We report photometric and spectroscopic observations of the nearby Type Ia Supernova ( SN Ia ) 2012ht from -15.8 days to +49.1 days after B -band maximum .
The decline rate of the light curve is \Delta m _ { 15 } ( B ) = 1.39 ~ { } \pm~ { } 0.05 mag , which is intermediate between normal and subluminous SNe Ia , and similar to that of the ‘ transitional ’ Type Ia SN 2004eo .
The spectral line profiles also closely resemble those of SN 2004eo .
We were able to observe SN 2012ht at very early phase , when it was still rising and was about three magnitudes fainter than at the peak .
The rise time to the B -band maximum is estimated to be 17.6 \pm 0.5 days and the time of the explosion is MJD 56277.98 \pm 0.13 .
SN 2012ht is the first transitional SN Ia whose rise time is directly measured without using light curve templates , and the fifth SN Ia overall .
This rise time is consistent with those of the other four SNe within the measurement error , even including the extremely early detection of SN 2013dy .
The rising part of the light curve can be fitted by a quadratic function , and shows no sign of a shock-heating component due to the interaction of the ejecta with a companion star .
The rise time is significantly longer than that inferred for subluminous SNe such as SN 1991bg , which suggests that a progenitor and/or explosion mechanism of transitional SNe Ia are more similar to normal SNe Ia rather than subluminous SNe Ia .