INFRANIGHT INFRARED SPECTRAL VARIABILITY IN OJ-287

We present results from five nights of pseudo-simultaneous JHK infrare d monitoring of the blazar OJ 287 taken with the Carlos Sanchez Telesc ope during March 1991 with a time resolution of approximately 20 min. The monitoring shows small amplitude variations in time scales of a fe w tens of minutes, which are largest in J. Considerable variations are seen in the spectra and the spectral index in time scales comparable with our time resolution. This is the first time that spectral variati ons have been observed at any wavelength on such short time scales in OJ 287. There is a strong correlation between the spectral index and t he intensity in the J-band which also shows significant night to night variations. We can rule out the cause of the variations being instrum ental, or due to sky transparency variations, with a high degree of co nfidence. The spectrum is seen to tilt about a point at approximately 1.9 mum, although the exact value of the wavelength of the pivotal poi nt varies from night to night. These observations could be explained b y a large, variable magnetic field, in which the energy spectrum of in jected relativistic electrons varies due to radiative decay on time sc ales of a few minutes. An alternative, but less viable explanation, is a two-component model in which a non-variable component dominates the flux in K and H bands, but the flux is dominated by a variable emissi on in the J-band.