Flares from the black hole

 

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

 

 

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.

 

 

 

<em>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</em>)
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)

 

Whether the substructure is due to an orbital motion is subject of current research - if it would, it would be an extremely valuable tool for constraining the spin of the black hole - the second and last parameter defining an astrophysical black hole, after the mass is known.

 

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. A simple model that can explain the data is of a “hotspot” orbiting the black hole (Hamaus et al. 2009), left side of the figure below. More physically motivated models fail to produce infrared flares with the observed characteristic, and predict a more chaotic motion (right side).

 

 

<p><em>Predicted near-infrared images of Sgr A*. Left: from a “hotspot” model. Right: from an MHD simulation of an accretion flow with angular momentum axis misaligned to the black hole spin axis. The orbital motion of the hotspot leads to a smooth track of centroid with time (bottom left), while the MHD simulation produces a more chaotic centroid track (right). Yet, the MHD simulation fails to produce realistic radiation characteristics in the near-infrared.</em></p> Zoom Image

Predicted near-infrared images of Sgr A*. Left: from a “hotspot” model. Right: from an MHD simulation of an accretion flow with angular momentum axis misaligned to the black hole spin axis. The orbital motion of the hotspot leads to a smooth track of centroid with time (bottom left), while the MHD simulation produces a more chaotic centroid track (right). Yet, the MHD simulation fails to produce realistic radiation characteristics in the near-infrared.

 

 

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.

 
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