  Hot Jupiters have been proposed as a likely population of low frequency radio
sources due to electron cyclotron maser emission of similar nature to that
detected from the auroral regions of magnetized solar system planets. Such
emission will likely be confined to specific ranges of orbital/rotational phase
due to a narrowly beamed radiation pattern. We report on GMRT 150 MHz radio
observations of the hot Jupiter Tau Bootis b, consisting of 40 hours carefully
scheduled to maximize coverage of the planet's 79.5 hour orbital/rotational
period in an effort to detect such rotationally modulated emission. The
resulting image is the deepest yet published at these frequencies and leads to
a 3-sigma upper limit on the flux density from the planet of 1.2 mJy, two
orders of magnitude lower than predictions derived from scaling laws based on
solar system planetary radio emission. This represents the most stringent upper
limits for both quiescent and rotationally modulated radio emission from a hot
Jupiter yet achieved and suggests that either a) the magnetic dipole moment of
Tau Bootis b is insufficient to generate the surface field strengths of > 50
Gauss required for detection at 150 MHz or b) Earth lies outside the beaming
pattern of the radio emission from the planet.
