1 Introduction

Active galactic nuclei hosting powerful relativistic jets aligned close to the line of sight to the observer are called blazars. Some of the defining properties of blazars are temporal and spectral flux variability, highly polarized radio and optical radiation, a flat radio spectrum, and superluminal motion. Blazars are classified as flat-spectrum radio quasars (FSRQs) and BL Lac objects with the former exhibiting broad and strong emission lines (rest-frame equivalent width >5 Å) in their optical spectra (Stickel et al. 1991).

Figure 1
figure 1

Optical spectra of Fermi BCUs reported in the SDSS-DR16. The left panel highlights the absorption features arising from the host galaxy of the BCU 4FGL J0212.2−0219. The lack of emission lines suggests a radiatively inefficient accretion which in turn hints this object to be of BL Lac type. On the other hand, prominent emission lines can be seen in the right panel, suggesting the BCU 4FGL J0030.6−0212 to be of FSRQ type.

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Blazars are strong \(\gamma \)-ray emitters and dominate the extragalactic high-energy \(\gamma \)-ray sky (cf. Ajello et al. 2020). Among the thousands of blazars detected with the Fermi-large area telescope (LAT), there are still \(\sim \)30% whose classification, i.e., FSRQs or BL Lacs, is unknown (Ajello et al. 2020). This is either due to lack of optical spectroscopy or due to extremely poor signal-to-noise ratio of the optical spectrum even if it is available. Such objects are called blazar candidates of uncertain type or BCU type (Ackermann et al. 2011). Several multi-frequency properties of BCUs are found to be similar to blazars, however, due to the lack of classification, these cannot be used to study various crucial research problems, e.g., blazar luminosity function. Optical spectroscopic followup of some of the BCUs has led to the identification of peculiar objects that have provided new insights on blazar phenomenon, e.g., the first \(z>3\) \(\gamma \)-ray detected BL Lac object (Paliya et al. 2020), thus highlighting the importance of characterizing BCUs. To this purpose, various spectroscopic campaigns have been carried out to classify these objects (Peña-Herazo et al. 2020).

Recently, Paliya et al. (2021) analyzed publicly available optical spectra of >1000 blazars included in the second data release of the fourth catalog of the Fermi-LAT detected AGN (4LAC-DR2; Ajello et al. 2020) and reported their black hole mass (\(M_\mathrm{BH}\)), and accretion disk luminosity (\(L_\mathrm{disk}\)). For sources with broad emission lines, these quantities were calculated following the single epoch virial relations (Shen et al. 2011). On the other hand, \(M_\mathrm{BH}\) values were estimated from the stellar velocity dispersion for blazars whose optical spectra are dominated by the host galaxy absorption features. By fitting their radio-to-\(\gamma \)-ray spectral energy distributions (SEDs) with polynomial functions, Paliya et al. (2021) also measured the ratio of the high- and low-energy peak luminosities, so-called the Compton dominance, and determined a strong correlation between the accretion rate in the Eddington unit and Compton dominance.

This work takes benefit of the physical parameters reported in Paliya et al. (2021) to classify BCUs present in the 4LAC-DR2 catalog. Section 2 describes the sample adopted in this study. Results are presented in Section 3 and summarized in Section 4.

2 Sample

The 4LAC-DR2 catalog contains 1499 BCUs including 1292 without the redshift information. Furthermore, Paliya et al. (2021) studied the optical spectra of 1020 4LAC-DR2 blazars, including BCUs, by taking the advantage of recent advancements in optical spectroscopy, e.g., sixteenth data release of the Sloan Digital Sky Survey (SDSS-DR16; Ahumada et al. 2020). Therefore, it is likely that many Fermi-BCUs might be present in the sample of Paliya et al. (2021). With this in mind, 1499 BCUs from 4LAC-DR2 were cross-matched with the Paliya et al. (2021) sample. This exercise has led to the identification of 70 BCUs whose optical spectra reveal broad/narrow emission lines. Additionally, 83 BCUs were also found whose optical spectra are dominated by the absorption features arising from the host galaxy.

3 Results

3.1 Emission and absorption line BCUs

There are 153 BCUs present in the Paliya et al. (2021) sample whose optical spectra show prominent emission/absorption lines. There are 83 of them whose optical spectra do not exhibit any emission lines and are rather dominated by absorption features. Paliya et al. (2021) modeled these spectra with penalized Pixel Fitting software (pPXF; Cappellari & Emsellem 2004) to determine the stellar velocity dispersion. Following the empirical relations of Gültekin et al. (2009), \(M_\mathrm{BH}\) values were measured. Furthermore, Paliya et al. (2021) also computed the 3\(\sigma \) upper limit in \(L_\mathrm{disk}\) and the Compton dominance from the broadband SEDs of these sources.

