Common Patterns in the Evolution between the Luminous Neutron Star Low-Mass X-ray Binary Subclasses

TL;DR

This study analyzes the evolution of neutron star low-mass X-ray binaries' spectral states, revealing that changes in mass accretion rate drive subclass transitions and that these behaviors are consistent across multiple sources.

Contribution

It demonstrates that secular evolution in NS-LMXBs is primarily driven by mass accretion rate changes, supported by detailed analysis of multiple sources over 16 years.

Findings

Cyg X-2 and Cir X-1 show similar CD/HID evolution to XTE J1701-462.

GX 13+1 exhibits unique but related CD/HID behavior, supporting its classification as a Z source.

Mass accretion rate is confirmed as the key parameter distinguishing NS-LMXB subclasses.

Abstract

The X-ray transient XTE J1701-462 was the first source observed to evolve through all known subclasses of low-magnetic-field neutron star low-mass X-ray binaries (NS-LMXBs), as a result of large changes in its mass accretion rate. To investigate to what extent similar evolution is seen in other NS-LMXBs we have performed a detailed study of the color-color and hardness-intensity diagrams (CDs and HIDs) of Cyg X-2, Cir X-1, and GX 13+1 -- three luminous X-ray binaries, containing weakly magnetized neutron stars, known to exhibit strong secular changes in their CD/HID tracks. Using the full set of Rossi X-ray Timing Explorer Proportional Counter Array data collected for the sources over the 16 year duration of the mission, we show that Cyg X-2 and Cir X-1 display CD/HID evolution with close similarities to XTE J1701-462. Although GX 13+1 shows behavior that is in some ways unique, it also exhibits similarities to XTE J1701-462, and we conclude that its overall CD/HID properties strongly indicate that it should be classified as a Z source, rather than as an atoll source. We conjecture that the secular evolution of Cyg X-2, Cir X-1, and GX 13+1 -- illustrated by sequences of CD/HID tracks we construct -- arises from changes in the mass accretion rate. Our results strengthen previous suggestions that within single sources Cyg-like Z source behavior takes place at higher luminosities and mass accretion rates than Sco-like Z behavior, and lend support to the notion that the mass accretion rate is the primary physical parameter distinguishing the various NS-LMXB subclasses.