An Optical Overview of Blazars with LAMOST. II. Gamma-Ray Blazar Candidates and Updated Classifications

Blazars are the dominantly known population in extragalactic γ-ray sources (Acero et al. 2015; Abdollahi et al. 2020) detected by the Fermi Large Area Telescope (LAT; Atwood et al. 2009). They are Active Galactic Nuclei (AGNs) with a powerful jet closely aligned with the observer's line of sight (Blandford & Rees 1978; Urry & Padovani 1995). Blazars are detected over the entire electromagnetic spectrum from radio up to TeV energies (Giommi et al. 1995; Fossati et al. 1998; Abdo et al. 2010a, 2010b; Mao et al. 2016), featuring a double-humped spectral energy distribution (SED; Abdo et al. 2010a; Fan et al. 2016; Paliya et al. 2017a).

Both the flux and spectral shape of blazars are variable with different timescales ranging from minutes to days (Gu et al. 2006; Aharonian et al. 2007; Gu & Ai 2011; Isler et al. 2013; León-Tavares et al. 2013; Hayashida et al. 2015; Paliya et al. 2017b; Patiño-Álvarez et al. 2017; Gupta 2018; Nalewajko et al. 2019; Sarkar et al. 2019; Chavushyan et al. 2020). Furthermore, blazars also feature flat radio spectra at GHz frequencies (Healey et al. 2007; Massaro et al. 2013; Petrov et al. 2013; Nori et al. 2014; Schinzel et al. 2015, 2017; Giroletti et al. 2016), coupled with polarized emission detected up to optical wavelengths (Poutanen 1994; Park et al. 2018; Liodakis & Blinov 2019; Mandarakas et al. 2019), and with peculiar mid-IR colors (Massaro et al. 2011, 2012b; D'Abrusco et al. 2012, 2013) not ascribable to dust emission.

Blazars are generally distinguished between BL Lac objects and flat-spectrum radio quasars (FSRQs), separated by an arbitrary threshold on the equivalent width of their optical emission lines corresponding to 5 Å (Stickel et al. 1991; Stocke et al. 1991). In particular, while FSRQs feature strong, broad emission lines with a rest-frame equivalent width EW > 5 Å, BL Lacs show optical spectra dominated by a blue continuum with weak absorption or emission lines (EW < 5 Å) making their redshift estimate extremely challenging (see, e.g., Paiano et al. 2020).

Adopting the Roma-BZCAT catalog nomenclature (Massaro et al. 2009) we label BL Lacs and FSRQs as BZBs and BZQs, respectively. In addition, Roma-BZCAT lists 227 sources classified as BZUs; that is, blazars of an uncertain type. In the Fifth release of the Roma-BZCAT (Massaro et al. 2015a) potential BL Lacs showing host galaxy emission dominance over the continuum were labeled as BZG. This classification is based on the Ca II break (C) contrast defined as the relative difference, C = (F₊ − F₋)/F₊, of fluxes at rest-frame wavelengths of 3750–3950 Å for F₋ and 4050–4250 Å for F₊, with BZGs having C ≥ 0.25 (Landt et al. 2002; Massaro et al. 2012a, 2015a).

Nearly 30% of the sources listed in the Fourth Fermi-LAT Point Source Catalog (4FGL; Abdollahi et al. 2020) lack an assigned lower-energy counterpart, being therefore labeled as unidentified or unassociated γ-ray sources (hereinafter UGSs; see Massaro et al. 2013a, 2015d; Peña-Herazo et al. 2020). Given the large sky density of blazars in the high energy sky (${0.12}_{-0.02}^{+0.03}\,{\deg }^{-2}$ at 100 MeV–100 GeV photon fluxes F₁₀₀ ≥ 10⁻⁹ ph cm⁻² s⁻¹; Abdo et al. 2010c), we could expect that large fraction of UGS are unknown blazars (Massaro et al. 2013a), not yet identified due to missing spectroscopic information. This hypothesis is also supported by the recent optical spectroscopic campaign we carried out that allowed to confirm the blazar-like nature of hundreds unassociated or unidentified Fermi sources (Peña-Herazo et al. 2020).

In the latest release of the 4FGL there are also Blazar Candidates of Uncertain Type (BCUs; see also Acero et al. 2015; Ackermann et al. 2015) accounting for nearly 25% of associated sources. The largest fraction of BCUs is currently unclassified mainly due to the lack of optical spectra, but they show the typical two-humped SED of known blazars coupled with a flat radio spectrum (Massaro et al. 2009, 2015a). In particular most BCUs observed during our spectroscopic campaign as well as by other groups were then classified as BL Lacs, thus confirming that this blazar subclass is the most elusive class of AGNs in the gamma-ray sky (Massaro et al. 2015b; Marchesini et al. 2019b; Peña-Herazo et al. 2020).

