BAT AGN Spectroscopic Survey. XVI. General Physical Characteristics of BAT Blazars

Active galactic nuclei (AGNs) are among the most energetic objects in the universe and are crucial players in the evolution of galaxies (see, e.g., Kormendy & Ho 2013, for a review). A subset of these interesting objects host relativistic jets and when the jet is closely aligned with the line of sight to the observer, the source is termed as a blazar (Blandford & Rees 1978). Due to their peculiar orientation, the emitted radiation from blazar jets is relativistically amplified. This phenomenon makes blazars visible even at very high redshifts (z > 2, e.g., Romani et al. 2004; Ackermann et al. 2017). Blazars are classified as flat spectrum radio quasars (FSRQs) and/or BL Lac objects based on the rest-frame equivalent width (EW) of the optical emission lines (Stickel et al. 1991) with FSRQs exhibiting broad emission lines (EW > 5 Å), while BL Lacs show weak or no emission lines in their optical spectra. This, in turn, suggests a radiatively efficient accretion process that illuminates the broad-line region (BLR) surrounding the central engine of FSRQs (see Ghisellini et al. 2011). The spectral energy distribution (SED) of a blazar is characterized by a double hump structure. The low-energy peak is understood to be due to a synchrotron process and is typically observed between the infrared and the X-ray band, whereas, the high-energy peak is typically explained by the inverse Compton scattering of the low-energy photons by relativistic electrons in the jet. Alternatively, hadronic models have also been invoked to explain the high-energy SEDs of blazars (e.g., Böttcher et al. 2013). Based on the location of the synchrotron peak frequency (${\nu }_{\mathrm{syn}}^{\mathrm{peak}}$), blazars are also classified as low- (${\nu }_{\mathrm{syn}}^{\mathrm{peak}}\lt {10}^{14}\,\mathrm{Hz}$), intermediate- (${10}^{14}\lt {\nu }_{\mathrm{syn}}^{\mathrm{peak}}\lt {10}^{15}\,\mathrm{Hz}$), and high- (${\nu }_{\mathrm{syn}}^{\mathrm{peak}}\gt {10}^{15}\,\mathrm{Hz}$) synchrotron peaked, or LSP, ISP, and HSP, respectively (Abdo et al. 2010). Typically FSRQs are LSP/ISP blazars, whereas, BL Lacs are mostly HSP ones.

The sky surveying capabilities of the currently operating hard X-ray and γ-ray instruments, namely the Burst Alert Telescope (BAT; Barthelmy et al. 2005) on board Neil Gehrels Swift observatory (Gehrels et al. 2004) and the Large Area Telescope (LAT; Atwood et al. 2009) on board Fermi Gamma-Ray Space Telescope, have been allowed to carry out the population studies of blazars and their luminosity-dependent evolution (e.g., Ajello et al. 2009, 2012, 2014). In particular, Ajello et al. (2009) studied 38 blazars detected in the first 36 months of the all-sky survey by Swift-BAT and used this sample to derive the 15–55 keV luminosity function (LF) of blazars. Interestingly, it was noted that the evolution of luminous FSRQs peaks at higher redshifts (∼4) compared to other classes of BAT detected AGNs. Ghisellini et al. (2010) used the LF reported in Ajello et al. (2009) to determine the space density of billion-solar-mass black holes residing in jetted systems and introduced an exponential cut-off in the blazar LF above z = 4.3 to remain consistent with the number of detected blazars in that redshift bin and also with the space density of massive halos. However, computation of the LF depends strongly on the number of sources detected in a particular redshift bin. For example, Ackermann et al. (2017) reported, for the first time, γ-ray detection of five z > 3.1 radio-loud quasars thus confirming their blazar nature. This allowed them to update the space density of 10⁹ M_⊙ black holes residing in radio-loud quasars at z ≈ 4 and to conclude that radio-loudness may be a crucial ingredient for the rapid black hole growth in the early universe (see also Volonteri et al. 2011). Moreover, by studying high-redshift (z > 2) Swift-BAT blazars, Ghisellini et al. (2010) found them to host more powerful jets and more luminous accretion disks compared to Fermi-LAT detected z > 2 sources.

