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Astron. Astrophys. 337, 815-818 (1998)

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1. Introduction

Quasi periodic oscillations (QPOs) observed in X-ray binaries are generally thought to be related to the rotation of the inner accretion disk. When the accretion disk can reach very close to the compact object, like in the case of black hole candidates and low magnetic field neutron star sources, the rotation of the inhomogeneities or hot blobs of material in the inner disk are reflected in the light curve as QPOs. In X-ray pulsars, however, the disk is interrupted at a large distance by the strong magnetic field of the neutron star, and the inner transition zone of the disk, which is at a large distance from the neutron star, does not emit in X-rays. Hence strong QPOs are believed to be rare in X-ray pulsars.

The hard X-ray transient XTE J1858+034 was discovered with the RXTE All Sky Monitor (ASM) in 1998 February (Remillard & Levine 1998). The spectrum was found to be hard, similar to the spectra of X-ray pulsars. Observations were made immediately after this with the Proportional Counter Array (PCA) of the RXTE and regular pulsations with a period of [FORMULA] s were discovered (Takeshima et al. 1998). The pulse profile is found to be nearly sinusoidal with a pulse fraction of [FORMULA]. From the transient nature of this source and pulsations they suggested that this is a Be-X-ray binary. The position of the X-ray source was refined by scanning the sky around the source with the PCA (Marshall et al. 1998).

From the XTE target of opportunity (TOO) public archival data of the observations of XTE J1858+034, made in 1998 February 20 and 24, we have discovered the presence of low frequency QPOs. We also have obtained the pulse profile of this source in two energy bands and the energy spectrum in one of the observations. In the following sections we describe the archival data that has been used, the analysis and results and discuss some implications of the detection of QPOs in this source.

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© European Southern Observatory (ESO) 1998

Online publication: August 27, 1998
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