Discovery of kilohertz quasi-periodic oscillations and state transitions in the low-mass X-ray binary 1E~1724-3045 (Terzan 2)

TL;DR

This study reports the discovery of kilohertz quasi-periodic oscillations in the low-mass X-ray binary 1E 1724-3045, confirming its atoll nature and analyzing its timing behavior across different states using RXTE data.

Contribution

First detection of kHz QPOs in 1E 1724-3045, expanding understanding of neutron-star LMXB timing properties and state transitions.

Findings

Detected lower and upper kHz QPOs, but not simultaneously.

Timing behavior similar to other neutron-star LMXBs.

Flux transitions linked to changes in accretion rate.

Abstract

We have studied the rapid X-ray time variability in 99 pointed observations with the Rossi X-ray Timing Explorer (RXTE)'s Proportional Counter Array of the low-mass X-ray binary 1E~1724--3045 which includes, for the first time, observations of this source in its island and banana states, confirming the atoll nature of this source. We report the discovery of kilohertz quasi-periodic oscillations (kHz QPOs). Although we have 5 detections of the lower kHz QPO and one detection of the upper kHz QPO, in none of the observations we detect both QPOs simultaneously. By comparing the dependence of the rms amplitude with energy of kHz QPOs in different atoll sources, we conclude that this information cannot be use to unambiguously identify the kilohertz QPOs as was previously thought. We find that Terzan~2 in its different states shows timing behavior similar to that seen in other neutron-star low mass X-ray binaries (LMXBs). We studied the flux transitions observed between February 2004 and October 2005 and conclude that they are due to changes in the accretion rate.