Unveiling the broad band X-ray continuum and iron line complex in Mrk 841

Context. Mrk 841 is a bright Seyfert 1 galaxy known to harbor a strong soft excess and a variable K alpha iron line. Historical UV, X and.- rays observations show clear variation of its spectrum in flux and in shape on a monthly time scale. Aims. Mrk 841 has been observed during 3 different periods ( January 2001, January 2005 and July 2005) by XMM-Newton for a total cumulated exposure time of similar to 108 ks. We present in this paper a broad band spectral analysis of the complete EPIC-pn data sets. These are the best observations so far for the study of the soft excess and iron line complex in this source. Methods. We use different methods of data analysis including model-independent methods ( spectral ratios, rms,...) as well as model fitting. We were able to test two different models for the soft excess, a relativistically blurred photoionized reflection (REF model) and a relativistically smeared ionized absorption (ABS model). The continuum is modeled by a simple cut-off power law and we also add a neutral reflection. Results. These observations confirm the presence of a soft excess and iron line and reveal extreme and puzzling spectral and temporal behaviors. The 0.5- 3 keV soft X-ray flux decreases by a factor 3 between 2001 and 2005 and the line shape appears to be a mixture of broad and narrow components, the former being variable on small ( ks) time scale while the later is consistent with being constant. The 2-10 keV spectrum also hardens between 2001 and 2005. We succeed in describing this complex broad-band 0.5- 10 keV spectral variability using either REF or ABS to fit the soft excess. Both models give statistically equivalent results even including simultaneous BeppoSAX data up to 200 keV. Both models are consistent with the presence of remote reflection characterized by a constant narrow component in the data. However they differ in the presence of a broad line component present in REF but not needed in ABS. Consequently the physical interpretation of the line profile variability is quite different, resulting from the variability of the broad line component in REF and from the variability of the absorbing medium in ABS. This study also reveals the sporadic presence of relativistically redshifted narrow iron lines, one of them being detected at 4.8 keV in the EPIC-pn instruments at more than 98.5% confidence level. If interpreted as the blue horn of a relativistically distorted neutral iron line, the large redshift implies the presence of a Kerr black hole.