In the quest of (i) searching for plausible UGSs counterparts and (ii) confirming the blazar nature of BCUs, several follow-up programs and archival searches have been carried out in the last decade. These were performed at radio frequencies above (e.g., Healey et al. 2007; Kovalev 2009; Petrov et al. 2013; Schinzel et al. 2015) and below 1 GHz (e.g., Massaro et al. 2013; Nori et al. 2014; Giroletti et al. 2016), in the submillimeter range (Giommi et al. 2012; López-Caniego et al. 2013), using infrared colors (Massaro et al. 2011; D'Abrusco et al. 2012), and with follow-up X-ray observations (Cheung et al. 2012; Acero et al. 2013; Paggi et al. 2013; Takeuchi et al. 2013; Landi et al. 2015; Kaur et al. 2018; Marchesini et al. 2019a, 2020; Kaur et al. 2019).

However, as previously stated, optical spectroscopic observations are the only way to firmly establish the blazar nature of UGS candidates and BCUs (for reviews see Massaro et al. 2016; Peña-Herazo et al. 2020). This was the underlying reason to carry out our optical spectroscopic campaign, mainly dedicated to observe target selected on the basis of peculiar mid-IR colors, similar to those of known γ-ray blazars (Massaro et al. 2011; D'Abrusco et al. 2012, 2019; Massaro & D'Abrusco 2016) started in 2014 (Paggi et al. 2014; Massaro et al. 2015c). Hundreds of new blazars were discovered (Landoni et al. 2015; Ricci et al. 2015; Álvarez Crespo et al. 2016a, 2016c; Peña-Herazo et al. 2017; Marchesini et al. 2019b; Peña-Herazo et al. 2019; de Menezes et al. 2020), also including searches in the archives of major spectroscopic surveys (Álvarez Crespo et al. 2016b; de Menezes et al. 2019) as the Sloan Digital Sky Survey (SDSS; Aguado et al. 2019), Large Sky Area Multi-Object Fibre Spectroscopic Telescope Data Release 5 (LAMOST DR 5; Zhao et al. 2012), and the Six-Degree Field Galaxy Survey (Jones et al. 2004, 2009).

Recently, in Peña-Herazo et al. (2021) we explored the use of LAMOST DR 5 survey to search for available optical spectra in addition to previous ones. Investigating 392 spectra of different blazar samples, we confirmed the blazar nature of 20 BCUs, obtained 15 new redshift estimates for known blazars, reported 26 changing-look blazars, and confirmed the blazar nature of six BL Lac candidates reported in the Roma-BzCAT.

Here we present an update search for blazars based on the LAMOST catalog's (Su et al. 1998; Cui et al. 2012; Zhao et al. 2012) sixth data release (DR6). ¹³ We aim at confirming the blazar-like nature of BCUs, measure redshifts for BZBs lacking such estimate, and search for new changing-look blazars. As in our previous works, we explored several samples as (i) blazars listed in the Roma-BZCAT, (ii) targets observed during our optical spectroscopic campaign of γ-ray blazars, (iii) BCUs listed in the latest release of the 4FGL, and (iv) we verified the presence of new spectra for those sources already analyzed in Peña-Herazo et al. (2021).

This paper is organized as follows: Section 2 is dedicated to an overview of the data used in the present analysis while in Section 3 we describe our sample selection. Then, Section 4 is entirely devoted to the procedure followed to carry out the spectral analysis with results presented in Section 5. Finally, Section 6 reports our summary, conclusions, and future perspectives.

Unless otherwise stated, we adopt cgs units for numerical results and we also assume spectral indices, α, defined by flux density, S_ν ∝ ν^−α .

The LAMOST (also known as Guo Shou Jing) telescope is a reflective Schmidt telescope located in Xinglong Station of National Astronomical Observatory, Chinese Academy of Sciences. LAMOST have an effective aperture of 3.6–4.9 m depending on the pointing direction, a focal length of 20 m, and a field of view of 5 × 5 deg². LAMOST has a configuration with the primary mirror and the focal surface fixed along the focal axis. The active aspherical secondary mirror has an alt-azimutal mount, and consists of 37 segments for 6.67 × 6.09 m² of total area. The fixed spherical primary mirror is composed of 24 segments, for a total area of 5.72 × 4.4 m². The sky covered by the LAMOST survey has a footprint in decl. −10 < δ < 90°. The LAMOST Telescope has a fixed focal plane equipped with 4000 fiber-positioning units. Each unit has a size of 33, feeding one of sixteen spectrographs throughout optical fibers (Su et al. 1998; Cui et al. 2012; Zhao et al. 2012). The LAMOST spectrographs consist of a blue and red arm, the blue camera covers 3700–5900 Å, and the red one 5700–9000 Å. The spectral resolution of the spectrograph is R = 1000 or 5000 depending on the slits and grating configuration, with a set of four gratings with spatial frequencies of 540, 800, 1680, and 2750 lines mm⁻¹.