For all of the 83 sources, only 3\(\sigma \) upper limit in the accretion rate could be measured, indicating a radiatively inefficient accretion process and supporting the BL Lac nature of these sources (Ghisellini et al. 2011). The left panel of Figure 1 shows the optical spectrum of one such BCU. Table 1 reports these 83 objects along with their central engine parameters.

Table 1 The list of Fermi BCUs whose optical spectra exhibit prominent absorption features, indicating them to belong to the BL Lac class of blazars. All of the parameters are adopted from Paliya et al. (2021). Both, \(M_\mathrm{BH}\) (in \(M_{\odot }\)) and 3\(\sigma \) upper limit in \(L_\mathrm{disk}\) (in erg s\(^{-1}\)) values are in log-scale. The second last column reports the Compton dominance and the last one refers to the proposed classification (bll).
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Figure 2
figure 2

Left: The \(\gamma \)-ray luminosity versus \(\gamma \)-ray photon index plane for Fermi-LAT detected blazars. The BCUs classified as FSRQs/BL Lacs tend to occupy the region populated mainly by known FSRQs/BL Lacs, supporting the classification scheme. Right: The Compton dominance versus accretion disk luminosity in Eddington units.

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There are 70 Fermi BCUs whose optical spectra reveal strong/faint broad emission lines. An example is demonstrated in the right panel of Figure 1. Since an equivalent width-based blazar classification is likely not physically motivated (cf. Ghisellini et al. 2011), Paliya et al. (2021) proposed another scheme. An FSRQ is a blazar with an accretion rate \(\ge \)1% of Eddington value and Compton dominance \(\ge \)1. On the other hand, sources with the accretion rate <1% of Eddington one and Compton dominance <1 can be classified as BL Lac objects. Following this classification scheme, 57 emission line BCUs turned out to be FSRQs and 13 as BL Lac sources (see Table 2). As also discussed in Paliya et al. (2021), sources with accretion rate \(\approx \)1% and Compton dominance \(\approx \)1 may belong to the peculiar class of masquerading BL Lac objectsFootnote 1 (Giommi et al. 2013).

Table 2 The list of Fermi BCUs whose optical spectra exhibit broad emission lines.
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3.2 A comparison with known FSRQs and BL Lac objects

In the left panel of Figure 2, the BCUs classified in this work are plotted in the \(\gamma \)-ray luminosity and \(\gamma \)-ray photon index diagram. For a comparison, known FSRQs and BL Lac objects present in the 4LAC-DR2 catalog are also shown. As can be seen, BCUs classified as FSRQs occupy a region populated mainly by known FSRQs. Similar results are obtained for objects identified as BL Lac type sources. This result further supports the blazar classification scheme proposed by Paliya et al. (2021).

The right panel of Figure 2 shows the variation of the Compton dominance as a function of the accretion luminosity in Eddington units. A majority of emission line BCUs have radiatively efficient accretion, i.e., their accretion luminosity is \(\ge \)1% of the Eddington luminosity. Furthermore, a few of them do exhibit low-level accretion activity and probably belong to the BL Lac class (see Table 2). It is possible that optical spectra of such blazars were taken during the low jet activity state, revealing the faint emission lines similar to the prototype of this class BL Lac itself (Vermeulen et al. 1995). Finally, BCUs identified in this work appear to follow the positive correlation between the plotted quantities as found by Paliya et al. (2021) for other blazars.

4 Summary

Taking the advantage of the recently published optical spectral parameters and Compton dominance computed from the multi-wavelength SEDs (Paliya et al. 2021), this work classifies 153 Fermi BCUs. Among the whole sample, 57 are found to exhibit FSRQs type behavior and the remaining 96 may belong to the BL Lac population. The results obtained also support the hypothesis that the Compton dominance can be used as a proxy for the accretion rate in Eddington units. Since current and next-generation all-sky surveys, e.g., e-ROSITA, and Square Kilometer Array, are going to provide flux measurements across the electromagnetic spectrum, a precise measurement of the Compton dominance would be possible for a large sample of blazars. In this regard, a Compton dominance-based blazar classification scheme can be very useful in identifying the FSRQ/BL Lac nature of BCUs, enabling a better understanding of the physics of relativistic jets.