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
Historical optical data are combined on the BL Lac object OQ 530. Verifying the existence of correlations among the flux variations in different bands serves as an important tool to investigate emission processes. To ...Historical optical data are combined on the BL Lac object OQ 530. Verifying the existence of correlations among the flux variations in different bands serves as an important tool to investigate emission processes. To examine the possible existence of a lag between variations in different optical bands from this source, a statistical analysis is performed through the discrete correlation function （DCF） method and the z-transformed discrete correlation function （ZDCF） method. Monte Carlo simulations called ＂flux redistribution/random subset selection＂ （FR/RSS） are performed to obtain statistically meaningful values for the cross-correlation time lags and their related uncertainties. The analysis confirms that the variations in different optical light curves are strongly correlated, with zero-lag within the errors. Long term variability of color indexes is also analyzed. The result suggests a strong correlation between color index and brightness. A clear bluer-when-brighter chromatism is found, in the sense that the spectrum steepens as the brightness decreases.展开更多
Long-term optical,X-ray and γ-ray data of blazar 3 C 279 have been compiled from Swift-XRT,RXTE-PCA,Fermi-LAT,SMARTS and literature.The source exhibits strong variability on long timescales.From the 1980 s until now,...Long-term optical,X-ray and γ-ray data of blazar 3 C 279 have been compiled from Swift-XRT,RXTE-PCA,Fermi-LAT,SMARTS and literature.The source exhibits strong variability on long timescales.From the 1980 s until now,the optical R band light curve spans more than 32 yr,and a possible 5.6-yr-long quasi-periodic variation component has been found in it.The optical spectral behavior has been investigated.In the optical band,the mean spectral index is –1.71.The source exhibits an obvious special spectral behavior.In the low state,the source manifests a clear bluer-when-brighter behavior in the sense that the optical spectrum turns harder(flatter) when the brightness increases.While in the high state,the optical spectrum is stable,which means the source spectral index does not vary with brightness.The correlation analysis has been performed among optical,X-ray and γ-ray energy bands.The result indicates that the variations of γ-ray and X-ray bands are well correlated without time delay on the timescale of days,and their variations exhibit weak correlations with those of the optical band.The variations,especially outbursts,are simultaneous,but the magnitude of variations is disproportionate.The detailed analysis reveals that the main outbursts exhibit strong correlations in different γ-ray,X-ray and optical bands.展开更多
The long-term optical and infrared color variability of blazars has been investigated with monitoring data from the Small and Moderate Aperture Research Telescope System（SMARTS）. The sample in this study consists of...The long-term optical and infrared color variability of blazars has been investigated with monitoring data from the Small and Moderate Aperture Research Telescope System（SMARTS）. The sample in this study consists of 49 flat spectrum radio quasars（FSRQs） and 22 BL Lacertae objects（BL Lacs）. The fractional variability amplitudes of each source have been calculated in both optical R band and infrared J band. Overall, the variability amplitudes of FSRQs are larger than those of BL Lacs. The results also suggest that the variability amplitude of most FSRQs is larger at a lower energy band（J band） than at a higher one（R band）, but the variability amplitude of BL Lacs is larger at the higher energy band. Both types of blazars display color variation along with variability in brightness. However, they show different variation behaviors in general. In the whole data set, 35 FSRQs exhibit redder-whenbrighter trends, and 11 FSRQs exhibit opposite trends; 11 BL Lacs follow bluer-whenbrighter trends, and seven BL Lacs follow opposite trends. A detailed examination indicates that there are 10 blazars showing redder-when-brighter trends in their low state, and bluer-when-brighter or stable-when-brighter trends in their high state. Some more complicated color behaviors have also been detected in several blazars. The nonthermal jet emission and the thermal emission from the accretion disk are employed to explain the observed color behaviors.展开更多
Oriented attachment of nanocrystals is an important route to constructing epitaxially-connected nanocrystal superlattices for various applications.During oriented attach me nt of semic on ductor nano crystals,neck can...Oriented attachment of nanocrystals is an important route to constructing epitaxially-connected nanocrystal superlattices for various applications.During oriented attach me nt of semic on ductor nano crystals,neck can be formed betwee n nan ocrystals and it strongly influe nces the properties of the resulting superlattice.However,the neck formation mechanism is poorly understood.Here,we use in situ liquid cell transmission electro n microscopy(TEM)to directly observe the initiatio n and growth of homoepitaxial n ecks betwee n PbSe nano crystals with atomic details.We find that neck initiatio n occurs slowly(~10 s)whe n two nano crystals approach to each other within an edge-to-edge dista nee of 0.6 nm.During neck initiation,Pb and Se atoms defuse from other facets into the gap,forming"dynamic reversible"filaments.Once the filament(n eck)width is larger than a critical size of 0.9 nm,it gradually(15 s)widens into a 3-nm-wide n eck.The atomic structure of the neck is further obtained using ex situ aberration-corrected seanning TEM imaging.Neck initiation and growth mechanisms are elucidated with density functional theory calculati ons.Our direct unveiling of the atomic pathways of neck formatio n duri ng oriented attach me nt shed light into the fabrication of nanocrystal superlattices with improved structural order and electronic properties.展开更多
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Supported by the National Natural Science Foundation of China (Grant No. 10975145)funded by Fuyang Normal College, the Research Foundation of the Department of Education of Anhui Province (KJ2010B159)
文摘Historical optical data are combined on the BL Lac object OQ 530. Verifying the existence of correlations among the flux variations in different bands serves as an important tool to investigate emission processes. To examine the possible existence of a lag between variations in different optical bands from this source, a statistical analysis is performed through the discrete correlation function （DCF） method and the z-transformed discrete correlation function （ZDCF） method. Monte Carlo simulations called ＂flux redistribution/random subset selection＂ （FR/RSS） are performed to obtain statistically meaningful values for the cross-correlation time lags and their related uncertainties. The analysis confirms that the variations in different optical light curves are strongly correlated, with zero-lag within the errors. Long term variability of color indexes is also analyzed. The result suggests a strong correlation between color index and brightness. A clear bluer-when-brighter chromatism is found, in the sense that the spectrum steepens as the brightness decreases.
