摘要：Context.Based onXMM-NewtonX-ray observations IGR J19552+0044 appears to be either a pre-polar or an asynchronous polar.Aims.We conducted follow-up optical observations to identify the sources and periods of variability precisely and to classify this X-ray source correctly.Methods.Extensive multicolor photometric and medium- to high-resolution spectroscopy observations were performed and period search codes were applied to sort out the complex variability of the object.Results.We found firm evidence of discording spectroscopic (81.29 ± 0.01 m) and photometric (83.599 ± 0.002 m) periods that we ascribe to the white dwarf (WD) spin period and binary orbital period, respectively. This confirms that IGR J19552+0044 is an asynchronous polar. Wavelength dependent variability and its continuously changing shape point at a cyclotron emission from a magnetic WD with a relatively low magnetic field below 20 MG.Conclusions.The difference between the WD spin period and the binary orbital period proves that IGR J19552+0044 is a polar with the largest known degree of asynchronism (0.97 or 3%).
关键词：enaccretion, accretion disksstars: magnetic fieldbinaries: close