A new catalog of Swift-BAT detected sources covering the first 105-month of the mission has recently been released (Oh et al. 2018) along with an extensive multiwavelength observations of BAT detected sources by the BAT AGN Spectroscopic Survey (BASS) collaboration (Koss et al. 2017; Lamperti et al. 2017; Ricci et al. 2017). Motivated by more than quadrupling the number of the sources compared to past studies and also by the larger redshift range it covers,¹⁵ we have carried out a multiwavelength study of all the blazars present in the 105-month Swift-BAT catalog. Our primary objectives are to explore the physical properties of Swift-BAT blazars by applying a simple leptonic radiative model and also to compare them with that known for the Fermi-LAT detected ones, which are yet to be detected by BAT.

In Section 2, we describe the sample adopted in this work and data analysis procedures are explained in Section 3. The adopted leptonic model is elaborated in Section 4 and we discuss the derived SED parameters in Section 5. In Section 6, we compare the accretion-jet connection observed in BAT blazars with Fermi-LAT detected sources, and Section 7 is devoted to the luminosity-dependent evolution of blazars. We summarize our findings in Section 8. Throughout this work, we use a flat cosmology with H₀ = 67.8 km s⁻¹ Mpc⁻¹ and Ω_M = 0.308 (Planck Collaboration et al. 2016).

Our blazar sample is based on the 1632 sources included in the 105-month Swift-BAT catalog. There are 158 sources classified as "beamed AGN," mainly based on their presence in the BZCAT and Combined Radio All-Sky Targeted Eight-GHz Survey (CRATES) catalogs (Healey et al. 2007; Massaro et al. 2015). We exclude those 32 objects that are reported as "beamed AGNs" (based on the BZCAT catalog) but, however, do not exhibit broadband properties typically observed from the blazar class of AGN. The full list of these sources is provided in Table 1. Additionally, we explore the broadband properties of the whole BAT sample to identify potential blazar candidates that might have been misclassified as Seyferts. This was done by examining their multifrequency SEDs generated using the ASI Data Center (ASDC) SED builder tool¹⁶ and physical properties (e.g., radio detection) reported in the literature (see Griffith & Wright 1993; Gregory et al. 1996). This led to the inclusion of 20 sources (Table 2). Admittedly, there could be a very few unidentified BAT objects that might be blazars though it may not be possible to pinpoint them due to lack of a confirmed optical/radio counterparts and also because of paucity of the multiwavelength information. The BAT survey completeness and redshift evolution of beamed AGN will be fully examined in a future study (L. Marcotulli et al. 2019, in preparation).

This paper makes use of redshift and black hole mass estimates from BASS, a large effort to collect optical spectra for all the Swift-BAT AGN. The Data Release 1 optical spectra were obtained from a large variety of telescopes (Koss et al. 2017) and can all be viewed on the BASS website¹⁷ . We also include 15 new Data Release 2 redshifts measurements, from Palomar, Southern Astrophysical Research (SOAR) telescope, and the Very Large Telescope (VLT) that will be published shortly (K. Oh et al. 2019, in preparation) as well as a comparison black hole mass measurement from this catalog. Altogether, our final sample consists of 146 BAT blazars.

3.1. Fermi-LAT

3.2. Swift-BAT, NuSTAR, and Archival Measurements

4.1. The Model

We adopt a simple one-zone leptonic emission model (e.g., Dermer et al. 2009; Ghisellini & Tavecchio 2009) to reproduce the multifrequency SEDs of BAT blazars. We consider a conical jet with a semi-opening angle of 0.1 rad. The emission region is assumed to be spherical and cover the whole cross-section of the jet. With this assumption, the dissipation distance (R_d) constrains the size of the emitting region. The observer receives the Doppler-boosted jet emission at an angle of θ_v from the jet. Furthermore, the radiating population of relativistic electrons has a broken power-law distribution of the following type:

