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 theM,K, andHbands 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 isνLν ∝ ν0.5 ± 0.2; the spectral slope observed in the X-ray band isνLν ∝ ν−0.7 ± 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 factorγmax, which differ by roughly two orders of magnitude. The synchrotron self-Compton model suggests that electrons are accelerated toγmax ∼ 500, while cooled synchrotron model requires acceleration up toγmax ∼ 5 × 104. The synchrotron self-Compton scenario requires electron densities of 1010cm−3that 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 factorγmax, implying that sustained particle acceleration is required to explain at least a part of the temporal evolution of the flare.
Highlights
It is believed that most galaxies harbor at least one supermassive black hole (BH) at their center (Kormendy & Ho 2013)
Either the flare requires acceleration of electrons by a factor of >103, or it requires electron densities increased by a factor of 102...3 ecm−3 and electron density changes with respect to the submillimeter ambient conditions that cannot be explained from the average accretion flow
For the synchrotron self-Compton (SSC)–SSC scenario, γmax regulates the width of the synchrotron spectrum, which in turn sets the width of the Compton component
Summary
It is believed that most galaxies harbor at least one supermassive black hole (BH) at their center (Kormendy & Ho 2013). Using a fully general relativistic model of a “hot spot”, the authors derived a typical orbital radius of around ∼4.5 RS, constrained the emission regions to ∼2.5 RS, and a viewing angle of i ∼ 140 deg (the inclination of the orbital plane to the line of sight) This model was extended by GRAVITY Collaboration (2020a), who showed that the flare light curves may be modulated by Doppler boosting on the order a few tens of percent. The radiative mechanism was consistent with synchrotron emission all the way from IR to X-ray, implying the presence of a powerful accelerator (with γmax > 105−6) and an evolving cooling break and high-energy cutoff in the distribution of accelerated particles. We report in this work the characterization and evolution of the IR to X-ray spectral energy distribution (SED) during the flare and the implications for our understanding of particle acceleration during the Sgr A flares
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