On the nature of the radio calibrator and gamma-ray emitting NLS1 galaxy 3C 286 and its multiwavelength variability

TL;DR

This study investigates the multiwavelength variability of the radio-loud NLS1 galaxy 3C 286, revealing its accretion disk-driven X-ray variability, confirming its role as a stable radio calibrator, and exploring its gamma-ray emission models.

Contribution

The paper provides new multiwavelength observations of 3C 286, analyzes the origin of its X-ray variability, and discusses its gamma-ray emission mechanisms, highlighting its unique properties among NLS1 galaxies.

Findings

X-ray variability is primarily driven by the accretion disk.

Radio flux density and polarization are stable, supporting its use as a calibrator.

No intrinsic reddening or absorption detected along the line-of-sight.

Abstract

The quasar 3C 286, a well-known calibrator source in radio astronomy, was found to exhibit exceptional multiwavelength properties. Its rich and complex optical emission-line spectrum revealed its narrow-line Seyfert 1 (NLS1) nature. Given its strong radio emission, this makes 3C 286 one of the radio-loudest NLS1 galaxies known to date. 3C 286 is also one of very few known compact steep-spectrum (CSS) sources detected in the gamma-ray regime. Observations in the X-ray regime, rarely carried out so far, revealed evidence for variability, raising the question if driven by the accretion disk or jet. 3C 286 is also well known for its damped Lyman alpha system from an intervening absorber at z = 0.692, triggering a search for the corresponding X-ray absorption along the line-of-sight. Here, we present new observations in the radio, X-ray, optical and UV band. The nature of the X-ray variability is addressed. Spectral evidence suggests that it is primarily driven by the accretion disk (not the jet), and the X-ray spectrum is well fit by a powerlaw plus soft excess model. The radio flux density and polarization remain constant at the Effelsberg telescope resolution, reconfirming the use of 3C 286 as radio calibrator. The amount of reddening/absorption along the line-of-sight {\em{intrinsic}} to 3C 286 is rigorously assessed. None is found, validating the derivation of a high Eddington ratio (L/L-Edd ~ 1) and of the very high radio-loudness index of 3C 286. Based on the first deep Chandra image of 3C 286, tentative evidence for hard X-ray emission from the SW radio lobe is reported. A large variety of models for the gamma-ray emission of 3C 286 is briefly discussed.