BL Lac Objects Research Articles

Study of correlation between optical flux and polarization variations in BL Lac objects

ABSTRACT Polarized radiation from blazars is one key piece of evidence for synchrotron radiation at low energy, which also shows variations. We present here our results on the correlation analysis between optical flux and polarization degree (PD) variations in a sample of 11 BL Lac objects using ∼10 yr of data from the Steward Observatory. We carried out the analysis on long-term (∼several months) as well as on short-term time-scales (∼several days). On long-term time-scales, for about 85 per cent of the observing cycles, we found no correlation between optical flux and PD. On short-term time-scales, we found a total of 58 epochs with a significant correlation between optical flux and PD, where both positive and negative correlation were observed. In addition, we also found a significant correlation between optical flux and γ-ray flux variations on long-term time-scales in 11 per cent of the observing cycles. The observed PD variations in our study cannot be explained by changes in the power-law spectral index of the relativistic electrons in the jets. The shock-in-jet scenario is favoured for the correlation between optical flux and PD, whereas the anticorrelation can be explained by the presence of multizone emission regions. The varying correlated behaviour can also be explained by the enhanced optical flux caused by the newly developed radio knots in the jets and their magnetic field alignment with the large-scale jet magnetic field.

Variability Signatures of a Burst Process in Flaring Gamma-Ray Blazars

Blazars exhibit stochastic flux variability across the electromagnetic spectrum, often exhibiting heavy-tailed flux distributions, commonly modeled as lognormal. However, Tavecchio et al. and Adams et al. found that the high-energy gamma-ray flux distributions of several of the brightest flaring Fermi-LAT flat-spectrum radio quasars (FSRQs) are well modeled by an even heavier-tailed distribution, which we show is the inverse gamma distribution. We propose an autoregressive inverse gamma variability model in which an inverse gamma flux distribution arises as a consequence of a shot-noise process. In this model, discrete bursts are individually unresolved and averaged over within time bins, as in the analysis of Fermi-LAT data. Stochastic variability on timescales longer than the time-bin duration is modeled using first-order autoregressive structure. The flux distribution becomes approximately lognormal in the limiting case of many weak bursts. The fractional variability is predicted to decrease as the time-bin duration increases. Using simulated light curves, we show that the proposed model is consistent with the typical gamma-ray variability properties of FSRQs and BL Lac objects. The model parameters can be physically interpreted as the average burst rate, the burst fluence, and the timescale of long-term stochastic fluctuations.

An Extensive Study of Blazar Broad Emission Line: Changing-look Blazars and the Baldwin Effect

It is known that the blazar jet emissions are dominated by nonthermal radiation, while the accretion disk jets are normally dominated by thermal emission. In this work, our aim is to study the connection between the two types of emission by investigating the correlation between the blazar emission-line intensity property, which embodies the nature of an accretion disk, and the γ-ray flux property, which is the representative of jet emission. We compiled a sample of 656 blazars with available emission-line equivalent widths (EWs), the GeV γ-ray flux, and the spectral energy distribution (SED) information from the literature. In this work, we found 55 previous blazar candidates of uncertain types (BCUs) that are now identified as flat-spectrum radio quasars (FSRQs), and found 52 “changing-look” blazars based on their EWs, 45 of which are newly confirmed. These changing-look blazars have a larger accretion ratio () than BL Lacertae (BL Lac) objects. In addition, we suggest that the lower synchrotron peak blazars (LSPs) could be the source of changing-look blazars because 90.7% of the changing-look blazars in this work are confirmed as LSPs. An anticorrelation between EW and continuum intensity, the so-called global “Baldwin effect” (BEff), has been confirmed. We suggest that the steeper global BEff observed for the blazar than for radio-quiet active galactic nuclei (RQ-AGNs) is caused by the inverse Compton scattering of broad-emission-line photons. This interpretation is further supported by the positive correlation between the emission-line EW and intrinsic inverse Compton luminosity.

