The Changing Look Blazar B2 1420+32

TL;DR

This paper documents the unprecedented 'changing look' phenomena in blazar B2 1420+32, showing spectral and flux variability that causes it to switch between FSRQ and BL Lac states, revealing complex jet and emission region dynamics.

Contribution

It provides the first detailed observation of a blazar transitioning between FSRQ and BL Lac states with spectral and flux variability, including the emergence of a strong Fe pseudo continuum.

Findings

Blazar B2 1420+32 exhibited dramatic multi-wavelength variability.

The blazar transitioned between FSRQ and BL Lac states multiple times.

Emission region separations are constrained to less than 800 and 130 gravitational radii.

Abstract

Blazars are active galactic nuclei with their relativistic jets pointing toward the observer, with two major sub-classes, the flat spectrum radio quasars and BL Lac objects. We present multi-wavelength photometric and spectroscopic monitoring observations of the blazar, B2 1420+32, focusing on its outbursts in 2018-2020. Multi-epoch spectra show that the blazar exhibited large scale spectral variability in both its continuum and line emission, accompanied by dramatic gamma-ray and optical variability by factors of up to 40 and 15, respectively, on week to month timescales. Over the last decade, the gamma-ray and optical fluxes increased by factors of 1500 and 100, respectively. B2 1420+32 was an FSRQ with broad emission lines in 1995. Following a series of flares starting in 2018, it transitioned between BL Lac and FSRQ states multiple times, with the emergence of a strong Fe pseudo continuum. Two spectra also contain components that can be modeled as single-temperature black bodies of 12,000 and 5,200 K. Such a collection of "changing look" features has never been observed previously in a blazar. We measure gamma-ray-optical and the inter-band optical lags implying emission region separations of less than 800 and 130 gravitational radii respectively. Since most emission line flux variations, except the Fe continuum, are within a factor of 2-3, the transitions between FSRQ and BL Lac classifications are mainly caused by the continuum variability. The large Fe continuum flux increase suggests the occurrence of dust sublimation releasing more Fe ions in the central engine and an energy transfer from the relativistic jet to sub-relativistic emission components.