Research Note

Orbital parameters of supergiant fast X-ray transients

¹ JAD Program, Universiti Industri Selangor, block 4, Jalan Zirkon A7/A, Seksyen 7, 40000 Shah Alam, Selangor, Malaysia
² Shibaura Institute of Technology, Toyosu 3-7-5, Koto, Tokyo 135-8548, Japan
e-mail: karino@sic.shibaura-it.ac.jp

Received: 3 May 2010
Accepted: 14 October 2010

Abstract

Context. Supergiant fast X-ray transient (SFXT) is a new class of the high-mass X-ray binary that shows short X-ray flares. The physical mechanism of SFXT short flares is still open for discussion. The accretion process of dense clumps in stellar wind onto neutron star (NS) has been proposed as the origin of such short flares.

Aims. In order to examine the applicability of the clumpy wind scenario, we focus on the accretion mode that depends on orbital parameters. Our goal is to impose restrictions on the orbital parameters of SFXT.

Methods. Assuming a simple analytic model of clumpy wind, we investigate the condition where the size of accretion cylinder overcomes the clump size.

Results. The allowed parameter region for SFXT is restricted in a relatively narrow window in P_orb − e diagram.

Conclusions. Binary systems with large eccentricities (e ≳ 0.4) and moderate orbital periods (P_orb ~ 10 d) are prone to show periodic X-ray outbursts, which are characteristic for SFXT. We confirm that systems with a long orbital period of more than 100 days cannot produce bright X-ray flares in the simple clumpy wind scenario.