We have collected a sample of 70 BL Lacs （33 radio-selected BL Lacs and 37 X-ray selected BL Lacs） with multi-waveband data for investigating the classifying criteria of BL Lacertae Objects. For each source, we esti...We have collected a sample of 70 BL Lacs （33 radio-selected BL Lacs and 37 X-ray selected BL Lacs） with multi-waveband data for investigating the classifying criteria of BL Lacertae Objects. For each source, we estimate its luminosities in radio, optical and X-ray, the broad-band spectral index from radio to X-ray and the peak frequency of the synchrotron emission, and make a statistical analysis of the data obtained. Our main results are as follows： （1） The broad-band spectral index and the peak frequency have no correlation with the redshift, while they are inversely correlated with each other and they could be regarded as equivalent classifying criteria of BL Lac objects. （2） There are significant effects of the luminosity/redshift relation on the observed luminosity distribution in our sample, hence, if the radio luminosity is to be used as a classifying criterion of BL Lac objects, it should not be regarded as equivalent to the broad-band spectral index or the peak frequency. （3） Our resuits supply a specific piece of evidence for the suggestion that the use of luminosities always introduces a redshift bias to the data and show that the location of the peak frequency is not always linked to the luminosity of any wave band.展开更多
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. Periodici...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.展开更多
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 C...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.展开更多
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 ...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.展开更多
The multi-frequency light curves of BL Lacertae during 1997.5 - 1999.5 have been modeled by four outbursts, each having a 3-stage evolution in the （Sm, vm） plane with distinct rising-plateau-decaying phases. It is s...The multi-frequency light curves of BL Lacertae during 1997.5 - 1999.5 have been modeled by four outbursts, each having a 3-stage evolution in the （Sm, vm） plane with distinct rising-plateau-decaying phases. It is shown that the observed light curves can be well fitted for the eight frequencies from 350 GHz to 4.8 GHz. The main characteristics of the model-fitting are; （1） the outbursts are found to have very flat spectra with an optically thin spectral index α （defined as Sv α u^-α） of about 0.15. This is consistent with the results previously obtained by Valtaoja et al. （1992）; （2） it is found that there is no spectral flattening between the rising-plateau phase and the decay phase. In other words, the optically thin spectral index does not change from the rising-plateau phase to the decay phase. These features are in contrast to the 3-stage shocked-in-jet model proposed by Marscher ＆ Gear （1985） for submm- IR-optical flares, in which a spectral flattening of △α = 0.5 is predicted when a transition occurs from the Compton/synchrotron phase （or rising-plateau phase） to the adiabatic phase （or decay phase） with α≥ 0.5 for the shock being non-radiative. We propose a new model to interpret the fitting results, suggesting that the 3-stage evolution of the mm-cm outbursts in BL Lacertae may be related to the process of shock formation and propagation in a highly collimated jet （for example, a ＇parabolic＇ jet）. In particular, during the rising phase, the thickness of the synchrotron-radiating region created by the shock may rapidly increase with time （relative to the jet width） due to the rapid injection of relativistic electrons and a magnetic field, and this leads to the observed behavior that the turnover flux density Sm rapidly increases while the turnover frequency um decreases. In the decay phase, the emitting plasma enters into a free expansion regime without further injection of relativistic electrons and a magnetic field （for example, when a transition from a collimated regime into a conical regime occurs）. The plateau phase is a short period between the two regimes with no distinct features determined.展开更多
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...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.展开更多
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 ...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.展开更多
The spectral energy distribution (SED) of the γ-ray flare observed inJuly 1997 in BL Lacertae is re-considered. It is pointed out that the optical observa-tions made by Webb et al. showed the associated optical flare...The spectral energy distribution (SED) of the γ-ray flare observed inJuly 1997 in BL Lacertae is re-considered. It is pointed out that the optical observa-tions made by Webb et al. showed the associated optical flare has a hard spectrum(the average spectral index αopt～ 0.48, F_v∝v^(-α)), and the ASCA observationsmade by Tanihata et al. showed very steep spectra in the soft X-ray band (0.7-1.5 keV) (α_x～3-4). We find that the flux densities and spectral indices in both theoptical and soft X-ray bands are closely consistent with a ‘canonical' synchrotronspectrum emitted by relativistic electrons of a power-law energy distribution witha high energy cutoff, and thus the peak of the SED of the synchrotron radiation (inrepresentation of vF_v) is located in the EUV -- soft X-ray bands. Therefore, theGeV γ-ray emission observed in the July 1997 outburst may be mainly due to thesynchrotron self-Compton (SSC) process, contrasting with the current explanationsin terms of external radiation Compton (ERC) process, in which the seed photonsare mostly taken to be the UV emission from the clouds of the broad emission lineregion. We argue that the hard optical spectra observed during the γ-ray outburstmay be an important signature for the acceleration of high energy electrons (γ_e～10~4)in the γ-ray emitting region.展开更多
