What did we do & why is this interesting? Deep technical dive ahead!
We learn about accreting #BlackHoles studying their spectra & short-term (~millisecond) variability, called timing. However, individually, both approaches leave us with a lot of puzzles - so we try to combine them in spectral-timing.
In the past, we could only do so above 3 keV using RXTE (and a bit using XMM, but not quiet that well - for this kind of study, one needs many short observations at different times and this is not the XMM observing strategy). Utilizing NICER, we can push down to 0.5 keV and thus finally, finally, probe details of the disk variability in different spectral states of the source!
Generally and kinda unsurprisingly, things are a lot more complex at low energies!
People have shown in the past (and M. Böck & I have really driven home for Cyg X-1 in 2011 & 2014) that power spectra (ways to measure contributions to overall variability at different timescales) are highly energy dependent. We confirm that this trend is crucial at low energies.
This means also that when calculating coherence and lags, we cannot work with a broad reference band (lots of literature does!). So this is going to be fun - people will need to change their methods.
We also see a clear change in how the noise at low and hard energies is connected depending on the spectral shape of the source: when the spectrum is hard, the noise at different energies is coherent, implying a connection between the processes producing the noise (or the same process). When the spectrum is soft, this is not the case anymore.
We also see a rather puzzling feature in the hard state at low energies- a jump in time lag, accompanied by a loss of coherence.
We have some ideas what it may be, but no definitive answer. But we do see it in other sources, too! (We haven't seen it before because there was no instrument with NICER's spectral and timing coverage!)
Yes, parts V to VII of this series were my PhD thesis back in the days (and there are two more PhD thesis in the earlier papers of the series). Ole is proudly carrying on the work!
And it's also fun to see how we keep adding more pieces to the puzzle but also how much there is that we still don't know when it comes to variability.
There is far more than one PhD thesis in this source still :D
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