$$\begin{eqnarray}&&S(\gamma )={S}_{0}\,\displaystyle \frac{{\left({\gamma }_{{\rm{b}}}\right)}^{-p}}{{\left(\gamma /{\gamma }_{{\rm{b}}}\right)}^{p}+{\left(\gamma /{\gamma }_{{\rm{b}}}\right)}^{q}}\end{eqnarray} \tag{ 1 }$$

where S₀ is the normalization constant (cm⁻³), γ is the the Lorentz factor of electrons, and p, q are the spectral indices below and above the break Lorentz factor γ_b, respectively. Note that our modeling technique does not include a time-dependent evolution of the electron spectrum considering particle injection/cooling.

The leptonic population emits synchrotron radiation in presence of uniform and randomly oriented magnetic field B. We also consider various photon fields, both inside and outside of the jet, to calculate the inverse Compton radiation from the emitting population (e.g., Rybicki & Lightman 1979). This includes thermal photons originated from the accretion disk, BLR, and dusty torus (the external Compton or EC process; e.g., Sikora et al. 1994) and also non-thermal synchrotron photons produced by the same electrons (synchrotron self Compton or SSC; Marscher & Gear 1985). Moreover, we assume the BLR and torus are spherical shells with radii ${R}_{\mathrm{blr}}={10}^{17}\sqrt{{L}_{{\rm{d}},45}}\,\mathrm{cm}$ and ${R}_{\mathrm{torus}}=2.5\times {10}^{18}\sqrt{{L}_{{\rm{d}},45}}\,\mathrm{cm}$, respectively, where L_d,45 is the luminosity of the accretion disk in units of 10⁴⁵ erg s⁻¹ (Ghisellini & Tavecchio 2009). The adopted emission profiles of these components follow a blackbody distribution peaking at Hydrogen Lyα frequency and the characteristic temperature of the torus (T_torus), respectively. We assume a geometrically thin and optically thick accretion disk (Shakura & Sunyaev 1973) whose radiative profile is considered to follow a multi-colored blackbody (see, e.g., Frank et al. 2002)

$$\begin{eqnarray}&&{F}_{\nu ,\mathrm{disk}}={\nu }^{3}\displaystyle \frac{4\pi h\cos {\theta }_{{\rm{v}}}}{{c}^{2}{{D}_{l}}^{2}}{\int }_{{R}_{{\rm{d}},\mathrm{in}}}^{{R}_{{\rm{d}},\mathrm{out}}}\displaystyle \frac{R\,{dR}}{{e}^{h\nu /{kT}(R)}-1}\end{eqnarray} \tag{ 2 }$$

where h is the Planck constant, D_l is the luminosity distance, k is the Boltzmann constant, c is the speed of light, and ${R}_{{\rm{d}},\mathrm{in}}$ and ${R}_{{\rm{d}},\mathrm{out}}$ are the inner and outer disk radii, assumed as 3R_Sch and 500R_Sch, respectively. R_Sch is the Schwarzschild radius. In the above equation, the disk temperature has the following dependence on the radius

$$\begin{eqnarray}&&T(R)=\displaystyle \frac{3{R}_{\mathrm{Sch}}{L}_{{\rm{d}}}}{16\pi {\eta }_{\mathrm{acc}}{\sigma }_{\mathrm{SB}}{R}^{3}}{\left[1-{\left(\displaystyle \frac{3{R}_{\mathrm{Sch}}}{R}\right)}^{1/2}\right]}^{1/4},\end{eqnarray} \tag{ 3 }$$

where σ_SB is the Stefan–Boltzmann constant and η_acc is the accretion efficiency, adopted here as 10%. The thermal emission originated from the X-ray corona is assumed to follow a power law with exponential cut-off (${L}_{\mathrm{cor}}(\nu )\propto {\nu }^{-1}\exp (-h\nu /150\,\mathrm{keV})$, see Ghisellini & Tavecchio 2009).