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 VERITA...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.展开更多
We present an analysis of the cross-correlation between optical brightness and polarization degree in different types of blazars.The aim is to identify objects with simultaneous and consistent changes in characteristi...We present an analysis of the cross-correlation between optical brightness and polarization degree in different types of blazars.The aim is to identify objects with simultaneous and consistent changes in characteristics and to determine if this behavior relates to the types of objects studied.The analysis includes 23 objects:11 Flat Spectrum Radio Quasars(FSRQs),11 BL Lacs,and 1 radio galaxy.Dense overlapping observation series in the R band were used,collected over more than 10 yr as part of a monitoring program for bright blazars at St.Petersburg State University.The cross-correlation analysis procedure is detailed,including a method for assessing significance based on Monte Carlo simulations of synthetic light curves modeled using a Damped Random Walk.Significant correlations were found for 5 FSRQs and 1 BL Lac.No significant correlation was detected for 10 BL Lacs and 6 FSRQs.One object did not yield a reliable estimate.Based on the current results,we cannot claim that the observed difference in the behavior of these emission characteristics for different classes of blazars is significant.It is possible that observed correlations may be explained by the contribution of simultaneous flare events to the changes in flux and polarization degree curves,which occur more frequently in FSRQs.展开更多
We present the results of the multi-epoch broadband spectral study of 1ES 2344+514 and study the evolution of physical parameters.We used nearly simultaneous data obtained from 2017 June 6 to 2022 August 6(MJD 57910–...We present the results of the multi-epoch broadband spectral study of 1ES 2344+514 and study the evolution of physical parameters.We used nearly simultaneous data obtained from 2017 June 6 to 2022 August 6(MJD 57910–59797)in optical,UV,X-ray andγ-ray wavebands from various instruments including Swift-UVOT,Swift-XRT,Nuclear Spectroscopic Telescope Array(NuSTAR),AstroSat(SXT and LAXPC),Fermi-LAT,and TeV flux from MAGIC.During 2017 July,1ES 2344+514 appeared to be in the highest flaring state compared to other periods.We used the 0.5–7.0 keV and 3.0–20.0 keV data,respectively,from SXT and LAXPC of AstroSat and also 0.3–8.0 keV and 3.0–79.0 keV data,respectively,from Swift-XRT and NuSTAR.A joint fit between SXT and LAXPC,and between Swift-XRT and NuSTAR has been done for constraining the synchrotron peak.A clear shift in the synchrotron peak has been observed,which included 1ES 2344+514 in the HSP BL Lac family.A“harder-when-brighter”trend is observed in X-rays,and the opposite trend,i.e.,“softer-when-brighter,”is seen in theγ-rays.The multi-epoch broadband spectral energy distributions(SEDs)of this source were built and studied to get an idea of the radiation processes.The SEDs were fitted using a steady-state leptonic one-zone synchrotron+SSC model,and the fitted parameters of the emission region are consistent with those of other TeV BL Lacs.In this study,we found a weak correlation tendency between bolometric luminosity and magnetic field(B),as well as between bolometric luminosity and the break Lorentz factor(γ_(break)).展开更多
We report our identification of three gigaelectronvoltγ-ray sources,4FGL J0502.6+0036,4FGL J1055.9+6507,and 4FGL J1708.2+5519,as Active Galactic Nuclei(AGNs).They are listed in the latest Fermi-Large Area Telescope s...We report our identification of three gigaelectronvoltγ-ray sources,4FGL J0502.6+0036,4FGL J1055.9+6507,and 4FGL J1708.2+5519,as Active Galactic Nuclei(AGNs).They are listed in the latest Fermi-Large Area Telescope source catalog as unidentified ones.We find that the sources all showedγ-ray flux variations in recent years.Using different survey catalogs,we are able to find a radio source within the error circle of each source's position.Further analysis of optical sources in the fields allows us to determine the optical counterparts,which showed similar variation patterns to those seen inγ-rays.The optical counterparts have reported redshifts of 0.6,1.5,and 2.3,respectively,estimated from photometric measurements.In addition,we also obtain an X-ray spectrum of 4FGL J0502.6+0036 and a flux upper limit on the X-ray emission of 4FGL J1055.9+6507 by analyzing the archival data.The broadband spectral energy distributions of the three sources from radio toγ-rays are constructed.Comparing mainly theγ-ray properties of the three sources with those of different sub-classes of AGNs,we tentatively identify them as blazars.Followup optical spectroscopy is highly warranted for obtaining their spectral features and thus verifying the identification.展开更多
