Astron. Astrophys. 358, 617-623 (2000)

Fourier power spectra at high frequencies: a way to distinguish a neutron star from a black hole

R. Sunyaev ^1,2 and M. Revnivtsev <sup>2,1

1</sup> Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Strasse 1, 85740 Garching bei München, Germany
² Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117810 Moscow, Russia

Received 17 April 2000 / Accepted 19 April 2000

Abstract

We analyzed the power density spectra of a sample of 9 neutron star and 9 black hole binaries in the low/hard spectral state. In the power density spectra of accreting neutron stars with a weak magnetic field a significant power is contained at frequencies close to one kHz. At the same time, most Galactic accreting black holes demonstrate a strong decline in the power spectra at the frequencies higher than 10-50 Hz. We propose to use this empirical fact as a method to distinguish the accreting neutron stars from black holes in X-ray transients. The X-ray transients that demonstrate significant noise in their X-ray flux at frequencies above Hz should be considered neutron stars. We propose to explain the observed difference as a result of the existence of a radiation dominated spreading layer on the neutron star surface (Inogamov & Sunyaev 1999). The possible very high frequency variabilities of this layer are discussed.

Key words: accretion, accretion disks instabilities stars: binaries: general stars: fundamental parameters stars: neutron X-rays: general

Send offprint requests to: revnivtsev@hea.iki.rssi.ru

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Contents

• 1. Introduction
• 2. Observations and data analysis
• 3. Results
• 4. On the nature of the difference in PDS
  • 4.1. The instabilities originated in the accretion disk
  • 4.2. The spreading layer and the highest possible frequencies in the PDS of NSs
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000
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