We have studied the face-on, barred spiral NGC 7424 (site of the rare Type
IIb SN 2001ig) with Chandra, Gemini and the Australia Telescope Compact Array.
After giving revised X-ray colours and luminosity of the supernova, here we
focus on some other interesting sources in the galaxy: in particular, our
serendipitous discovery of two ultraluminous X-ray sources (ULXs). The brighter
one (~ 10^{40} erg/s) has a power-law-like spectrum with photon index Gamma ~
1.8. The other ULX shows a spectral state transition or outburst between the
two Chandra observations, 20 days apart. Optical data show that this ULX is
located in a young (age ~ 7-10 Myr), bright complex rich with OB stars and
clusters. An exceptionally bright, unresolved radio source (0.14 mJy at 4.79
GHz, implying a radio luminosity twice as high as Cas A) is found slightly
offset from the ULX (~ 80 pc). Its radio spectral index alpha ~ -0.7 suggests
optically-thin synchrotron emission, either from a young supernova remnant or
from a radio lobe powered by a ULX jet. An even brighter, unresolved radio
source (0.22 mJy at 4.79 GHz) is found in another young, massive stellar
complex, not associated with any X-ray sources: based on its flatter radio
spectral index (alpha ~ -0.3), we suggest that it is a young pulsar wind
nebula, a factor of 10 more radio luminous than the Crab.Comment: 12 pages, 11 figures. Accepted by MNRAS. High-resolution colour
images of NGC 7424 from the Gemini observations are available at
http://www.gemini.edu/2001igp
