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

We present the results of six BeppoSAX observations of 1ES 2344+514, five of 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 ≈50 keV. During the first five closely spaced observations 1ES 2344+514 showed large-amplitude luminosity variability, associated with spectacular spectral changes: in particular, the last observation found the source to be several times fainter, with a much steeper X-ray spectrum. The energy-dependent shape of the light curve and the spectral changes both imply a large frequency shift (by a factor of 30 or more) of the peak of synchrotron emission. At maximum flux the peak was located at, or above, 10 keV, making 1ES 2344+514 the second blazar (after Mrk 501) 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 brightest.