A powerful flare from Sgr A* confirms the synchrotron nature of the X-ray emission

Abstract

We present the first fully simultaneous fits to the NIR and X-ray spectral\nslope (and its evolution) during a very bright flare from Sgr A*, the\nsupermassive black hole at the Milky Way's center. Our study arises from\nambitious multi-wavelength monitoring campaigns with XMM-Newton, NuSTAR and\nSINFONI. The average multi-wavelength spectrum is well reproduced by a broken\npower-law with $\\Gamma_{NIR}=1.7\\pm0.1$ and $\\Gamma_X=2.27\\pm0.12$. The\ndifference in spectral slopes ($\\Delta\\Gamma=0.57\\pm0.09$) strongly supports\nsynchrotron emission with a cooling break. The flare starts first in the NIR\nwith a flat and bright NIR spectrum, while X-ray radiation is detected only\nafter about $10^3$ s, when a very steep X-ray spectrum\n($\\Delta\\Gamma=1.8\\pm0.4$) is observed. These measurements are consistent with\nsynchrotron emission with a cooling break and they suggest that the high energy\ncut-off in the electron distribution ($\\gamma_{max}$) induces an initial\ncut-off in the optical-UV band that evolves slowly into the X-ray band. The\ntemporal and spectral evolution observed in all bright X-ray flares are also in\nline with a slow evolution of $\\gamma_{max}$. We also observe hints for a\nvariation of the cooling break that might be induced by an evolution of the\nmagnetic field (from $B\\sim30\\pm8$ G to $B\\sim4.8\\pm1.7$ G at the X-ray peak).\nSuch drop of the magnetic field at the flare peak would be expected if the\nacceleration mechanism is tapping energy from the magnetic field, such as in\nmagnetic reconnection. We conclude that synchrotron emission with a cooling\nbreak is a viable process for Sgr A*'s flaring emission.\n