BL Lacertae objects and the extragalactic γ-ray background

A tight correlation between γ-ray and radio emission is found for a sample of BL Lacertae （BL Lac） objects detected by the Fermi Gamma-ray Space Telescope （Fermi） and the Energetic Gamma-Ray Experiment Telescope （EGRET）. The γ-ray emission of BL Lac objects exhibits strong variability, and the detection rate of γ-ray BL Lac objects is low, which may be related to the γ-ray duty cycle of BL Lac objects. We estimate the γ-ray duty cycle, δγ ≌ 0.11, for BL Lac objects detected by EGRET and Fermi. Using the empirical relation of γ-ray emission with radio emission and the estimated γ-ray duty cycle δγ, we derive the γ-ray luminosity function （LF） of BL Lac objects from their radio LE Our derived γ-ray LF of BL Lac objects can almost reproduce that calculated with the recently released Fermi bright active galactic nuclei （AGN） sample. Comparison of the derived LF of the γ-ray BL Lac objects in this work with that derived by Abdo et al. （2009a） requires the γ-ray duty cycle of BL Lac objects to be almost luminosity-independent. We find that - 45% of the extragalactic diffuse γ-ray background （EGRB） is contributed by BL Lac objects. Combining the estimate of the quasar contribution to the EGRB in the previous work, we find that 77% of the EGRB is contributed by BL Lac objects and radio quasars.

Is BZB J1450+5201 the most distant gamma-ray BL Lacertae object?

BL Lacertae （BL Lac） objects at high redshifts （z ≥ 2） are rarely detected. Through careful analysis of its Sloan Digital Sky Survey spectrum, BZB J1450＋5201 is confirmed to be a high-z BL Lac object with z ≥ 2.471 by identifying the Lyα 1216 and CIV 1548/1550 absorption lines. This indicates that BZB J1450＋5201 is the most distant BL Lac object discovered to date. Careful analysis of the five-year Fermi-LAT data of 2FGL J1451.0＋5159 shows that its γ-ray emission is robust with a confidence level of 6.2σ at 1-3 GeV and 6.7σ at 3-10 GeV. This analysis with the five-year data overcomes confusion with its bright neighbor, which is a problem when analyzing the two-year data. In addition, 2FGL J 1451.0＋5159 is confirmed to be associated with BZB J1450＋5201 using the five-year data. The analysis of multiwavelength data, from radio to γ-ray energies, indicates that BZB J1450＋5201 is an intermediate synchrotron peaked （ISP） source. Its multiwavelength properties are consistent with distributions of other ISP sources at lower redshifts in the second Fermi-LAT AGN catalog. The pure synchrotron self-Compton （SSC） model seems to be disfavored, but the scattering of weak external emission plus the SSC process can provide a satisfactory description of the broadband emission.

On the weak Lyα emission line of BL Lacertae objects

Recent studies reveal that weak Lyα emission line may be a ubiquitous feature of nearby BL Laceratae objects(BL Lacs). We present a survey of the Lyα emission lines in BL Lacs, with a focus on data obtained by the Cosmic Origins Spectrograph onboard the Hubble Space Telescope. Among the11 selected targets, seven show a clear detection of the intrinsic Lyα emission line, while one manifests a marginal detection. Two of these intrinsic Lyα lines are newly detected in this work(FBQS J1217+3007 and3 C 66 A). Most selected BL Lacs are either high or intermediate-frequency peaked BL Lacs, with the only exception being one low-frequency peaked BL Lac. A strong anti-correlation between the line equivalent width and the continuum luminosity was found. The possible sources of ionizing photons that are responsible for production of the Lyα emission line are studied based on a widely accepted detailed accretion-jet model. It is found that the clouds of the broad line region are unlikely to be located in the jet cone region with an inclination angle of less than 10 degrees. Contributions from the jet and disk to the emission lines are found to be comparable. Additionally, a possible way to constrain the accretion rate from the emission line is proposed.

