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Aims. We present a multiwavelength study of the Be/X-ray binary system SAXJ2103.5+4545 with the goal of better characterizing the transient behaviour ofthis source.  Methods. SAX J2103.5+4545 was observed by Swift-XRT four times in 2007 fromApril 25 to May 5, and during quiescence in 2012 August 31. In addition, thissource has been monitored from the ground-based astronomical observatories ofEl Teide (Tenerife, Spain), Roque de los Muchachos (La Palma, Spain) and SierraNevada (Granada, Spain) since 2011 August, and from the TUBITAK NationalObservatory (Antalya, Turkey) since 2009 June. We have performed spectral andphotometric temporal analyses in order to investigate the different statesexhibited by SAX J2103.5+4545.  Results. In X-rays, an absorbed power law model provided the best fit for allthe XRT spectra. An iron-line feature at ~6.42 keV was present in all theobservations except for that taken during quiescence in 2012. The photonindexes are consistent with previous studies of SAX J2103.5+4545 in high/lowluminosity states. Pulsations were found in all the XRT data from 2007(2.839(2) mHz; MJD 54222.02), but not during quiescence. Both optical outburstsin 2010 and 2012 lasted for about 8/9 months (as the one in 2007 probably didand the current one in 2014 might do) and were most probably caused by massejection events from the Be star that eventually fed the circumstellar disc.All of these outbursts started about 3 months before the triggering of theX-ray activity, and about the same period before the maximum of the H_alphaline equivalent width (in emission) was reached at only ~ -5 \AA. In this workwe found that the global correlation between the BV variability and the X-rayintensity was also observed at longer wavelengths in the IR domain.

Recent activity of the Be/X-ray binary system SAX J2103.5+4545