The red blazar PMN J2345-1555 becomes blue

TL;DR

This paper reports a dramatic spectral shift in the blazar PMN J2345-1555 during a flare, indicating a change in the emission region's location that results in a bluer spectral energy distribution and higher energy emissions.

Contribution

It provides a detailed analysis of the spectral evolution of a blazar during a flare, linking the shift to the emission region moving outside the broad line region.

Findings

Spectral peaks shifted from IR/MeV to optical-UV and >1 GeV during the flare.

The emission region moved outside the broad line region.

Electrons achieved higher energies due to decreased radiation energy density.

Abstract

The Flat Spectrum Radio Quasar PMN J2345-1555 is a bright gamma-ray source, that recently underwent a flaring episode in the IR, UV and gamma-ray bands. The flux changed quasi simultaneously at different frequencies, suggesting that it was produced by a single population of emitting particles, hence by a single and well localized region of the jet. While the overall Spectral Energy Distribution (SED) before the flare was typical of powerful blazars (namely two broad humps peaking in the far IR and below 100 MeV bands, respectively), during the flare the peaks moved to the optical-UV and to energies larger than 1 GeV, to resemble low power BL Lac objects, even if the observed bolometric luminosity increased by more than one order of magnitude. We interpret this behavior as due to a change of the location of the emission region in the jet, from within the broad line region, to just outside. The corresponding decrease of the radiation energy density as seen in the comoving frame of the jet allowed the relativistic electrons to be accelerated to higher energies, and thus produce a "bluer" SED.