Hysteresis in the spectral states of the neutron star low-mass X-ray binary EXO 1745-248

TL;DR

This study analyzes the spectral and timing evolution of the neutron star binary EXO 1745-248, revealing unique hysteresis behavior and oscillations that challenge typical atoll source patterns, offering insights into black hole and neutron star accretion.

Contribution

It reports the first detailed spectral-timing analysis of EXO 1745-248, identifying unusual hysteresis and oscillation features that bridge black hole and neutron star accretion behaviors.

Findings

Discovery of a ~0.45 Hz QPO and ~10 Hz peaked noises.

Unusual clockwise hysteresis in color-color and hardness-intensity diagrams.

Extended transition to softer states over 2.5 months.

Abstract

We study the low-frequency timing properties and the spectral state evolution of the transient neutron star low-mass X-ray binary EXO 1745-248 using the entire Rossi X-ray Timing Explorer Proportional Counter Array data. We tentatively conclude that EXO 1745-248 is an atoll source, and report the discovery of a ~ 0.45 Hz low-frequency quasi-periodic oscillation and ~ 10 Hz peaked noises. If it is an atoll, this source is unusual because (1) instead of a `C'-like curve, it traced a clear overall clockwise hysteresis curve in each of the colour-colour diagram and the hardness-intensity diagram; and (2) the source took at least 2.5 months to trace the softer banana state, as opposed to a few hours to a day, which is typical for an atoll source. The shape of the hysteresis track was intermediate between the characteristic `q'-like curves of several black hole systems and `C'-like curves of atolls, implying that EXO 1745-248 is an important source for the unification of the black hole and neutron star accretion processes.