The outburst of a 60 min AM CVn reveals peculiar color evolution: implications for outbursts in long period double white dwarfs

We report on multi-wavelength observations taken in quiescence and during the first detected outburst of the ~60 min orbital period AM CVn system SDSS J113732+405458. Using X-ray and UV observations we determined an upper limit on the duration of the event, which was around one year. The amplitude of the outburst was remarkably small, of around one magnitude in r and 0.5 magnitudes in g near the peak. We have also investigated the color variations of SDSS J113732+405458 and other long period AM CVns in outbursts and identified a track in the color-magnitude diagram that is not compatible with the one expected under the disk instability model, suggesting that the outbursts in long period AM CVns do not have a disk instability origin, but rather are caused by enhanced mass-transfer. To our knowledge, these are the first studies of the color evolution in AM CVn systems. During quiescence we detected SDSS J113732+405458 as an X-ray source, with an X-ray luminosity of ~3E29 erg/s in the 0.5-10 keV band. This indicates a very low accretion rate, in agreement with the disk instability model for long period systems. In the same state we identified an infrared excess that can be attributed to the donor star. This makes SDSS J113732+405458 the second AM CVn for which the donor has been directly detected.