The nearest known microquasar and black
hole to the Sun, at a distance of about 1,500 light-years. V4641 Sgr
is a binary system consisting of a black hole primary of about 10 solar
masses and a normal secondary star of 5-8 solar masses. The pair has an
orbital period of 2.8 days and at quiescence has an apparent magnitude of
13.8. In 1999 V4641 Sgr underwent a very strong but very short outburst.
Radio observations revealed relativistic jets associated with this outburst.
During another outburst in 2002, the RXTE satellite
observed a strong X-ray flux. Amateur astronomers
observed it at optical wavelengths, and afterwards when it returned to the
quiescent state. This period was marked by very short, powerful flares,
particularly at quiescence. Makato Uemura and his colleagues (Pub Ast.
Soc. Japan, May 2002) have suggested this was due to synchrotron radiation
in the inner disk near the black hole's event horizon, as the powerful and
turbulent magnetic field of the black hole underwent recombination.
|V4641 Sgr imaged just after emitting an outburst
in the radio band. Jets, which lasted only minutes, are visible. Image: