X-ray variability of transitional millisecond pulsars: a faint, stable, and fluctuating disc
Transitional millisecond pulsars (tMSPs) have emerged in the last decade as a unique class of neutron stars at the crossroads between accretion- and rotation-powered phenomena. In their (sub-luminous) accretion disc state, with X-ray luminosities of order 1033–1034 erg s-1, they switch rapidly betwe...
| Autores: | , , , |
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| Tipo de recurso: | artículo |
| Fecha de publicación: | 2022 |
| País: | España |
| Institución: | Universitat Politècnica de Catalunya (UPC) |
| Repositorio: | UPCommons. Portal del coneixement obert de la UPC |
| Idioma: | inglés |
| OAI Identifier: | oai:upcommons.upc.edu:2117/378246 |
| Acceso en línea: | https://hdl.handle.net/2117/378246 https://dx.doi.org/10.1093/mnras/stac720 |
| Access Level: | acceso abierto |
| Palabra clave: | Neutron stars X-ray binaries Accretion Accretion discs Stars: neutron Pulsars: individual: (PSR J1824–2452I PSR J1023 + 0038 PSR J1227-4853) Globular clusters: individual: (M28) X-rays: binaries X-rays: individual: (IGR J18245-2452 2XMM J102347.6 + 003841 XSS J12270-4859) Estels de neutrons Estels binaris de raigs X Àrees temàtiques de la UPC::Física::Astronomia i astrofísica |
| Sumario: | Transitional millisecond pulsars (tMSPs) have emerged in the last decade as a unique class of neutron stars at the crossroads between accretion- and rotation-powered phenomena. In their (sub-luminous) accretion disc state, with X-ray luminosities of order 1033–1034 erg s-1, they switch rapidly between two distinct X-ray modes: the disc-high (DH) and disc-low (DL) states. We present a systematic XMM–Newton and Chandra analysis of the aperiodic X-ray variability of all three currently known tMSPs, with a main focus on their disc state and separating DH and DL modes. We report the discovery of flat-topped broad-band noise in the DH state of two of them, with break frequencies of 2.8 mHz (PSR J1023 + 0038) and 0.86 mHz (M28-I). We argue that the lowest frequency variability is similar to that seen in disc-accreting X-ray binaries in the hard state, at typical luminosities at least two orders of magnitude higher than tMSPs. We find strong variability in the DH state around 1 Hz, not typical of hard state X-ray binaries, with fractional rms amplitudes close to 30 per¿cent. We discuss our results and use them to constrain the properties of the accretion disc, assuming that the X-ray variability is produced by fluctuations in mass accretion rate, and that the break frequency corresponds to the viscous time-scale at the inner edge of the disc. In this context, we find that the newly found break frequencies are broadly consistent with a disc truncated close to the light cylinder with M¿¿1013-5×1014 g s-1 and a viscosity parameter a ¿ 0.2. |
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