The LAMOST archive provides spectra with relative flux calibration, reduced using the LAMOST 2D pipeline (Luo et al. 2015). The procedure is similar to that of SDSS (Stoughton et al. 2002). The LAMOST 2D pipeline performs flat-fielding corrections using twilight and screen flat fields, sky background modeling and subtraction, wavelength calibration using arc lines, and relative flux calibration using standard stars. In the overlapping wavelength region of both arms, between 5300 and 6500 Å data are scaled to match (Luo et al. 2015). The artifact of matching both arms can be noticeable in a low signal-to-noise ratio (S/N) spectra. The mismatch is due to differences in flat fielding and sky subtraction between both channels.

The two major LAMOST surveys started on 2012, the LAMOST Experiment for Galactic Understanding and Exploration focused on the Galaxy stellar structure (Zhao et al. 2012), and the LAMOST ExtraGAlactic Surveys (LEGAS; Kong & Su 2010) with a scientific objective aimed at studying the large scale structure of the universe (Zhao et al. 2012). LEGAS is constituted of three subsurveys: two dedicated to galaxies, LEGAS-deep with 3.5 million targets observed during dark nights up to r = 19.5 mag, and LEGAS-shallow with 2.4 million targets up to r = 18.7 mag. The third survey, LEGAS-QSOs, aims at observing 0.6 million quasars, up to i = 20.5 mag. LAMOST's most recent public data release includes the sixth year of operations (DR6) and contains spectra acquired between 2017 September 19 and 2018 May 18, with new spectra with respect to previous releases for 20,180 out of a total of 177,270 observed galaxies, and 7783 out of a total of 62,168 observed quasars (Yao et al. 2019).

We searched for spectra available in the LAMOST DR6 for three different samples or catalogs. The first catalog includes the 1312 BCUs and 1825 known blazars, either identified or associated, listed in the 4FGL (Abdollahi et al. 2020). These sources are part of the 5064 sources detected by the Fermi-LAT as reported in the latest release of the 4FGL emitting in the energy range between 50 MeV to 1 TeV.

The second sample is constituted of 517 blazars observed during our optical spectroscopic campaign (Cowperthwaite et al. 2013; Paggi et al. 2014; Landoni et al. 2015; Massaro et al. 2015c, 2016; Ricci et al. 2015; Álvarez Crespo et al. 2016a, 2016c; Peña-Herazo et al. 2017, 2019, 2020; Marchesini et al. 2019b; de Menezes et al. 2020), or collected from the literature (see, e.g., Shaw et al. 2013; Paiano et al. 2017a, 2017b, 2019; Klindt et al. 2017; Desai et al. 2019). This sample is distinguished in 417 BZBs, 30 BZGs, 44 BZQs, and 24 quasars.

The third and last sample analyzed here lists 3561 blazars belonging to the Fifth release of the Roma-BZCAT (Massaro et al. 2015a) classified as: BZBs (1151, 369 of which having a firm redshift estimate), BZQs (1909), and BZGs (274).

To perform the LAMOST spectral analysis we selected those sources (i) lying in the LAMOST footprint and (ii) having optical spectra with an S/N larger than 10 between 4000 and 9000 Å. To carry out this selection we also considered only those targets having a maximum angular separation of 2'' between the known position of the catalog and their counterpart in LAMOST DR6, as used for LAMOST DR 5 (Peña-Herazo et al. 2021). This choice of the association radius is based on the statistical procedure described in Massaro et al. (2014), corresponding to a chance probability of spurious associations lower than ∼1%. Thus the final samples lists:

• 1.  
  From the 4FGL catalog 35 sources, classified as 20 BCUs, seven BL Lacs, seven FSRQs, and one AGN.
• 2.  
  From our optical spectroscopic campaign 12 sources, being eight BZBs, two BZGs, and two quasars.
• 3.  
  From the Roma-BZCAT 399 sources, distinguished as: five BZBs candidates, 139 BZBs, 42 BZGs, and 198 BZQs.

