomical Society, 2008
We present results of 3D simulations of magnetohydrodynamics (MHD) instabilities at the accretion... more We present results of 3D simulations of magnetohydrodynamics (MHD) instabilities at the accretion disc-magnetosphere boundary. The instability is Rayleigh-Taylor, and develops for a fairly broad range of accretion rates and stellar rotation rates and magnetic fields. It manifests itself in the form of tall, thin tongues of plasma that penetrate the magnetosphere in the equatorial plane. The shape and number of the tongues changes with time on the inner disc dynamical timescale. In contrast with funnel flows, which deposit matter mainly in the polar region, the tongues deposit matter much closer to the stellar equator. The instability appears for relatively small misalignment angles, 30 • , between the star's rotation and magnetic axes, and is associated with higher accretion rates. The hotspots and light curves during accretion through instability are generally much more chaotic than during stable accretion. The unstable state of accretion has possible implications for quasi-periodic oscillations and intermittent pulsations from accreting systems, as well as planet migration.
Arxiv preprint arXiv: …, 2009
We present results of global 3D MHD simulations of disk accretion to a rotating star with dipole ... more We present results of global 3D MHD simulations of disk accretion to a rotating star with dipole and more complex magnetic fields using a Godunovtype code based on the "cubed sphere" grid developed earlier in our group. We describe the code and the grid and show examples of simulation results.
Advances in Space Research, 2006
We performed 2D and full 3D magnetohydrodynamic simulations of disk accretion to a rotating star ... more We performed 2D and full 3D magnetohydrodynamic simulations of disk accretion to a rotating star with an aligned or misaligned dipole magnetic field. We investigated the rotational equilibrium state and derived from simulations the ratio between two main frequencies: the spin frequency of the star and the orbital frequency at the inner radius of the disk. In 3D simulations we observed different features related to the non-axisymmetry of the magnetospheric flow. These features may be responsible for high-frequency quasi-periodic oscillations (QPOs). Variability at much lower frequencies may be connected with restructuring of the magnetic flux threading the inner regions of the disk. Such variability is specifically strong at the propeller stage of evolution.
Amoebic Liver Abscess (ALA) is a most common extraintestinal manifestation of amoebiasis which is... more Amoebic Liver Abscess (ALA) is a most common extraintestinal manifestation of amoebiasis which is most commonly present with high grade fever with right upper quadrant abdominal pain. Here we present a case of 32 year male patient newly diagnosed as People Living with HIV/AIDS (PLHIV) with ALA. Patient was treated with Metronidazole (500 mg 8 hourly) and therapeutic drainage.
International Journal of Contemporary Medicine, Surgery and Radiology, 2019
AIP Conference Proceedings, 2008
The appearance and time variability of accreting millisecond X-ray pulsars (hereafter AMXPs, e.g.... more The appearance and time variability of accreting millisecond X-ray pulsars (hereafter AMXPs, e.g. Wijnands & van der Klis 1998) depends strongly on the accretion rate, the effective viscosity and the effective magnetic diffusivity of the disk-magnetosphere boundary. The accretion rate is the main parameter which determines the location of the magnetospheric radius of the star for a given stellar magnetic field. We introduce a classification of accreting neutron stars as a function of the accretion rate and show the corresponding stages obtained from our global 3D magnetohydrodynamic (MHD) simulations and from our axisymmetric MHD simulations. We discuss the expected variability features in each stage of accretion, both periodic and quasi-periodic (QPOs). We conclude that the periodicity may be suppressed at both very high and very low accretion rates. In addition the periodicity may disappear when ordered funnel flow accretion is replaced by disordered accretion through the interchange instability.
