Kilohertz quasi-periodic oscillations from neutron star spreading layers

When the accretion disc around a weakly magnetised neutron star (NS) meets the stellar surface, it should brake down to match the rotation of the NS, forming a boundary layer. As the mechanisms potentially responsible for this braking are apparently inefficient, it is reasonable to consider this layer as a spreading layer (SL) with negligible radial extent and structure. We perform hydrodynamical 2D spectral simulations of an SL, considering the disc as a source of matter and angular momentum. Interaction of new, rapidly rotating matter with the pre-existing, relatively slow material co-rotating with the star leads to instabilities capable of transferring angular momentum and creating variability on dynamical timescales. For small accretion rates, we find that the SL is unstable for heating instability that disrupts the initial latitudinal symmetry and produces large deviations between the two hemispheres. This instability also results in breaking of the axial symmetry as coherent flow structures are formed and escape from the SL intermittently. At enhanced accretion rates, the SL is prone to shearing instability and acts as a source of oblique waves that propagate towards the poles, leading to patterns that again break the axial symmetry. We compute artificial light curves of an SL viewed at different inclination angles. Most of the simulated light curves show oscillations at frequencies close to 1kHz. We interpret these oscillations as inertial modes excited by shear instabilities near the boundary of the SL. Their frequencies, dependence on flux, and amplitude variations can explain the high-frequency pair quasi-periodic oscillations observed in many low-mass X-ray binaries.Comment: accepted to A&A; 22 pages, 21 figur

Models of neutron star atmospheres enriched with nuclear burning ashes

Low-mass X-ray binaries hosting neutron stars (NS) exhibit thermonuclear (type-I) X-ray bursts, which are powered by unstable nuclear burning of helium and/or hydrogen into heavier elements deep in the NS "ocean". In some cases the burning ashes may rise from the burning depths up to the NS photosphere by convection, leading to the appearance of the metal absorption edges in the spectra, which then force the emergent X-ray burst spectra to shift toward lower energies. These effects may have a substantial impact on the color correction factor $f_c$ and the dilution factor $w$, the parameters of the diluted blackbody model $F_E \approx w B_E(f_c T_{eff})$ that is commonly used to describe the emergent spectra from NSs. The aim of this paper is to quantify how much the metal enrichment can change these factors. We have developed a new NS atmosphere modeling code, which has a few important improvements compared to our previous code required by inclusion of the metals. The opacities and the internal partition functions (used in the ionization fraction calculations) are now taken into account for all atomic species. In addition, the code is now parallelized to counter the increased computational load. We compute a detailed grid of atmosphere models with different exotic chemical compositions that mimic the presence of the burning ashes. From the emerging model spectra we compute the color correction factors $f_c$ and the dilution factors $w$ that can then be compared to the observations. We find that the metals may change $f_c$ by up to about 40%, which is enough to explain the scatter seen in the blackbody radius measurements. The presented models open up the possibility for determining NS mass and radii more accurately, and may also act as a tool to probe the nuclear burning mechanisms of X-ray bursts.Comment: 14 pages, 7 figures, to be published in A&

