Constraining particle acceleration in Sgr A⋆ with simultaneous GRAVITY, Spitzer, NuSTAR, and Chandra observations
This paper (with a much too long title) uses observations from 4 different telescopes to measure the near infrared and X-ray emission of Sgr A*. One instrument is the GRAVITY interferometer at the VLTI in Chile. The three others are Space Telescopes which measure the light of Sgr A* in the mid-infrared (Spitzer), and the X-ray (NuSTAR and Chandra). With this setup we caught one very bright flare of Sgr A* and can constrain how it's emission changes across the different wavelengths as function of time.
Above: The Spectral Energy Distribution (SED) of the bright flare of Sgr A* which we observed. Our measurements show how it changes as function of time.
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Left: The plot shows the constraints on the emission zone which we derived from our observations.
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Right: The underling data that lead to the measurements: Five light curves obtained on three different spaces crafts and the GRAVITY instrument at the Paranal observatory.
Scientific Abstract
We report the time-resolved spectral analysis of a bright near-infrared and moderate X-ray flare of Sgr A⋆. We obtained light curves in the M, K, and H bands in the mid- and near-infrared and in the 2 − 8 keV and 2 − 70 keV bands in the X-ray. The observed spectral slope in the near-infrared band is νLν ∝ ν0.5 ±â€…0.2; the spectral slope observed in the X-ray band is νLν ∝ ν−0.7 ±â€…0.5. Using a fast numerical implementation of a synchrotron sphere with a constant radius, magnetic field, and electron density (i.e., a one-zone model), we tested various synchrotron and synchrotron self-Compton scenarios. The observed near-infrared brightness and X-ray faintness, together with the observed spectral slopes, pose challenges for all models explored. We rule out a scenario in which the near-infrared emission is synchrotron emission and the X-ray emission is synchrotron self-Compton. Two realizations of the one-zone model can explain the observed flare and its temporal correlation: one-zone model in which the near-infrared and X-ray luminosity are produced by synchrotron self-Compton and a model in which the luminosity stems from a cooled synchrotron spectrum. Both models can describe the mean spectral energy distribution (SED) and temporal evolution similarly well. In order to describe the mean SED, both models require specific values of the maximum Lorentz factor γmax, which differ by roughly two orders of magnitude. The synchrotron self-Compton model suggests that electrons are accelerated to γmax ∼ 500, while cooled synchrotron model requires acceleration up to γmax ∼ 5 × 104. The synchrotron self-Compton scenario requires electron densities of 1010 cm−3 that are much larger than typical ambient densities in the accretion flow. Furthermore, it requires a variation of the particle density that is inconsistent with the average mass-flow rate inferred from polarization measurements and can therefore only be realized in an extraordinary accretion event. In contrast, assuming a source size of 1 RS, the cooled synchrotron scenario can be realized with densities and magnetic fields comparable with the ambient accretion flow. For both models, the temporal evolution is regulated through the maximum acceleration factor γmax, implying that sustained particle acceleration is required to explain at least a part of the temporal evolution of the flare.