for their brevity. The July X-ray lightcurve indicates that the time spent above $10^{35}$  was no more than 4 days, and perhaps only 2.5 days (Fig. \[fig:inset\]), one of the shortest transient LMXB outbursts so far recorded [cf. @Natalucci00; @intZand04; @Wijnands07]. The August outburst lightcurve indicates $<3.5$ days spent above $10^{35}$ , and the lightcurves from the other outbursts, while less constraining, are consistent with this timescale (Fig. \[fig:inset\]). We note that another ultracompact AMXP, XTE J1751-305, has shown similarly short and faint outbursts [@Markwardt07; @Linares07; @Markwardt09].

Why does NGC 6440 X-2 show such faint and brief outbursts? The obvious drivers are accretion disk instabilities or magnetospheric instabilities. However, magnetospheric instabilities occur on much faster timescales (e.g. the Rapid Burster, Lewin 1993), and would require that the disk remain viscous (and thus ionized) between outbursts, which seems unlikely. Standard accretion disk instability models (e.g. Lasota 2001) predict quiescent periods 10 times longer, and brighter and longer outbursts, for systems with 57-minute orbital periods. Naively, we expect longer intervals between outbursts for a 57-minute system than for the other known ultracompact systems, if NGC 6440 X-2 is indeed an evolutionary descendant of systems like them, as it will have a larger 