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 slope (and its evolution) during a very bright flare from Sgr A ⋆ , the supermassive black hole at the Milky Way's center. Our study arises from ambitious multi-wavelength monitoring campaigns with XMMNewton, NuSTAR and SINFONI. The average multi-wavelength spectrum is well reproduced by a broken power-law with Γ N IR = 1.7 ± 0.1 and Γ X = 2.27 ± 0.12. The difference in spectral slopes (∆Γ = 0.57 ± 0.09) strongly supports synchrotron emission with a cooling break. The flare starts first in the NIR with a flat and bright NIR spectrum, while X-ray radiation is detected only after about 10 3 s, when a very steep X-ray spectrum (∆Γ = 1.8 ± 0.4) is observed. These measurements are consistent with synchrotron emission with a cooling break and they suggest that the high energy cut-off in the electron distribution (γ max ) induces an initial cut-off in the optical-UV band that evolves slowly into the X-ray band. The temporal and spectral evolution observed in all bright X-ray flares are also in line with a slow evolution of γ max . We also observe hints for a variation of the cooling break that might be induced by an evolution of the magnetic field (from B ∼ 30 ± 8 G to B ∼ 4.8 ± 1.7 G at the X-ray peak). Such drop of the magnetic field at the flare peak would be expected if the acceleration mechanism is tapping energy from the magnetic field, such as in magnetic reconnection. We conclude that synchrotron emission with a cooling break is a viable process for Sgr A ⋆ 's flaring emission.

X-ray spectra of Sagittarius A East and diffuse X-ray background near the Galactic center

“…using Γ =2.3 (Ponti et al 2017). The observed luminosity of Sgr A East is ∼ 7 × 10 34 erg s −1 in the 5-10 keV band (Koyama et al 2007b).…”

A possible model for the long-term flares of Sgr A*

“…They showed the quiescent model with a strong magnetic field of | B |∼ 20 G at R ≤ 10R g and the X-ray flare model with a weak magnetic field | B |≤ 1 G in the emitting region. From the best fit parameters of the mean spectrum of very bright flares, Ponti et al (2017) showed that large magnetic field amplitude (| B |∼ 30 G) is observed at the start of the X-ray flare and then drops to | B |∼ 4.8 G at the peak of the X-ray flare. This scenario to the rapid flares may be adaptable also to the long-term flares of Sgr A*.…”

“…The observations of Sgr A* showed that the durations of the X-ray and IR flares are typically of 1 -3 hrs and the flare events usually occur a few times per day and that the observed emission at radio and IR flares roughly vary by factors of 1/2 and 1 − 5 (Genzel et al 2003;Ghez et al 2004;Eckart et al 2006;Meyer et al 2006a,b;Trippe et al 2007;Yusef-Zadeh et al 2009, 2011. While the observed flare emission at X-ray wavelength varies by more than two orders of magnitude with respect to the quiescent state (Ponti et al 2017).…”

“…Motivated by their works, we examined the low angular 3 momentum flow model for Sgr A* using 2D time-dependent hydrodynamic calculations and discussed the implication of their results on the activity of Sgr A* (Okuda & Molteni 2012;Okuda 2014;Okuda & Das 2015). On the other hand, the observational spectra of Sgr A* show a synchrotron emission component which is presumably driven by the magnetic field around Sgr A* (Ponti et al 2017). Therefore, a necessary complementary step to these studies is to examine the magnetohydrodynamical accretion flow with low angular momentum.…”

A Nonthermal Bomb Explains the Near-infrared Superflare of Sgr A*

“…X-ray flares usually follow the NIR ones after a few tens of minutes, but there are multiple NIR flares without a X-ray counterpart (e.g. Eckart et al 2006;Yusef-Zadeh et al 2012;Ponti et al 2017) (but see Fazio et al 2018). Flares are also observed in mm and submm wavelengths (e.g.…”