Discovery of a second kilohertz quasi-periodic oscillation peak in 4U 1608-52

• Autores: Méndez, Mariano R.; Klis, M. van der; Paradijs, J. van; Lewin, Walter H. G.; Vaughan, Brian A.; Kuulkers, Erik; Zhang, W. W.; Lamb, Frederick K.; Psaltis, Dimitrios
• Año de publicación: 1998
• Idioma: inglés
• Tipo de recurso: artículo
• Materia: Astronomía; Física; accretion, accretion disks; stars: neutron; stars: individual (4U 1608252); X-rays: stars
• Institución: Universidad Nacional de La Plata

Using a new technique to improve the sensitivity to weak quasi-periodic oscillations (QPOs), we discovered a new QPO peak at about 1100 Hz in the March 1996 outburst observations of 4U 1608-52, simultaneous with the ~600-900 Hz peak previously reported from these data. The frequency separation between the upper and the lower QPO peaks varied significantly from 232.7±11.5 Hz on March 3 to 293.1±6.6 Hz on March 6. This is the first case of a variable kilohertz peak separation in an atoll source. We discuss to what extent this result could be accommodated in beat-frequency models such as proposed for the kilohertz QPOs. We measured the rms fractional amplitude of both QPOs as a function of energy, and we found that the relation is steeper for the lower than for the upper frequency peak. This is the first source where such a difference between the energy spectrum of the two kilohertz QPOs could be measured.
Facultad de Ciencias Astronómicas y Geofísicas