Unstable cyclic modulation during the main outburst of KS 1731-260

作者：Simon Vojtech^*

来源：New Astronomy, 2012, 17(8): 697-704.

DOI：10.1016/j.newast.2012.04.006

• X-rays: binaries
• Binaries: close
• Circumstellar matter
• Stars: individual: KS 1731-260

We analyze the intensity modulation in the final, very broad peak of the main outburst of the neutron star low-mass X-ray binary KS 1731-260. We use ASM/RXTE observations for a time-series analysis of the long-term variations. We also investigate the X-ray color (hardness ratio) changes in the 1.5-12 keV band. The modulation with the mean cycle-length of 37 days is transient and is detected only in several time segments. It underwent significant variations of both the cycle-length and the amplitude. This cycle cannot be caused by transitions of the outer disk region between the hot and cool state that gave rise to the subsequent series of the echo outbursts. Because of its high X-ray luminosity (L-x approximate to 0.1 of the Eddington luminosity), KS 1731-260 is a promising candidate for having its accretion disk tilted and warped. The properties of the modulation can therefore be explained as due to variable (multimodal), but still detectable superorbital periods caused by a disk precession with mode switching and unstable warps. We find that a variable L-x is not the sole parameter that governs the presence of the cycle. Variable absorption of X-rays cannot be dominant in producing the modulation of the ASM flux during the cycle. Variations of the stream impact on the tilted and warped disk, hence affecting the mass flow in the inner disk region, consequently the emission components, are a promising mechanism for the observed cycle. In this scenario, the true cycle-length can be twice as long because of the double-wave profile.

• 出版日期2012-11