Ryhmäliikuntamalli työikäisille sepelvaltimo-operoiduille

Suomessa sydän- ja verenkiertoelimistönsairaudet ovat naisten ja miesten yleisin kuolinsyy. Opinnäytetyön tarkoituksena oli pilotoida ryhmäliikuntamalli työikäisille sepelvaltimo-operoiduille henkilöille ja selvittää, miten ohjattu ryh-mäliikunta vaikuttaa kohdehenkilöiden fyysiseen suorituskykyyn, kehonkoostumukseen ja elämänlaatuun. Opinnäytetyön yhteistyökumppanina toimi Kaakkois-Suomen Sydänpiiri. Tutkimukseen osallistuneet kohdehenkilöt (n=3) olivat pallolaajennuksen läpi-käyneitä ja iältään 40 - 58-vuotiaita. Yksi kohdehenkilö joutui jättäytymään pois loppumittauksista. Tutkimuksen alku- ja loppumittauksissa käytettiin kuuden minuutin kävelytestiä, kehonkoostumusmittausta, MacNew-elämänlaatukyselyä ja kyselylomaketta. Alku- ja loppumittausten välisen harjoittelujakson pituus oli neljä viikkoa. Ohjattuja harjoituskertoja suoritettiin kahdesti viikossa. Harjoitusohjelma sisälsi tasavauhtista aerobista liikuntaa, lihasvoima- ja intervalliharjoittelua. Pienen otannan vuoksi alku- ja loppumittausten tuloksia tarkasteltiin yksilötasolla. Tuloksissa tapahtui positiivisia muutoksia mittausten välillä prosentuaalisella tasolla. Kuuden minuutin kävelytestissä kävelymatka kasvoi 9,7 % (689 m vs. 756 m) ja 22,6 % (517 m vs. 634 m). Lepoverenpaine laski 142/94 mmHg:sta 119/85 mmHg:aan ja 152/82 mmHg:sta 120/72 mmHg:aan. MacNew-elämänlaatukyselyn kokonaispisteet kasvoivat 2,9 %. Tuloksia ei voida yleistää tilastollisen merkitsevyyden puutteen vuoksi. Jatkossa liikuntaryhmän toimintamallia voisi tutkia pidemmällä harjoitusjaksolla ja suuremmalla otannalla. Liikuntaryhmämalliin voisi myös liittää ruokavalio-neuvonnan. Tulevaisuudessa voisi tutkia liikuntaryhmämallin käyttöä sydän-potilaiden jatkokuntouksen vaihtoehtona.Cardiovascular diseases are the most common cause of death among men and women in Finland. The purpose of this Barchelor’s Thesis was to pilot a group exercise model for working age people after coronary artery operation and to examine how the controlled exercise programme influences their physical performance, body composition and quality of life. The study was commissioned by Kaakkois-Suomen Sydänpiiri. Persons who participated in this study (n=3) had undergone angioplasty and were 40 to 58 years old. One person did not participate in the final measurements. A six minute walk test, body composition measurement (InBody 720), MacNew quality of life questionnaire and our own questionnaire were used as the measurements in this study. The intervention period lasted for four weeks. Controlled exercise sessions were accomplished twice in a week. The intervention programme included aerobic exercises, muscle strengthening exercises and interval training. Due to the small sample size the initial and the final results were examined at individual level. There were some positive changes between the initial and final results on the percent level. In the six minute walk test the walking distance increased 9,7 % (689 m vs. 756 m) and 22,6 % (517 m vs. 634 m). The resting blood pressure decreased from 142/94 mmHg to 119/85 mmHg and from 152/82 mmHg to 120/72 mmHg. The total score of the MacNew quality of life questionnaire increased 2,9 %. The results cannot be generalized due to the lack of statistical significance. In the future, the examination of the exercise group model could include a longer training period and a bigger sample size. Also diet counselling could be a part of the exercise group model. Future studies could investigate the use of the exercise group model as an alternative form of cardiac rehabilitation

Oblate Schwarzschild approximation for polarized radiation from rapidly rotating neutron stars

We have developed a complete theory for the calculation of the observed Stokes parameters for radiation emitted from the surface of a rapidly rotating neutron star (NS) using the oblate Schwarzschild approximation. We accounted for the rotation of the polarization plane due to relativistic effects along the path from the stellar surface to the observer. The results were shown to agree with those obtained by performing full numerical general relativistic ray-tracing with the \textsc{arcmancer} code. We showed that the obtained polarization angle (PA) profiles may differ substantially from those derived for a spherical star. We demonstrated that assuming incorrect shape for the star can lead to biased constraints for NS parameters when fitting the polarization data. Using a simplified model, we also made a rough estimate of how accurately the geometrical parameters of an accreting NS can be determined using the X-ray polarization measurements of upcoming polarimeters like the Imaging X-ray Polarimeter Explorer (IXPE) or the enhanced X-ray Timing and Polarimetry (eXTP) mission.Comment: 11 pages, 10 figures, accepted in A&

