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We continuously monitored the blazar S5 0716+714 in the optical $g$, $r$ and $i$ bands from Nov. 10, 2017 to Jun. 06, 2019. The total number of observations is 201 nights including 26973 data points. This is a very large quasi-simultaneous multicolor sample for the blazar. The average time spans and time resolutions are 3.4 hours and 2.9 minutes per night, respectively. During the period of observations, the target source in the $r$ band brightens from $14^{\rm m}.16$ to $12^{\rm m}.29$ together with five prominent sub-flares, and then first becomes fainter to $14^{\rm m}.76$ and again brightens to $12^{\rm m}.94$ with seven prominent sub-flares. For the long-term variations, we find a strong flatter when brighter (FWB) trend at a low flux state and then a weak FWB trend at a higher flux state. A weak FWB trend at a low flux state and then a strong FWB trend at a higher flux state are also reported. Most of sub-flares show the strong FWB trends, except for two flares with a weak FWB trend. The particle acceleration and cooling mechanisms together with the superposition of different FWB-slopes from sub-flares are likely to explain the optical color behaviours. A scenario of bent jet is discussed., Comment: 27 pages, 9 figures, 4 tables, accepted for publication in ApJS

The production site of gamma-rays in blazar jet is an unresolved problem. We present a method to locate gamma-ray emission region in the framework of one-zone emission model. From measurements of core-shift effect, the relation between the magnetic field strengths ($B'$) in the radio cores of jet and the distances ($R$) of these radio cores from central supermassive black hole (SMBH) can be inferred. Therefore once the magnetic field strength in gamma-ray emission region ($B'_{\rm diss}$) is obtained, one can use the relation of $B'$-$R$ to derive the distance ($R_{\rm diss}$) of gamma-ray emission region from SMBH. Here we evaluate the lower limit of $B'_{\rm diss}$ by using the criteria that the optical variability timescale $t_{\rm var}$ should be longer or equal to the synchrotron radiation cooling timescale of the electrons that emit optical photons. We test the method with the observations of PSK 1510-089 and BL Lacertae, and derive $R_{\rm diss}<0.15\delta_{\rm D}^{1/3}(1+A)^{2/3}\ $pc for PSK 1510-089 with $t_{\rm var}\sim$ a few hours, and $R_{\rm diss}<0.003\delta_{\rm D}^{1/3}(1+A)^{2/3}\ $pc for BL Lacertae with $t_{\rm var}\sim$ a few minutes. Here $\delta_{\rm D} $ is the Doppler factor and $A$ is the Compton dominance (i.e., the ratio of the Compton to the synchrotron peak luminosities)., Comment: 5 pages, 0 figures, accepted by ApJ

We model simultaneous or quasi-simultaneous multi-band spectral energy distributions (SEDs) for a sample of 25 blazars that have radio core-shift measurements, where a one-zone leptonic model and Markov chain Monte Carlo technique are adopted. In the SED fitting for 23 low-synchrotron-peaked (LSP) blazars, the seed photons from broad line region (BLR) and molecular torus are considered respectively in the external Compton process. We find that the SED fitting with the seed photons from the torus are better than those utilizing BLR photons, which suggests that the $\gamma$-ray emitting region may be located outside the BLR. Assuming the magnetic field strength in the $\gamma$-ray emitting region as constrained from the SED fitting follows the magnetic field distribution as derived from the radio core-shift measurements (i.e., $B(R)\simeq B_{\rm 1 pc}(R/\rm 1pc)^{-1}$, $R$ is the distance from the central engine and $B_{\rm 1 pc}$ is the magnetic field strength at 1 pc), we further calculate the location of the $\gamma$-ray emitting region, $R_{\gamma}$, for these blazars. We find that $R_{\gamma}\sim2\times10^{4}R_{\rm S}\simeq10 R_{\rm BLR}$ ($R_{\rm S}$ is the Schwarzschild radius and $R_{\rm BLR}$ is the BLR size), where $R_{\rm BLR}$ is estimated from the broad line luminosities using the empirical correlations obtained using the reverberation mapping methods., Comment: 20 pages, 10 Figures, ApJ, in press

NGC 4993, as a host galaxy of the electromagnetic counterpart of the first gravitational-wave detection of a binary neutron-star merger, was observed by many powerful telescopes from radio to $\gamma$-ray waveband. The weak nuclear activities of NGC 4993 suggest that it is a low-luminosity active galactic nuclear (LLAGN). We build the multi-waveband spectral energy distributions (SEDs) of NGC 4993 from literatures. We find that the radio spectrum at $\sim 100-300$ GHz is much steeper than that of low-frequency waveband (e.g., 6-100 GHz), where this break was also found in the supermassive black holes in our galaxy center (Sgr A*), and in some other nearby AGNs. The radio emission above and below this break may has different physical origins, which provide an opportunity to probe the accretion and jet properties. We model the multi-waveband SEDs of NGC 4993 with an advection-dominated accretion flow (ADAF)-jet model. We find that the high-frequency steep radio emission at millimeter waveband is consistent with the prediction of the ADAF, while the low-frequency flat radio spectrum is better fitted by the jet. Further more, the X-ray emission can be also explained by the ADAF model simultaneously. From the model fits, we estimate important parameters of the central engine (e.g., accretion rate near the horizon of the black hole and mass-loss rate in jet) for NGC 4993. This result strengthen that the millimiter, submillimer and deep X-ray observations are crucial to understand the weak or quiescent activities in supermassive black-hole systems. Further simultaneous millimeter and X-ray monitoring of this kind of LLAGNs will help us to better understand the physical origin of multiwaveband emission., Comment: 5 pages, 1 figure, ApJ in press