For more than >50% of sources, the near-infrared-to-ultraviolet (IR-to-UV) SED exhibits a bump which we interpret as due to accretion disk emission. By modeling this bump with a standard Shakura & Sunyaev (1973) disk, we are able to constrain both the accretion disk luminosity (L_d) and the mass of the central black hole (M_bh) with the assumption of the accretion efficiency η_a = 0.1 (see also, Frank et al. 2002; Ghisellini et al. 2010). In order to get independent measurements, we use results from our own ongoing optical spectroscopic campaign (Koss et al. 2017; J. Mejia-Restrepo et al. 2019, in preparation) and also search for the availability of M_bh and L_d derived from single-epoch optical spectroscopy available in the literature (e.g., Shaw et al. 2012). We note that such commonly used single-epoch M_bh estimates carry systematic uncertainties of order 0.3–0.5 dex (see, e.g., Shen 2013; Peterson 2014, for reviews of these methods), and that the usage of the broad C iv λ1549 emission line may carry additional uncertainties related to the virialized nature of the BLR (e.g., Trakhtenbrot & Netzer 2012; Mejía-Restrepo et al. 2018, and references therein). Furthermore, if optical spectral parameters (e.g., line and continuum luminosities) are reported (see Torrealba et al. 2012), we use them to constrain M_bh following empirical relations commonly used for blazars (e.g., Shen et al. 2011; Shaw et al. 2012). We compute the luminosity of the BLR from the emission line luminosities by considering the quasar emission line weights provided by Francis et al. (1991). Assuming that the BLR reprocesses 10% of the disk radiation, we determine L_d.

4.2. Modeling Constraints

In the left panel of Figure 1, we show the 14–195 keV rest-frame luminosity of various types of BAT detected AGNs as a function of their redshifts. As can be seen, blazars dominate the 105-month BAT catalog above z ≈ 0.5. This is likely a selection effect due to relativistic beaming, which makes blazar jets brighter compared to other classes of astrophysical objects. Considering only blazars, the photon index versus the luminosity distribution is shown in the middle panel, and we highlight z > 2 blazars by showing them with pink stars. As can be seen, these are the most powerful ones with luminosities exceeding 10⁴⁷ erg s⁻¹. Furthermore, there is a hint of an anti-correlation (the Spearmann coefficient of ρ = −0.29 ± 0.08, probability of no correlation or PNC = 0.01) where more luminous objects, mostly FSRQs, tend to have harder BAT spectra. This is because the X-ray to γ-ray emission in FSRQs primarily originates via an EC process that has a hard spectral shape in X-rays (see Figure 2), which becomes more prominent at high redshifts due to the shift of SED peaks to lower frequencies and K-correction. Less luminous HSP blazars generally have a falling synchrotron emission in the BAT energy range and thus exhibit a steep spectrum with a 14–195 keV photon index >2. We plot the γ-ray luminosity as a function of the hard X-ray one in the right panel of Figure 1. Interestingly, the Fermi-LAT undetected objects share the same range of the 14–195 keV luminosity with γ-ray detected ones. Since all of them are FSRQs, this observation indicates their SED peaks to lie at lower frequencies with respect to Fermi-LAT sources.

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View figure set (146 panels)

Tarball of figure set images

The modeled SEDs of BAT blazars are shown in Figure 2 and we provide associated parameters in Table 4. Note that there are 28 sources whose broadband SEDs are well represented with synchrotron SSC processes, i.e., without invoking an EC mechanism. All of them are HSP blazars and their names and redshifts are provided in Table 6.

Particle energy distribution. The histograms of the spectral indices of the electron energy distribution are shown in Figure 3 (panels (a) and (b)). The low-energy index (p) distribution peaks at 1.90 and when fitted with a Gaussian function, the dispersion is σ = 0.20. On the other hand, the high-energy index (q) has a rather broad range. The distribution peaks at $\langle q\rangle =4.30$ with σ = 0.49 and its tail extends up to q ∼ 6. This is primarily due to the inclusion of γ-ray undetected blazars whose falling part of the inverse Compton spectrum needs to be steep to remain below the Fermi-LAT detection threshold. Additionally, the steep index can also be due to a few HSP blazars with falling BAT spectra. The distribution of the break energy, γ_b, which is derived from the synchrotron peak frequency location, peaks at 100 and tails to very large values, ≳1000. The very large γ_b is probably due to inefficient cooling of relativistic electrons in HSP blazars, and overall the distribution has a broad width (σ = 0.55, Figure 3, panel (c)).