Based on the collected multiwavelength data, namely in the radio(NVSS, FIRST, RATAN-600), IR(WISE),optical(Pan-STARRS), UV(GALEX), and X-ray(ROSAT, Swift-XRT) ranges, we have performed a cluster analysis for the blaza...Based on the collected multiwavelength data, namely in the radio(NVSS, FIRST, RATAN-600), IR(WISE),optical(Pan-STARRS), UV(GALEX), and X-ray(ROSAT, Swift-XRT) ranges, we have performed a cluster analysis for the blazars of the Roma-BZCAT catalog. Using two machine learning methods, namely a combination of PCA with k-means clustering and Kohonen's self-organizing maps(SOMs), we have constructed an independent classification of the blazars(five classes) and compared the classes with the known Roma-BZCAT classification(FSRQs, BL Lacs, galaxy-dominated BL Lacs, and blazars of an uncertain type) as well as with the high synchrotron peaked(HSP) blazars from the 3HSP catalog and blazars from the TeVCat catalog. The obtained groups demonstrate concordance with the BL Lac/FSRQ classification along with a continuous character of the change in the properties. The group of HSP blazars stands out against the overall distribution. We examine the characteristics of the five groups and demonstrate distinctions in their spectral energy distribution shapes. The effectiveness of the clustering technique for objective analysis of multiparametric arrays of experimental data is demonstrated.展开更多
We analyze the optical light curve data,obtained with the Zwicky Transient Facility(ZTF)survey,for 47 γ-ray blazars monitored by the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope(Fermi).These 47sou...We analyze the optical light curve data,obtained with the Zwicky Transient Facility(ZTF)survey,for 47 γ-ray blazars monitored by the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope(Fermi).These 47sources are selected because they are among the Fermi blazars with the largest optical variations in the ZTF data.Two color-magnitude variation patterns are seen in them,with one being redder-to-stable-when-brighter(RSWB;in 31 sources)and the other being stable when brighter(in 16 sources).The patterns fit with the results recently reported in several similar studies with different data.Moreover,we find that the colors in the stable state of the sources share similar values,for which(after being corrected for the Galactic extinction)most sources are in a range of 0.4–0.55.This feature could be intrinsic and may be applied in,for example,study of the intragalactic medium.We also determine the turning points for the sources showing the RSWB pattern,after which the color changes saturate and become stable.We find a correlation between optical fluxes and γ-ray fluxes at the turning points.The physical implications of the correlation remain to be investigated,probably better with a sample of high-qualityγ-ray flux measurements.展开更多
A Very High-energy(VHE) flare was observed by Major Atmospheric Gamma Imaging Cherenkov Telescopes on MJD 57788. This VHE flare was characterized by increased VHE flux and short timescales. In this study, we used one-...A Very High-energy(VHE) flare was observed by Major Atmospheric Gamma Imaging Cherenkov Telescopes on MJD 57788. This VHE flare was characterized by increased VHE flux and short timescales. In this study, we used one-zone synchrotron self-Compton, two-zone, and spine/layer models to explore this VHE flare's origin. The results indicate that this γ-ray flare can be explained by Inverse Compton Scattering radiation from the layer contributed gamma radiation and results in the γ-ray flare in a different way than the two-zone model. The difference between the two models lies in the performance of spectral energy distribution after γ-ray flare as well as a possible difference in the timescales of the flare.展开更多
With the assumption that the optical variability timescale is dominated by the cooling time of the synchrotron process for BL Lac objects,we estimate time dependent magnetic field strength of the emission region for t...With the assumption that the optical variability timescale is dominated by the cooling time of the synchrotron process for BL Lac objects,we estimate time dependent magnetic field strength of the emission region for two BL Lac objects.The average magnetic field strengths are consistent with those estimated from core shift measurement and spectral energy distribution modelling.Variation of magnetic field strength in the dissipation region is discovered.Variability of flux and magnetic field strength shows no clear correlation,which indicates the variation of magnetic field is not the dominant reason of the variability origin.The evolution of magnetic field strength can provide another approach to constrain the energy dissipation mechanism in jets.展开更多