AIP Conference Proceedings, 2008
We present results of 3D simulations of MHD instabilities at the accretion diskmagnetosphere boun... more We present results of 3D simulations of MHD instabilities at the accretion diskmagnetosphere boundary. The instability is Rayleigh-Taylor, and develops for a fairly broad range of accretion rates and stellar rotation rates and magnetic fields. It produces tall, thin tongues of plasma that penetrate the magnetosphere in the equatorial plane. The shape and number of the tongues changes with time on the inner-disk dynamical timescale. In contrast with funnel flows, which deposit matter mainly in the polar region, the tongues deposit matter much closer to the stellar equator. The instability appears for relatively small misalignment angles, Θ 30 • , between the star's rotation and magnetic axes, and is associated with higher accretion rates. The hot spots and light curves during accretion through instability are generally much more chaotic than during stable accretion. The unstable state of accretion has possible implications for quasi-periodic oscillations and intermittent pulsations from accreting systems.
We investigate the photometric variability of magnetized stars, particularly neutron stars, accre... more We investigate the photometric variability of magnetized stars, particularly neutron stars, accreting through a magnetic Rayleigh-Taylor-type instability at the disk-magnetosphere interface, and compare it with the variability during stable accretion, with the goal of looking for possible quasi-periodic oscillations. The lightcurves during stable accretion show periodicity at the star's frequency and sometimes twice that, due to the presence of two funnel streams that produce antipodal hotspots near the magnetic poles. On the other hand, lightcurves during unstable accretion through tongues penetrating the magnetosphere are more chaotic due to the stochastic behaviour of the tongues, and produce noisier power spectra. However, the power spectra do show some signs of quasi-periodic variability. Most importantly, the rotation frequency of the tongues and the resulting hotspots is close to the inner-disk orbital frequency, except in the most strongly unstable cases. There is theref...
We performed full 3D MHD simulations of disk accretion to a rotating star with a misaligned dipol... more We performed full 3D MHD simulations of disk accretion to a rotating star with a misaligned dipole magnetic field, with the magnetic axis of the star misaligned relative to its rotational axis at an angle Theta . We observed that even at very small misalignment angles matter accretes from the disk to the star through the streams, and forms hot spots at the surface of the star. The shape of the spots reflects the shape of the funnel streams and depends on Theta . At small Theta , they have a shape of a bow, which curves around the magnetic pole. At large Theta , the spots have a shape of an elongated bar, which crosses the magnetic pole. The variability curves were calculated for different misalignment angles and different inclination angles relative to the observer. The spots may rotate faster/slower than the star, thus leading to possible quasi-periodic oscillations. Such oscillations are probable during periods of fast, non-stationary accretion. This research was supported by NASA...
Proceedings of the International Astronomical Union, 2007
We discuss a number of topics relevant to disk-magnetosphere interaction and how numerical simula... more We discuss a number of topics relevant to disk-magnetosphere interaction and how numerical simulations illuminate them. The topics include: (1) disk-magnetosphere interaction and the problem of disk-locking; (2) the wind problem; (3) structure of the magnetospheric flow, hot spots at the star's surface, and the inner disk region; (4) modeling of spectra from 3D funnel streams; (5) accretion to a star with a complex magnetic field; (6) accretion through 3D instabilities; (7) magnetospheric gap and survival of protoplanets. Results of both 2D and 3D simulations are discussed.
3D Magnetohydrodynamic simulations show that when matter accretes onto neutron stars, in particul... more 3D Magnetohydrodynamic simulations show that when matter accretes onto neutron stars, in particular if the misalignment angle is small, it does not constantly fall at a fixed spot. Instead, the location at which matter reaches the star moves. These moving hot spots can be produced both during stable accretion, where matter falls near the magnetic poles of the star, and unstable accretion, characterized by the presence of several tongues of matter which fall on the star near the equator, due to Rayleigh-Taylor instabilities. Precise modeling with Monte Carlo simulations shows that those movements could be observed as high frequency Quasi Periodic Oscillations. We performed a number of new simulation runs with a much wider set of parameters, focusing on neutron stars with a small misalignment angle. In most cases we observe oscillations whose frequency is correlated with the mass accretion rateṀ. Moreover, in some cases double QPOs appear, each of them showing the same correlation withṀ.