A synchrotron self-Compton scenario for the very high energy γ-ray emission of the intermediate BL Lacertae object W Comae

W Comae has significant variability in multi-wavelengthes, from radio to gamma-ray bands. A bright outburst in optical and X-ray bands was observed in 1998, and most recently, a strong TeV flare was detected by VERITAS in 2008. It is the first TeV intermediate-frequency-peaked BL Lacertae source. I find that both the broadband spectral energy distributions （SEDs） which were quasi-simultaneously obtained during the TeV flare and during the optical/X-ray outburst are well fit by using a single-zone synchrotron ＋ synchrotron-self-Compton model. The satisfactory fitting requires a large beaming factor, i.e., δ- 25 and δ- 20 for the TeV flare and the optical/X-ray outburst, respectively, suggesting that both the optical/X-ray outburst and the TeV flare are from a relativistic jet. The size of the emission region of the TeV flare is three times larger than that of the optical/X-ray outburst, and the strength of the magnetic field for the TeV flare is - 14 times smaller than that of the X-ray/optical outburst, likely indicating that the region of the TeV flare is more distant from the core than that of the X-ray/optical outburst. The inverse Compton component of the TeV flare peaks around 1.3 GeV, but it is around 20 MeV for the X-ray/optical outburst, lower than that for the TeV flare by two orders of magnitude. The model predicts that the optical/X-ray outburst might be accompanied by a strong MeV/GeV emission, but the TeV flare may be not associated with the X-ray/optical outburst. The GeV emission is critical for characterizing the SEDs of the optical/X-ray outburst and the TeV flare. The predicted GeV flux is above the sensitivity of Fermi/LAT, and it could be verified with the observations by Fermi/LAT in the near future.

Comparison between TeV and non-TeV BL Lac Objects

BL Lacertae objects（BL Lacs） are the dominant population of Te V emitting blazars. In this work, we investigate whether there are any special observational properties associated with Te V sources.To do so, we will compare the observational properties of Te V detected BL Lacs（Te V BLs） with non-Te V detected BL Lac objects（non-Te V BLs）. From the 3rd Fermi/LAT source catalog（3FGL）, we can obtain662 BL Lacs, out of which 47 are Te V BLs and 615 are non-Te V BLs. Their multi-wavelength flux densities（FpR, FO, FX and Fγ）, photon spectral indexes（α^phXand α^phγ）, and effective spectral indexes（αRO and αOX）are compiled from the available literature. Then the luminosities（log ν LR, log ν LO, log ν LX, log ν Lγ）are calculated. From comparisons, we find that Te V BLs are different from low synchrotron peaked BLs and intermediate synchrotron peaked BLs, but Te V BLs show similar properties to high synchrotron peaked（HSP） BLs. Therefore, we concentrated on a comparison between Te V HSP BLs and non-Te V HSP BLs.Analysis results suggest that Te V HSP BLs and non-Te V HSP BLs exhibit some differences in their αRO and αphγ, but their other properties are quite similar.

The Binary Black Hole Scenario for the BL Lacertae Object AO 0235+16

Recent analysis of the long term radio light curve of the extremely variableBL Lacertae object AO 0235+16 by Raiteri et al. have revealed the presence of recurrent outburstswith a period of ～ 5.7 +- 0.5 yr. Periodicity analysis of the optical light curve also showsevidence for a shorter period. Here we discuss whether such a behavior can be explained by a binaryblack hole model where the accretion disk of one of the supermassive black holes is precessing dueto the tidal effects of the companion. We estimate the mass of the accreting hole and analyzeconstraints on the secondary mass and the orbital parameters of the system. It is possible toprovide a viable interpretation of the available multiwavelength data.