The Classification of Blazar Candidates of Uncertain Types

In this work, the support vector machine (SVM) method is adopted to separate BL Lacertae objects (BL Lacs) and flat spectrum radio quasars (FSRQs) in the plots of the photon spectrum index against the photon flux, αph∼logF, those of the photon spectrum index against the variability index, αph∼logVI and those of the variability index against the photon flux, logVI∼logF. Then, we used the dividing lines to distinguish BL Lacs from FSRQs in the blazar candidates of uncertain types from the Fermi/LAT catalogue. Our main conclusions are: 1. We separate BL Lacs and FSRQs by αph=−0.123logF+1.170 in the αph∼logF plot, αph=−0.161logVI+2.594 in the αph∼logVI plot and logVI=0.792logF+9.203 in the logVI∼logF plot. 2. We obtain 932 BL Lac candidates and possible BL Lac candidates, and 585 FSRQ candidates and possible FSRQ candidates. 3. Discussion is given regarding comparisons with the literature.

Photohadronic interpretations of the different incarnations of 1ES 2344+514

ABSTRACT Since its discovery in 1995, the high-energy peaked blazar 1ES 2344+514 has undergone several episodes of GeV–TeV flaring and has been observed in the multiwavelength by several telescopes. The observed X-ray spectrum of 1996 and the flaring event of 2016 establish that 1ES 2344+514 has a temporary behaviour like that of an extremely high-energy peaked BL Lacertae object (EHBL). Such behaviour has also been observed in several nearby high-energy peaked blazars. We use the photohadronic model to account for the GeV–TeV flaring observed events of 1995 and 2007. Also, a recently proposed two-zone photohadronic model, which is successful in explaining the multi-TeV flaring events of many transient EHBL-like sources, is employed to explain the GeV–TeV flaring spectra of MJD 57611 and MJD 57612. We find that the zone-2 parameters of the two-zone photohadronic model play a central role in explaining these spectra. This is probably an indication of a new type of transient EHBL-like source. We find that our fits to the observed spectra are comparable or better than the other leptonic and hadronic models employed in the literature to address the same issue.

Revisiting the evidences for spectral anomalies in distant blazars: New data on the photon-ALP mixing

ABSTRACT We re-examine possible dependencies on redshift of the spectral parameters of blazars observed at very-high energies (VHEs) with Imaging Atmospheric Cherenkov telescopes. This is relevant to assess potential effects with the source distance of the photon to axion-like particle mixing that would deeply affect the propagation of VHE photons across the Universe. We focus our spectral analysis on 38 BL Lac objects (32 high-peaked and 6 intermediate-peaked) up to redshift z ≃ 0.5, and a small sample of 5 Flat Spectrum Radio Quasars up to z = 1 treated independently to increase the redshift baseline. The 78 independent spectra of these sources are first of all carefully corrected for the gamma–gamma interaction with photons of the Extragalactic Background Light that are responsible for the major redshift-dependent opacity effect. Then, the corrected spectra are fitted with simple power laws to infer the intrinsic spectral indices Γem at VHE, to test the assumption that such spectral properties are set by the local rather than the global cosmological environment. We find some systematic anticorrelations with redshift of Γem that might indicate, although with low-significance, a spectral anomaly potentially requiring a revision of the photon propagation process. More conclusive tests with higher statistical significance will require the observational improvements offered by the forthcoming new generation of Cherenkov arrays (CTA, ASTRI, LHAASO).

