Contemporaneous broadband observations of three high-redshift BL Lac Objects

TL;DR

This study presents simultaneous broadband observations of three high-redshift BL Lac objects, modeling their spectral energy distributions to understand jet emission, particle properties, and extragalactic background light absorption effects.

Contribution

It provides the first simultaneous broadband SEDs of high-redshift BL Lacs, constrains jet emission models, and investigates EBL absorption effects at high redshift.

Findings

EBL absorption is evident in the highest redshift source.

Synchrotron self-Compton models fit the SEDs well.

Discovery of a nearby X-ray source possibly contributing to Fermi flux.

Abstract

We have collected broadband spectral energy distributions (SEDs) of three BL Lac objects, 3FGL J0022.1$-$1855 (z=0.689), 3FGL J0630.9$-$2406 (z >~ 1.239), and 3FGL J0811.2$-$7529 (z=0.774), detected by Fermi with relatively flat GeV spectra. By observing simultaneously in the near-IR to hard X-ray band, we can well characterize the high end of the synchrotron component of the SED. Thus, fitting the SEDs to synchro-Compton models of the dominant emission from the relativistic jet, we can constrain the underlying particle properties and predict the shape of the GeV Compton component. Standard extragalactic background light (EBL) models explain the high-energy absorption well, with poorer fits for high UV models. The fits show clear evidence for EBL absorption in the Fermi spectrum of our highest redshift source 3FGL J0630.9$-$2406. While synchrotron self-Compton models adequately describe the SEDs, the situation may be complicated by possible external Compton components. For 3FGL J0811.2$-$7529, we also discover a nearby serendipitous source in the X-ray data, which is almost certainly another lower synchrotron peak frequency ($\nu^{sy}_{pk}$) BL Lac, that may contribute flux in the Fermi band. Since our sources are unusual high-luminosity, moderate $\nu^{sy}_{pk}$ BL Lacs we compare these quantities and the Compton dominance, the ratio of peak inverse-Compton to peak synchrotron luminosities ($L^{IC}_{pk}/L^{sy}_{pk}$), with those of the full Fermi BL Lac population.