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core - collapse - supernovae ( cc - sne ) are believed to be the explosions that mark the end - point in the evolution of massive stars ( @xmath8 m@xmath9 ; see e.g. * ? ? ? they are spectroscopically divided in different groups according mainly to the presence of the h and he lines in their optical spectra . while type ii sne show prominent h features , type i do not , and can be further subdivided , according to the presence or absence of he , as types ib and ic respectively . there are also hybrid objects which undergo a transition from being h dominated at early phases of their spectral evolution , to he dominated at later times . these are termed type iib sne . the first sn to have shown this spectral transition was sn  1987k , and its progenitor was proposed to be a star that had lost most of its hydrogen envelope before exploding @xcite . the best studied type iib sn to date is sn  1993j ( e.g. @xcite ) , that is considered the prototype of the subclass . its lc showed an early peak , followed by a rapid decline thought to be the consequence of the cooling of the progenitor s stellar envelope after shock breakout . subsequently it presented a secondary maximum attributed to input from the radioactive decay of @xmath7ni . the star that exploded as sn  1993j was detected in archival images @xcite , and its disappearance was confirmed by @xcite . two mechanisms have been proposed by which type iib sn progenitors lose part of their h envelope before exploding , namely stripping by a close companion after roche lobe overflow , and wind - driven stellar mass loss in a single star . the former scenario has gained strength over the past years , for example with the detection of signatures of the possible hot blue companion star of sn  1993j ( @xcite ; see also @xcite ) , and the modelling of a progenitor binary system and likely detection of the stellar companion of sn  2011dh @xcite . moreover , although a wolf - rayet ( wr ) wind - like spectrum was obtained for the type iib sn  2013cu a few hours after explosion @xcite , recent work has shown that the progenitor star for sn  2013cu was unlikely a wr @xcite . an important question is which of the observed properties of sne iib can be connected with the characteristics of their progenitor systems . for example , @xcite proposed that bright early luminosity from the shock - heated progenitor stellar envelope , low radio shell velocities , and thermal x - ray emission were characteristic of extended progenitors with r  @xmath10  r@xmath4 , while compact progenitors with r  @xmath11  r@xmath4 have faint early optical lcs , have high radio shell velocities and non - thermal x - ray emission . however , this link is not straightforward and needs to be revised since the type iib sne  2011dh and 2011hs present fast expanding radio shells but the hydrodynamical modelling of their lc @xcite , and in the case of sn  2011dh the detection of its progenitor in archival images @xcite and its disappearance @xcite , point to the explosion of extended stars . to further complicate the picture , sne iib show a variety of continuum flux excess in early uv spectra @xcite . strong uv - excess suggest significant interaction between the sn ejecta and circumstellar material .    to date , few type iib sne have been thoroughly studied since they are relatively rare events . @xcite estimated the fraction of type iib over a volume limited sample of 81 type ii sne to be @xmath12 . in this paper we present optical and near infrared ( nir ) data for the type iib sn  2011fu . sn 2011fu , with coordinates @xmath13 and @xmath14 ( j2000 ) , was discovered in a spiral arm of the galaxy ugc  1626 by f. ciabattari and e. mazzoni of the italian supernovae search project ( issp ) , on 2011 september 21.04 ut and classified by @xcite as a young type ii sn 2011 september 23.84 ut . a previous analysis of optical data of sn  2011fu was presented by @xcite , which confirmed it was a type iib sn . in this manuscript we contribute with data probing the whole sn evolution . in section [ host ] we discuss the distance , reddening , and explosion epoch of sn  2011fu . in section [ data ] we summarize our observations and the reduction process of the data . in section [ phot ] we present the optical and nir lcs of the object , while in section [ spec ] we present and discuss the spectral evolution . in section [ discussion ] we discuss the results obtained from the hydrodynamical modelling of the pseduo - bolometric lc and we compare our nebular spectra with published models for late time type iib sn spectra . finally , in section [ conclusion ] , we present the conclusions of our work . the rise to a first maximum in the _ v _ and _ r _ lcs ( see the inset of figure [ fig : apparentlcs ] ) implies that sn  2011fu was discovered very soon after explosion . in fact , models show that the evolution of the sn lc during this rise should be very fast , and lasts @xmath11  d for a number of configurations ( e.g. @xcite ) . by adopting @xmath15 as the explosion date of sn  2011fu , we obtain the best fit of the early phase of the pseudo - bolometric lc with the hydrodynamical models presented in section [ lcmodelling ] . for this reason we have chosen @xmath15 as the explosion date of sn  2011fu , which is in agreement with the discovery epoch , the last non detection of the object which was 2011 august 10 ( @xmath16 ) , the pre - discovery image taken by xing gao 2011 september 20 ( @xmath17 ) , and the classification as a young object . note that this assumed explosion date also agrees , within the uncertainties , with the explosion epoch adopted by @xcite . all phases in the rest of this manuscript are given with respect to @xmath15 . ugc  1626 is a sab(rs)c type galaxy whose recessional velocity given by the nasa / ipac extragalactic database ( ned ) is @xmath18  km  s@xmath19 . we have performed measurements of the sn redshift from the narrow h@xmath20 emissions in its spectra and have obtained the same redshift as for its host galaxy , which we adopt in the rest of this paper . the redshift derived distance to ugc  1626 , also provided by ned assuming h@xmath21 km s@xmath19 mpc@xmath19 , @xmath22 , @xmath23 , and accounting for the virgo , great attractor and shapley infalls is @xmath24 mpc , i.e. @xmath25 mag , @xcite . the reddening in the line of sight of ugc  1626 due to the milky way is @xmath26  mag @xcite . we detect a narrow absorption likely due na i d @xmath27@xmath28 , @xmath29 at the host galaxy redshift . the equivalent width of this line is @xmath30   that using the relations for unresolved na i d given by @xcite , provides an estimate of the reddening in the host galaxy as : @xmath31  mag . in fact , a similar value of reddening due to the host galaxy is obtained using the relations of @xcite . in the rest of this manuscript we adopt @xmath32  mag as the total reddening towards sn  2011fu . we note that in @xcite the relations of @xcite were used to estimate the contribution to the reddening due to the host galaxy , and they obtained @xmath33 , which is consistent with the value we have adopted within the uncertainties . the observational follow - up data of sn 2011fu presented in this paper covers the period between a few days post discovery , until the sn disappeared behind the sun ( end of february 2012 ) . after that , one more spectrum and one epoch of _ gri _ photometric data were obtained well into the nebular phase of the sn , 2012 july 20 ut . thanks to a large collaboration between many european institutions , optical photometry and spectroscopy , as well as some nir photometric data of sn 2011fu , have been collected at several sites . some amateur data taken by f.ciabattari and e. mazzoni from italy , and xing gao from china were also included in our analysis . a summary of the characteristics of the instruments used to acquire our photometric data can be seen in table [ photometric_log ] .    the list with our spectroscopic observations and the instrumental configuration used to acquire the data is presented in table [ spectroscopic_log ] . all the data were corrected for overscan , bias , and flat - field within the iraf environment , except for data obtained at the liverpool telscope ( lt ) which were automatically reduced with the instrument specific pipeline . the instrumental magnitudes of the sn were derived via point spread function ( psf ) fitting which was done with snoopy @xcite . in the case of the _ u _ and _ u _ photometry , the measurements were done after performing template subtraction . for all other bands , psf photometry at late time was verified by performing template subtractions and no major differences were found . a set of fifteen local stars in the sn field ( figure [ fig:11fusequence ] ) were used to trace the photometric calibration . the stellar sequence was calibrated with the zero points and colour terms derived for each site thanks to observations of landolt standards ( tables [ sequence_magsjc ] and [ sequence_magssloan ] ) and the sn magnitudes were calculated relative to these . the errors of the magnitudes of the stellar sequence were obtained as the rms deviation with respect to their mean values . given the fact that several instruments with different passbands were used to acquire our data , we performed s - corrections of our _ bvrrii _ photometry to the bessell system @xcite . in order to derive the s - correction terms we used our spectral sequence of the sn . note that we were not able to perform s - corrections to the _ u _ , _ u _ , and _ z _ band photometry given the fact that our spectra do not cover completely these passbands . the sn magnitudes together with their errors , calculated by adding in quadrature the uncertainties associated with the calibration and the psf fit , are reported in tables [ ubvri_phot ] , [ ugz_phot ] , and [ jhk_phot ] . the unfiltered photometry provided by f. ciabattari