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Magnetic field. The magnetic field and Compton dominance²⁰ distributions are shown in panels (d) and (e) of Figure 3. There is a hint of bi-modality that can be understood keeping in mind that a major fraction of BAT blazars studied here are FSRQs and the remaining are mostly HSP blazars with synchrotron dominated SEDs. FSRQs are known to have Compton-dominated (CD > 1) SEDs with a larger magnetic field compared to HSP blazars (see, e.g., Tavecchio et al. 2010; Paliya et al. 2017). Therefore, the distributions are skewed toward larger CD and B with $\langle \mathrm{log}\,\mathrm{CD}\rangle =0.79$ and $\langle B\rangle =1.10$ Gauss, respectively.

Bulk Lorentz factor and Doppler factor. In panels (f) and (g) of Figure 3, we show the histograms of the bulk Lorentz factor (Γ) and Doppler factor (δ). Both of them have narrow distributions peaking at $\langle {\rm{\Gamma }}\rangle =11$ and $\langle \delta \rangle =16.5$. These values are in agreement with that found from radio studies (e.g., Savolainen et al. 2010), from the SED modeling of a large sample of γ-ray emitting blazars (Paliya et al. 2017), and also from LF studies (Ajello et al. 2012).

Dissipation distance. The distribution of the location of the emission region peaks at R_d ∼ 3 × 10¹⁷ cm with a majority of sources having R_d > 10¹⁷ cm (see Figure 3, panel (h)). To understand the emission region location with respect to BLR, we plot R_d/R_BLR as a function of L_d in Eddington units in Figure 4. The R_BLR is estimated following Ghisellini & Tavecchio (2009), i.e., R_BLR = 10¹⁷(L_d/10⁴⁵ erg s⁻¹)^1/2 cm. The location is outside BLR, i.e., above the horizontal line, mostly for blazars with less luminous accretion disks, which is typically the case of BL Lacs. Moreover, a majority of sources have R_d comparable to or slightly larger than R_BLR, thus indicating the jet environment surrounding the emission region to be transparent enough for γ-rays to avoid absorption via pair production with BLR photons (see also, Böttcher & Els 2016; Paliya et al. 2018 for quantitative discussions). Blazars with most luminous accretion disks, i.e., with the largest BLRs, tend to have emission region well within it. These are the objects with the largest Compton dominance.

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Central engine. We have derived M_bh in 92 objects using the disk modeling approach, which shows the signature of the accretion disk emission at optical–UV energies. In 20 sources, we use the optical spectroscopic M_bh since their optical–UV SED is synchrotron dominated. The multiwavelength SEDs of 28 HSP blazars (Table 6) are well reproduced with a synchrotron SSC model and thus no M_bh value was estimated/used. Remaining six sources neither exhibit the big blue bump nor have the spectroscopic M_bh. In these objects, we suitably assume a M_bh value based on the available observations. The different methods adopted to compute M_bh are tabulated in Table 4.

The left and middle panels of Figure 5 represent the M_bh and L_d histograms for BAT blazars. The M_bh distribution has a rather narrow range (∼10⁸–10¹⁰ M_⊙) and peaks at $\langle \mathrm{log}{M}_{\mathrm{bh}}\rangle =9.0$ M_⊙. Fitting it with a log-normal function returns a width of σ = 0.44. On the other hand, L_d peaks at 10⁴⁶ erg s⁻¹ and has a rather broad range with σ = 1.14.