Intra-night optical variability monitoring of $$\gamma $$-ray emitting blazars

We present the results obtained from our campaign to characterize the intra-night optical variability properties of blazars detected by the Fermi large area telescope. This involves R-band monitoring observations of a sample of 18 blazars, which includes five flat spectrum radio quasars (FSRQs) and thirteen BL Lac objects (BL Lacs) covering the redshift range $$z = 0.085-1.184$$ . Our observations carried out using the 1.3m J. C. Bhattacharya telescope cover a total of 40 nights ( $$\sim $$ 200 h) between the period December 2016 and March 2020. We characterized variability using the power enhanced F-test. We found duty cycle (DC) variability of about 11% for FSRQs and 12% for BL Lacs. Dividing the sample into different sub-classes based on the position of the synchrotron peak in their broadband spectral energy distribution (SED), we found DC of $$\sim $$ 16%, $$\sim $$ 10% and $$\sim $$ 7% for low synchrotron peaked (LSP), intermediate synchrotron peaked (ISP) and high synchrotron peaked (HSP) blazars. Such high DC of variability in LSP blazars could be understood in the context of the R-band tracing the falling part (contributed by high energy electrons) of the synchrotron component of the broadband SED. Also, the R-band tracing the rising synchrotron part (produced by low energy electrons) in the case of ISP and HSP blazars, could cause lesser variability in them. Thus, the observed high DC of variability in LSP blazars relative to ISP and HSP blazars is in accordance with the leptonic model of emission from blazar jets.

Classification and Jet Power of Fermi Blazars

In this work, we compile a sample of 449 Fermi blazars with the luminosity of the broadline region, the black hole mass, the beam radio luminosity, and the jet power; obtain the beam power and the black hole spin; investigate the dividing line between BL Lacertae objects (BL Lacs) and flat-spectrum radio quasars (FSRQs) to identify the discrepancies in their dominant mechanisms; and revisit the dependence of the jet power on the disk accretion luminosity, the black hole mass, and the black hole spin. We come to the following conclusions. (1) A boundary of log (L BLR/L Edd) = −3.14, separating the BL Lacs and the FSRQs, is obtained from the Bayesian analysis, which is consistent with the results from the literature. We employ the boundary to divide the blazar candidates of uncertain types into candidates for BL Lacs or FSRQs, and we find five changing-look blazars at the same time. (2) A strong correlation is found between black hole mass and intrinsic γ-ray luminosity, but a weaker correlation is found between black hole mass and observed γ-ray luminosity. The latter is weakened by jet effects: it is apparently weak for BL Lacs that have disordered amplification of the Doppler factor, since their mechanism is dominated by jets, while it is moderate for FSRQs, since their mechanism is dominated by accretion processes. (3) The jets of both FSRQs and BL Lacs are likely governed by the Blandford–Znajek mechanism.

The dichotomy between low-frequency-peaked BL Lacs and flat-spectrum radio quasars

ABSTRACT By comparing the Fourth Fermi Large Area Telescope active galactic nuclei (AGNs) Catalog Data Release 2 (4LAC-DR2) and the Radio Fundamental Catalog (RFC-2021b), we obtain an AGN sample containing 2816 AGNs. We calculate the γ-ray loudness (Gγ) of the sample sources, and establish the relevant relationship between several gamma/radio properties. The results show that: 1) the synchrotron peak frequency ($\nu _\mathrm{peak}^S$) strongly correlates with the γ-ray photon spectral index (Γγ), γ-ray luminosity (Lγ), and radio luminosity (LR); 2) the low-synchrotron-peaked BL Lac objects (LBLs) and the flat-spectrum radio quasars (FSRQs) partially overlap in log $\nu _\mathrm{peak}^S\!-\!\Gamma _\gamma$, log Lγ–Γγ, log LR–Γγ, log Gγ–Γγ, log $\nu _\mathrm{peak}^S\!-\!$log Lγ, log $\nu _\mathrm{peak}^S\!-\!$log LR, and log $\nu _\mathrm{peak}^S\!-\!$log Gγ space. We plotted the distribution of redshift (z), $\nu _\mathrm{peak}^S$, Γγ, Lγ, VLBI 8.4 GHz flux density ($S_{\rm {8.4\, GHz}}$), and Gγ to distinguish FSRQs and LBLs. We note that $\nu _\mathrm{peak}^S$, Γγ, and $S_{8.4\, {\rm GHz}}$ show similar bimodal distributions. To further determine whether FSRQs are misjudged as LBLs, we examined the Compton dominance (CD), and found that CD < 1 for only two out of 39 sources. This measurement provides further evidence that FSRQs may be mistaken for LBLs.