On the Geometry of Broad-Line Regions in BL Lac Objects

The geometry of broad-line regions （BLRs） in active galactic nuclei （AGNs） is still controversial. We use a sample of BL Lac objects, of which the black hole masses Mbh are estimated from their host galaxy absolute magnitude at R-band, MR, by using the empirical relation between MR and black hole mass Mbh. The sizes of the broad-line regions for MglI are derived from the widths of MglI lines and the black hole masses. Compared with the empirical relation between BLR size RBLR and MgII line luminosity LMglI, it is found the BLR sizes in the BL Lac objects derived in this paper are 2-3 orders of magnitude higher. If the BLR geometry of these sources is disklike, then the viewing angle between the axis and the line of sight is in the range of - 2° - 15°, which is consistent with the unification scheme.

Intra-day variability of BL Lacertae from 2016 to 2018

We monitored BL Lacertae in the B,V,R and I bands for 14 nights during the period of 2016-2018.The source showed significant intra-day variability on 12 nights.We performed color-magnitude analysis and found that the source exhibited bluer-when-brighter chromatism.This bluer-when-brighter behavior is at least partly caused by the larger variation amplitude at shorter wavelengths.The variations at different wavelengths are correlated well and show no inter-band time lag.

Explaining the Multiwavelength Emission of Hard-Te V BL Lac Objects Using a Truncated Conical Jet Model

Hard-Te V BL Lac objects are newly identified populations of active galactic nuclei with the emittedγ-ray photons well above Te V energies.In this paper,we explain the multiwavelength emission of six Hard-Te V BL Lac objects by using a truncated conical emission region of the jet,where the electron distribution is obtained by numerically solving the evolution equation along the jet self-consistently.For comparison,we also apply the model to Mrk 421and Mrk 501,which are the potential candidates for the hard Te V emissions.We demonstrate that the model can satisfactorily reproduce the spectral energy distributions of eight sources,particularly of six Hard-Te V sources,where no extreme minimum Lorentz factor of the electron population is required.In contrast with Mrk 421 and Mrk 501,six Hard-Te V sources have rather low magnetization in emitting regions and high cutoff energies of the electron distributions.

Gamma-ray Emission from the γ-ray-loud BL Lac Objects

Using the HST observation data of BL Lac objects by Urry et al. and γ-ray observation data, we find that there is a correlation between Fγ and Fonuclei forγ-ray-loud BL Lac objects (correlation coefficients: 7γ=0.63,p = 4.0 × 10-2), but no correlation between Fγ and Fohost, where Fonuclei and Fohost are the fluxes of nuclei and host galaxy in V-band. For 19γ-ray-loud BL Lac objects with observed spectral index in multi-wavebands, the spectral index correlations between any two bands are as follow: (1) there is a strong correlation between aγ and aK for 15 BL Lac objects(γ = 0.84,p = 3.11 × 10-4); (2) the correlation between aγ and ao for 12 BL Lac objects isγ = 0.82, p = 1.5 × 10-3; (3) there is no correlation between a7 and ax for 16 BL Lac objects. The results, together with characteristic double-humped shape of their SEDs, show that the synchrotron self-Compton mechanism might be a main mechanism for theγ-ray emission of the BL Lac objects. The electrons emitting JR and optical radiation via synchrotron are also responsible for upscattering these photons toγ-rays, and a variability in IR-optical regime should be accompanied by a change in theγ-rays.

射电选和X射线选BL Lacertae天体的射电性质(英文)

收集了射电选和X射线选BL Lac天体的射电数据(包括核和延展光度),并计算了它们的核主导系数R。研究显示:射电选BL Lac(RBLs)天体的总射电光度是X射线选BL Lac(XBLs)天体的2个量级。详细分析得到这种差别主要是来自于核光度的差别因为延展光度差别只有1个量级。研究RBLs和XBLs的核(延展元)光度与核主导系数之间的关系,发现延展元光度与核主导系数负相关,而核(总)光度几乎与核主导系数没有相关。

Properties of BL Lac Objects with Redshift ≤ 0.2

We present a large sample which includes 82 BL Lac objects with red- shifts below 0.2 from recent literature. We find strong correlations in both flux and luminosity between the radio (5 GHz) and optical bands (5500 A). The correlations in other bands are very weak. Five TeV BL Lacs and two suspect sources are found to have similar properties as high- frequency- peaked BL Lacs (HBLs). Our results suggest that both the radio and optical emissions are from the same radiation mech- anism in the SSC model. The TeV BL Lac candidates should be HBLs or HBL-like objects with small redshifts.