Proceedings of the International Astronomical Union, 2007
We present results of 3D simulations of MHD instabilities at the accretion disk-magnetosphere bou... more We present results of 3D simulations of MHD instabilities at the accretion disk-magnetosphere boundary. The instability is Rayleigh-Taylor, and develops for a large range of parameter values. It manifests itself in the form of tall, thin tongues of plasma that reach the star by penetrating through the magnetosphere in the equatorial plane. The tongues rotate around the star in the equatorial plane, and their shape and number changes with time on inner-disk dynamical timescales. In contrast with funnel flows, which deposit matter mainly in the polar region, the tongues deposit matter much closer to the stellar equator.
The Astrophysical Journal, 2008
We report on the first global three-dimensional (3D) MHD simulations of disk accretion onto a rot... more We report on the first global three-dimensional (3D) MHD simulations of disk accretion onto a rotating magnetized star through the Rayleigh-Taylor instability. The star has a dipole field misaligned relative to the rotation axis by a small angle Θ. Simulations show that, depending on the accretion rate, a star may be in the stable or unstable regime of accretion. In the unstable regime, matter penetrates deep into the magnetosphere through several elongated "tongues" which deposit matter at random places on the surface of the star, leading to stochastic light-curves. In the stable regime, matter accretes in ordered funnel streams and the light-curves are almost periodic. A star may switch between these two regimes depending on the accretion rate and may thus show alternate episodes of ordered pulsations and stochastic light-curves. In the intermediate regime, both stochastic and ordered pulsations are observed. For Θ > 30 • , the instability is suppressed and stable accretion through funnel streams dominates.
The Astrophysical Journal, 2005
We model the variability profiles of millisecond-period X-ray pulsars. We performed three-dimensi... more We model the variability profiles of millisecond-period X-ray pulsars. We performed three-dimensional magnetohydrodynamic simulations of disk accretion to millisecond-period neutron stars with a misaligned magnetic dipole moment, using the pseudo-Newtonian Paczyński-Wiita potential to model general relativistic effects. We found that the shapes of the resulting funnel streams of accreting matter and the hot spots on the surface of the star are quite similar to those for more slowly rotating stars obtained from earlier simulations using the Newtonian potential. The funnel streams and hot spots rotate approximately with the same angular velocity as the star. The spots are bowshaped (bar-shaped) for small (large) misalignment angles. We found that the matter falling on the star has a higher Mach number when we use the Paczyński-Wiita potential than in the Newtonian case. Having obtained the surface distribution of the emitted flux, we calculated the variability curves of the star, taking into account general relativistic, Doppler, and light-travel time effects. We found that general relativistic effects decrease the pulse fraction (flatten the light curve), while Doppler and light-travel time effects increase it and distort the light curve. We also found that the light curves from our hot spots are reproduced reasonably well by spots with a Gaussian flux distribution centered at the magnetic poles. We also calculated the observed image of the star in a few cases and saw that for certain orientations, both the antipodal hot spots are simultaneously visible, as noted by earlier authors.
The Astrophysical Journal, 2007
Thermonuclear burning on the surface of a neutron star causes the expansion of a thin outer layer... more Thermonuclear burning on the surface of a neutron star causes the expansion of a thin outer layer of the star, ∆R(t). The layer rotates slower than the star due to angular momentum conservation. The shear between the star and the layer acts to twist the star's dipole magnetic field giving at first a trailing spiral field. The twist of the field acts in turn to 'torque up' the layer increasing its specific angular momentum. As the layer cools and contracts, its excess specific angular momentum causes it to rotate faster than the star which gives a leading spiral magnetic field. The process repeats, giving rise to torsional oscillations. We derive equations for the angular velocity and magnetic field of the layer taking into account the diffusivity and viscosity which are probably due to turbulence. The magnetic field causes a nonuniformity of the star's photosphere (at the top of the heated layer), and this gives rise to the observed X-ray oscillations. The fact that the layer periodically rotates faster than the star means that the X-ray oscillation frequency may "overshoot" the star's rotation frequency. Comparison of the theory is made with observations of Chakrabarty et al. (2003) of an X-ray burst of SAX J1804.4-3658.