Flux and spectral variability of Mrk 421 during its moderate activity state usingNuSTAR: Possible accretion disc contribution?

Context.The X-ray emission in BL Lac objects is believed to be dominated by synchrotron emission from their relativistic jets. However, when the jet emission is not strong, one could expect signatures of X-ray emission from inverse Compton scattering of accretion disc photons by hot and energetic electrons in the corona. Moreover, the observed X-ray variability can also originate in the disc, and get propagated and amplified by the jet.Aims.Here, we present results on the BL Lac object Mrk 421 using the Nuclear Spectroscopic Telescope Array data acquired during 2017 when the source was in a moderate X-ray brightness state. For comparison with high jet activity state, we also considered one epoch of data in April 2013 when the source was in a very high X-ray brightness state. Our aim is to explore the possibility of the signature of accretion disc emission in the overall X-ray emission from Mrk 421 and also examine changes in accretion parameters considering their contribution to spectral variations.Methods.We divided each epoch of data into different segments in order to find small-scale variability. Data for all segments were fitted using a simple power-law model. We also fitted the full epoch data using the two component advective flow (TCAF) model to extract the accretion flow parameters. Furthermore, we estimated the X-ray flux coming from the different components of the flow using the lowest normalisation method and analysed the relations between them. For consistency, we performed the spectral analysis using models available in the literature.Results.The simple power-law function does not fit the spectra well, and a cutoff needs to be added. The spectral fitting of the data using the TCAF model shows that the data can be explained with a model where (a) the size of the dynamic corona at the base of the jet is from ∼28 to 10 rs, (b) the disc mass accretion rate is from 0.021 to 0.051ṀEdd, (c) the halo mass accretion rate is from 0.22 to 0.35ṀEdd, and (d) the viscosity parameter of the Keplerian accretion disc from 0.18 to 0.25. In the assumed model, the total flux, disc and jet flux correlate with the radio flux observed during these epochs.Conclusions.From the spectral analysis, we conclude that the spectra of all the epochs of Mrk 421 in 2017 are well described by the accretion-disc-based TCAF model. The estimated disc and jet flux relations with radio flux show that accretion disc can contribute to the observed X-ray emission, when X-ray data (that cover a small portion of the broad band spectral energy distribution of Mrk 421) are considered in isolation. However, the present disc-based models are disfavoured with respect to the relativistic jet models when considering the X-ray data in conjunction with data at other wavelengths.

Multiwavelength Variation Phenomena of PKS 0735+178 on Diverse Timescales

The BL Lac object PKS 0735+178 has shown some complex multiwavelength variation phenomena in previous studies, especially in its color behavior. Bluer-when-brighter, redder-when-brighter, and achromatic behavior were all found to be possible long-term trends of PKS 0735+178. In this work, we collected long-term multiwavelength data on PKS 0735+178, and also performed a multicolor optical monitoring on intraday timescales. Intraday variability was detected on one night. On long timescales, a possible 22 day time lag was found between the R and γ-ray bands. The results of a cross-correlation analysis exhibited strong correlations between various optical bands on both intraday and long timescales. However, only a mild correlation was found between the long-term γ-ray and R-band light curves, which could be interpreted as different emission mechanisms for the γ-ray and optical emissions. PKS 0735+178 showed a significant harder-when-brighter (HWB) behavior in the γ-ray band, which is consistent with the observed optical bluer-when-brighter (BWB) trend on both long-term and intraday timescales. We found that the HWB and BWB trends will be enhanced during active states, especially for the historical low state. Such a phenomenon indicates a special activity-dependent color behavior of PKS 0735+178, and it could be well interpreted by the jet emission model.

