A new multiwavelength census of blazars

Abstract

Context. Blazars are the rarest and most powerful active galactic nuclei. They play a crucial and expanding role in the multifrequency and multimessenger astrophysics of today. Dominating the high-energy extragalactic sky, they have been recently associated with 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 they are depleted, in particular, at low Galactic latitudes. Aims. Our study is aimed 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 γ-ray sources and sources of high-energy neutrino emission or ultra-high energy cosmic rays. Methods. Based on the ALMA Calibrator Catalog, we compiled a selection of 1580 ALMA Blazar Candidates (ABCs) for which we collected multiwavelength information, including Gaia photometric, parallax, and proper motion data, as well as 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, such as 4FGL, 3HSP, WIBRaLS2, and the KDEBLLACS. Results. The ABC catalog fills in the gaps with regard to low Galactic latitude sources in current blazar catalogs. By cross-matching this catalog with Gaia DR2, SDSS DR12, LAMOST DR5, AllWISE, and 4FGL catalogs, we obtained 805, 295, 31, 1311, and 259 matches, respectively. ALMA Blazar Candidates are significantly dimmer than known blazars in the Gaia g band, while the difference in the Gaia b − r color between the two populations is less pronounced. Also, ABC sources appear bluer in SDSS than known blazars, although with low statistical significance. Most ABCs classified as QSO and BL Lac fall into the SDSS color 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 turned out to be galactic objects. Regarding the WISE colors, we found that ABC sources are significantly bluer than known blazars in the w2 − w3 and w3 − w4 colors. In X-ray, we detected 173 ABCs and we were able to extract X-ray spectra for 92 of them. Our sources are, on average, similar in the X-ray band to known blazars in terms of count rates and spectral slopes, implying that our sample covers the same region of the blazar parameter space in this band. A comparison of γ-ray properties shows that the ABCs are, on average, dimmer and that their γ-ray spectra are softer than known blazars, indicating a significant contribution on the part of FSRQ sources. Making use of WISE colors, we classified 715 ABCs as candidate γ-ray blazar of different classes. Conclusions. We built a new catalog of 1580 candidate blazars with a rich multiwavelength data-set, filling in the gaps for low Galactic latitude sources in current blazar catalogs. This will be particularly important for the identification of 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 for investigating the possible blazar nature of these sources.

Probing the physical properties of the intergalactic medium using blazars

“…The total number of confirmed blazars by means of published spectra as of 2020 was 2968 (1909 FSRQ and1059 BL Lac) per the Roma-BZCAT Multifrequency Catalogue (Roma), which is regarded as the most comprehensive list of blazars (Massaro et al 2015;Paggi et al 2020). The vast majority of blazars are at z < 2, with less than 4 per cent at z > 2 and 1.2 per cent at z > 2.5 (Sahakyan et al 2020).…”

The JCMT Transient Survey: Single-epoch Transients and Variability of Faint Sources

“…More importantly, the source revealed no measurable parallax, implying a location more distant than Orion. The source is therefore almost certainly a background, extragalactic, source, which, following the mid-IR color selection by Paggi et al (2020), we identify as a blazar.…”

“…It is well known (e.g., Harvey et al 2007;Gutermuth et al 2009;Kryukova et al 2014) that YSOs have infrared colors similar to background galaxies and AGN (for a review, see Megeath et al 2022). More recently, attempts to catalog galaxies using their mid-IR colors (Rakshit et al 2019;Paggi et al 2020) have had to deal with YSO contamination: especially for blazars and BL Lac objects. Due to its mid-IR colors, Source 2864 was included in the initial ALMA blazar catalog by Paggi et al (2020) but subsequently culled due to its pre-existing classification as a YSO.…”

“…More recently, attempts to catalog galaxies using their mid-IR colors (Rakshit et al 2019;Paggi et al 2020) have had to deal with YSO contamination: especially for blazars and BL Lac objects. Due to its mid-IR colors, Source 2864 was included in the initial ALMA blazar catalog by Paggi et al (2020) but subsequently culled due to its pre-existing classification as a YSO.…”

Probing the physical properties of the intergalactic medium using blazars

“…The total number of confirmed blazars by means of published spectra as of 2020 was 2,968 (1,909 FSRQ and 1,059 BL Lac) per the Roma-BZCAT Multifrequency Catalogue (Roma), which is regarded as the most comprehensive list of blazars (Paggi et al 2020;Massaro et al 2015). The vast majority of blazars are at z < 2, with less than 4% at z > 2 and 1.2% at z > 2.5 (Sahakyan et al 2020).…”