标题： 通过使用dropout技术，利用{\it 斯威夫特}和SARA观测发现高-$z$BL Lacs 显示英文标题

标题： Discover high-$z$ BL Lacs by {\it Swift} and SARA observations with the dropout technique

摘要： 光谱红移测量BL Lac，一种耀变体类别，是具有挑战性的，因为它的光谱没有或有弱发射线 ($\leqslant5\AA$). 我们通过光度dropout技术对64个耀变体样本（59个BL Lacs和五个不确定类型的耀变体候选者）估计红移。 两种望远镜被用于观测该样本： {\it Swift} 空间望远镜在 $uvw2,\ uvm2,\ uvw1,\ u,\ b,\ v$ 滤镜下观测源，而地基望远镜SARA-CT/RM在 $g',\ r,' \ i',\ z'$ 滤镜下观测源。 光度数据使用 {\it photozpy} 包获得。 我们通过LePhare软件包拟合光度数据，并报告在$z$ ($z>1.3$) 处发现的四个新的高 BL Lacs，在$2.03^{+0.07}_{-0.05}$, $1.84^{+0.10}_{-0.03}$, $2.04^{+0.16}_{-0.14}$, $2.93^{+0.01}_{-0.04}$以及对50个源的上限。 这项工作将通过此方法发现的高$z$ BL Lacs的数量增加到了23个。 高-$z$源在宇宙伽马射线视界、耀变体序列、费米耀变体分割和伪装BL Lacs的背景下进行讨论。 显示英文摘要

摘要： The spectroscopic redshift measurement of BL Lac, a class of blazar, is challenging because its spectrum has no or weak emission lines ($\leqslant5\AA$). We estimate the redshift by the photometric dropout technique for a sample of 64 blazars (59 BL Lacs and five blazar candidates of uncertainty type). Two telescopes are utilized to observe the sample: the {\it Swift} space telescope observes sources in $uvw2,\ uvm2,\ uvw1,\ u,\ b,\ v$ filters, while the ground-based telescopes SARA-CT/RM observed sources in $g',\ r,' \ i',\ z'$ filters. The photometric data are obtained using the {\it photozpy} package. We fit the photometric data by the LePhare package and report four new high-$z$ ($z>1.3$) BL Lacs at $2.03^{+0.07}_{-0.05}$, $1.84^{+0.10}_{-0.03}$, $2.04^{+0.16}_{-0.14}$, $2.93^{+0.01}_{-0.04}$ as well as upper limits for 50 sources. The 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.