Multicolor Infrared Observations of SN 2006aj. I. The Supernova Associated with XRF 060218

We report simultaneous multicolor near-infrared (NIR) observations of thesupernova associated with x-ray Flash 060218 during the first 16 days after thehigh energy event. We find that the light curve rises and peaks relatively fastcompared to other SN Ic, with the characteristic broad NIR peak seen in allthree bands. We find that the rise profile before the peak is largelyindependent of NIR wavelength, each band appearing to transition into a plateauphase around day 10--13. Since the light curve is in the plateau phase when ourobservations end at day 16, we can only place limits on the peak absolutemagnitudes, but we estimate that SN 2006aj is one of the lowest NIR luminosityXRF/GRB associated SNe observed to date. The broad peaks observed in the {\emJHK$_s$} bands point to a large increase in the NIR contribution of the totalflux output from days 10--16. This evolution can be seen in the broad color andSED diagrams constructed using {\em UBVRIJHK$_s$} monochromatic fluxmeasurements for the first 16 days of the event. Ultimately, a 10-day rise timewould make SN 2006aj an extremely fast rise SN Ic event, faster than SN 1998bwand SN 2003dh, which combined with its underluminous nature, indicates a loweramount of $^{56}$Ni ejected by the progenitor compared to other XRF/GRB-SNe.Furthermore, the lack of significant color change during the rise portion ofthe burst points to little or no spectral evolution over the first 10 days ofactivity in the NIR.Comment: 25 pages, 7 figures (6 color, 1 bw), 3 table