# Radio Weak BL Lac objects in the Fermi era

## TL;DR

This paper investigates the existence of radio weak BL Lac objects (RWBLs) in the Fermi era, proposing a new method using WISE mid-IR colors to distinguish them from similar objects and confirming two candidates as genuine RWBLs.

## Contribution

It introduces a novel IR color-color diagnostic to identify RWBLs and confirms two new RWBLs associated with Fermi sources using multifrequency data.

## Key findings

- Identified two new genuine RWBLs using IR color diagnostics.
- Demonstrated the effectiveness of WISE colors in classifying BL Lac objects.
- Confirmed spectral behavior consistent with BL Lac population for the candidates.

## Abstract

The existence of "radio weak BL Lac objects" (RWBLs) has been an open question, still unsolved, since the discovery that quasars could be radio-quiet or radio-loud. Recently several groups identified RWBL candidates, mostly found while searching for low energy counterparts of the unidentified/unassociated gamma-ray sources listed in the Fermi catalogs. Confirming RWBLs is a challenging task since they could be confused with white dwarfs (WDs) or weak emission line quasars (WELQs) when there are not sufficient data to precisely draw their broad band spectral energy distribution and their classification is mainly based on a featureless optical spectra. Motivated by the recent discovery that Fermi BL Lacs appear to have very peculiar mid-IR emission, we show that it is possible to distinguish between WDs, WELQs and BL Lacs using the [3.4]-[4.6]-[12]$\mu$m color-color plot built using the WISE magnitudes when the optical spectrum is available. On the basis of this analysis, we identify WISE J064459.38+603131 and WISE J141046.00+740511.2 as the first two genuine RWBLs, both potentially associated with Fermi sources. Finally, to strengthen our identification of these objects as true RWBLs, we present multifrequency observations for these two candidates to show that their spectral behavior is indeed consistent with those of the BL Lac population.