We present extensive spectroscopic observations for one of the closest type Ia supernovae ( SNe Ia ) , SN 2014J discovered in M82 , ranging from 10.4 days before to 473.2 days after B -band maximum light .
The diffuse interstellar band ( DIB ) features detected in a high-resolution spectrum allow an estimate of line-of-sight extinction as A _ { \textrm { v } } \sim 1.9 \pm 0.6 mag .
Spectroscopically , SN 2014J can be put into the high-velocity ( HV ) subgroup in Wang ’ s classification with a velocity of Si ii \lambda 6355 at maximum light of v _ { 0 } = 1.22 \pm 0.01 \times 10 ^ { 4 } km s ^ { -1 } , but has a low velocity gradient ( LVG , following Benetti ’ s classification ) of \dot { v } = 41 \pm 2 km s ^ { -1 } day ^ { -1 } , which is inconsistent with the trend that HV SNe Ia generally have larger velocity gradients .
We find that the HV SNe Ia with LVGs tend to have relatively stronger Si iii ( at \sim 4400 Å ) absorptions in early spectra , larger ratios of S ii \lambda 5468 to S ii \lambda 5640 , and weaker Si ii 5972 absorptions compared to their counterparts with similar velocities but high velocity gradients .
This shows that the HV+LVG subgroup of SNe Ia may have intrinsically higher photospheric temperature , which indicates that their progenitors may experience more complete burning in the explosions relative to the typical HV SNe Ia .