Constraining particle acceleration in Sgr A^⋆with simultaneous GRAVITY,Spitzer,NuSTAR, andChandraobservations

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 ± 0.2; the spectral slope observed in the X-ray band is νLν ∝ 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 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.

Multiwavelength Variability of Sagittarius A* in 2019 July

“…The baseline flux density of these IR light curves is unknown, though the value has been inferred to be 1.9 mJy from the cumulative distributions of flux densities of Sgr A * (Witzel et al 2018). As in Abuter et al (2021), we added an offset of 1.9 mJy and dereddened the resulting values with the extinction A Ks = 2.42 ± 0.002 from Fritz et al (2011) to produce the light curves plotted in Figure 1.…”

“…The X-ray light curves are plotted in purple in Figure 1. Flux densities for SED modeling (Section 4) were corrected for dust scattering and absorption as described in Abuter et al (2021).…”

“…The K-band (2.1−2.4 μm) GRAVITY light curve was derived from the coherent flux measurement of Sgr A * as described by Gravity Collaboration et al (2020b) and Abuter et al (2021). We derived the flux ratios relative to S2 using separate observations.…”

“…The H-band light curve was also reduced but not used here, as the lower signal-to-noise ratio provided negligible improvement over the K-band data in constraining the timing. See Abuter et al (2021) for details.…”

“…Here we extend our original study by investigating the physical and temporal correlations between X-ray and IR variability with Spitzer and Chandra observations of Sgr A * in the summer of 2019, alongside simultaneous NuSTAR, GRAVITY, and Atacama Large Millimeter/submillimeter Array (ALMA) monitoring. To constrain the particle accretion responsible for flaring, Abuter et al (2021) analyzed the Spitzer, GRAVITY, NuSTAR, and Chandra data of July 17 −18 in the context of time-resolved SED modeling and found that the NIR and X-ray flare can be best modeled with a nonthermal synchrotron source. Michail et al (2021) combined the Spitzer NIR measurements with the 340 GHz ALMA measurements on July 18 to explore models that describe the NIR as SSC of a synchrotron source responsible for delayed submillimeter emission adiabatically expanding.…”