基金the support from National Natural Science Foundation of China (NSFC,Nos.U1831124 and 11273008)the Natural Science Foundation of Anhui Province of China (Grant No.1908085MA28)。
文摘Long-term optical,X-ray and γ-ray data of blazar 3 C 279 have been compiled from Swift-XRT,RXTE-PCA,Fermi-LAT,SMARTS and literature.The source exhibits strong variability on long timescales.From the 1980 s until now,the optical R band light curve spans more than 32 yr,and a possible 5.6-yr-long quasi-periodic variation component has been found in it.The optical spectral behavior has been investigated.In the optical band,the mean spectral index is –1.71.The source exhibits an obvious special spectral behavior.In the low state,the source manifests a clear bluer-when-brighter behavior in the sense that the optical spectrum turns harder(flatter) when the brightness increases.While in the high state,the optical spectrum is stable,which means the source spectral index does not vary with brightness.The correlation analysis has been performed among optical,X-ray and γ-ray energy bands.The result indicates that the variations of γ-ray and X-ray bands are well correlated without time delay on the timescale of days,and their variations exhibit weak correlations with those of the optical band.The variations,especially outbursts,are simultaneous,but the magnitude of variations is disproportionate.The detailed analysis reveals that the main outbursts exhibit strong correlations in different γ-ray,X-ray and optical bands.
基金supported by the National Natural Science Foundation of China (Grant No. 11273008)partly by the Foundation for Young Talents in College of Anhui Province (Grant No. 2012SQRL116)
文摘The long-term optical and infrared color variability of blazars has been investigated with monitoring data from the Small and Moderate Aperture Research Telescope System（SMARTS）. The sample in this study consists of 49 flat spectrum radio quasars（FSRQs） and 22 BL Lacertae objects（BL Lacs）. The fractional variability amplitudes of each source have been calculated in both optical R band and infrared J band. Overall, the variability amplitudes of FSRQs are larger than those of BL Lacs. The results also suggest that the variability amplitude of most FSRQs is larger at a lower energy band（J band） than at a higher one（R band）, but the variability amplitude of BL Lacs is larger at the higher energy band. Both types of blazars display color variation along with variability in brightness. However, they show different variation behaviors in general. In the whole data set, 35 FSRQs exhibit redder-whenbrighter trends, and 11 FSRQs exhibit opposite trends; 11 BL Lacs follow bluer-whenbrighter trends, and seven BL Lacs follow opposite trends. A detailed examination indicates that there are 10 blazars showing redder-when-brighter trends in their low state, and bluer-when-brighter or stable-when-brighter trends in their high state. Some more complicated color behaviors have also been detected in several blazars. The nonthermal jet emission and the thermal emission from the accretion disk are employed to explain the observed color behaviors.
文摘Oriented attachment of nanocrystals is an important route to constructing epitaxially-connected nanocrystal superlattices for various applications.During oriented attach me nt of semic on ductor nano crystals,neck can be formed betwee n nan ocrystals and it strongly influe nces the properties of the resulting superlattice.However,the neck formation mechanism is poorly understood.Here,we use in situ liquid cell transmission electro n microscopy(TEM)to directly observe the initiatio n and growth of homoepitaxial n ecks betwee n PbSe nano crystals with atomic details.We find that neck initiatio n occurs slowly(~10 s)whe n two nano crystals approach to each other within an edge-to-edge dista nee of 0.6 nm.During neck initiation,Pb and Se atoms defuse from other facets into the gap,forming"dynamic reversible"filaments.Once the filament(n eck)width is larger than a critical size of 0.9 nm,it gradually(15 s)widens into a 3-nm-wide n eck.The atomic structure of the neck is further obtained using ex situ aberration-corrected seanning TEM imaging.Neck initiation and growth mechanisms are elucidated with density functional theory calculati ons.Our direct unveiling of the atomic pathways of neck formatio n duri ng oriented attach me nt shed light into the fabrication of nanocrystal superlattices with improved structural order and electronic properties.
