A first detection of Sgr A* in the midinfrared

credit CfA/Mel Weiss

JWST reveals Sgr A*'s midinfrared emission and provides a new handle on the black holes magnetic field

JWST's observations reveal a bright flare which we detect from 5 almost 20 microns. The flare is reminiscent of flares observed in the NIR, for instance, it shows a triple-peaked structure, which is often interpreted as Doppler modulation from a hot spot orbiting the black hole. Since we do not have astrometric, or polarimetric observations, we cannot directly interpret this light curve shape.

However, the flare shows another interesting feature, which is readily identified in the video above. If you look closely, you will find that while the first two peaks in the light curve are almost the same height in both bands, the third peak is higher in the channel 2 data, at 8.6 microns!

This is important, as it indicates that the spectral shape changes during the flare!

Explicitly, when we look at the full data set below, one can spot that this trend is also present in the other light curves at longer wavelengths (12 and 19 microns).

Why is this relevant?

The reason that such a spectral index is important to measure lies in the nature of the emission that we observe: synchrotron emission. Synchrotron is generated when very fast electrons are embedded in a magnetic field, which lets them spiral whereby they generate light. Due to the emission of this light, the photons cool, i.e. they lose energy. Now if you dig in your favourite physics handbook you will find that in a situation where a fraction of electrons are constantly accelerated while also cooling through the emission of synchrotron radiation a so-called cooling break will appear in the spectrum. The cool thing about the cooling break is that it only depends on the strength of the magnetic field in which the electrons are embedded, and on nothing else.

Now in our case, everything is a bit more complicated, but can still be evaluated on a standard issue laptop. When we do this, and generate model light curves which we optimize to best reproduce the observed light curve we can use this dependence of the cooling break on the magnetic field to measure it in Sgr A*'s accretion flow (or more precisely the region where the electrons are accelerated. This is what we do, and find a magnetic field strength of 40-100 Gauss!

Here's an illustration that I made, take it with a grain of salt!

Here's a link to the pre-print of the accepted paper (ApJL).