1999A&A...345L..35W

We report for the first time on the rapid X-ray variability of the galactic bulge source and X-ray burster SLX 1735-269. The power spectrum as observed with the Rossi X-ray Timing Explorer is characterized by a strong band-limited noise component which is approximately flat below a 0.1-2.3Hz break frequency; above this frequency the power spectrum declines as a power law of index 0.9. At the highest observed count rate a broad bump is superimposed on this band-limited noise. The power spectrum is very similar to that of other low-luminosity neutron-star low-mass X-ray binaries (LMXBs) and to black-hole candidates when these types of source accrete at their lowest observed mass accretion rates. However, we identify one unusual aspect of the X-ray variability of SLX 1735-269: the break frequency increases when the inferred mass accretion rate decreases. This is the opposite to what is normally observed in other sources. The only source for which the same behavior has been observed is the accretion-powered millisecond X-ray pulsar SAX J1808.4-3658. No coherent millisecond pulsations were observed from SLX 1735-269 with an upper limit on the amplitude of 2.2% rms. Observing this behavior in SLX 1735-269 increases the similarities between SAX J1808.4-3658 and the other neutron star LMXBs for which so far no coherent pulsations have been observed. We expect that other sources will show the same behavior when these sources are studied in detail at their lowest mass accretion rates.