Multiwavelength Observations of the Blazar VER J0521+211 during an Elevated TeV Gamma-Ray State

We report on a long-lasting, elevated gamma-ray flux state from VER J0521+211 observed by VERITAS, MAGIC, and Fermi-LAT in 2013 and 2014. The peak integral flux above 200 GeV measured with the nightly binned light curve is (8.8 ± 0.4) × 10−7 photons m−2 s−1, or ∼37% of the Crab Nebula flux. Multiwavelength observations from X-ray, UV, and optical instruments are also presented. A moderate correlation between the X-ray and TeV gamma-ray fluxes was observed, and the X-ray spectrum appeared harder when the flux was higher. Using the gamma-ray spectrum and four models of the extragalactic background light (EBL), a conservative 95% confidence upper limit on the redshift of the source was found to be z ≤ 0.31. Unlike the gamma-ray and X-ray bands, the optical flux did not increase significantly during the studied period compared to the archival low-state flux. The spectral variability from optical to X-ray bands suggests that the synchrotron peak of the spectral energy distribution (SED) may become broader during flaring states, which can be adequately described with a one-zone synchrotron self-Compton model varying the high-energy end of the underlying particle spectrum. The synchrotron peak frequency of the SED and the radio morphology of the jet from the MOJAVE program are consistent with the source being an intermediate-frequency-peaked BL Lac object.

Kinematics of Parsec-scale Jets of Gamma-Ray Blazars at 43 GHz during 10 yr of the VLBA-BU-BLAZAR Program

We analyze the parsec-scale jet kinematics from 2007 June to 2018 December of a sample of γ-ray bright blazars monitored roughly monthly with the Very Long Baseline Array (VLBA) at 43 GHz under the VLBA-BU-BLAZAR program. We implement a novel piecewise linear fitting method to derive the kinematics of 521 distinct emission knots from a total of 3705 total intensity images in 22 quasars, 13 BL Lacertae objects, and 3 radio galaxies. Apparent speeds of these components range from 0.01c to 78c, and 18.6% of knots (other than the “core”) are quasi-stationary. One-fifth of moving knots exhibit nonballistic motion, with acceleration along the jet within 5 pc of the core (projected) and deceleration farther out. These accelerations occur mainly at locations coincident with quasi-stationary features. We calculate the physical parameters of 273 knots with statistically significant motion, including their Doppler factors, Lorentz factors, and viewing angles. We determine the typical values of these parameters for each jet and the average for each subclass of active galactic nuclei. We investigate the variability of the position angle of each jet over the 10 yr of monitoring. The fluctuations in position of the quasi-stationary components in radio galaxies tend to be parallel to the jet, while no directional preference is seen in the components of quasars and BL Lacertae objects. We find a connection between γ-ray states of blazars and their parsec-scale jet properties, with blazars with brighter 43 GHz cores typically reaching higher γ-ray maxima during flares.

A possible origin for the very-high-energy photons from BL Lac object S4 0954+65 as observed on Feb 14 2015

A possible origin for the very-high-energy photons from BL Lac object S4 0954+65 as observed on Feb 14 2015

A 13-yr-long broad-band view of BL Lac

ABSTRACT We present the results of an extensive analysis of the optical, ultraviolet, X-ray, and γ-ray data collected from the observations of the BL Lac objects prototype BL Lacertae carried out over a period of nearly 13 yr, between 2008 August and 2021 March. The source is characterized by strongly variable emission at all frequencies, often accompanied by spectral changes. In the γ-ray band several prominent flares have been detected, the largest one reaching the flux of Fγ(> 196.7 MeV) = (4.39 ± 1.01) × 10−6 photon cm−2 s−1. The X-ray spectral variability of the source during the brightest flare on MJD 59128.18 (2020 October 6) was characterized by a softer-when-brighter trend due to a shift of the synchrotron peak to ∼1016 Hz, well into the HBL domain. The widely changing multiwavelength emission of BL Lacertae was systematically investigated by fitting leptonic models that include synchrotron self-Compton and external Compton components to 511 high-quality and quasi-simultaneous broad-band spectral energy distributions (SEDs). The majority of selected SEDs can be adequately fitted within a one-zone model with reasonable parameters. Only 46 SEDs with soft and bright X-ray spectra and when the source was observed in very high energy γ-ray bands can be explained in a two-zone leptonic scenario. The HBL behaviour observed during the brightest X-ray flare is interpreted as due to the emergence of synchrotron emission from freshly accelerated particles in a second emission zone located beyond the broad-line region.

