Revealing high-z Fermi-LAT BL Lacs using Swift and SARA data with photometric analysis

Y. Sheng, M. Rajagopal, A. Kaur, M. Ajello, A. Dominguez, A. Rau, B. Cenko, J. Greiner, D. H. Hartmann, I. Cox, S. Joffre, C. Karwin, A. Mcdaniel, R. Silver, N. Torres-Alba

Submitted on 20 September 2022


BL Lacertae (BL Lac) objects are one type of blazar, distinguished by their featureless optical spectrum. This presents a challenge in measuring the redshift of the BL Lacs. This paper uses the photometric dropout technique to measure the redshifts of BL Lac objects. Space-based telescope \emph{Swift} and ground-based SARA telescopes are employed to provide magnitudes in the uvw2, uvm2, uvw1, u, b, v, g, r, i, z filters. We observe 60 BL Lacs and report reliable redshift upper limits for 41 sources. We discover four high-z BL Lacs (z>1.3), bringing the number of high-z BL Lacs found by this method up to 20. We also discuss the blazar sequence, the \emph{Fermi} blazar divide, and the gamma-ray horizon using the 4LAC catalog and all high-z BL Lacs discovered with the photo-z technique.


Comment: 16pages, 7 figures

Subject: Astrophysics - High Energy Astrophysical Phenomena