General relativistic effects and the near-infrared and X-ray variability of Sgr A* I

This paper aims at better understanding the temporal properties of Sgr A*'s brightes flares in the near infrared (NIR) and X-ray. These flares always occur together in the NIR and X-ray, implying some causal connection. In oder to understand what the flares are like, we pick 25 bright NIR and 24 X-ray flares, and stacked them on top of each other. Using an advanced denoising method called Principle Component Analysis, we determined the average flare shape, and found, quite suprisingly that they are indistinguishable in the NIR and X-ray!

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Why is this sur[rising? The associated energies with these observing band is very different, and generally one expects that radiative mechanisms are much faster in the X-ray. For instance, the time it takes an electron to cool of its energry through radiation (synchtroton cooling) is a roughly 1000 faster.

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Why are the shape the same then? We propose that achromatic effects, like relativstic Doppler amplifcation are responsible for much of the observed variability.

 

This also tells us something about the incilination under which we view the black hole: If we were seeing Sgr A*'s accretion flow edge-on, we'd expect that emission is lensed around the black hole, which would cause significant assymetry in the shape of the flares. We don't see this, thus we assume that we see the black hole more face-on, something which is also supported by the EHT radio image, as well as NIR observations of flares by the GRAVITY collaboration!