  The majority of short gamma-ray bursts (SGRBs) are thought to originate from
the merger of compact binary systems collapsing directly to form a black hole.
However, it has been proposed that both SGRBs and long gamma-ray bursts (LGRBs)
may, on rare occasions, form an unstable millisecond pulsar (magnetar) prior to
final collapse. GRB 090515, detected by the Swift satellite was extremely
short, with a T_90 of 0.036 +/- 0.016 s, and had a very low fluence of 2 x
10^-8 erg cm^-2 and faint optical afterglow. Despite this, the 0.3 - 10 keV
flux in the first 200 s was the highest observed for a SGRB by the Swift X-ray
Telescope (XRT). The X-ray light curve showed an unusual plateau and steep
decay, becoming undetectable after ~500 s. This behaviour is similar to that
observed in some long bursts proposed to have magnetars contributing to their
emission. In this paper, we present the Swift observations of GRB 090515 and
compare it to other gamma-ray bursts (GRBs) in the Swift sample. Additionally,
we present optical observations from Gemini, which detected an afterglow of
magnitude 26.4 +/- 0.1 at T+ 1.7 hours after the burst. We discuss potential
causes of the unusual 0.3 - 10 keV emission and suggest it might be energy
injection from an unstable millisecond pulsar. Using the duration and flux of
the plateau of GRB 090515, we place constraints on the millisecond pulsar spin
period and magnetic field.
