We established in an earlier study that articles listed at or near the top of the daily arXiv: astro-ph mailings receive on average significantly more citations than articles further down the list. In our earlier work we were not able to decide whether this positional citation effect was due to author self-promotion of intrinsically more citable papers or whether papers are cited more often simply because they are at the top of the astro-ph listing. Using new data we can now disentangle both effects. Based on their submission times we separate articles into a self-promoted sample and a sample of articles that achieved a high rank on astro-ph by chance and compare their citation distributions with those of articles in lower astro-ph positions. We find that the positional citation effect is a superposition of self-promotion and visibility bias.