All sources included in the present analysis are listed in Table 1. It is worth noting that sources belonging to more than one sample but having a Roma-BZCAT counterpart are only listed in the third sample reported above, while targets observed during our spectroscopic campaign, and thus being associated to a 4FGL source, are not listed in the 4FGL subsample.

In the present analysis we included all 364 sources previously analyzed in Peña-Herazo et al. (2021), using the LAMOST DR5. We checked them to verify if new available spectra have better signal-to-noise ratio that would allow us to obtain a more precise classification or a redshift estimate.

We retrieved LAMOST DR6 spectra for the sources in the three samples described in Section 3 and we then identified their emission or absorption lines. We searched for typical quasar emission line such as C IV, C III], Mg II, [O II], [O III], [N II], and [S II] doublets along with Balmer emission lines. For those objects showing absorption lines we searched for Ca II H&K doublet, G band, Mg I, or Mg II doublet in absorption.

The spectral measurements were done using IRAF standard packages (Tody 1986). We measured the lines central positions by fitting a Gaussian profile. To avoid shifted spectral components we fitted the Gaussian near the peak of the line. We defined a local continuum to measure the equivalent widths used for discriminating between BZQs and BZBs, using the same classification scheme used in our previous analyses (Massaro & D'Abrusco 2016; de Menezes et al. 2020; Peña-Herazo et al. 2020).

We adopted the same classification criteria of the Roma-BZCAT, using the same naming convention; that is, BZBs for BL Lac objects, and BZQs for FSRQs. We classify as BZBs sources showing featureless spectra or with emission lines of EW < 5 Å. On the other hand, BZQs show typical quasar-like optical spectra, with broad emission lines and EW > 5 Å. BZGs are those blazars having a nonnegligible host galaxy emission in both their optical spectra and in their broadband SED (Massaro et al. 2012c), similar to elliptical galaxies in the optical band (Massaro et al. 2012c, 2015d; Shaw et al. 2012).

The main objective of our work includes confirming the blazar nature of BCUs, measuring the redshift of known BZBs, and searching for changing-look blazars. As explained in Section 3, our sample is constituted of the BCUs in the 4FGL, targets observed during our optical spectroscopic campaign, and sources in the Roma-BzCAT. Here we describe the results obtained in our analysis. In particular, we found LAMOST DR6 spectra for 39 sources in our samples. In addition, three out of the 392 objects previously analyzed in LAMOST DR5 (Peña-Herazo et al. 2021) are also present in DR6. In total, we then collected spectra for 42 sources listed in Table 1.

We present in Figure 1 an example of each source class studied: BZB, BZQ, and BZG; together with a classical elliptical galaxy spectrum as comparison.

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Our results can be summarized as follows:

• 1.  
  From the 4FGL catalog we found four sources with LAMOST DR6 spectra. These sources are: 4FGL J0651.4+6525, 4FGL J0927.2+2454, 4FGL J1057.2+55, and 4FGL J1242.9+7315, all classified as BCUs in the 4FGL. Based on the collected spectra, we classify all of them as BZB but 4FGL J1242.9+7315 that we classified as BZG. In particular, we estimated a redshift of z = 0.074 for the source 4FGL J1242.9+7315, compatible (Marcha et al. 1996) with the literature value of z = 0.075. In Figure 2 we present the LAMOST spectra of these four sources.
• 2.  
  From the targets of the optical campaign we found three sources with LAMOST DR6 spectra. These sources are J202155.46+062913.6, J224436.69+250342.5, and J232352.09+421058.4, all previously classified as BZBs. We confirm the BZB classification for all these three sources.
• 3.  
  For targets from the Roma-BzCAT we found 35 sources with a spectrum in the DR6. In particular, three of them (namely 5BZQ J0842+4018, 5BZB J1101+4108, and 5BZG J1132+0515) were already present in the LAMOST DR5 analysis presented in Peña-Herazo et al. (2021). For these sources we report the same classification as in DR5. In addition, for 5BZQ J0842+4018 and 5BZG J1132+0515, we estimate the same redshifts as the literature ones. We present the spectra of both sources in Figure 3. We rescaled the spectra with their corresponding flux at 4500 Å to compare the LAMOST and SDSS spectra. We did not find significant spectral variability for both sources. The difference observed between the LAMOST and SDSS spectrum of 5BZQ J0842+4018 is due to different calibration of blue and red channels in LAMOST spectrum. The other 32 sources from the Roma-BzCAT are 14 BZBs, four BGZs, 13 BZQs, and one BZU. We confirm the classification for all these sources, with the exception of 5BZG J0758+2705, now classified as BZBs according to its Ca II break, and the BZU (5BZU J0909+4253), now classified as a BZB. In addition we were able to estimate the redshift for 19 of these sources, all confirming the literature values. We present the spectrum of 5BZU J0909+4253 in the second panel of Figure 2 and the spectrum of 5BZG J0758+2705 in the first panel of Figure 3.
• 4.  
  We did not find changing-look blazars in our selected samples in the LAMOST DR 6. We considered as candidates to changing-look blazars objects with different optical spectral classifications (BZBs, BZQs, or BZGs) reported in two different epochs. For the case of 5BZG J0758+2705, a.k.a. BZB J0758+2705, we cannot confirm its changing-look nature as we did not find an optical BZG-like spectrum in the literature. On the contrary, the available archival SDSS spectrum has a Ca II break value (C = 0.095 ± 0.01) corresponding to a BZB.

