BL Lacertae Research Articles

Simultaneous optical monitoring of BL Lacertae object S5 0716+714 with high temporal resolution

We have monitored the BL Lacertae object S5 0716+714 simultaneously in the B, R and I bands on three nights in November 2014. The average time resolution is quite high (73s, 34s, 58s for the filters B, R and I), which can help us trace the profile of the variation and search for the short inter-band time delay. Intra-day variability was about 0.1 mag on the first two nights and more than 0.3 mag on the third. A bluer-when-brighter color behavior was found. An clear loop path can be seen on the color-magnitude diagram of the third night, revealing possible time delays between variations at high and low energies. It is the first time that the intra-day spectral hysteresis loop has been found so obviously in the optical band. We used the interpolated cross-correlation function method to further confirm the time delay and calculated the values of lag between light curves at different wavelengths on each night. On the third night, variations in the R and B bands is approximately 1.5 minutes lagging behind the I band. Such optical time delay is probably due to the interplay of different processes of electrons in the jet of the blazar.

CONSTRAINTS ON THE INTERGALACTIC MAGNETIC FIELD WITH GAMMA-RAY OBSERVATIONS OF BLAZARS

Distant BL Lacertae objects emit $\gamma$ rays which interact with the extragalactic background light (EBL), creating electron-positron pairs, and reducing the flux measured by ground-based imaging atmospheric Cherenkov telescopes (IACTs) at very-high energies (VHE). These pairs can Compton-scatter the cosmic microwave background, creating a $\gamma$-ray signature at slightly lower energies observable by the \fermi\ Large Area Telescope (LAT). This signal is strongly dependent on the intergalactic magnetic field (IGMF) strength ($B$) and its coherence length ($L_B$). We use IACT spectra taken from the literature for 5 VHE-detected BL Lac objects, and combine it with LAT spectra for these sources to constrain these IGMF parameters. Low $B$ values can be ruled out by the constraint that the cascade flux cannot exceed that observed by the LAT. High values of $B$ can be ruled out from the constraint that the EBL-deabsorbed IACT spectrum cannot be greater than the LAT spectrum extrapolated into the VHE band, unless the cascade spectrum contributes a sizable fraction of the LAT flux. We rule out low $B$ values ($B< 10^{-19}$ G for $L_B\ge1$\ Mpc) at $>5\sigma$ in all trials with different EBL models and data selection, except when using $>1$ GeV spectra and the lowest EBL models. We were not able to constrain high values of $B$.

Multiband optical variability of the blazar S5 0716+714 in outburst state during 2014–2015

We analyzed the multi-band optical behaviour of the BL Lacertae object, S5 0716+714, during its outburst state from 2014 November - 2015 March. We took data on 23 nights at three observatories, one in India and two in Bulgaria, making quasi-simultaneous observations in B, V, R, and I bands. We measured multi-band optical fluxes, colour and spectral variations for this blazar on intraday and short timescales. The source was in a flaring state during the period analyzed and displayed intense variability in all wavelengths. R band magnitude of 11.6 was attained by the target on 18 Jan 2015, which is the brightest value ever recorded for S5 0716+714. The discrete correlation function method yielded good correlation between the bands with no measurable time lags, implying that radiation in these bands originate from the same region and by the same mechanism. We also used the structure function technique to look for characteristic timescales in the light curves. During the times of rapid variability, no evidence for the source to display spectral changes with magnitude was found on either of the timescales. The amplitude of variations tends to increase with increasing frequency with a maximum of $\sim$ 22% seen during flaring states in B band. A mild trend of larger variability amplitude as the source brightens was also found. We found the duty cycle of our source during the analyzed period to be $\sim$ 90%. We also investigated the optical spectral energy distribution of S5 0716+714 using B, V, R, and I data points for 21 nights. We briefly discuss physical mechanisms most likely responsible for its flux and spectral variations.

Auto- and cross-correlation analysis of the QSOs radio wave intensity

We discuss here the Flicker-Noise Spectroscopy approach to studying astrophysical systems, for example the radio wave intensity of quasi-stellar object (QSO) 1641+399 and BL Lacertae (BL Lac) 0215+015 in different frequency ranges. The presented method allows to parameterize the study dynamics using a short set of characteristics. The considering sources have a significant differences in manifesting the non-stationary effects, dynamical intermittency and synchronization. The radio wave intensity dynamics of the BL Lac 0215+015 is characterized by well-defined set of natural frequencies, persistent behavior with low effects of non-stationarity and high level of frequency-phase synchronization. For dynamics of the QSO 1641+399 reverse occurs including the asymmetrical structure of cross-correlator. Our findings show that using the flicker-noise spectroscopy approach to studying astrophysical objects allows to carry out the more detail analysis of their behavior and evolution.

