The hybrid nature of 0846+51W1: a BL Lac object with a narrow line seyfert 1 nucleus

We found a NLS1 nucleus in the extensively studied eruptive BL Lac object, 0846+51W1, out of a large sample of NLS1s 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) similar or equal to 1710 km s(-1) and [OIII]lambda 5007/H beta similar or equal to 0.32 from its SDSS spectrum observed when it was in the faint state, fulfil the conventional definition of NLS1. Strong Fell emission is detected in the SDSS spectrum, which is also typical of NLS1s. We tried to estimate its central black hole mass using various techniques and found that 0846+51W1 is very likely emitting at a few x 10% of the Eddington luminosity. We speculate that Seyfert-like nuclei, including NLS1s, might be concealed in a significant fraction of BL Lacs. but have remained largely unobserved because, often, their optical-UV continuum is overwhelmed by the synchrotron emission.