  We report on observations of Markarian 421 made with the Whipple 10m Cerenkov
telescope and the RXTE satellite during a multi-wavelength campaign in December
2002 and January 2003, initiated by a Whipple target of opportunity. The
observations revealed several flares with flux levels between 1 and 2 times the
flux of the Crab Nebula. We will discuss the temporal properties, including
evidence of X-ray/TeV gamma-ray flux correlation.

Astrophysics