Supernovae ( SNe ) embedded in dense circumstellar material ( CSM ) may show prominent emission lines in their early-time spectra ( \leq 10 days after the explosion ) , owing to recombination of the CSM ionized by the shock-breakout flash .
From such spectra ( “ flash spectroscopy ” ) , we can measure various physical properties of the CSM , as well as the mass-loss rate of the progenitor during the year prior to its explosion .
Searching through the Palomar Transient Factory ( PTF and iPTF ) SN spectroscopy databases from 2009 through 2014 , we found 12 Type II SNe showing flash-ionized ( FI ) signatures in their first spectra .
All are younger than 10 days .
These events constitute 14 % of all 84 SNe in our sample having a spectrum within 10 days from explosion , and 18 % of SNe II observed at ages < 5 days , thereby setting lower limits on the fraction of FI events .
We classified as “ blue/featureless ” ( BF ) those events having a first spectrum which is similar to that of a black body , without any emission or absorption signatures .
It is possible that some BF events had FI signatures at an earlier phase than observed , or that they lack dense CSM around the progenitor .
Within 2 days after explosion , 8 out of 11 SNe in our sample are either BF events or show FI signatures .
Interestingly , we found that 19 out of 21 SNe brighter than an absolute magnitude M _ { R } = -18.2 belong to the FI or BF groups , and that all FI events peaked above M _ { R } = -17.6 mag , significantly brighter than average SNe II .