Abstract
The first low radio frequency (<1.4 GHz) detection of the outburst of the recurrent nova RS Ophiuchi is presented in this Letter. Radio emission was detected at 0.61 GHz on day 20 with a flux density of ~48 mJy and at 0.325 GHz on day 38 with a flux density of ~44 mJy. This is in contrast with the 1985 outburst, when it was not detected at 0.327 GHz even on day 66. The emission at low radio frequencies is clearly nonthermal and is well explained by a synchrotron spectrum of index α ~ -0.8 (S ∝ να) suffering foreground absorption due to the preexisting, ionized, warm, clumpy red giant wind. The absence of low-frequency radio emission in 1985 and the earlier turn-on of the radio flux in the current outburst are interpreted as being due to higher foreground absorption in 1985 compared to that in 2006, suggesting that the overlying wind densities in 2006 are only ~30% of those in 1985.
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