Monthly Notices of the Royal Astronomical Society, 2013
We present an analytical formula for the position and shape of the spots on the surface of accret... more We present an analytical formula for the position and shape of the spots on the surface of accreting magnetized stars in cases where a star has a dipole magnetic field tilted at a small misalignment angle 30 • about the rotational axis, and the magnetosphere is 2.5-5 times the radius of the star. We observed that the azimuthal position of the spots varies significantly when the position of the inner disc varies. In contrast, the polar position of the spots varies only slightly because of the compression of the magnetosphere. The azimuthal width of the spots strongly varies with : spots have the shape of an arc at larger misalignment angles, and resemble a ring at very small misalignment angles. The polar width of the spots varies only slightly with changes in parameters. The motion of the spots in the azimuthal direction can provide phase-shifts in accreting millisecond pulsars, and the 'drift' of the period in Classical T Tauri stars. The position and shape of the spots are determined by three parameters: misalignment angle ; normalized corotation radius, r c /R * , and normalized magnetospheric radius, r m /R * , where R * is the stellar radius. We also use our data to check the formula for the Alfvén radius, where the main dependences are r m ∼ (μ 2 /Ṁ) 2/7 , where μ is the magnetic moment of the star, andṀ is the accretion rate. We found that the dependence is more gradual, r m ∼ (μ 2 /Ṁ) 1/5 , which can be explained by the compression of the magnetosphere by the disc matter and by the non-dipole shape of the magnetic field lines of the external magnetosphere.
Monthly Notices of the Royal Astronomical Society, 2009
We investigate the photometric variability of magnetized stars, particularly neutron stars, accre... more We investigate the photometric variability of magnetized stars, particularly neutron stars, accreting through a magnetic Rayleigh-Taylor-type instability at the diskmagnetosphere interface, and compare it with the variability during stable accretion, with the goal of looking for possible quasi-periodic oscillations. The lightcurves during stable accretion show periodicity at the star's frequency and sometimes twice that, due to the presence of two funnel streams that produce antipodal hotspots near the magnetic poles. On the other hand, lightcurves during unstable accretion through tongues penetrating the magnetosphere are more chaotic due to the stochastic behaviour of the tongues, and produce noisier power spectra. However, the power spectra do show some signs of quasi-periodic variability. Most importantly, the rotation frequency of the tongues and the resulting hotspots is close to the inner-disk orbital frequency, except in the most strongly unstable cases. There is therefore a high probability of observing QPOs at that frequency in longer simulations. In addition, the lightcurves in the unstable regime show periodicity at the star's rotation frequency in many of the cases investigated here, again except in the most strongly unstable cases which lack funnel flows and the resulting antipodal hotspots. The noisier power spectra result in the fractional rms amplitudes of the Fourier peaks being smaller. We also study in detail the effect of the misalignment angle between the rotation and magnetic axes of the star on the variability, and find that at misalignment angles 25 • , the star's period always appears in the lightcurves.
Monthly Notices of the Royal Astronomical Society, 2011
Recent spectropolarimetric observations of the classical T Tauri star BP Tau and analysis of its ... more Recent spectropolarimetric observations of the classical T Tauri star BP Tau and analysis of its surface magnetic field have shown that the magnetic field can be approximated as a superposition of slightly tilted dipole and octupole moments with respective strengths of the polar magnetic fields of 1.2 kG and 1.6 kG (Donati et al. 2008, hereafter D08). We adopt the measured properties of BP Tau and model the disc accretion onto the star by performing global three-dimensional magnetohydrodynamic simulations. We observed in simulations that the disc is disrupted by the dipole component and matter flows towards the star in two funnel streams which form two accretion spots below the dipole magnetic poles. The octupolar component becomes dynamically important very close to the star and it redirects the matter flow to higher latitudes and changes the distribution and shape of the accretion spots. The spots are meridionally elongated and are located at higher latitudes, compared with the pure dipole case, where crescent-shaped, latitudinally elongated spots form at lower latitudes. The position and shape of the spots are in good agreement with observations. The disk-magnetosphere interaction leads to the inflation of the field lines and to the formation of magnetic towers above and below the disk. The magnetic field of BP Tau is close to the potential inside the magnetospheric surface, where magnetic stress dominates over the matter stress. However, it strongly deviates from the potential at larger distances from the star. A series of simulation runs were performed at different accretion rates. In one of them, the disk is truncated at r ≈ (6 − 7)R which is close to the corotation radius, R cor ≈ 7.5R. However, the accretion rate, 1.4 × 10 −9 M yr −1 , is lower than that obtained from most of the observations. In a sample model with a higher accretion rate 8.5 × 10 −9 M yr −1 , the disk is truncated at r ≈ 3.6R , but such a state can not be a typical state for the slowly rotating BP Tau if it is in the rotational equilibrium. However, torque acting on the star is also small: it is about an order of magnitude lower than that which is required for the rotational equilibrium. We suggest that a star could lose most of its angular momentum at earlier stages of its evolution.
