Constraints on the magnetic field structure in accreting compact objects from aperiodic variability

Monthly Notices of the Royal Astronomical Society, vol. 515, pp. 571–580

Abstract

ABSTRACT We investigate the aperiodic variability for a relatively large sample of accreting neutron stars and intermediate polars, focusing on the properties of the characteristic break commonly observed in power spectra of accreting objects. In particular, we investigate the relation of the break frequency and the magnetic field strength, both of which are connected to the size of the magnetosphere. We find that for the majority of objects in our sample the measured break frequency values indeed agree with estimated inner radii of the accretion disc, which allows to use observed break frequencies to independently assess the magnetic field strength and structure in accreting compact objects. As a special case, we focus on Hercules X-1 which is a persistent, medium-luminosity X-ray pulsar accreting from its low-mass companion. In the literature, it has been suggested that the complex pulse profiles, the spin-up behaviour and the luminosity-correlation of the cyclotron energy seen in Her X-1 can be explained with a complex magnetic field structure of the neutron star. Here, we connect the measured break frequency to the magnetospheric radius and show that the magnetic field strength derived assuming a dipole configuration is nearly an order of magnitude smaller than the magnetic field strength corresponding to the cyclotron energy. Accordingly, this discrepancy can be explained with the magnetic field having strong multipole components. The multipolar structure would also increase the accreting area on the neutron star surface, explaining why the critical luminosity for accretion column formation is puzzlingly high in this source.

Authors 6

1. Juhani Mönkkönen corresponding

   University of Turku

   Affiliation as printed

   Department of Physics and Astronomy , FI-20014 University of Turku, Finland

2. Sergey S. Tsygankov

   University of Turku · Space Research Institute

   Affiliation as printed

   Department of Physics and Astronomy , FI-20014 University of Turku, Finland

   Space Research Institute of the Russian Academy of Sciences , Profsoyuznaya Str. 84/32, Moscow 117997, Russia

3. Alexander A. Mushtukov Aachen Leiden Observatory

   Leiden University · University of Oxford · Leiden Observatory

   Affiliation as printed

   Astrophysics, Department of Physics, University of Oxford , Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK

   Leiden Observatory, Leiden University , NL-2300RA Leiden, the Netherlands

4. Victor Doroshenko

   University of Tübingen

   Affiliation as printed

   Institut für Astronomie und Astrophysik, University of Tübingen , Sand 1, D-72076 Tübingen, Germany

5. В. Ф. Сулейманов

   University of Tübingen

   Affiliation as printed

   Institut für Astronomie und Astrophysik, University of Tübingen , Sand 1, D-72076 Tübingen, Germany

6. Juri Poutanen

   University of Turku · Space Research Institute

   Affiliation as printed

   Department of Physics and Astronomy , FI-20014 University of Turku, Finland

   Space Research Institute of the Russian Academy of Sciences , Profsoyuznaya Str. 84/32, Moscow 117997, Russia