PKS 2155-304 is a well studied BL Lac object in the southern sky. The historical optical data during different periods have been collected and compiled. Light curves spanning 35 yr have been constructed. The R-band li...PKS 2155-304 is a well studied BL Lac object in the southern sky. The historical optical data during different periods have been collected and compiled. Light curves spanning 35 yr have been constructed. The R-band light curve has been ana- lyzed by means of three methods： the epoch folding method, the Jurkevich method and the discrete correlation function method. It is derived that there is an evident periodic component of 317 d （i.e. 0.87 yr） superposed on a long-term trend with large- amplitude variation in the light curve. The variability of this source is accompanied by a slight color variation, and the brightness and color index are correlated with each other. On a long-term time scale, PKS 2155-304 exhibits a tendency of bluer-when- brighter, which means the spectrum becomes flatter when the source brightens.展开更多
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 us...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.展开更多
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...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.展开更多
基金the National Natural Science Foundation of China
文摘We have collected a sample of 70 BL Lacs （33 radio-selected BL Lacs and 37 X-ray selected BL Lacs） with multi-waveband data for investigating the classifying criteria of BL Lacertae Objects. For each source, we estimate its luminosities in radio, optical and X-ray, the broad-band spectral index from radio to X-ray and the peak frequency of the synchrotron emission, and make a statistical analysis of the data obtained. Our main results are as follows： （1） The broad-band spectral index and the peak frequency have no correlation with the redshift, while they are inversely correlated with each other and they could be regarded as equivalent classifying criteria of BL Lac objects. （2） There are significant effects of the luminosity/redshift relation on the observed luminosity distribution in our sample, hence, if the radio luminosity is to be used as a classifying criterion of BL Lac objects, it should not be regarded as equivalent to the broad-band spectral index or the peak frequency. （3） Our resuits supply a specific piece of evidence for the suggestion that the use of luminosities always introduces a redshift bias to the data and show that the location of the peak frequency is not always linked to the luminosity of any wave band.
基金Supported by the National Natural Science Foundation of China
文摘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.
基金supported by the National Key R&D Program of China (2017YFA0402600)the National Natural Science Foundation of China (Grant Nos. 11525312 and U1531130)
文摘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.
基金Supported by the National Natural Science Foundation of China(Grant Nos. 10773020, 10821302 and 10833002)the CAS (Grant No. KJCX2-YWT03)+1 种基金the Science and Technology Commission of Shanghai Municipality (10XD1405000)the National Basic Research Program of China (Grant No. 2009CB824800)
文摘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.
基金This research has made use of data from the University of Michigan Radio Astronomy Observatory which is supported by funds from the University of Michigan and by a series of grants from the NSF.This paper is partly based on observations carried out at the 30 m telescope of IRAM,which is supported by INSU/CNRS (France),MPG (Germany) and IGN (Spain)
文摘The multi-frequency light curves of BL Lacertae during 1997.5 - 1999.5 have been modeled by four outbursts, each having a 3-stage evolution in the （Sm, vm） plane with distinct rising-plateau-decaying phases. It is shown that the observed light curves can be well fitted for the eight frequencies from 350 GHz to 4.8 GHz. The main characteristics of the model-fitting are; （1） the outbursts are found to have very flat spectra with an optically thin spectral index α （defined as Sv α u^-α） of about 0.15. This is consistent with the results previously obtained by Valtaoja et al. （1992）; （2） it is found that there is no spectral flattening between the rising-plateau phase and the decay phase. In other words, the optically thin spectral index does not change from the rising-plateau phase to the decay phase. These features are in contrast to the 3-stage shocked-in-jet model proposed by Marscher ＆ Gear （1985） for submm- IR-optical flares, in which a spectral flattening of △α = 0.5 is predicted when a transition occurs from the Compton/synchrotron phase （or rising-plateau phase） to the adiabatic phase （or decay phase） with α≥ 0.5 for the shock being non-radiative. We propose a new model to interpret the fitting results, suggesting that the 3-stage evolution of the mm-cm outbursts in BL Lacertae may be related to the process of shock formation and propagation in a highly collimated jet （for example, a ＇parabolic＇ jet）. In particular, during the rising phase, the thickness of the synchrotron-radiating region created by the shock may rapidly increase with time （relative to the jet width） due to the rapid injection of relativistic electrons and a magnetic field, and this leads to the observed behavior that the turnover flux density Sm rapidly increases while the turnover frequency um decreases. In the decay phase, the emitting plasma enters into a free expansion regime without further injection of relativistic electrons and a magnetic field （for example, when a transition from a collimated regime into a conical regime occurs）. The plateau phase is a short period between the two regimes with no distinct features determined.