Magnetospheric return-current-heated atmospheres of rotation-powered millisecond pulsars

We computed accurate atmosphere models of rotation-powered millisecond pulsars in which the polar caps of a neutron star (NS) are externally heated by magnetospheric return currents. The external ram pressure, energy losses, and stopping depth of the penetrating charged particles were computed self-consistently with the atmosphere model, instead of assuming a simplified deep-heated atmosphere in radiative equilibrium. We used exact Compton scattering formalism to model the properties of the emergent X-ray radiation. The deep-heating approximation was found to be valid only if most of the heat originates from ultra-relativistic bombarding particles with Lorentz factors of $\gamma \gtrsim 100$. In the opposite regime, the atmosphere attains a distinct two-layer structure with an overheated optically thin skin on top of an optically thick cool plasma. The overheated skin strongly modifies the emergent radiation: it produces a Compton-upscattered high-energy tail in the spectrum and alters the radiation beaming pattern from limb darkening to limb brightening for emitted hard X-rays. This kind of drastic change in the emission properties can have a significant impact on the inferred NS pulse profile parameters as performed, for example, by Neutron star Interior Composition ExploreR. Finally, the connection between the energy distribution of the return current particles and the atmosphere emission properties offers a new tool to probe the exact physics of pulsar magnetospheres.Comment: 13 pages, 10 figures, published in A&

Kilohertz quasi-periodic oscillations from neutron star spreading layers

When the accretion disc around a weakly magnetised neutron star (NS) meets the stellar surface, it should brake down to match the rotation of the NS, forming a boundary layer. As the mechanisms potentially responsible for this braking are apparently inefficient, it is reasonable to consider this layer as a spreading layer (SL) with negligible radial extent and structure. We perform hydrodynamical 2D spectral simulations of an SL, considering the disc as a source of matter and angular momentum. Interaction of new, rapidly rotating matter with the pre-existing, relatively slow material co-rotating with the star leads to instabilities capable of transferring angular momentum and creating variability on dynamical timescales. For small accretion rates, we find that the SL is unstable for heating instability that disrupts the initial latitudinal symmetry and produces large deviations between the two hemispheres. This instability also results in breaking of the axial symmetry as coherent flow structures are formed and escape from the SL intermittently. At enhanced accretion rates, the SL is prone to shearing instability and acts as a source of oblique waves that propagate towards the poles, leading to patterns that again break the axial symmetry. We compute artificial light curves of an SL viewed at different inclination angles. Most of the simulated light curves show oscillations at frequencies close to 1 kHz. We interpret these oscillations as inertial modes excited by shear instabilities near the boundary of the SL. Their frequencies, dependence on flux, and amplitude variations can explain the high-frequency pair quasi-periodic oscillations observed in many low-mass X-ray binaries

Oblate Schwarzschild approximation for polarized radiation from rapidly rotating neutron stars

We have developed a complete theory for the calculation of the observed Stokes parameters for radiation emitted from the surface of a rapidly rotating neutron star (NS) using the oblate Schwarzschild approximation. We accounted for the rotation of the polarization plane due to relativistic effects along the path from the stellar surface to the observer. The results were shown to agree with those obtained by performing full numerical general relativistic ray-tracing with the ARCMANCER code. We showed that the obtained polarization angle profiles may differ substantially from those derived for a spherical star. We demonstrated that assuming incorrect shape for the star can lead to biased constraints for NS parameters when fitting the polarization data. Using a simplified model, we also made a rough estimate of how accurately the geometrical parameters of an accreting NS can be determined using the X-ray polarization measurements of upcoming polarimeters like the Imaging X-ray Polarimeter Explorer or the enhanced X-ray Timing and Polarimetry mission