Discovering High- <i>z</i> BL Lacs Using Swift/UVOT and SARA Observations with the Dropout Technique

Abstract Measuring spectroscopic redshifts for BL Lacertae (BL Lac) objects, a class of blazars, is challenging because their optical spectrum lacks, or has weak, emission lines (equivalent width ≤5 Å). In this situation, alternative techniques are necessary for the estimation of distances to these sources. In this paper, we estimate the redshift by the photometric dropout technique for a sample of 64 blazars (59 BL Lacs and 5 blazar candidates of uncertain type). Two telescopes are utilized to observe the sample. The Ultraviolet/Optical Telescope on board Swift observes sources in uvw 2, uvm 2, uvw 1, u , b , and v filters, while the ground-based SARA telescopes at Roque de los Muchachos, La Palma, Spain and Cerro Tololo, Chile observed sources in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi>g</mml:mi> </mml:mrow> <mml:mrow> <mml:mo accent="true">′</mml:mo> </mml:mrow> </mml:msup> <mml:mo>,</mml:mo> <mml:mspace width="0.25em"/> <mml:mi>r</mml:mi> <mml:msup> <mml:mrow> <mml:mo>,</mml:mo> </mml:mrow> <mml:mrow> <mml:mo accent="true">′</mml:mo> </mml:mrow> </mml:msup> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mi>i</mml:mi> </mml:mrow> <mml:mrow> <mml:mo accent="true">′</mml:mo> </mml:mrow> </mml:msup> <mml:mo>,</mml:mo> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mi>z</mml:mi> </mml:mrow> <mml:mrow> <mml:mo accent="true">′</mml:mo> </mml:mrow> </mml:msup> </mml:math> filters. We fit the photometric data with the LePHARE package and report four new high- z ( z &gt; 1.3) BL Lacs at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>2.0</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.05</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.07</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>1.8</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.03</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.10</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>2.0</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.14</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.16</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>2.9</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.04</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.01</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> as well as upper limits for 50 sources. This work increased the number of high- z BL Lacs found by this method up to 23. The high- z sources are discussed in the context of the cosmic gamma-ray horizon, blazar sequence, Fermi blazar divide, and masquerading BL Lacs.