Multiband study of NGC 7424 and its two newly discovered ultraluminous X-ray sources

We have studied the face-on, barred spiral NGC7424 (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 (∼10⁴⁰ erg s^-1) has a power-law-like spectrum with photon index Γ ≈ 1.8. The other ULX shows a spectral state transition or outburst between the two Chandra observations, 20-d 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 α ≈ -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 (α ≈ -0.3), we suggest that it is a young pulsar wind nebula, a factor of 10 more radio luminous than the Crab.