English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Tools annual

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Presents the access of premium content as premium feature

Premium Content

Global: Zendy Plus annual

Global: Zendy Free Plan

We present the analysis of the X-ray variability and spectral timingproperties of the ultraluminous X-ray source (ULX) NGC 5408 X-1, one of themost variable ULXs known so far. The variability properties are used as adiagnostic of the accretion state of the source and to derive estimates of theblack hole (BH) mass. The observed high level of fast X-ray variability(fractional root-mean-square variability - rms - amplitude of $\sim$ 30 percentin the hard energy band), the hardening of the fractional rms spectrum, and theproperties of the QPO, all resemble those of a source in a hard-intermediateaccretion state. We confirm the previous detection of a soft lag in the X-raylight curves of the source during 2006 and 2008 observations and find that thesoft lag is still present in the more recent 2010/2011 observations. Using theentire available XMM-Newton data set (public as of February 2012) we observethat the soft lag (of few seconds amplitude) is detected over a relativelylarge range of frequencies ($\nu \sim 5-90$ mHz), which always includes the QPOfrequencies. The soft lag displays energy-dependence, with the (absolute)amplitude increasing as a function of energy separation. We find closeanalogies with soft lags associated to type-C QPOs in BH binary systems(although an association to other types of QPOs cannot be completely excluded),as well as with reverberation lags observed in AGN. In both cases anintermediate mass BH (IMBH) solution appears the most plausible.

Time lags in the ultraluminous X-ray source NGC 5408 X-1: implications for the black hole mass