Long-term optical-infrared color variability of blazars

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-when-brighter trends, and 11 FSRQs exhibit opposite trends; 11 BL Lacs follow bluer-when-brighter 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 non-thermal jet emission and the thermal emission from the accretion disk are employed to explain the observed color behaviors.

A SEARCH FOR AGN INTRA-DAY VARIABILITY WITH KVN

Active galactic nuclei (AGN) are known for irregular variability on all time scales, down to intra-day variability with relative variations of a few percent within minutes to hours. On such short timescales, unexplored territory, such as the possible existence of a shortest characteristic time scale of activity and the shape of the high frequency end of AGN power spectra, still exists. We present the results of AGN single-dish fast photometry performed with the Korean VLBI Network (KVN). Observations were done in a "anti-correlated" mode using two antennas, with always at least one antenna pointing at the target. This results in an effective time resolution of less than three minutes. We used all four KVN frequencies, 22, 43, 86, and 129 GHz, in order to trace spectral variability, if any. We were able to derive high-quality light curves for 3C 111, 3C 454.3, and BL Lacertae at 22 and 43 GHz, and for 3C 279 at 86 GHz, between May 2012 and April 2013. We performed a detailed statistical analysis in order to assess the levels of variability and the corresponding upper limits. We found upper limits on flux variability ranging from $\sim$1.6% to $\sim$7.6%. The upper limits on the derived brightness temperatures exceed the inverse Compton limit by three to six orders of magnitude. From our results, plus comparison with data obtained by the University of Michigan Radio Astronomy Observatory, we conclude that we have not detected source-intrinsic variability which would have to occur at sub-per cent levels.

Optical and radio variability of BL Lacertae

We observed the prototype blazar, BL Lacertae, extensively in optical and radio bands during an active phase in the period 2010--2013 when the source showed several prominent outbursts. We searched for possible correlations and time lags between the optical and radio band flux variations using multifrequency data to learn about the mechanisms producing variability. During an active phase of BL Lacertae, we searched for possible correlations and time lags between multifrequency light curves of several optical and radio bands. We tried to estimate any possible variability timescales and inter-band lags in these bands. We performed optical observations in B, V, R and I bands from seven telescopes in Bulgaria, Georgia, Greece and India and obtained radio data at 36.8, 22.2, 14.5, 8 and 4.8 GHz frequencies from three telescopes in Ukraine, Finland and USA. Significant cross-correlations between optical and radio bands are found in our observations with a delay of cm-fluxes with respect to optical ones of ~250 days. The optical and radio light curves do not show any significant timescales of variability. BL Lacertae showed many optical 'mini-flares' on short time-scales. Variations on longer term timescales are mildly chromatic with superposition of many strong optical outbursts. In radio bands, the amplitude of variability is frequency dependent. Flux variations at higher radio frequencies lead the lower frequencies by days or weeks. The optical variations are consistent with being dominated by a geometric scenario where a region of emitting plasma moves along a helical path in a relativistic jet. The frequency dependence of the variability amplitude supports an origin of the observed variations intrinsic to the source.

RADIO VARIABILITY AND RANDOM WALK NOISE PROPERTIES OF FOUR BLAZARS

We show the results of a time series analysis of the long-term light curves of four blazars: 3C 279, 3C 345, 3C 446, and BL Lacertae. We used densely sampled light curves spanning 32 years at three frequency bands (4.8, 8, 14.5 GHz), provided by the University of Michigan Radio Astronomy Observatory monitoring program. The spectral indices of our sources are mostly flat or inverted (-0:5 < α < 0), which is consistent with optically thick emission. Strong variability was seen in all light curves on various time scales. From the analyses of time lags between the light curves from different frequency bands and the evolution of the spectral indices with time, we find that we can distinguish high-peaking flares and lowpeaking flares according to the Valtaoja et al. classification. The periodograms (temporal power spectra) of the light curves are in good agreement with random-walk power-law noise without any indication of (quasi-)periodic variability. We note that random-walk noise light curves can originate from multiple shocks in jets. The fact that all our sources are in agreement with being random-walk noise emitters at radio wavelengths suggests that such behavior is a general property of blazars. We are going to generalize our approach by applying our methodology to a much larger blazar sample in the near future.