基金Supported by the National Natural Science Foundation of China
文摘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.
基金supported by the National Natural Science Foundation of China(Grant No.11973017)。
文摘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.
文摘The spectral energy distribution (SED) of the γ-ray flare observed inJuly 1997 in BL Lacertae is re-considered. It is pointed out that the optical observa-tions made by Webb et al. showed the associated optical flare has a hard spectrum(the average spectral index αopt～ 0.48, F_v∝v^(-α)), and the ASCA observationsmade by Tanihata et al. showed very steep spectra in the soft X-ray band (0.7-1.5 keV) (α_x～3-4). We find that the flux densities and spectral indices in both theoptical and soft X-ray bands are closely consistent with a ‘canonical' synchrotronspectrum emitted by relativistic electrons of a power-law energy distribution witha high energy cutoff, and thus the peak of the SED of the synchrotron radiation (inrepresentation of vF_v) is located in the EUV -- soft X-ray bands. Therefore, theGeV γ-ray emission observed in the July 1997 outburst may be mainly due to thesynchrotron self-Compton (SSC) process, contrasting with the current explanationsin terms of external radiation Compton (ERC) process, in which the seed photonsare mostly taken to be the UV emission from the clouds of the broad emission lineregion. We argue that the hard optical spectra observed during the γ-ray outburstmay be an important signature for the acceleration of high energy electrons (γ_e～10~4)in the γ-ray emitting region.
基金supported by the National Natural Science Foundation of China under grants 10533050the National Basic Research Program ("973" Program) of China under Grant 2009CB824800
文摘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.
基金the National Science Foundation grant AST-2108622several NASA Fermi vip Investigator grants,the latest is 80NSSC23K1507supported by Fermi vip Investigator grants NNX08AW56G,NNX09AU10G,NNX12AO93G,and NNX15AU81G.
文摘We present an analysis of the cross-correlation between optical brightness and polarization degree in different types of blazars.The aim is to identify objects with simultaneous and consistent changes in characteristics and to determine if this behavior relates to the types of objects studied.The analysis includes 23 objects:11 Flat Spectrum Radio Quasars(FSRQs),11 BL Lacs,and 1 radio galaxy.Dense overlapping observation series in the R band were used,collected over more than 10 yr as part of a monitoring program for bright blazars at St.Petersburg State University.The cross-correlation analysis procedure is detailed,including a method for assessing significance based on Monte Carlo simulations of synthetic light curves modeled using a Damped Random Walk.Significant correlations were found for 5 FSRQs and 1 BL Lac.No significant correlation was detected for 10 BL Lacs and 6 FSRQs.One object did not yield a reliable estimate.Based on the current results,we cannot claim that the observed difference in the behavior of these emission characteristics for different classes of blazars is significant.It is possible that observed correlations may be explained by the contribution of simultaneous flare events to the changes in flux and polarization degree curves,which occur more frequently in FSRQs.