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In our sample, there are a total of 82 blazars that have M_bh measurements from both the disk-fitting and optical spectroscopy methods. In the right panel of Figure 5, we compare M_bh derived using these two approaches. As can be seen, both methods provide reasonably similar M_bh for BAT blazars and thus confirm the findings earlier reported for Fermi-LAT detected sources (Paliya et al. 2017).

It may be of a great interest to compare the accretion and jet powers of BAT blazars with that detected by Fermi-LAT sources. The powers that the jet carries in the form of the bulk motion of electrons (P_e), "cold" protons (P_p, assuming one proton per electron, i.e., no pairs), radiation (P_r), and magnetic field (P_m) are calculated following Celotti & Ghisellini (2008). The derived jet powers, assuming a two-sided jet, are reported in Table 5. For a comparison, we use the results reported for a large sample of Fermi blazars in Ghisellini et al. (2014) and Paliya et al. (2017).

P_r versus L_d. The most robust estimate of the jet power that we can have is the one that the jet produces in the form of radiation. This is because P_r is directly proportional to the observed bolometric luminosity. In the left panel of Figure 6, we show the behavior of P_r as a function of L_d. The low P_r–L_d end is mostly occupied by low-power HSP sources, whereas, luminous BAT FSRQs dominate at the high end. We compute the partial Spearmann's coefficient, ρ (Padovani 1992), and PNC to quantify the strength of the correlation and derive the parameters independent of the common redshift effect. The derived coefficients are ρ = 0.40 ± 0.08 with PNC < 10⁻¹⁰ and ρ = 0.33 ± 0.18 with PNC <10⁻¹⁰ for Fermi-LAT and Swift-BAT blazar populations, respectively. A positive P_r − L_d correlation has been reported previously for Fermi-LAT detected sources (e.g., Ghisellini et al. 2014; Paliya et al. 2017), and our findings establishes the fact that Swift-BAT blazars also follow the same positive trend.

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P_j versus L_d. In the middle panel of Figure 6, we show the total jet power (P_j = P_p + P_e + P_m) versus L_d. A positive correlation is observed, which is statistically confirmed with ρ = 0.53 ± 0.08 with PNC < 10⁻¹⁰ and ρ = 0.69 ± 0.05 with PNC <10⁻¹⁰ for Fermi-LAT and Swift-BAT blazars, respectively. Similar to LAT blazars, BAT detected ones to host jets with P_j > L_d. Interestingly, at the high end of the jet power (P_j > 10⁴⁸ erg s⁻¹), BAT blazars seems to have more extreme jets compared to LAT blazars. This is probably a selection effect due to higher detection threshold of Swift-BAT, which leads to the identification of the most luminous sources. In a more physical scenario, the observational shift of the SED peaks to lower frequencies with an increase in the bolometric luminosity (e.g., Fossati et al. 1998) can also explain this observation. BAT blazars that likely have inverse Compton peaks located at lower energies with respect to LAT detected ones, host more powerful jets, and luminous accretion disks.

It is instructive to determine the average SEDs of BAT blazars with the motivation to explore their luminosity-dependent evolution (Ajello et al. 2012). We divide the sources in different BAT luminosity (L_BAT) bins, and also based on their SED appearances, to identify FSRQs/BL Lacs. The latter is necessary since the 105-month BAT catalog does not provide FSRQ/BL Lac classification. Moreover, we consider only those sources that have both Fermi-LAT and Swift-BAT detections. We take the arithmetic average of the broadband SEDs in each L_BAT bin and show in Figure 7. Note that the adopted choice of L_BAT bins are mainly driven to distinguish BAT blazars of various powers and does not carry any other physical meaning.

