Pulse Profiles of Accreting Neutron Stars from GRMHD Simulations
The pulsed X-ray emission from the neutron star surface acts as a window to study the state of matter in the neutron star interior. For accreting millisecond pulsars, the surface X-ray emission is generated from the “hotspots” formed due to the magnetically channeled accretion flow hitting the stell...
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2025-01-01
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| Online Access: | https://doi.org/10.3847/1538-4357/add472 |
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| author | Pushpita Das Tuomo Salmi Jordy Davelaar Oliver Porth Anna Watts |
| author_facet | Pushpita Das Tuomo Salmi Jordy Davelaar Oliver Porth Anna Watts |
| author_sort | Pushpita Das |
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| description | The pulsed X-ray emission from the neutron star surface acts as a window to study the state of matter in the neutron star interior. For accreting millisecond pulsars, the surface X-ray emission is generated from the “hotspots” formed due to the magnetically channeled accretion flow hitting the stellar surface. The emission from these hotspots is modulated by stellar rotation, giving rise to pulsations. Using global three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulations of the star-disk system, we investigate the accretion hotspots and the corresponding X-ray pulse properties of accreting millisecond pulsars with dipolar magnetic fields. The accretion spot morphologies in our simulations are entirely determined by the accretion columns and vary as a function of the stellar magnetic inclination. For lower magnetic inclinations, the hotspots are shaped like crescents around the magnetic axis and are transformed into elongated bars for higher inclinations. We model the X-ray pulses resulting from the simulated hotspots using general-relativistic ray-tracing calculations and quantify the variability of the pulsed signal. The pulse amplitudes in our simulations usually range between 1% and 12% rms and are consistent with the observed values. We find that the turbulent accretion flow in the GRMHD simulations introduces significant broadband variability on a timescale similar to the stellar rotational period. We also explore the impact of electron scattering absorption and show that, along with being a key factor in determining the pulse characteristics, this also introduces significant additional variability and higher harmonics in the bolometric light curve of the accreting sources. |
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| institution | Kabale University |
| issn | 1538-4357 |
| language | English |
| publishDate | 2025-01-01 |
| publisher | IOP Publishing |
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| spelling | doaj-art-4a4a493bf3d34f72922ac9a6e6c8a04e2025-08-20T03:24:14ZengIOP PublishingThe Astrophysical Journal1538-43572025-01-0198713410.3847/1538-4357/add472Pulse Profiles of Accreting Neutron Stars from GRMHD SimulationsPushpita Das0https://orcid.org/0000-0002-4764-6189Tuomo Salmi1https://orcid.org/0000-0001-6356-125XJordy Davelaar2https://orcid.org/0000-0002-2685-2434Oliver Porth3https://orcid.org/0000-0002-4584-2557Anna Watts4https://orcid.org/0000-0002-1009-2354Anton Pannekoek Institute for Astronomy, University of Amsterdam , Science Park 904, 1098 XH, The Netherlands ; p.das2@uva.nl; Department of Astronomy, Columbia University , New York, NY 10027, USAAnton Pannekoek Institute for Astronomy, University of Amsterdam , Science Park 904, 1098 XH, The Netherlands ; p.das2@uva.nl; Department of Physics, University of Helsinki , P.O. Box 64, FI-00014, FinlandDepartment of Astrophysical Sciences, Peyton Hall, Princeton University , Princeton, NJ 08544, USA; NASA Hubble Fellowship Program, Einstein FellowAnton Pannekoek Institute for Astronomy, University of Amsterdam , Science Park 904, 1098 XH, The Netherlands ; p.das2@uva.nlAnton Pannekoek Institute for Astronomy, University of Amsterdam , Science Park 904, 1098 XH, The Netherlands ; p.das2@uva.nlThe pulsed X-ray emission from the neutron star surface acts as a window to study the state of matter in the neutron star interior. For accreting millisecond pulsars, the surface X-ray emission is generated from the “hotspots” formed due to the magnetically channeled accretion flow hitting the stellar surface. The emission from these hotspots is modulated by stellar rotation, giving rise to pulsations. Using global three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulations of the star-disk system, we investigate the accretion hotspots and the corresponding X-ray pulse properties of accreting millisecond pulsars with dipolar magnetic fields. The accretion spot morphologies in our simulations are entirely determined by the accretion columns and vary as a function of the stellar magnetic inclination. For lower magnetic inclinations, the hotspots are shaped like crescents around the magnetic axis and are transformed into elongated bars for higher inclinations. We model the X-ray pulses resulting from the simulated hotspots using general-relativistic ray-tracing calculations and quantify the variability of the pulsed signal. The pulse amplitudes in our simulations usually range between 1% and 12% rms and are consistent with the observed values. We find that the turbulent accretion flow in the GRMHD simulations introduces significant broadband variability on a timescale similar to the stellar rotational period. We also explore the impact of electron scattering absorption and show that, along with being a key factor in determining the pulse characteristics, this also introduces significant additional variability and higher harmonics in the bolometric light curve of the accreting sources.https://doi.org/10.3847/1538-4357/add472Neutron starsPulsarsCompact objectsAccretionLow-mass x-ray binary starsMagnetohydrodynamics |
| spellingShingle | Pushpita Das Tuomo Salmi Jordy Davelaar Oliver Porth Anna Watts Pulse Profiles of Accreting Neutron Stars from GRMHD Simulations The Astrophysical Journal Neutron stars Pulsars Compact objects Accretion Low-mass x-ray binary stars Magnetohydrodynamics |
| title | Pulse Profiles of Accreting Neutron Stars from GRMHD Simulations |
| title_full | Pulse Profiles of Accreting Neutron Stars from GRMHD Simulations |
| title_fullStr | Pulse Profiles of Accreting Neutron Stars from GRMHD Simulations |
| title_full_unstemmed | Pulse Profiles of Accreting Neutron Stars from GRMHD Simulations |
| title_short | Pulse Profiles of Accreting Neutron Stars from GRMHD Simulations |
| title_sort | pulse profiles of accreting neutron stars from grmhd simulations |
| topic | Neutron stars Pulsars Compact objects Accretion Low-mass x-ray binary stars Magnetohydrodynamics |
| url | https://doi.org/10.3847/1538-4357/add472 |
| work_keys_str_mv | AT pushpitadas pulseprofilesofaccretingneutronstarsfromgrmhdsimulations AT tuomosalmi pulseprofilesofaccretingneutronstarsfromgrmhdsimulations AT jordydavelaar pulseprofilesofaccretingneutronstarsfromgrmhdsimulations AT oliverporth pulseprofilesofaccretingneutronstarsfromgrmhdsimulations AT annawatts pulseprofilesofaccretingneutronstarsfromgrmhdsimulations |