基金support of the Department of Atomic Energy,Government of India,under project identification No.RT4002supported by Fermi vip Investigator grants NNX08AN56G,NNX08AN56G,NNX09AV10G,and NNX15AU81G。
文摘We present the results of the multi-epoch broadband spectral study of 1ES 2344+514 and study the evolution of physical parameters.We used nearly simultaneous data obtained from 2017 June 6 to 2022 August 6(MJD 57910–59797)in optical,UV,X-ray andγ-ray wavebands from various instruments including Swift-UVOT,Swift-XRT,Nuclear Spectroscopic Telescope Array(NuSTAR),AstroSat(SXT and LAXPC),Fermi-LAT,and TeV flux from MAGIC.During 2017 July,1ES 2344+514 appeared to be in the highest flaring state compared to other periods.We used the 0.5–7.0 keV and 3.0–20.0 keV data,respectively,from SXT and LAXPC of AstroSat and also 0.3–8.0 keV and 3.0–79.0 keV data,respectively,from Swift-XRT and NuSTAR.A joint fit between SXT and LAXPC,and between Swift-XRT and NuSTAR has been done for constraining the synchrotron peak.A clear shift in the synchrotron peak has been observed,which included 1ES 2344+514 in the HSP BL Lac family.A“harder-when-brighter”trend is observed in X-rays,and the opposite trend,i.e.,“softer-when-brighter,”is seen in theγ-rays.The multi-epoch broadband spectral energy distributions(SEDs)of this source were built and studied to get an idea of the radiation processes.The SEDs were fitted using a steady-state leptonic one-zone synchrotron+SSC model,and the fitted parameters of the emission region are consistent with those of other TeV BL Lacs.In this study,we found a weak correlation tendency between bolometric luminosity and magnetic field(B),as well as between bolometric luminosity and the break Lorentz factor(γ_(break)).
基金supported by the Basic Research Program of Yunnan Province(No.202201AS070005)the National Natural Science Foundation of China(NSFC,grant No.12273033)+1 种基金the Original Innovation Program of the Chinese Academy of Sciences(E085021002)support of the science research program for graduate students of Yunnan University(KC-23234629)。
文摘We report our identification of three gigaelectronvoltγ-ray sources,4FGL J0502.6+0036,4FGL J1055.9+6507,and 4FGL J1708.2+5519,as Active Galactic Nuclei(AGNs).They are listed in the latest Fermi-Large Area Telescope source catalog as unidentified ones.We find that the sources all showedγ-ray flux variations in recent years.Using different survey catalogs,we are able to find a radio source within the error circle of each source's position.Further analysis of optical sources in the fields allows us to determine the optical counterparts,which showed similar variation patterns to those seen inγ-rays.The optical counterparts have reported redshifts of 0.6,1.5,and 2.3,respectively,estimated from photometric measurements.In addition,we also obtain an X-ray spectrum of 4FGL J0502.6+0036 and a flux upper limit on the X-ray emission of 4FGL J1055.9+6507 by analyzing the archival data.The broadband spectral energy distributions of the three sources from radio toγ-rays are constructed.Comparing mainly theγ-ray properties of the three sources with those of different sub-classes of AGNs,we tentatively identify them as blazars.Followup optical spectroscopy is highly warranted for obtaining their spectral features and thus verifying the identification.
文摘Based on the collected multiwavelength data, namely in the radio(NVSS, FIRST, RATAN-600), IR(WISE),optical(Pan-STARRS), UV(GALEX), and X-ray(ROSAT, Swift-XRT) ranges, we have performed a cluster analysis for the blazars of the Roma-BZCAT catalog. Using two machine learning methods, namely a combination of PCA with k-means clustering and Kohonen's self-organizing maps(SOMs), we have constructed an independent classification of the blazars(five classes) and compared the classes with the known Roma-BZCAT classification(FSRQs, BL Lacs, galaxy-dominated BL Lacs, and blazars of an uncertain type) as well as with the high synchrotron peaked(HSP) blazars from the 3HSP catalog and blazars from the TeVCat catalog. The obtained groups demonstrate concordance with the BL Lac/FSRQ classification along with a continuous character of the change in the properties. The group of HSP blazars stands out against the overall distribution. We examine the characteristics of the five groups and demonstrate distinctions in their spectral energy distribution shapes. The effectiveness of the clustering technique for objective analysis of multiparametric arrays of experimental data is demonstrated.
