We found a NLS1 nucleus in the extensively studied eruptive BL Lac , 0846+51W1 , out of a large sample of NLS1 compiled from the spectroscopic dataset of SDSS DR1 .
Its optical spectrum can be well decomposed into three components , a power law component from the relativistic jet , a stellar component from the host galaxy , and a component from a typical NLS1 nucleus .
The emission line properties of 0846+51W1 , FWHM ( H \beta ) \simeq 1710 ~ { } km~ { } s ^ { -1 } and \frac { [ OIII ] \lambda 5007 } { H \beta } \simeq 0.32 according to its SDSS spectrum observed when it was in faint state , fulfil the conventional definition of NLS1 .
Strong FeII emission is detected in the SDSS spectrum , which is also typical of NLS1s .
We try to estimate its central black hole mass using various techniques and find that 0846+51W1 is very likely emitting at a~ { } few \times 10 \% of Eddington luminosity .
We speculate that Seyfert-like nuclei , including NLS1s , might be concealed in a significant fraction of BL Lacs but have not been sufficiently explored due to the fact that , by definition , the optical-UV continuum of such kind of objects are often overwhelmed by the synchrotron emission .