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Mathematisch-Naturwissenschaftliche FakultätAstrophysik I.Physik

NIR and X-ray variability and polarisation

Sgr A* as seen in X-Rays, near-infrared and longer wavelengths, is a highly variable source associated with a supermassive black hole at the center of the Milky Way. Its flux-density excursions, also called flares, are strongest in the X-Ray regime where values reaching to two orders of magnitude those at faintest states can be observed nearly once a day. In contrast, approximately four near-infrared flares are detected per day, but the brightest are maximum one order of magnitude above the quiescent level. With statistical investigations of CHANDRA data we try to understand the nature of these flares, respectively the physical link between the X-ray and lower frequency flares. We find indications of a synchrotron self-Compton process being responsible for the observed X-ray outbursts.

Sgr A* is also strongly polarized in the near-infrared with typical polarisation degrees of 10 – 20% and a preferred polarisation angle of 13 ± 15 degrees. The distributions of polarized and total flux-densities are both consistent with a single-state power-law distribution (with slope of ~4). These facts indicate that the source geometry and the energetics of the accretion process within the Sgr A* system are quite stable. The preferred polarisation angle may be associated with the jet components close-to or at the foot point of the jet.  

We intensively monitor the SgrA* variable activity particularly in the near-infrared, but also through multiwavelength campaigns as part of international collaborations.

Lineal polarisation direction of the 2.2micron emission of SgrA*. Bottom points show the near-infrared flux variations of SgrA* observed between 2006 and 2009 as a concatenated light curve. The background L-band image showing the central 20 x 20 arcseconds of the Milky Way was obtained with NACO VLT(ESO).

Credits: Eckart group, Ph1-UniKoeln.

Selected publications: