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The third Fermi source catalog lists 3033 γ\\(\\gamma \\)-ray sources above 4σ\\(4\\sigma \\) significance. More than 30% are classified as either unidentified/unassociated Gamma-ray sources (UGSs), with about 20% classified as Blazar candidates of uncertain types (BCUs). To confirm the blazar-like nature of candidate counterparts of UGSs and BCUs, we started in 2012 an optical spectroscopic follow up campaign. We report here the spectra of 36 targets with observations from the Observatorio Astronómico Nacional San Pedro Mártir, the Southern Astrophysical Research Observatory, the Kitt Peak National Observatory and the Northern Optical Telescope, between 2016 and 2017. We confirm the BL Lac nature of 23 sources, and the flat spectrum radio quasar nature of other 7 ones. We also provide redshift estimates for 19 out of these 30 confirmations, with only one being a lower limit due to spectral features ascribable to intervening systems along the line of sight. As in previous analyses, the largest fraction of now-classified BCUs belong to the class of BL Lac objects, that appear to be the most elusive class of active galactic nuclei. One of the BL Lacs identified in this work, associated with 3FGL J2213.6-4755, lies at a redshift of z>1.529\\(z>1.529\\), making it one of the few distant gamma-ray BL Lac objects.

The fourth Fermi Large Area Telescope Source Catalog (4FGL) lists over 5000 γ-ray sources with statistical significance above 4σ. About 23% of the sources listed in this catalog are unidentified/unassociated γ-ray sources while ∼26% of the sources are classified as blazar candidates of uncertain type (BCUs), lacking optical spectroscopic information. To probe the blazar nature of candidate counterparts of UGSs and BCUs, we started our optical spectroscopic follow up campaign in 2012, which up to date account for more than 350 observed sources. In this paper, the tenth of our campaign, we report on the spectroscopic observations of 37 sources, mostly BCUs, whose observations were carried out predominantly at the Observatorio Astronómico Nacional San Pedro Mártir and the Southern Astrophysical Research Observatory between August 2018 and September 2019. We confirm the BL Lac nature of 27 sources and the flat spectrum radio quasar nature of three sources. The remaining ones are classified as six BL Lacs galaxy-dominated and one normal galaxy. We were also able to measure the redshifts for 20 sources, including 10 BL Lacs. As in previous analyses, the largest fraction of BCUs revealed to be BL Lac objects.

The well-sampled optical light curves of S5 0716+714 in BVRI bands spanning about 20 years were built. The variability asymmetry could provide independent constraints on physical models for blazars variability. To perform analyses of variability properties, individual flares were decomposed and fitted from the light curves with functions of exponential rise and decay. In order to investigate whether the short term variability (STV) and intraday variability (IDV) had different mechanisms of origin, the light curves of S5 0716+714 with different timescales were analyzed, respectively. Results indicate that both STV and IDV flares of S5 0716+714 are predominantly asymmetric profiles. And there also exist a very weak tendency of positive asymmetry in both timescales variabilities, i.e., rapid rises and gradual decays. The estimated sizes of emission region are 2.4 × 10 16 ∼ 3.3 × 10 18 cm and 6.6 × 10 14 ∼ 4.8 × 10 16 cm for STV and IDV, respectively. Based on the individual flares decomposed from STV and IDV light curves, the distribution of the peak flux and duration time of flares are consistent with log-normal and Gamma distributions. Minijet-in-a-jet model could interpret the log-normal distribution. Gamma distribution could demonstrate that the emission regions in the jet may be distributed in a Poisson way.

We report on the optical observations and analysis of the high-energy peaked BL Lac object (HBL), Mrk 501, at redshift z = 0.033. We can confirm microvariable behavior over the course of minutes on several occasions per night. As an alternative to the commonly understood dynamical model of random variations in intensity of the AGN, we develop a relativistic beaming model with a minimum of free parameters, which allows us to infer changes in the line of sight angles for the motion of the different relativistic components. We hope our methods can be used in future studies of beamed emission in other active microvariable sources, similar to the one we explored.

Nearly one third of the sources in the Fermi-LAT catalogs lacks a lower energy counterpart, hence being referred as unidentified/unassociated gamma-ray sources (UGSs). In order to firmly classify them, dedicated multifrequency follow-up campaigns are necessary. These will permit to unveil their nature and identify the fraction that could belong to the class of active galaxies known as blazars that is the largest population of extragalactic γ\\(\\gamma \\)-ray sources. In Fermi-LAT catalogs there are also gamma-ray sources associated with multifrequency blazar-like objects known as Blazars Candidates of Uncertain type (i.e., BCUs) for which follow up spectroscopic campaigns are mandatory to confirm their blazar nature. Thus, in 2013 we started an optical spectroscopic campaign to identify blazar-like objects potential counterparts of UGSs and BCUs. Here we report the spectra of 31 additional targets observed as part of our follow up campaign. Thirteen of them are BCUs for which we acquired spectroscopic observations at Observatorio Astrofísico Guillermo Haro (OAGH) and at Southern Astrophysical Research Observatory (SOAR) telescopes, while the rest has been identified thanks to the archival observations available from the Sloan Digital Sky Survey (SDSS). We confirm the blazar nature of all BCUs: three of them are in blazar of quasar type (BZQs) while the remaining ones can be spectroscopically classified as BL Lac objects (BZBs). Then we also discovered 18 BL Lac objects lying within the positional uncertainty regions of UGSs that could be their potential counterparts.

