Discovery of a ∼1 Hz Quasi‐periodic Oscillation in the Low‐Mass X‐Ray Binary 4U 1746−37

We have discovered a ~1 Hz quasi-periodic oscillation (QPO) in the persistentX-ray emission and during type I X-ray bursts of the globular cluster source,dipper and low-mass X-ray binary (LMXB) 4U 1746-37. The QPO properties resemblethose of QPOs found recently in the LMXB dippers 4U 1323-62, and EXO 0748-676,which makes 4U 1746-37 the third source known to exhibit this type of QPOs. Wepresent evidence for X-ray spectral changes in this source similar to thoseobserved in LMXBs referred to as atoll sources. We detect two states, a lowintensity and spectrally hard state, and a higher intensity and spectrally softstate. This may explain the different spectral characteristics reported for 4U1746-37 earlier. The high intensity state resembles the banana branch state ofatoll sources. The QPOs are only seen in the low intensity state, and areabsent when the source is in the banana branch. This strongly suggests thateither the accretion disk or an extended central source change shape betweenthe low intensity state and the banana branch. Twelve bursts were detected, ofwhich 5 took place while the source was on the banana branch and 7 when thesource was in the low intensity state. The bursts occurring on the bananabranch had an e-folding time ~3 times longer than those which occurred in thelow intensity state. Whereas previously detected dips showed only a decrease incount rate of ~15%, we found in one observation a dip in which the count ratedropped from ~200 counts per second to ~20 counts per second. This dip lastedonly ~250 seconds, during which clear spectral hardening occured. This is thefirst time strong evidence for spectral changes during a dip are reported forthis source.Comment: 17 pages, accepted for publication in Ap