Pulsating ULXs: large pulsed fraction excludes strong beaming

Monthly Notices of the Royal Astronomical Society, vol. 501, pp. 2424–2429

Abstract

ABSTRACT The recent discovery of pulsating ultraluminous X-ray sources (ULXs) shows that the apparent luminosity of accreting neutron stars can exceed the Eddington luminosity by a factor of 100s. The relation between the actual and apparent luminosity is a key ingredient in theoretical models of ULXs, but it is still under debate. A typical feature of the discovered pulsating ULXs is a large pulsed fraction (PF). Using Monte Carlo simulations, we consider a simple geometry of accretion flow and test the possibility of simultaneous presence of a large luminosity amplification due the geometrical beaming and a high PF. We argue that these factors largely exclude each other and only a negligible fraction of strongly beamed ULX pulsars can show PF above 10 per cent. Discrepancy between this conclusion and current observations indicates that pulsating ULXs are not strongly beamed and their apparent luminosity is close to the actual one.

Authors 5

1. Alexander A. Mushtukov corresponding Aachen Leiden Observatory

   Leiden University · Space Research Institute · Leiden Observatory

   Affiliation as printed

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

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

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

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

2. Simon Portegies Zwart Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

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

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

3. Sergey S. Tsygankov

   University of Turku · Space Research Institute

   Affiliation as printed

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

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

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

4. Dmitrij I. Nagirner

   St Petersburg University

   Affiliation as printed

   Sobolev Astronomical Institute, Saint Petersburg State University, Saint-Petersburg 198504, Russia

5. Juri Poutanen

   Nordic Institute for Theoretical Physics · University of Turku · Stockholm University · Space Research Institute · KTH Royal Institute of Technology

   Affiliation as printed

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

   Nordita, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden

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

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