Nature of intranight optical variability of BL Lacertae

We present the results of extensive multi-band intra-night optical monitoring of BL Lacertae during 2010--2012. BL Lacertae was very active in this period and showed intense variability in almost all wavelengths. We extensively observed it for a total for 38 nights; on 26 of them observations were done quasi-simultaneously in B, V, R and I bands (totaling 113 light curves), with an average sampling interval of around 8 minutes. BL Lacertae showed significant variations on hour-like timescales in a total of 19 nights in different optical bands. We did not find any evidence for periodicities or characteristic variability time-scales in the light curves. The intranight variability amplitude is generally greater at higher frequencies and decreases as the source flux increases. We found spectral variations in BL Lacertae in the sense that the optical spectrum becomes flatter as the flux increases but in several flaring states deviates from the linear trend suggesting different jet components contributing to the emission at different times.

The broadband spectral energy distributions of SDSS blazars**Supported by the National Natural Science Foundation of China.

We compiled the radio, optical and X-ray data of blazars from the Sloan Digital Sky Survey database, and presented the distribution of luminosities and broadband spectral indices. The distribution of luminosities shows that the averaged luminosity of flat spectrum radio quasars (FSRQs) is larger than that of BL Lacertae (BL Lac) objects. On the other hand, the broadband spectral energy distribution reveals that FSRQs and low energy peaked BL Lac objects have similar spectral properties, but high energy peaked BL Lac objects have a distinct spectral property. This may be due to the fact that different subclasses of blazars have different intrinsic environments and are at different cooling levels. Even so, a unified scheme is also revealed from the color-color diagram, which hints that there are similar physical processes operating in all objects under a range of intrinsic physical conditions or beaming parameters.

Short timescale photometric and polarimetric behavior of two BL Lacertae type objects

Blazars are astrophysical sources whose emission is dominated by non-thermal processes, typically interpreted as synchrotron and inverse Compton emission. Although the general picture is rather robust and consistent with observations, many aspects are still unexplored. Polarimetric monitoring can offer a wealth of information about the physical processes in blazars. Models with largely different physical ingredients can often provide almost indistinguishable predictions for the total flux, but usually are characterized by markedly different polarization properties. We explore, with a pilot study, the possibility to derive structural information about the emitting regions of blazars by means of a joint analysis of rapid variability of the total and polarized flux at optical wavelengths. Short timescale (from tens of seconds to a couple of minutes) optical linear polarimetry and photometry for two blazars, BL Lacertae and PKS 1424+240, was carried out with the PAOLO polarimeter at the 3.6m Telescopio Nazionale Galileo. Several hours of almost continuous observations were obtained for both sources. Our intense monitoring allowed us to draw strongly different scenarios for BL Lacertae and PKS 1424+240, with the former characterized by intense variability on time-scales from hours to a few minutes and the latter practically constant in total flux. Essentially the same behavior is observed for the polarized flux and the position angle. The variability time-scales turned out to be as short as a few minutes, although involving only a few percent variation of the flux. The polarization variability time-scale is generally consistent with the total flux variability. Total and polarized flux appear to be essentially uncorrelated. However, even during our relatively short monitoring, different regimes can be singled out. (abridged)

Connection between inner jet kinematics and broadband flux variability in the BL Lacertae object S5 0716+714

We present a high-frequency very long baseline interferometry (VLBI) kinematical study of the BL Lac object S5 0716+714 over the time period of September 2008 to October 2010. The aim of the study is to investigate the relation of the jet kinematics to the observed broadband flux variability. We find significant non-radial motions in the jet outflow of the source. In the radial direction, the highest measured apparent speed is \sim37 c, which is exceptionally high, especially for a BL Lac object. Patterns in the jet flow reveal a roughly stationary feature \sim0.15 mas downstream of the core. The long-term fits to the component trajectories reveal acceleration in the sub-mas region of the jet. The measured brightness temperature, T_{B}, follows a continuous trend of decline with distance, T_B \propto r_{jet}^{-(2.36\pm0.41)}, which suggests a gradient in Doppler factor along the jet axis. Our analysis suggest that a moving disturbance (or a shock wave) from the base of the jet produces the high-energy (optical to \gamma-ray) variations upstream of the 7 mm core, and then later causes an outburst in the core. Repetitive optical/\gamma-ray flares and the curved trajectories of the associated components suggest that the shock front propagates along a bent trajectory or helical path. Sharper \gamma-ray flares could be related to the passage of moving disturbances through the stationary feature. Our analysis suggests that the \gamma-ray and radio emission regions have different Doppler factors.

