Detection of exceptional X-ray spectral variability in the TeVBL Lac 1ES 2344+514

We present the results of six BeppoSAX observations of 1ES 2344+514, five o f which were taken within one week. 1ES 2344+514, one of the few known TeV BL Lac objects, was detected by the BeppoSAX Narrow Field Instruments (NFI) in the range 0.1 and approximate to 50 keV. During the first five closely spaced observations 1ES 2344+514 showed large-amplitude luminosity variabil ity, associated with spectacular spectral changes: in particular, the last observation found the source to be several times fainter, with a much steep er X-ray spectrum. The energy-dependent shape of the light curve and the sp ectral changes both imply a large frequency shift (by a factor of 30 or mor e) of the peak of synchrotron emission. At maximum flux the peak was locate d at, or above, 10 keV, making 1ES 2344+514 the second blazar (after Mrk 50 1) to have a synchrotron peak in the hard X-ray band. The frequency shift, and the corresponding increase in luminosity, might be caused by the onset of a second synchrotron component extending from the soft to the hard X-ray band where most of the power is emitted. Rapid variability on a time-scale of approximately 5000 s has also been detected when the source was brighte st.