2001MNRAS.328..451W

We present a detailed study of the 5-Hz quasi-periodic oscillation (QPO) recently discovered in the bright X-ray transient and black hole candidate (BHC) GRS 1739-278 (Borozdin & Trudolyubov) during a Rossi X-ray Timing Explorer observation taken on 1996 March 31. In total 6.6ksec of on-source data were obtained, divided in two data sets of 3.4 and 3.2ksec which were separated by ∼2.6ksec. The 5-Hz QPO was only present during the second data set. The QPO increased in strength from below 2 per cent rms amplitude for photon energies below 4keV to ∼5 per cent rms amplitude for energies above 10keV. The soft QPO photons (below 5keV) lagged the hard ones (above 10keV) by almost 1.5rad. Besides the QPO fundamental, its first overtone was detected. The strength of the overtone increased with photon energy (from <2 per cent rms below 5keV to ∼8 per cent rms above 10keV). Although limited statistics did not allow for an accurate determination of the lags of the first overtone, indications are that also for this QPO the soft photons lagged the hard ones. When the 5-Hz QPO was not detected (i.e., during the first part of the observation), a broad noise component was found for photon energies below 10keV but it became almost a true QPO (with a Q value of ∼1.9) above that energy, with a frequency of ∼3Hz. Its hard photons preceded the soft ones in a way reminiscent of the 5-Hz QPO, strongly suggesting that both features are physically related. We discuss our finding in the framework of low-frequency QPOs and their properties in BHCs.