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We present the UV, optical, X-ray, and radio properties of the Type IIb SN2008ax discovered in NGC 4490. The observations in the UV are one of theearliest of a Type IIb supernova (SN). On approximately day four after theexplosion, a dramatic upturn in the u and uvw1 (lambda_c = 2600 Angstroms)light curves occurred after an initial rapid decline which is attributed toadiabatic cooling after the initial shock breakout. This rapid decline andupturn is reminiscent of the Type IIb SN 1993J on day six after the explosion.Optical/near-IR spectra taken around the peak reveal prominent H-alpha, HeI,and CaII absorption lines. A fading X-ray source is also located at theposition of SN 2008ax, implying an interaction of the SN shock with thesurrounding circumstellar material and a mass-loss rate of the progenitor ofM_dot = (9+/-3)x10^-6 solar masses per year. The unusual time evolution (14days) of the 6 cm peak radio luminosity provides further evidence that themass-loss rate is low. Combining the UV, optical, X-ray, and radio data withmodels of helium exploding stars implies the progenitor of SN 2008ax was anunmixed star in an interacting-binary. Modeling of the SN light curve suggestsa kinetic energy (E_k) of 0.5x10^51 ergs, an ejecta mass (M_ej) of 2.9 solarmasses, and a nickel mass (M_Ni) of 0.06 solar masses.

Multi-Wavelength Properties of the Type IIb SN 2008ax