  We study the dependence of citation counts of e-prints published on the
arXiv:astro-ph server on their position in the daily astro-ph listing. Using
the SPIRES literature database we reconstruct the astro-ph listings from July
2002 to December 2005 and determine citation counts for e-prints from their ADS
entry. We use Zipf plots to analyze the citation distributions for each
astro-ph position. We find that e-prints appearing at or near the top of the
astro-ph mailings receive significantly more citations than those further down
the list. This difference is significant at the 7 sigma level and on average
amounts to two times more citations for papers at the top than those further
down the listing. We propose three possible non-exclusive explanations for this
positional citation effect and try to test them. We conclude that
self-promotion by authors plays a role in the observed effect but cannot
exclude that increased visibility at the top of the daily listings contributes
to higher citation counts as well. We can rule out that the positional
dependence of citations is caused by the coincidence of the submission deadline
with the working hours of a geographically constrained set of intrinsically
higher cited authors. We discuss several ways of mitigating the observed
effect, including splitting astro-ph into several subject classes, randomizing
the order of e-prints, and a novel approach to sorting entries by relevance to
individual readers.
