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Description
Blazars are the rarest and most powerful active galactic nuclei, playing a crucial and growing role in today multi-frequency and multi-messenger astrophysics. They dominate the high-energy extragalactic sky and recently have been associated to high-energy astrophysical neutrinos, and they may be among the accelerators of the ultra-high energy cosmic rays. Current blazar catalogs, however, are incomplete and particularly depleted at low Galactic latitudes. We aim at augmenting the current blazar census starting from a sample of ALMA calibrators that provides more homogeneous sky coverage, especially at low Galactic latitudes, to build a catalog of blazar candidates that can provide candidate counterparts to unassociated gamma-ray sources and to sources of high-energy neutrino emission or ultra-high energy cosmic rays. Starting from the ALMA Calibrator Catalog we built a catalog of 1580 blazar candidates (ALMA Blazar Candidates) for which we collect multi-wavelength information, including Gaia photometric, parallax and proper motion data, SDSS and LAMOST photometric and spectral data, WISE photometric data, X-ray (Swift-XRT, Chandra-ACIS and XMM-Newton-EPIC) count-rates and spectra, and Fermi-LAT fluxes and spectral slopes. We also compared our ALMA Blazar Candidates with existing blazar catalogs, like 4FGL, 3HSP, WIBRaLS2 and the KDEBLLACS. The ALMA Blazar Candidates catalogue fills the lack of low Galactic latitude sources in current blazar catalogues. Cross-matching this catalog with Gaia DR2, SDSS DR12, LAMOST DR5, AllWISE and 4FGL catalogues we obtained 805, 295, 31, 1311 and 259 matches, respectively. ALMA Blazar Candidates are significantly dimmer than known blazars in Gaia g band, while the difference in the Gaia b-r colour between the two populations is less pronounced. Also, ALMA Blazar Candidates sources appear bluer in SDSS than known blazars, although with low statistical significance. Most ALMA Blazar Candidates classified as QSO and BL Lac fall into the SDSS colour regions of low redshift quasars, with some QSOs entering the regions of higher redshift quasars. We collected 110 optical spectra in SDSS DR12 and LAMOST DR5, which mostly classify the corresponding sources as QSO (98), while 12 sources resulted galactic objects. Regarding WISE colours, we found that ABC sources are significantly bluer than known blazars in the w2-w3 and w3-w4 colours. In X-rays we detected 173 ALMA Blazar Candidates, and we were able to extract a X-ray spectra for 92 of them. Our sources are on average similar in X-rays to known blazar in terms of count-rates and spectral slopes, implying that our sample is covering the same region of the blazar parameter space in this band. A comparison of gamma-ray properties shown that ALMA Blazar Candidates are on average dimmer, and their gamma-ray spectra are softer than known blazars, indicating a significant contribution of FSRQ sources. Making use of WISE colours, we classified 715 ALMA Blazar Candidates as candidate gamma-ray blazar of different classes. We built a new catalogue of 1580 candidate blazars with a rich multi-wavelength data-set, filling the lack of low Galactic latitude sources in current blazar catalogues. This will be particularly important to identify the source population of high energy neutrinos or ultra-high energy cosmic rays, or to verify the Gaia optical reference frame. In addition, ALMA Blazar Candidates can be investigated both through optical spectroscopic observation campaigns or through repeated photometric observations for variability studies. In this context, the data collected by the upcoming LSST surveys will provide a key tool to investigate the possible blazar nature of these sources.
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