A BL Lacertae Object at a Cosmic Age of 800 Myr

Abstract We report the discovery of the BL Lacertae object FIRST J233153.20+112952.11 at redshift >6 using near-infrared spectroscopy and broadband observations in the infrared, submillimeter, and radio wavelengths. The detection of the continuum break at ∼0.92 μm in the near-infrared spectrum of FIRST J233153.20+112952.11 provides an approximate redshift of 6.57, corresponding to an age of the universe of ∼800 Myr. The rest-frame UV/optical spectrum of FIRST J233153.20+112952.11 shows no emission lines detected with equivalent width >1.5 Å. The nondetection of the C iv λ1549 broad emission line constrains the luminosity of the accretion disk to be ≲1045 erg s−1, which corresponds to the radiatively inefficient accretion regime of BL Lacertae objects. The spectral index of the rest-frame UV/optical continuum, α ν,opt = 1.43 ± 0.23, is consistent with the expected spectral index of the synchrotron emission spectrum of the relativistic jet. The flat radio continuum (α ν,r ≈ 0) in a rest-frame frequency interval of 7–23 GHz is similar to that of the typical BL Lacertae objects and also consistent with originating from the synchrotron jet emission. The rest-frame UV/optical and radio fluxes of FIRST J233153.20+112952.11 show significant (15%–40%) variations. The shortest recorded rest-frame timescale of these variations is ∼8 days. Assuming the local scaling relation between the radio luminosity and black hole mass, the 5 GHz radio luminosity of FIRST J233153.20+112952.11, 1.6 × 1033 erg s−1 Hz−1, suggests a billion solar mass central supermassive black hole.

Classification of Fermi-LAT blazars with Bayesian neural networks

The use of Bayesian neural networks is a novel approach for the classification of γ-ray sources. We focus on the classification of Fermi-LAT blazar candidates, which can be divided into BL Lacertae objects and Flat Spectrum Radio Quasars. In contrast to conventional dense networks, Bayesian neural networks provide a reliable estimate of the uncertainty of the network predictions. We explore the correspondence between conventional and Bayesian neural networks and the effect of data augmentation. We find that Bayesian neural networks provide a robust classifier with reliable uncertainty estimates and are particularly well suited for classification problems that are based on comparatively small and imbalanced data sets. The results of our blazar candidate classification are valuable input for population studies aimed at constraining the blazar luminosity function and to guide future observational campaigns.

MALS SALT-NOT Survey of MIR-selected Powerful Radio-bright AGN at 0 < z < 3.5

We present results of an optical spectroscopic survey using SALT and the Nordic Optical Telescope to build a Wide-field Infrared Survey Explorer mid-infrared color-based, dust-unbiased sample of powerful radio-bright (>200 mJy at 1.4 GHz) active galactic nuclei (AGN) for the MeerKAT Absorption Line Survey (MALS). Our sample has 250 AGN (median z = 1.8) showing emission lines, 26 with no emission lines, and 27 without optical counterparts. Overall, our sample is fainter (Δi = 0.6 mag) and redder (Δ(g−i) = 0.2 mag) than radio-selected quasars, and representative of fainter quasar population detected in optical surveys. About 20% of the sources are narrow-line AGN (NLAGN)–65% of these, at z < 0.5 are galaxies without strong nuclear emission, and 10% at z > 1.9, have emission line ratios similar to radio galaxies. The farthest NLAGN in our sample is M1513-2524 (z em = 3.132), and the largest radio source (size ∼330 kpc) is M0909-3133 (z em = 0.884). We discuss in detail 110 AGN at 1.9 < z < 3.5. Despite representing the radio loudest quasars (median R = 3685), their Eddington ratios are similar to the Sloan Digital Sky Survey quasars having lower R. We detect four C iv broad-absorption line (BAL) QSOs, all among AGN with least R, and highest black hole masses and Eddington ratios. The BAL detection rate (%) is consistent with that seen in extremely powerful (L 1.4GHz > 1025 W Hz−1) quasars. Using optical light curves, radio polarization, and γ-ray detections, we identify seven high-probability BL Lacertae objects. We also summarize the full MALS footprint to search for H i 21 cm and OH 18 cm lines at z < 2.

