State-Dependent Orbital Modulation of X-rays in CYG X-3

TL;DR

This study analyzes RXTE data of Cyg X-3, revealing state-dependent orbital X-ray modulation patterns, challenging existing models of wind absorption and jet contributions, and providing new insights into its complex state behavior.

Contribution

First comprehensive analysis of orbital X-ray modulation across all states of Cyg X-3, revealing unique energy-dependent behaviors and challenging previous assumptions about its emission mechanisms.

Findings

Orbital X-ray modulation amplitude increases from hard to soft states.

In the soft non-thermal state, hard X-ray modulation exceeds soft X-ray modulation.

Results challenge wind absorption and jet origin models for X-ray modulation.

Abstract

We analyze all of the available {\it RXTE} observations of Cyg X-3 in order to investigate the connection between the central X-ray source and its surrounding environment. The hardness--intensity diagram of Cyg X-3 displays a "shoe" shape rather than the Q-type shape commonly seen in other black hole X-ray binary, and exhibits no apparent hysteresis effect. During the $\gamma$-ray outbursts, no existing data are located in the hard and intermediate states, which suggest the absence of a significant population of non-thermal electrons when the source is in these states. For the first time, we present the orbital modulation of the X-ray light curve (LC) of all five states. The different energy band LCs are in phase with each other in all five states, and the modulation amplitude of both soft and hard X-ray LCs monotonously increases with decreasing hardness from hard to soft non-thermal states. We confirm that the modulation depth decreases with increasing energy in the hard, intermediate, and very high states, as originally reported by Zdziarski et al. However, in the soft non-thermal state, the hard X-ray modulation strength significantly increases and is even larger than the soft X-ray one. Our results rule out both wind absorption and jet origins of the hard X-ray LC modulation in the soft non-thermal state, and challenge our understanding of the states of Cyg X-3.