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3C 286 is a well-known calibrator source in radio astronomy. It is also oneof very few compact steep-spectrum sources (CSS) detected in $\gamma$-rays.Here, we perform a detailed spectroscopic and variability analysis and presentthe first quasi-simultaneous optical to X-ray spectral energy distribution inorder to reveal physical mechanisms which dominate its emission at differentwavelengths, and arrive at a reliable optical source classification. The firstmain result of our study reveals several pitfalls when applying simple broad-or narrow-line Seyfert 1 (BLS1 or NLS1) classification criteria which only lookat the [OIII]-H$\beta$ complex. [OIII] and H$\beta$ can be dominated by thesame outflow components, in which case FWHM(H$\beta$) is no reliableclassification criterion, and extinction by intrinsic or intervening materialcan make the highest-velocity H$\beta$ component undetectable. After carefulcombination of all information from UV-optical spectra along withmulti-wavelength data, we confirm that 3C 286 can be classified as NLS1 galaxy,with line properties and SMBH mass (of order 10$^{8}$ M$_{\odot}$ and accretingnear the Eddington limit) close to the BLS1 regime, making it an importantborderline object. The quasi-simultaneous SED taken with $swift$ shows a sharprise in the optical-UV, implying the presence of a strong accretion-diskcomponent with EUV excess, consistent with emission-line diagnostics. Finally,we report the discovery of X-ray variability of 3C 286, plausibly dominated byjet emission, and variable by at least a factor $\sim4$. This result suggeststo exercise caution when using 3C 286 as radio calibrator in high-resolutionradio VLBI observations.

Spectroscopic classification, variability and SED of the Fermi-detected CSS 3C 286: the radio-loudest NLS1 galaxy?