Multiband optical variability studies of BL Lacertae

We monitored BL Lacertae for 13 nights in optical B, V, R, and I bands during 2014 October and November including quasi-simultaneous observations in V and R bands using two optical telescopes in India. We have studied multiband optical flux variations, colour variation and spectral changes in this blazar. Source was found to be active during the whole monitoring period and showed significant intraday variability on three nights in V and R filters while displayed hints of variability on six other dates in R passband and on two nights in V filter. From the colour–magnitude analysis of the source, we found that the spectra of the target get flatter as it becomes brighter on intranight time-scale. Using discrete correlation technique, we found that intraday light curves in both V and R filters are almost consistent and well correlated with each other. We also generated spectral energy distribution of the target using the B, V, R, and I data sets for all 13 nights which could help us investigate the physical process responsible for the observed variations in BL Lacertae objects. We also discuss possible physical causes of the observed spectral variability.

Is BZB J1450+5201 the most distant gamma-ray BL Lacertae object?**Supported by the National Natural Science Foundation of China.

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 J1451.0+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.

Longterm optical monitoring of bright BL Lacertae objects with ATOM: Spectral variability and multiwavelength correlations

Blazars are the established sources of an intense and variable non-thermal radiation extending from radio wavelengths up to HE and VHE gamma-rays. Understanding the spectral evolution of blazars in selected frequency ranges, as well as multi-frequency correlations in various types of blazar sources, is of a primary importance for constraining the blazar physics. Here we present the results of a long-term optical monitoring of a sample of 30 blazars of the BL Lac type. We study the optical color-magnitude correlation patterns emerging in the analyzed sample, and compare the optical properties of the targets with the high-energy gamma-ray and high-frequency radio data. The optical observations were carried out in R and B filters using ATOM telescope. Each object was observed during at least 20 nights in the period 2007-2012. We find significant global color-magnitude correlations in 40 % of the sample. The sources which do not display any clear chromatism often do exhibit bluer-when-brighter (bwb) behavior but only in isolated shorter time intervals. We also discovered spectral state transitions at optical wavelengths in several sources. Finally, we find that the radio, optical, and gamma-ray luminosities of the sources obey almost linear correlations, which seem however induced, at least partly, by the redshift dependance, and may be also affected by non-simultaneousness of the analyzed dataset. We argue that the observed bwb behavior is intrinsic to the jet emission regions, at least for some of the analyzed blazars, rather than resulting from the contamination of the measured flux by the starlight of host galaxies. We also conclude that the significance of color-magnitude scalings does not correlate with the optical color, but instead seems to depend on the source luminosity, in a sense that these are the lowest-luminosity BL Lac objects which display the strongest correlations.

QUANTITATIVE EVALUATION OF THE HYPOTHESIS THAT BL LACERTAE OBJECTS ARE QSO REMNANTS

We evaluate with numerical simulations the hypothesis that BL Lacertae objects (BLLs) are the remnants of quasi-stellar objects. This hypothesis is based on their highly peculiar redshift evolution. They have a comoving space density that increases with decreasing redshift, contrary to all other active galactic nuclei. We assume that relativistic jets are below detection in young radio-quiet quasars and increase in strength with cosmic time so that they eventually are detected as BLLs. Our numerical simulations fit very well the observed redshift distributions of BLLs. There are strong indications that only the high-synchrotron-peaked BLLs could be QSO remnants.

Fermi-LAT γ-ray anisotropy and intensity explained by unresolved radio-loud active galactic nuclei

Radio-loud active galactic nuclei (AGN) are expected to contribute substantially to both the intensity and anisotropy of the isotropic γ-ray background (IGRB). In turn, the measured properties of the IGRB can be used to constrain the characteristics of proposed contributing source classes. We consider individual subclasses of radio-loud AGN, including low-, intermediate-, and high-synchrotron-peaked BL Lacertae objects, flat-spectrum radio quasars, and misaligned AGN. Using updated models of the γ-ray luminosity functions of these populations, we evaluate the energy-dependent contribution of each source class to the intensity and anisotropy of the IGRB. We find that collectively radio-loud AGN can account for the entirety of the IGRB intensity and anisotropy as measured by the Fermi Large Area Telescope (LAT). Misaligned AGN provide the bulk of the measured intensity but a negligible contribution to the anisotropy, while high-synchrotron-peaked BL Lacertae objects provide the dominant contribution to the anisotropy. In anticipation of upcoming measurements with the Fermi-LAT and the forthcoming Cherenkov Telescope Array, we predict the anisotropy in the broader energy range that will be accessible to future observations.

SIX-YEAR OPTICAL MONITORING OF THE BL LACERTAE OBJECT 1ES 0806+52.4

We present the results of the first systematic long-term multi-color optical monitoring of the BL Lacertae object 1ES 0806+52.4. The monitoring was performed in multiple passbands with a 60/90 cm Schmidt telescope from December 2005 to February 2011. The overall brightness of this object decreased from 2005 December to 2008 December, and regained after that. A sharp outburst probably occurred around the end of our monitoring program. Overlapped on the long-term trend are some short-term small-amplitude oscillations. No intra-night variability was found in the object, which is in accord with the historical observations before 2005. By investigating the color behavior, we found strong bluer-when-brighter chromatism for the long-term variability of 1ES 0806+52.4. The total amplitudes at the c, i and o bands are 1.18, 1.12, and 1.02 mags, respectively. The amplitudes tend to increase toward shorter wavelength, which may be the major cause of bluer-when-brighter. Such bluer-when-brighter is also found in other blazars like S5 0716+714, OJ 287, etc. The hard X-ray data collected from the Swift/BAT archive was correlated with our optical data. No positive result was found, the reason of which may be that the hard X-ray flux is a combination of the synchrotron and inverse Compton emission but with different timescales and cadences under the leptonic Synchrotron-Self-Compton (SSC) model.

ANHST/COS OBSERVATION OF BROAD Lyα EMISSION AND ASSOCIATED ABSORPTION LINES OF THE BL LACERTAE OBJECT H 2356-309

Weak spectral features in BL Lacertae objects (BL Lac) often provide a unique opportunity to probe the inner region of this rare type of active galactic nucleus. We present a Hubble Space Telescope/Cosmic Origins Spectrograph observation of the BL Lac H 2356-309. A weak Ly$\alpha$ emission line was detected. This is the fourth detection of a weak Ly$\alpha$ emission feature in the ultraviolet (UV) band in the so-called "high energy peaked BL Lacs", after Stocke et al. Assuming the line-emitting gas is located in the broad line region (BLR) and the ionizing source is the off-axis jet emission, we constrain the Lorentz factor ($\Gamma$) of the relativistic jet to be $\geq 8.1$ with a maximum viewing angle of 3.6$^\circ$. The derived $\Gamma$ is somewhat larger than previous measurements of $\Gamma \approx 3 - 5$, implying a covering factor of $\sim$ 3% of the line-emitting gas. Alternatively, the BLR clouds could be optically thin, in which case we constrain the BLR warm gas to be $\sim 10^{-5}\rm\ M_{\odot}$. We also detected two HI and one OVI absorption lines that are within $|\Delta v| < 150\rm\ km\ s^{-1}$ of the BL Lac object. The OVI and one of the HI absorbers likely coexist due to their nearly identical velocities. We discuss several ionization models and find a photoionization model where the ionizing photon source is the BL Lac object can fit the observed ion column densities with reasonable physical parameters. This absorber can either be located in the interstellar medium of the host galaxy, or in the BLR.

Long-term optical and radio variability of BL Lacertae

Well-sampled optical and radio light curves of BL Lacertae in B,V,R,I bands and 4.8, 8.0, 14.5GHz from 1968 to 2014 were presented in this paper. A possible 1.26±0.05yr period in optical bands and a 7.50±0.15yr period in radio bands were detected based on discrete correlation function, structure function as well as Jurkevich method. Correlations among different bands were also analyzed and no reliable time delay was found between optical bands. Very weak correlations were detected between V band and radio bands. However, in radio bands the variation at low frequency lagged that at high frequency obviously. The spectrum of BL Lacertae turned mildly bluer when the object turned brighter, and stronger bluer-when-brighter trends were found for short flares. A scenario including a precessing helical jet and periodic shocks was put forward to interpret the variation characteristics of BL Lacertae.