A Quasi-periodic Oscillation of ~4.6 yr in the Radio Light Curves of Blazar PKS 0607-157

We present periodicity search analyses on long-term radio light curves at 4.8,8,and 14.5 GHz of blazar PKS 0607–157 observed by the University of Michigan Radio Astronomical Observatory telescope.The highly variable radio emissions are approximately distributed as a log-normal probability distribution function.The Power Spectral Density for the radio light curves can be well characterized by a power-law model.Using the Weighted Wavelet Z-transform and Lomb-Scargle periodogram methods,significant Quasi-periodic Oscillation(QPO)of∼4.6 yr in the radio light curve has been observed above the 3σconfidence level,which presents an interesting case among blazar QPO phenomena.We explore three plausible physical models to explain the observed QPOs:a supermassive binary black hole system,Lense-Thirring precession of the disk,and helical motion of plasma blobs within the jet.

The debeamed luminosity, sychrotron peak frequency and black hole mass of BL Lac objects

We estimate the intrinsic luminosities and synchrotron peak frequencies using the derived Doppler factor for a sample of 170 BL Lac objects, of which the synchrotron peak frequency is derived by fitting the SED constructed with the collected multi-band data from the literature. We find that the debeamed radio and optical core luminosities follow the same correlation found for FR I radio galaxies, which is in support of the unification of the BL Lac objects and the FR I galaxies based on orientation. For the debeamed luminosity at the synchrotron peak frequency, we find a significant positive correlation between the luminosity and intrinsic synchrotron peak frequency. This implies that the more powerful sources may have the majority of jet emission at higher frequency. At the synchrotron peak frequency, the intrinsic luminosity and black hole mass show strong positive correlation, while mild correlation is found in the case of jet power, indicating that the more powerful sources may have more massive black holes.

Comprehensive Study of the Blazars from Fermi-LAT LCR: The Log-Normal Flux Distribution and Linear rms–Flux Relation

Fermi-LAT LCR provides continuous and regularly sampled gamma-ray light curves, spanning about 14 yr, for a large sample of blazars. The log-normal flux distribution and linear rms–flux relation of the light curves for a few Fermi blazars have been examined in previous studies. However, the probability that blazars exhibit the log-normal flux distribution and linear rms–flux relation in their gamma-ray light curves has not been systematically explored.In this study, we comprehensively research the distribution of γ-ray flux and the statistical characteristics on a large sample of 1414 variable blazars from the Fermi-LAT LCR catalog, including 572 FSRQs, 477 BL Lacs, and 365BCUs, and statistically compare their flux distributions with normal and log-normal distributions. The results indicate that the probability of not rejecting log-normal is 42.05% for the large sample, and there is still a 2.05%probability of not rejecting normality, based on the joint of Kolmogorov–Smirnov, Shapiro–Wilk, and Normality tests. We further find that the probability that BL Lacs conform to the log-normal distribution is higher than that of FSRQs. Besides, after removing sources with less than 200 data points from this large sample, a sample of 549blazars, which is still a large sample compared to the previous studies, was obtained. Based on dividing the light curves into segments every 20 points(or 40 points, or one year), we fitted the linear rms–flux relation of these three different sets and found that the Pearson correlation coefficients are all close to 1 for most blazars. This result indicates a strong linear correlation between the rms and the flux of these 549 blazars. The log-normal distribution and linear rms–flux relation indicate that the variability of the γ-ray flux for most blazars is a non-linear and multiplicative process.

