RossiXTE monitoring of 4U 1636-53: I. Long-term evolution and kHz Quasi-Periodic Oscillations

TL;DR

This study monitored the neutron star binary 4U 1636-53 over 1.5 years, revealing long-term oscillations and variable kHz QPOs that depend on spectral states, providing insights into the source's timing behavior.

Contribution

First long-term RXTE monitoring campaign of 4U 1636-53 revealing state transitions and detailed kHz QPO frequency distributions.

Findings

Detected ~30-40 day long-term oscillations between spectral states.

Observed kHz QPOs predominantly in the soft state.

Found the lower kHz QPO frequency distribution differs from previous observations.

Abstract

We have monitored the atoll-type neutron star low-mass X-ray binary 4U 1636-53 with the Rossi X-Ray Timing Explorer (RXTE) for more than 1.5 years. Our campaign consisted of short (~2 ks) pointings separated by two days, regularly monitoring the spectral and timing properties of the source. During the campaign we observed a clear long-term oscillation with a period of ~30-40 days, already seen in the light curves from the RXTE All-Sky Monitor, which corresponded to regular transitions between the hard (island) and soft (banana) states. We detected kHz QPOs in about a third of the observations, most of which were in the soft (banana) state. The distribution of the frequencies of the peak identified as the lower kHz QPO is found to be different from that previously observed in an independent data set. This suggests that the kHz QPOs in the system shows no intrinsically preferred frequency.