was rescaled to the _ r _ band considering that the detector sensitivity peaks at this wavelength . with the same argument the data provided by x. gao were rescaled to _ note that stars 2 , 6 , 9 , 12 , and 14 from our sequence coincide with stars 2 , 1 , 4 , 3 and 5 used for photometric calibration by @xcite . the greatest difference between the two sequences is 0.1 mag found for star 2 in the _ u _ band . for the rest of the bands differences are found to be less than 0.1 mag . the nir data was reduced within iraf with the help of caindr , a package developed by jos a. acosta pulido and rafael barrena for the instrument cain3 at the tcs . the images of each specific filter were corrected for flat field , and bad pixel corrections were applied . then background images were constructed and subtracted from the science frames , and finally the images were combined taking into account the dithering pattern . similarly to the optical data , we measured the instrumental sn magnitudes by psf fitting . the sn calibration was done relative to 2-mass stars in the field ( @xcite ; see table [ sequence_magsjc ] ) . the spectra were corrected for overscan , bias , and flat - field within iraf . next they were variance - weighted extracted and wavelength calibrated with the use of arc lamp spectra . the wavelength calibration was cross - checked with respect to sky emission lines . finally a flux calibration was applied using sensitivity functions obtained from the observation of spectrophotometric standards . the flux calibration was cross - checked with the photometry of the sn at the nearest epoch . telluric lines were corrected dividing the sn spectra by telluric line spectra obtained from the spectrophotometric standards . the optical - nir lcs of sn  2011fu are presented in figure [ fig : apparentlcs ] . in addition to the data presented in the figure we obtained two epochs of sloan _ g _ band data , which are listed in table [ ugz_phot ] . in the optical , the lcs clearly present two peaks in all bands , but not in the nir since the data do not cover the early phases . the rise to primary peak in the _ v _ and _ r _ band can be seen thanks to the early time data provided by amateur astronomers ( see inset in the top - right of the figure ) .        by making low order polynomial fits to the optical lcs we have estimated the phases and magnitudes at which the minima after first peak and secondary maxima take place . the results are presented in table [ min_max_decay_rise ] . note that we obtain different absolute magnitudes at secondary peak than @xcite due to the fact that we have adopted different distance and reddening towards the host , however , our values agree within the uncertainties . we also fitted low order polynomials to the first peaks in the _ r _ and _ v _ lcs . in the _ v _ band the first peak at 16.3 mag is reached @xmath34  d after explosion while in the _ r _ band the 16.0 mag first peak is obtained @xmath35  d post explosion . we have also estimated the decline rates of the tails of the _ bvriz _ lcs ( table [ min_max_decay_rise ] ) . these rates are quite similar to those estimated for sn  2013df by @xcite , and are steeper than expected from @xmath7co decay . this is a common characteristic to stripped envelope sne ( e.g. , sn 2008ax , @xcite ) and is possibly due to increasing transparency for @xmath36 rays in their lower mass ejecta .
we present optical and near infrared observations of the type iib supernova ( sn )  2011fu from a few days to @xmath0  d after explosion . 
 the sn presents a double - peaked light curve ( lc ) similar to that of sn  1993j , although more luminous and with a longer cooling phase after the primary peak . 
 the spectral evolution is also similar to sn  1993j s , with hydrogen dominating the spectra to @xmath1  d , then helium gaining strength , and nebular emission lines appearing from @xmath2  d post - explosion . 
 the velocities derived from the p - cygni absorptions are overall similar to those of other type iib sne . 
 we have found a strong similarity between the oxygen and magnesium line profiles at late times , which suggests that these lines are forming at the same location within the ejecta . 
 the hydrodynamical modelling of the pseudo - bolometric lc and the observed photospheric velocities suggest that sn  2011fu was the explosion of an extended star ( @xmath3 r@xmath4 ) , in which 1.3 @xmath5 erg of kinetic energy were released and 0.15  m@xmath6 of @xmath7ni were synthesised . 
 in addition , a better reproduction of the observed early pseudo - bolometric lc is achieved if a more massive h - rich - envelope than for other type iib sne is considered ( 0.3  m@xmath6 ) . 
 the hydrodynamical modelling of the lc and the comparison of our late - time spectra with nebular spectral models for type iib sne , point to a progenitor for sn  2011fu with a zams mass of 13 - 18  m@xmath6 . 
 [ firstpage ]    supernovae : general , supernovae : individual : sn  2011fu .