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Clarifying the nature of blazar candidates and determining their redshift is crucial since it (i) helps to obtain more stringent constraints on their luminosity function (Ajello et al. 2014; Ackermann et al. 2016), (ii) allows us to study the imprint of the extragalactic background light in the blazar γ-ray spectra (e.g., Domínguez Sánchez et al. 2011; Ackermann et al. 2012; Sandrinelli et al. 2013), (iii) helps select potential targets for TeV observatories (Massaro et al. 2013c; Arsioli et al. 2015), (iv) searches for new classes of γ-rays sources (Massaro et al. 2017; Bruni et al. 2018), and (vi) sets more stringent limits on the annihilation of dark matter in subhalos (see e.g., Zechlin et al. 2012; Ackermann et al. 2014; Berlin & Hooper 2014).

In this work we analyzed 42 sources present in the LAMOST DR6 archive and selected from three samples: (i) the Fourth Fermi-LAT Point Source Catalog (Abdollahi et al. 2020), (ii) objects observed during the follow-up spectroscopic campaign of unidentified or unassociated γ-ray sources or reported in the literature (see e.g., Massaro et al. 2013b; Shaw et al. 2013; D'Abrusco et al. 2014; Klindt et al. 2017; Paiano et al. 2017a, 2017b, 2019; Marchesini et al. 2019b; Peña-Herazo et al. 2019, 2020; de Menezes et al. 2020), and (iii) sources listed in the Roma-BZCAT (Massaro et al. 2009, 2015a).

This work represents an extension of our analysis on LAMOST DR5 data set (Peña-Herazo et al. 2021). Summing up the combined results of the LAMOST DR5 and DR6 analysis, we obtained the following:

• 1.  
  We classified five sources without a clear blazar classification (four BCUs and one BZU) as blazars, namely four BZBs and one BZG.
• 2.  
  We confirm the blazar classification of 25 BZBs, 14 BZQs, and three BZGs.
• 3.  
  We confirmed the blazar nature of the BZU J090933.49+425346.5, classifying it as a BZB at z = 0.670.

BZBs have proven to be the most elusive type of blazars (de Menezes et al. 2020; Peña-Herazo et al. 2021), given the challenges to measure their redshift due to the nonthermal dominant continuum. From the samples analyzed in this work and present in LAMOST DR6, only four out of 23 BZBs have redshift measurements.

LAMOST surveys continue to represent an extremely useful tool to investigate blazars, their variability, classification, and redshift measurement. The upcoming public release of LAMOST DR7 will yield an increase of more than 3,200,000 spectra and will therefore represent an invaluable resource to clarify the nature of blazar candidates and investigate the properties of the known blazar population.

H.P.-H. and V.C. acknowledge support from CONACyT research grant No. 280789. M.F.G. acknowledges support from the National Science Foundation of China (grant 11873073). This work is supported by the "Departments of Excellence 2018-2022" Grant awarded by the Italian Ministry of Education, University, and Research (L. 232/2016). This research has made use of resources provided by the Ministry of Education, Universities, and Research for the grant MASF_FFABR_17_01. A.P. acknowledges financial support from the Consorzio Interuniversitario per la fisica Spaziale under the agreement related to the grant MASF_CONTR_FIN_18_02.

This work is based on data from the Guoshoujing Telescope (the Large Sky Area Multi-Object Fiber Spectroscopic Telescope) and is a National Major Scientific Project built by the Chinese Academy of Sciences. Funding for the project has been provided by the National Development and Reform Commission. LAMOST is operated and managed by the National Astronomical Observatories, the Chinese Academy of Sciences.