Discovery of a variable broad absorption line in the BL Lac object PKS B0138-097

We report the discovery of a broad absorption line （BAL） of - 10^4 km s^-1 in width in the previously known BL Lac object PKS 0138-097, which we tentatively identified as an Mg II BAL. This is the first detection of a BAL, which is sometimes seen in powerful quasars with high accretion rates, in a BL Lac object. The BAL was clearly detected in its spectra spanning two epochs at a high luminosity state taken in the Sloan Digital Sky Survey （SDSS）, while it disappeared in three SDSS spectra taken at a low luminosity state. The BAL and its variability pattern were also found in its historical multi-epoch spectra in the literature, but have been overlooked previously. In its high resolution radio maps, PKS 0138-097 shows a core plus a one- sided parsec-scale jet. The BAL variability can be interpreted as follows： The optical emission is dominated by the core in a high state and by the jet in a low state and the BAL material is located between the core and jet so that the BAL appears only when the core is shining. Our discovery suggests that outflows may also be produced in active galactic nuclei at a low accreting state.

The intra-day optical monitoring of BL Lacerate object 1ES 1218+304 at its highest X-ray flux level

We here report a monitor of the BL Lac object 1 ES 1218+304 in both B-and R-bands by the GWAC-F60 A telescope in eight nights,when it was triggered to be at its highest X-ray flux in history by the VERITAS Observatory and Swift follow-ups.Both ANOVA and χ^(2)-test enable us to clearly reveal an intraday variability in optical wavelengths in seven out of the eight nights.A bluer-when-brighter chromatic relationship has been clearly identified in five out of the eight nights,which can be well explained by the shock-in-jet model.In addition,a quasi-periodic oscillation phenomenon in both bands could be tentatively identified in the first night.A positive delay between the two bands has been revealed in three out of the eight nights,and a negative one in the other nights.The identified minimum time delay enables us to estimate the MBH=2.8 × 10^(7) M_(⊙) that is invalid.

A Possible 456-Day Optical Period of OQ 530

The post-1994 observations of the blazar OQ 530 in optical BVRI bands, and radio observations at 22 GHz and 37 GHz were collected. The date compensated discrete Fourier transform （DCDFr） and CLEANest methods were used to search for possible periodicities. Two possible periods of 456±100 days and 258±29 days were found in the optical bands. The existence of possible correlations between the optical and radio emissions was investigated by means of discrete correlation function （DCF） analysis, and no significant correlation was found. Some possible mechanisms for the periodic variability are discussed.

The intrinsic γ-ray emissions of Fermi blazars

The beaming effect is important for understanding the observational properties of blazars.In this work, we collect 91 Fermi blazars with available radio Doppler factors. γ-ray Doppler factors are estimated and compared with radio Doppler factors for some sources. The intrinsic（de-beamed）γ-ray flux density（fγin）, intrinsic γ-ray luminosity（Lγin） and intrinsic synchrotron peak frequency（VPin）are calculated. Then we study the correlations between finγand redshift and find that they follow the theoretical relation： log f =-2.0 log z ＋ const. When the subclasses are considered, we find that stationary jets are perhaps dominant in low synchrotron peaked blazars. Sixty-three Fermi blazars with both available short variability time scales（？T） and Doppler factors are also collected. We find that the intrinsic relationship between Lγin and ？Tinobeys the Elliot ＆ Shapiro and Abramowicz ＆ Nobili relations. Strong positive correlation between fγinand VPinis found, suggesting that synchrotron emissions are highly correlated with γ-ray emissions.

The Correlation between Gamma-ray and Radio Emissions in γ-ray Loud Blazars

We collect 119 γ-ray-loud blazars (97 flat spectrum radio quasars (FSRQs) and 22 BL Lacertae objects (BL Lacs)), and investigate possible correlations between their γ-ray emission (maximum, minimum and average values) at 1 GeV and the radio emission at 8.4 GHz. Our main results are as follows. For the lower state γ-ray data, there is no correlation between the γ-ray and radio flux density; For the high state γ-ray data, there are good correlations for the whole 119 blazars and 97 FSRQs, and a weak correlation for the 22 BL Lac objects; For the average γ-ray data, there are good correlations. According to our analysis, we propose that the γ-rays are associated with the radio emission from the jet, and that the γ-ray emission is likely from the SSC process in this case.