Anomalous Low States and Long Term Variability in the Black Hole Binary LMC X-3

TL;DR

This paper reports on unprecedented long and low luminosity X-ray states in the black hole binary LMC X-3, analyzing their characteristics and suggesting a common mechanism with neutron star Her X-1 involving a precessing accretion disk.

Contribution

It identifies and characterizes two extended low states in LMC X-3, compares their variability with Her X-1, and links the long-term variability to a possible warped, precessing accretion disk mechanism.

Findings

First low state lasted over 3 months with minimal variability.

Second low state lasted nearly twice as long with significant variability.

Long-term variability amplitude correlates with timescale, similar to Her X-1.

Abstract

Rossi X-ray Timing Explorer observations of the black hole binary LMC X-3 reveal an extended very low X-ray state lasting from 2003 December 13 until 2004 March 18, unprecedented both in terms of its low luminosity (>15 times fainter than ever before seen in this source) and long duration (~3 times longer than a typical low/hard state excursion). During this event little to no source variability is observed on timescales of ~hours-weeks, and the X-ray spectrum implies an upper limit of 1.2x10^35 erg s^-1. Five years later another extended low state occurs, lasting from 2008 December 11 until 2009 June 17. This event lasts nearly twice as long as the first, and while significant variability is observed, the source remains reliably in the low/hard spectral state for the ~188 day duration. These episodes share some characteristics with the "anomalous low states" in the neutron star binary Her X-1. The average period and amplitude of the variability of LMC X-3 have different values between these episodes. We characterize the long-term variability of LMC X-3 before and after the two events using conventional and nonlinear time series analysis methods, and show that, as is the case in Her X-1, the characteristic amplitude of the variability is related to its characteristic timescale. Furthermore, the relation is in the same direction in both systems. This suggests that a similar mechanism gives rise to the long-term variability, which in the case of Her X-1 is reliably modeled with a tilted, warped precessing accretion disk.