The aperiodic variability of the Ultraluminous X-ray source in NGC 5408

TL;DR

This paper analyzes X-ray timing data from the ULX in NGC 5408 to study its variability, providing insights into its nature and potential evidence for intermediate mass black holes.

Contribution

It presents the first detailed analysis of broad-band noise variability in the ULX in NGC 5408, revealing patterns similar to black-hole binaries in hard states.

Findings

Anti-correlation between variability and intensity.

Similarity to black-hole binaries during hard states.

Supports the black hole accretion model for ULXs.

Abstract

Ultra-Luminous X-ray sources are accreting black holes that might represent strong evidence of the Intermediate Mass Black Holes (IMBH), proposed to exist by theoretical studies but with no firm detection (as a class) so far. We analyze the best X-ray timing and spectral data from the ULX in NGC 5408 provided by XMM-Newton. The main goal is to study the broad-band noise variability of the source. We found an anti-correlation of the fractional root-mean square variability versus the intensity of the source, similar to black-hole binaries during hard states.