{
  "id": "2409.16236",
  "version": "v1",
  "published": "2024-09-24T16:52:41.000Z",
  "updated": "2024-09-24T16:52:41.000Z",
  "title": "Discover high-$z$ BL Lacs by {\\it Swift} and SARA observations with the dropout technique",
  "authors": [
    "Y. Sheng",
    "K. Imam",
    "A. Kaur",
    "M. Ajello",
    "A. Domínguez",
    "A. Rau",
    "S. B. Cenko",
    "J. Greiner",
    "D. H. Hartmann",
    "I. Cox",
    "S. Joffre",
    "A. Mcdaniel",
    "R. Silver",
    "N. Torres-Albà"
  ],
  "comment": "Updated the CGRH plot",
  "categories": [
    "astro-ph.HE"
  ],
  "abstract": "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.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2024-09-24T16:52:41.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "bl lac",
      "sara observations",
      "photometric data",
      "cosmic gamma-ray horizon",
      "spectroscopic redshift measurement"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 0,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}