Flares from the black hole


Our group has discovered the (sporadic) infrared emission of Sgr A* (Genzel et al. 2003).



Example of a Sgr A* flare light curve. Top: The infrared light curve showing substructure on a time-scale of 20 to 30 minutes. Bottom: Simultaneously, an X-ray flare has been observed (Dodds-Eden et al. 2009)

The light curves show excursions that typically last an hour, and often exhibit tantalizing variations on the orbital time scale of the last stable orbit around the massive black hole - 20 to 30 minutes.

During the flares, a small region in the accretion flow appears to heat up electrons much beyond equilibrium, the synchrotron emission of which shines in the near-infrared and even up to the X-ray regime (Dodds-Eden et al. 2009)


The energy released in flares suggests that they originate in the inner ~ 10 Schwarzschild radii, but their physical origin remains uncertain. The high amplitudes and short timescales (factors ~ 10 with timescales ~ 30 minutes), non-linear flux distribution (Dodds-Eden et al. 2011), short time scale changes in the polarization, and possible quasi-periodicities in bright flares (Genzel et al. 2003) suggest a compact region responsible for accelerating electrons and emitting the observed synchrotron radiation.

Observations during the summer of 2018 with the new GRAVITY instrument revealed that the emission during an infrared flare consistently moves in a clockwise loop a few times bigger than the event horizon of the black hole (Gravity Collaboration et al. 2018). These loops are consistent with a small region of heated electrons (a "hotspot"), moving in an orbit around the black hole.



Motion of the centroid of emission during a flare on July 22, 2018 (Gravity Collaboration etal. 2018). From lower left, the panels show (a) The X and Y positions during the flare (blueand red points, respectively) as well as the lightcurve (black points). (b) The motionprojected onto the plane of the sky, color coded by the time proceeding from red to blue.(c) The X and Y data along with a model consisting of a hotspot on a relativistic orbit nearthe black hole. (d) The astrometric data and model fit on the plane of the sky.

Measured polarization during a flare on July 28, 2018. Over the course of the flare, the polarization angle completed a full loop in the Q-U plane.

In addition to the astrometric signature of an orbit, GRAVITY observations also revealed changes in the polarization angle over the course of the flare.In particular, as the centroid of the emission region completes one orbit around the black hole, the polarization angle also makes a single loop in the Q-U plane. These polarization measurements indicate the presence of a strong magnetic field in the immediate vicinity of the black hole.



The motion of the centroid of the flare emission might well move by around 100 micro-arcseconds during half an hour - a tiny angle, yet large enough such that in the future we might be able to see this motion using GRAVITY.

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

Sgr A*, the presumed massive black hole at the centre of the Milky Way, emits light in a wide range of frequencies of the electromagnetic spectrum. In what is known as the “quiescent state”, most the flux is emitted at submillimetre wavelengths. However, there are “flaring” events in which the Near-Infrared (NIR) and X-ray fluxes increase by factors of up to 10 and 100, respectively. Studying the properties of this flaring state can help us understand the emission mechanisms at play, magnetic field structure and the properties of the accretion flow.

In August 2014, for the first time, Sgr A* was observed in a bright flaring state at different energies simultaneously: in X-rays with XMM–Newton, hard X-rays with NuSTAR and in the NIR with SINFONI.

Observed simultaneous flare in three spectral bands: black and red points show the XMM-Newton and NuSTAR observations respectively, squares are the NIR extinction corrected SINFONI data. The flux has been renormalised to that at 2.2 μm in mJy units.

By doing a simultaneous fit to the mean properties of the flare at these different energies and its evolution in time, it was found that the multi-wavelength spectrum is well reproduced with an electron distribution given by broken power law, and that the difference in spectral slopes strongly supports synchrotron emission with a cooling break.

The spectral and temporal evolution of the flare flux maximum point to a variation of the cooling break induced by decrease in magnetic field strength. Such drop would be expected if the acceleration mechanism of the electrons is tapping energy from the magnetic field, like in magnetic reconnection scenarios. It is concluded that synchrotron emission with a cooling break is a viable process for Sgr A*’s flaring emission.

NIR (red) and X-ray (black) emission during the flare. The dotted red and black straight lines show the uncertainties on the determination of the NIR and X-ray power-law slope, respectively. The black solid line shows the best fit Synchrotron with cooling break model while the blue solid line shows the best fit Synchrotron self Compton model.
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