We report on seven nights of optical observation taken over a two-week period, and the resultant analysis of the intermediate-frequency peaked BL Lac object (IBL), BL Lac itself, at redshift z = 0.069. The microvariable behavior can be confirmed over the course of minutes for each night. A relativistic beaming model was used in our analysis, to infer changes to the line of sight angles for the motion of the different relativistic components. This model has very few free parameters. The light curves we generated show both high and moderate frequency cadence to the variable behavior of BL Lac itself, in addition to the well documented long-term variability.

We report the detection of a quasi-periodic oscillation (QPO) in the optical R-band light curve of the blazar S5 0716+714 spanning approximately 32 years. Using both the Weighted Wavelet Z-transform and Lomb-Scargle Periodogram methods, we identified a prominent QPO of 1060±50 days at a 5σ confidence level against the red noise background. Our findings align well with previous studies. We introduce a supermassive binary black hole (SMBBH) model to account for the observed QPO and estimate the binary system’s Keplerian orbit to be 0.0053 parsec, indicating S5 0716+714 as a notable subject for upcoming nanohertz gravitational wave studies.

Extragalactic very-high-energy (VHE; E > 100 GeV) sources are unique objects to study the most powerful particle accelerators in nature, as active galactic nuclei are likely sources of ultra-high-energy cosmic rays. BL Lacertae blazars are the most frequent extragalactic objects found in the VHE gamma-ray catalogs. It is very difficult to estimate their redshifts, considering they have no strong enough optical spectral features, hence ∼20% of them have unknown or poorly constrained redshifts. KUV 00311-1938 is a VHE BL Lacertae blazar, with an uncertain redshift in the range of 0.5 < z < 0.98. We have obtained deep spectroscopy using Gemini and the GTC telescopes of KUV 00311-1938 and its surroundings with high signal-to-noise ratio in 2016-2017. The lack of features did not allow us to determine the spectroscopic redshift of KUV 00311-1938; nevertheless, we obtain a lower limit of z 0.475. We determined the redshifts of 41 galaxies observed in the field-of-view of the blazar and through a population study, identified three pairs and four groups consisting of 3 or 4 members with redshifts in the range z = 0.1468-0.4756. Due to the absence of a large group of galaxies, we could not associate KUV 00311-1938 with any of the groups detected.

Since 1978, when BL Lac objects and violently variable quasars were married to become “blazars”, physical interpretations of the phenomenon have evolved. Remarkably, though, the general picture of relativistic jets beaming their radiation in our direction, proposed that year by Blandford and Rees, remains intact. The main stress on theoretical models has come from observations that reveal ever more extreme variability requiring rampant particle acceleration on time-scales of minutes, often parsecs away from the central black hole. Here the author reviews many of the observations and theoretical ideas that have shaped his studies of blazars over about 40 years. This leads to his preferred scenario that blazar jets contain a helical magnetic field close to the black hole, turbulent plasma on parsecs scales, and both standing and moving shock waves. Particle acceleration can then occur in multiple stages involving the second-order Fermi process, magnetic reconnections, and modest jumps in energy at shock fronts. The most extreme variability, as well as brightness temperatures ~100 times the inverse Compton limit, probably require occasional exceptionally high bulk Lorentz factors. These can result, for example, from supersonic, relativistic turbulence, or ultra-relativistic flows propelled from sites of magnetic reconnection. Future efforts in these and other areas can determine whether these potential solutions are valid.

Mrk 501 is one of our long-term photometric monitoring objects with the optical telescopes at Yunnan Astronomical Observatory (YAO). In this work, observations were performed from 2010 March 11 to 2017 June 7 in the BVRI bands. We detected minute-scale rapid variabilities in the optical multi-band light curves. Durations of these rapid variabilities are from 5 to 77 minutes. The minimal decay timescale is about 3 minutes (2.330 1.572 minutes on 2012 May 22 and 3.172 1.626 minutes on 2017 May 20). After subtracting the host-galaxy component, The minimal decay timescale was about 8 minutes on 2017 June 3, and the corresponding duration is about 10 minutes. This fast variability requires the strength of the co-moving magnetic field in the jet larger than 38.60 G (19.69 G for corrected results, with Doppler factor δD ∼ 3). The discrete correlation function (DCF) analysis between B and I band short-term light curves show significant correlated variability; however, no significant time lags are detected. The color-magnitude variation shows significant negative correlation on intranight timescales, which is due to the superposition of multiple components.