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

3C 286 is a well-known calibrator source in radio astronomy. It is also one of very few compact steep-spectrum (CSS) sources detected in gamma-rays. Here, we perform a detailed spectroscopic and variability analysis and present the first quasi-simultaneous optical to X-ray spectral energy distribution (SED) in order to reveal physical mechanisms that dominate its emission at different wavelengths, and arrive at a reliable optical source classification. The first main result of our study reveals several pitfalls when applying simple broad- or narrow-line Seyfert 1 (BLS1 or NLS1) classification criteria that only look at the [OIII]-H beta complex. [O III] and H beta can be dominated by the same outflow components, in which case full width at half-maximum (H beta) is no reliable classification criterion, and extinction by intrinsic or intervening material can make the highest velocity H beta component undetectable. After careful combination of all information from ultraviolet (UV)-optical spectra along with multiwavelength data, we confirm that 3C 286 can be classified as NLS1 galaxy, with line properties and supermassive black hole mass (of order 10(8) M-circle dot and accreting near the Eddington limit) close to the BLS1 regime, making it an important borderline object. The quasi-simultaneous SED taken with Swift shows a sharp rise in the optical-UV, implying the presence of a strong accretion-disc component with extreme UV excess, consistent with emission-line diagnostics. Finally, we report the discovery of X-ray variability of 3C 286, plausibly dominated by jet emission, and variable by at least a factor of similar to 4. This result suggests to exercise caution when using 3C 286 as radio calibrator in high-resolution radio very long baseline interferometry observations.

Automated summary

3C 286 is classified as a NLS1 galaxy with characteristics close to the BLS1 regime, revealing pitfalls in applying simple classification criteria. The quasi-simultaneous SED shows a sharp rise in the optical-UV, implying the presence of a strong accretion-disc component with extreme UV excess. X-ray variability discovery suggests caution when using 3C 286 as a radio calibrator in high-resolution very long baseline interferometry observations.