基金supported by the National Science Foundation under Grant No.AST-2034437 and a collaboration including Caltech,IPACsupported by the Basic Research Program of Yunnan Province(No.202201AS070005)+2 种基金the National Natural Science Foundation of China(NSFC,grant No.12273033)the Original Innovation Program of the Chinese Academy of Sciences(E085021002)the support of the science research program for graduate students of Yunnan University(KC-23234629)。
文摘We analyze the optical light curve data,obtained with the Zwicky Transient Facility(ZTF)survey,for 47 γ-ray blazars monitored by the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope(Fermi).These 47sources are selected because they are among the Fermi blazars with the largest optical variations in the ZTF data.Two color-magnitude variation patterns are seen in them,with one being redder-to-stable-when-brighter(RSWB;in 31 sources)and the other being stable when brighter(in 16 sources).The patterns fit with the results recently reported in several similar studies with different data.Moreover,we find that the colors in the stable state of the sources share similar values,for which(after being corrected for the Galactic extinction)most sources are in a range of 0.4–0.55.This feature could be intrinsic and may be applied in,for example,study of the intragalactic medium.We also determine the turning points for the sources showing the RSWB pattern,after which the color changes saturate and become stable.We find a correlation between optical fluxes and γ-ray fluxes at the turning points.The physical implications of the correlation remain to be investigated,probably better with a sample of high-qualityγ-ray flux measurements.
基金partially supported by the National Natural Science Foundation of China (NSFC, grant Nos. 12433004, U2031201, and 11733001)Guangdong Major Project of Basic and Applied Basic Research (grant No. 2019B030302001)+2 种基金the science research grants from the China Manned Space Project with No. CMS-CSST-2021-A06the support for Astrophysics Key Subjects of Guangdong Province and Guangzhou CityThe Key Laboratory for Astronomical Observation and Technology of Guangzhou also supports the work。
文摘A Very High-energy(VHE) flare was observed by Major Atmospheric Gamma Imaging Cherenkov Telescopes on MJD 57788. This VHE flare was characterized by increased VHE flux and short timescales. In this study, we used one-zone synchrotron self-Compton, two-zone, and spine/layer models to explore this VHE flare's origin. The results indicate that this γ-ray flare can be explained by Inverse Compton Scattering radiation from the layer contributed gamma radiation and results in the γ-ray flare in a different way than the two-zone model. The difference between the two models lies in the performance of spectral energy distribution after γ-ray flare as well as a possible difference in the timescales of the flare.
基金supported by the National Natural Science Foundation of China(NSFCGrant Nos.11803081,11947099,U1931203,and 12003014)supported by the CAS Youth Innovation Promotion Association and Basic Research Program of Yunnan Province(202001AW070013)。
文摘With the assumption that the optical variability timescale is dominated by the cooling time of the synchrotron process for BL Lac objects,we estimate time dependent magnetic field strength of the emission region for two BL Lac objects.The average magnetic field strengths are consistent with those estimated from core shift measurement and spectral energy distribution modelling.Variation of magnetic field strength in the dissipation region is discovered.Variability of flux and magnetic field strength shows no clear correlation,which indicates the variation of magnetic field is not the dominant reason of the variability origin.The evolution of magnetic field strength can provide another approach to constrain the energy dissipation mechanism in jets.
基金supported by the National Natural Science Foundation of China (Grant No.11273008)
文摘PKS 2155-304 is a well studied BL Lac object in the southern sky. The historical optical data during different periods have been collected and compiled. Light curves spanning 35 yr have been constructed. The R-band light curve has been ana- lyzed by means of three methods： the epoch folding method, the Jurkevich method and the discrete correlation function method. It is derived that there is an evident periodic component of 317 d （i.e. 0.87 yr） superposed on a long-term trend with large- amplitude variation in the light curve. The variability of this source is accompanied by a slight color variation, and the brightness and color index are correlated with each other. On a long-term time scale, PKS 2155-304 exhibits a tendency of bluer-when- brighter, which means the spectrum becomes flatter when the source brightens.
基金the National Natural Science Foundation of China
文摘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.
基金support from the National Key R & D Program of China (Grant No.2020YFE0202100)supported by the National Natural Science Foundation of China (Grant 11773036)+2 种基金by the Strategic Pioneer Program on Space Science,Chinese Academy of Sciences (Grant Nos.XDA15052600 and XDA15016500)supported by the Natural Science Foundation of Guangxi (2018GXUSFGA281007 & 2020GXNSFDA238018)by the Bagui Young Scholars Program。
文摘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.