Monthly Notices of the Royal Astronomical Society, 2010
We present recent results of 3D magnetohydrodynamic simulations of neutron stars with small misal... more We present recent results of 3D magnetohydrodynamic simulations of neutron stars with small misalignment angles, as regards the features in light curves produced by regular movements of the hot spots during accretion on to the star. In particular, we show that the variation of position of the hot spot created by the infalling matter, as observed in 3D simulations, can produce high-frequency quasi-periodic oscillations (QPOs) with frequencies associated with the inner zone of the disc. Previously reported simulations show that the usual assumption of a fixed hot spot near the polar region is valid only for misalignment angles relatively large. Otherwise, two phenomena challenge the assumption: one is the presence of Rayleigh-Taylor instabilities at the disc-magnetospheric boundary, which produce tongues of accreting matter that can reach the star almost anywhere between the equator and the polar region; the other one is the motion of the hot spot around the magnetic pole during stable accretion. In this paper, we start by showing that both phenomena are capable of producing short-term oscillations in the light curves. We then use Monte Carlo techniques to produce model light curves based on the features of the movements observed, and we show that the main features of kHz QPOs can be reproduced. Finally, we show the behaviour of the frequencies of the moving spots as the mass accretion rate changes, and propose a mechanism for the production of double QPO peaks.
Monthly Notices of the Royal Astronomical Society, 2008
We present results of 3D simulations of magnetohydrodynamics (MHD) instabilities at the accretion... more We present results of 3D simulations of magnetohydrodynamics (MHD) instabilities at the accretion disc-magnetosphere boundary. The instability is Rayleigh-Taylor, and develops for a fairly broad range of accretion rates and stellar rotation rates and magnetic fields. It manifests itself in the form of tall, thin tongues of plasma that penetrate the magnetosphere in the equatorial plane. The shape and number of the tongues changes with time on the inner disc dynamical timescale. In contrast with funnel flows, which deposit matter mainly in the polar region, the tongues deposit matter much closer to the stellar equator. The instability appears for relatively small misalignment angles, 30 • , between the star's rotation and magnetic axes, and is associated with higher accretion rates. The hotspots and light curves during accretion through instability are generally much more chaotic than during stable accretion. The unstable state of accretion has possible implications for quasi-periodic oscillations and intermittent pulsations from accreting systems, as well as planet migration.
Arxiv preprint arXiv: …, 2009
We present results of global 3D MHD simulations of disk accretion to a rotating star with dipole ... more We present results of global 3D MHD simulations of disk accretion to a rotating star with dipole and more complex magnetic fields using a Godunovtype code based on the "cubed sphere" grid developed earlier in our group. We describe the code and the grid and show examples of simulation results.
Advances in Space Research, 2006
We performed 2D and full 3D magnetohydrodynamic simulations of disk accretion to a rotating star ... more We performed 2D and full 3D magnetohydrodynamic simulations of disk accretion to a rotating star with an aligned or misaligned dipole magnetic field. We investigated the rotational equilibrium state and derived from simulations the ratio between two main frequencies: the spin frequency of the star and the orbital frequency at the inner radius of the disk. In 3D simulations we observed different features related to the non-axisymmetry of the magnetospheric flow. These features may be responsible for high-frequency quasi-periodic oscillations (QPOs). Variability at much lower frequencies may be connected with restructuring of the magnetic flux threading the inner regions of the disk. Such variability is specifically strong at the propeller stage of evolution.