Dual-high-frequency VLBI study of blazar-jet brightness-temperature gradients and collimation profiles

Context.On the kiloparsec scale, extragalactic radio jets show two distinct morphologies related to their power: collimated high-power jets ending in a bright termination shock and low-power jets opening up close to the core and showing a more diffuse surface brightness distribution. The emergence of this morphological dichotomy on the parsec scale at the innermost jet regions can be studied with very-long-baseline interferometry (VLBI) radio observations of blazars in which the jet emission is strongly Doppler boosted due to relativistic bulk motion at small angles between the jet direction and the line of sight.Aims.We seek to characterize the geometry and emission profiles of the parsec-scale radio jets of flat-spectrum radio quasars (FSRQs) and BL Lacertae objects (BL Lacs) on parsec scales to derive properties of the magnetic field, environment, and energetics for different classes of extragalactic jets.Methods.We analyze the VLBI radio data of 15 FSRQs, 11 BL Lacs, and two radio galaxies contained in both the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments data archive and the Boston University blazar group sample archive at 15 GHz and 43 GHz, repectively. We derived the brightness-temperature and jet-width gradients along the jet axis from parameterizations of the jets using 2D Gaussian brightness distributions.Results.In most BL Lac objects, the diameter and brightness-temperature gradients along the jet axis can generally be described well by single power laws, while the jets of FSRQs show more complex behavior and remain more strongly collimated on larger physical scales. We find evidence for a transition of the global jet geometry from a parabolic to a conical shape in the BL Lac objects 3C 66A, Mrk 421 and BL Lacertae, the radio galaxy 3C 111 and the FSRQs CTA 26, PKS 0528+134, 4C +71.07, 4C +29.45, and 3C 279 outside the Bondi sphere.Conclusions.Our results combined with findings from kinematic VLBI studies that the jets of FSRQs exhibit larger bulk Lorentz factors than BL Lacs are in agreement with relativistic magnetohydrodynamical jet-disk simulations in which the flattening of the jet magnetization profile due to magnetic fields from the accretion disk leads to a more persistent collimation in high-accretion-rate blazars.

Optical Spectral Variations of a Large Sample of Fermi Blazars

Abstract We have investigated the optical spectral behavior of a large sample of Fermi blazars (40 flat-spectrum radio quasars (FSRQs) and 13 BL Lacertae objects (BL Lacs)), and found two new universal optical spectral behaviors. In the low state the optical spectrum gradually becomes softer (steeper) or harder (flatter), but more and more slowly as the brightness increases, and then tends to stabilize in the high state. These are briefly named the redder-stable-when-brighter (RSWB) and bluer-stable-when-brighter (BSWB) behaviors, respectively. Thirty-four FSRQs and 7 BL Lacs exhibit clear RSWB behavior, and 2 FSRQs and 5 BL Lacs show distinct BSWB behavior, which means that FSRQs favor more RSWB than BSWB behavior, while BL Lacs have no clear preference between both behaviors. We have put forward a unified nonlinear formula to quantitatively characterize the optical spectral behaviors of FSRQs and BL Lacs, which can fit both kinds of behaviors very well. We argue that the RSWB and BSWB behaviors originate from the same mechanism, and that they are the universal optical spectral behaviors for blazars. The frequently observed redder-when-brighter and bluer-when-brighter trends can be considered to be the approximations of the RSWB and BSWB behaviors, respectively. The rarely observed stable-when-brighter trend can also be viewed as an approximation or a special case of the RSWB or BSWB behaviors. We have developed a model with two constant-spectral-index components, which cannot only explain the two kinds of optical spectral behaviors well, but also successfully interpret the differential behaviors between FSRQs and BL Lacs.