Study of the Temporal Behavior of 4U 1728−34 as a Function of Its Position in the Color‐Color Diagram

Abstract

We study the timing properties of the bursting atoll source 4U 1728[34 as a function of its position in the X-ray color-color diagram. In the island part of the color-color diagram (corresponding to the hardest energy spectra), the power spectrum of 4U 1728[34 shows several features such as a band-limited noise component present up to a few tens of Hz, a low-frequency quasi-periodic oscillation (LFQPO) at frequencies between 20 and 40 Hz, a peaked noise component around 100 Hz, and one or two QPOs at kHz frequencies. In addition to these, in the lower banana (corresponding to softer energy spectra) we also Ðnd a very low frequency noise (VLFN) component below D1 Hz. In the upper banana (corresponding to the softest energy spectra), the power spectra are dominated by the VLFN, with a peaked noise component around 20 Hz. We Ðnd that the frequencies of the kHz QPOs are well corre-lated with the position in the X-ray color-color diagram. For the frequency of the LFQPO and the break frequency of the broadband noise component, the relation appears more complex. Both of these frequencies increase when the frequency of the upper kHz QPO increases from 400 to 900 Hz, but at this frequency a jump in the values of the parameters occurs. We interpret this jump in terms of the gradual appearance of a QPO at the position of the break at high inferred mass accretion rate, while the previous LFQPO disappears. Simultaneously, another kind of noise appears with a break frequency of D7 Hz, similar to the NBO of Z sources. The 100 Hz peaked noise does not seem to correlate with the position of the source in the color-color diagram but remains relatively constant in frequency. This com-ponent may be similar to several 100 Hz QPOs observed in black hole binaries. Subject headings : accretion, accretion disks È stars : individual (4U 1728[34) È stars : neutron È X-rays : stars 1. INTRODUCTION Low-mass X-ray binaries (LMXBs) are usually character-ized by weak magnetic Ðelds. In these systems, the accretion disk can reach regions close to the compact object, where the matter is subject to a strong gravitational Ðeld. This results in complex power spectra with characteristic fre-quencies, determined by the dynamical timescales close to the compact object, up to more than a thousand Hz. These timing structures can be used to probe the physics near the compact object, and timing is a key tool to perform such studies. Because several features are usually simultaneously present in the power spectra of LMXBs, we need to under-stand the entire timing structure in order to construct reli-able theories for the di erent timing signals. Power spectra of LMXBs show signiÐcant changes as a function of the source intensity and spectral hardness. Based on their spectral and temporal behavior, LMXBs can be divided into two classes, the so-called Z and atoll sources (Hasinger & van der Klis 1989), the names of which are derived from the tracks they trace out in an X-ray color-color diagram. The Z sources are thought to have higher mass accretion rates and perhaps higher magnetic Ðelds than the atoll sources. These two classes of sources show a temporal behavior that strongly depends on their position in the color-color diagram (Hasinger & van der Klis 1989).