X-ray variability and energy spectra from NGC5408 X-1 with XMM-Newton

TL;DR

This study analyzes X-ray timing and spectral data from NGC 5408 X-1, revealing similarities to black hole binaries and suggesting the source is in a bright hard-intermediate accretion state.

Contribution

It applies state classification tools from black hole binaries to ULX sources, providing new insights into their spectral and timing properties.

Findings

Energy spectra fit by cold disc plus curved high-energy component

Anti-correlation between variability and intensity in 0.0001-0.2Hz range

Indication of a bright hard-intermediate accretion state

Abstract

The notion of source states characterizing the X-ray emission from black hole binaries has revealed to be a very useful tool to disentangle the complex spectral and aperiodic phenomenology displayed by those classes of accreting objects. We seek to use the same tools for Ultra-Luminous X-ray (ULX) sources. We analyzed the data from the longest observations obtained from the ULX source in NGC 5408 (NGC 5408 X-1) taken by XMM-Newton. We performed a study of the timing and spectral properties of the source. In accordance with previous studies on similar sources, the intrinsic energy spectra of the source are well described by a cold accretion disc emission plus a curved high-energy emission component. We studied the broad-band noise variability of the source and found an anti-correlation between the root mean square variability in the 0.0001-0.2Hz and intensity, similarly to what is observed in black-hole binaries during the hard states. We discuss the physical processes responsible for the X-ray features observed and suggest that NGC 5408 X-1 harbors a black hole accreting in an unusual bright hard-intermediate state.