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In the left panel, we show averaged SEDs for FSRQs in four L_BAT bins and note the increasing dominance of the inverse Compton peak (i.e., more Compton-dominated SED) with the increasing L_BAT and L_d. Also, there is an indication for the shift of the inverse Compton peak to lower frequencies as their power increases, especially in the highest L_BAT bin. In the right-hand panel of Figure 7, we plot the SEDs for BL Lacs. There are no BL Lacs in our sample with L_BAT > 10⁴⁷ erg s⁻¹ and only one (J0428.6−3788 or PKS 0426−380, z = 1.11) has log L_BAT in the bin 46.5–47, which is hence not plotted. The lowest L_BAT bin is dominated by HSP BL Lacs, and a pronounced shift of the SED peaks to lower frequencies can be observed in the higher luminosity bin. These results are aligned with our current understanding about the blazar sequence where the most powerful objects are LSP FSRQs with Compton-dominated SEDs, and the luminosity-dependent SED evolution is more pronounced in BL Lacs (see Ghisellini et al. 2017, for latest results).

In this work, we study 146 blazars detected in the first 105-month all-sky survey of Swift-BAT. We analyze the Fermi-LAT data and 14–195 keV BAT spectra and supplement them with multiwavelength archival spectral measurements. We determine the physical properties of the jet and central engine by modeling the broadband SED with a simple leptonic emission model. Our main findings are summarized below.

• 1.  
  We nearly quadruple the number of Swift-BAT detected blazars and also the number of Fermi-LAT undetected ones compared to earlier studies (146 versus 38 and 45 versus 12, respectively; Ajello et al. 2009).
• 2.  
  In the photon index versus 14–195 keV luminosity plane, we find tentative evidences (ρ = −0.29 ± 0.08, PNC = 0.01) of inverse correlation with more luminous blazars that tend to have flatter spectra.
• 3.  
  The overall physical properties of BAT blazars resemble well with that known for the LAT blazar population studied in Paliya et al. (2017).
• 4.  
  The mass of the central black hole derived from the single-epoch optical spectroscopy matches reasonably well with that derived by modeling the optical–UV bump with a standard accretion disk.
• 5.  
  BAT blazars exhibit a positive correlation between the power that the jet produces in the form of radiation and L_d. Blazars with most luminous accretion disks host the most radiatively powerful jets.
• 6.  
  Comparing the averaged SEDs of BAT blazars binned in different 14–195 keV luminosity bins, we find that more luminous sources have more Compton-dominated SEDs. There is also a hint of the shift of SED peaks to lower frequencies as the power of the blazars increases and it is found to be more pronounced in low-luminosity BL Lac objects.

We are grateful to the journal referee for a constructive criticism. M.K. acknowledges support from NASA through ADAP award NNH16CT03C. C.R. acknowledges support from the CONICYT+PAI Convocatoria Nacional subvencion a instalacion en la academia convocatoria año 2017 PAI77170080. K.O. is an International Research Fellow of the Japan Society for the Promotion of Science (JSPS, ID: P17321). We are grateful to the Fermi-LAT Collaboration internal referee Raniere Menezes for useful suggestions and Alberto Domínguez, Judith Recusin and Philippe Bruel for their critical reading of the manuscript. The Fermi LAT Collaboration acknowledges generous ongoing support from a number of agencies and institutes that have supported both the development and the operation of the LAT as well as scientific data analyses. These include the National Aeronautics and Space Administration and the Department of Energy in the United States; the Commissariat à l'Energie Atomique and the Centre National de la Recherche Scientifique/Institut National de Physique Nucléaire et de Physique des Particules in France; the Agenzia Spaziale Italiana and the Istituto Nazionale di Fisica Nucleare in Italy; the Ministry of Education, Culture, Sports, Science and Technology (MEXT); High Energy Accelerator Research Organization (KEK) and Japan Aerospace Exploration Agency (JAXA) in Japan; and the K. A. Wallenberg Foundation, the Swedish Research Council and the Swedish National Space Board in Sweden. Additional support for science analysis during the operations phase is gratefully acknowledged from the Istituto Nazionale di Astrofisica in Italy and the Centre National d'Études Spatiales in France. This work performed in part under DOE Contract DE-AC02-76SF00515. We acknowledge the work of the Swift-BAT team to make this study possible.

This research has made use of data obtained through the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA/Goddard Space Flight Center. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Part of this work is based on archival data, software, or online services provided by the ASI Data Center (ASDC).

Software: fermiPy (Wood et al. 2017).