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

The discovery of source states in the X-ray emission of black-hole binariesand neutron-star low-mass X-ray binaries constituted a major step forward inthe understanding of the physics of accretion onto compact objects. While thereare numerous studies on the correlated timing and spectral variability of thesesystems, very little work has been done on high-mass X-ray binaries, the thirdmajor type of X-ray binaries. The main goal of this work is to investigatewhether Be accreting X-ray pulsars display source states and characterise thosestates through their spectral and timing properties. We have made a systematicstudy of the power spectra, energy spectra and X-ray hardness-intensitydiagrams of nine Be/X-ray pulsars. The evolution of the timing and spectralparameters were monitored through changes over two orders of magnitude inluminosity. We find that Be/X-ray pulsars trace two different branches in thehardness-intensity diagram: the horizontal branch corresponds to alow-intensity state of the source and it is characterised by fast colour andspectral changes and high X-ray variability. The diagonal branch is ahigh-intensity state that emerges when the X-ray luminosity exceeds a criticallimit. The photon index anticorrelates with X-ray flux in the horizontal branchbut correlates with it in the diagonal branch. The correlation between QPOfrequency and X-ray flux reported in some pulsars is also observed if the peakfrequency of the broad-band noise that accounts for the aperiodic variabilityis used. The two branches may reflect two different accretion modes, dependingon whether the luminosity of the source is above or below a critical value.This critical luminosity is mainly determined by the magnetic field strength,hence it differs for different sources.

Patterns of variability in Be/X-ray pulsars during giant outbursts