Discovery of 800 Hz Quasi-periodic Oscillations in 4U 1608−52

Abstract

We present results of RXTE observations of the low-mass X-ray binary and atoll source 4U 1608−52 made over 9 days during the decline of an X-ray intensity outburst in March 1996. A fast-timing analysis shows a strong and narrow quasi periodic oscillation (QPO) peak at frequencies between 850 and 890 Hz on March 3 and 6, and a broad peak around 690 Hz on March 9.Observations on March 12 show no significant signal. On March 3, the X-ray spectrum of the QPO is quite hard; its strength increases steadily from 5 % at ∼2 to ∼20 % at ∼12 keV. The QPO frequency varies between 850 and 890 Hz on that day, and the peak widens and its rms decreases with centroid frequency in a way very similar to the well-known horizontal branch oscillations (HBO) in Z-sources. We apply the HBO beat frequency model to atoll sources, and suggest that, whereas the model could produce QPOs at the observed frequencies, the lack of correlation we observe between QPO properties and X-ray count rate is hard to reconcile with this model. Subject headings: stars: individual (4U 1608−52) -stars: neutron -accretion, accretion disks However, in spite of these arguments, the absence of a correlation between X-ray intensity on the one hand and QPO frequency and rms amplitude on the other hand poses a major problem for a beat frequency model interpretation. The QPO frequency remains constant near 850 Hz between 3 and 6 March while the count rate drops by more than a factor 4, from ∼3200 to ∼600 counts/sec (Sect. 2). The beat frequency model predicts that if the QPO frequency remains constant, the mass flow through the inner edge of the disk should remain constant as well. Our data therefore are inconsistent with a beat frequency model interpretation if all accretion takes place via the inner edge of the disk. We note that the presence of a hypothetical additional (non-disk) mass flow component contributing more than 75% of the total flux on March 3 and much weaker on March 6 can not easily resolve this discrepancy. The rms amplitude of the QPO only increases by a factor 2 from March 3 to 6 while the X-ray intensity drops by a factor of more than 4, whereas a similar fractional change would be predicted in this explanation. Only by invoking rather large and entirely ad-hoc changes in the beaming or bolometric correction could the model be maintained. The 150 Hz drop in QPO from March 6 to March 9 without a change in count rate presents similar difficulties. Further observations of 4U 1608−52 during other outbursts, and study of its X-ray bursts are required to shed further light on the relation of the 800 Hz QPO in 4U 1608−52 with those in 4U 1728−34, and with the QPO in Sco X-1. Sztajno M. 1985, Nature 316, 225 van der Klis M., 1989. NATO ASI C262: Timing Neutron Stars,Ögelman and van den Heuvel (eds.), Kluwer, p. 27. van der Klis M. 1995, in X-ray Binaries, Van Paradijs and Van den Heuvel (eds.), Cambridge University Press, p. 252.

Probing X-ray burst – accretion disk interaction in low mass X-ray binaries through kilohertz quasiperiodic oscillations

“…The burst emission dominates in the soft X-ray band while the QPO modulated photons have a harder spectrum (Berger et al 1996).…”

New class of low frequency QPOs: Signature of nuclear burning or accretion disk instabilities?

“…Typically, the fractional rms amplitude of the variability (at least at frequencies ∼Hz or higher) increases with energy for accreting NS LMXBs (see e.g. Lewin et al 1992;Berger et al 1996;Zhang et al 1996;Homan et al 1999). The same is true for the Very Low Frequency Noise (VLFN) in 4U1608-52 and 4U1636-536 at frequencies comparable to the mQPO frequency.…”

Drop of coherence of the lower kilo‐Hz QPO in neutron stars: Is there a link with the innermost stable circular orbit?

“…We have selected 4U 1608-522 because in a rather limited data set, a loss of coherence of the lower QPO was reported by Berger et al (1996) and Barret et al (2005a). The choice of 4U 1735-44 is motivated by the fact that not much is known about its kilo-Hz QPOs (see however Wijnands et al 1998 andFord et al 1998), while the source has been rather extensively observed with RXTE.…”