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We have monitored the BL Lac object Mrk 501 in the optical V, R, and I bands from 2010 to 2015. For Mrk 501, the presence of a strong host galaxy component can affect the results of photometry. After subtracting the host galaxy contributions, the source shows intraday and long-term variabilities for optical flux and color indices. The average variability amplitudes of the V, R, and I bands are 22.05%, 22.25%, and 23.82%, respectively, and the value of the duty cycle is 14.87%. A minimal variability timescale of 106 minutes is detected. No significant time lag between the V and I bands is found on one night. The bluer-when-brighter (BWB) trend is dominant for Mrk 501 on intermediate, short, and intraday timescales, which supports the shock-in-jet model. For the long timescale, Mrk 501, in different states, can have different BWB trends. The corresponding results for non-correcting host galaxy contributions are also presented.

We present a detailed analysis on the spectral lags of the short gamma‐ray bursts (GRBs) and compare them with that of the long GRBs by using the CGRO (Compton Gamma‐ray Observatory)/BATSE GRB catalogue. Our sample includes 308 short GRBs and 1008 long GRBs. The light curves of these GRBs are in 64‐ms time bin and they have at least one long and intense pulse, which satisfies δT≥ 0.512 s at c= 1σ and cmax≥ 6σ, where δT is the pulse duration, c is the photon counts and σ is the standard error of the background. We calculate the cross‐correlation function (CCF) for the light curves in 25–55 and 110–300 keV bands and derive the spectral lag by fitting the CCF with the Gaussian model. Our results are as follows. (i) The spectral lag distribution of the short GRBs is significantly different from that of the long GRBs. Excluding the statistical fluctuation effect, a proportion of ∼17 per cent of the short GRBs has a negative spectral lag, i.e. the hard photons are being lag behind the soft photons. We do not find any peculiar features from their light curves to distinguish these bursts from those with a positive spectral lag. We argue that a more physical mechanism dominated the hard lag may be hid behind the morphological features of the light curves. This should be a great challenge to the current GRB models. We note that this proportion is consistent with the proportion of short GRBs correlated with nearby galaxies newly discovered by Tanvir et al., although it is unclear if these short GRBs are indeed associated with the sources originated at low redshift. (ii) While the spectral lags of the long GRBs are strongly correlated with the pulse durations, they are not for the short GRBs. However, the ratios of the spectral lag to the pulse duration for the short and long GRBs are normal distributions at 0.023 and 0.046, respectively, with the sample width, indicating that the curvature effect alone could not explain the difference of the spectral lags between the two types of GRBs. The hydrodynamic time‐scales of the outflows and the radiative processes at work in GRBs might also play an important role as suggested by Daigne and Mochkovitch.

Recently, Shen et al. have studied the contributions of the curvature effect of fireballs to the spectral lag and have shown that the observed lags can be accounted for by the effect. Here, we check their results by performing a more precise calculation with both formulae presented by Shen et al. and Qin et al. Several other aspects which were not considered by Shen et al. are investigated. We find that in the case of ultrarelativistic motions, both formulae are identical as long as the whole fireball surface is concerned. In our analysis, the previous conclusion that the detected spectral lags can be accounted for by the curvature effect is confirmed, while the conclusion that the lag has no dependence on the radius of fireballs is not true. We find that introducing extreme physical parameters is not the only outlet to explain these observed large lags. Even for the larger lags (∼5 s), a wider local pulse (Δtθ,FWHM= 107 s) can account for it. Some conclusions not presented in Shen et al. or those modified in our analysis are listed below: (i) lag ∝Γ−ε with ε > 2; (ii) lag is proportional to the local pulse width and the full width at half‐maximum of the observed light curves; (iii) a large lag requires a large α0 and a small β0 as well as a large E0,p; (iv) when the rest‐frame spectrum varies with time, the lag would become larger; (v) lag decreases with the increase of Rc; (vi) lag ∝E within the certain energy range for a given Lorentz factor; (vii) lag is proportional to the opening angle of uniform jets when θj < 0.6Γ−1.

The γ-ray emission properties of CTD 135, a typical compact symmetric object (CSO), are investigated with ∼11-year Fermi/LAT observations. We show that it has bright and significantly variable GeV emission, with the γ-ray luminosity of Lγ ∼ 1047 erg s−1 and a variation index of TSvar = 1002. A quasi-periodic oscillation (QPO) with a periodicity of ∼460 days is detected in the global 95% false-alarm level. These γ-ray emission features are similar to that of blazars. Its broadband spectral energy distribution (SED) can be attributed to the radiations of the relativistic electrons accelerated in the core region and the extended region. The SED modeling shows that the γ-rays are from the core region, which has a Doppler boosting factor of δ ∼ 10.8 and relativistically moves with a small viewing angle, being similar to blazar jets. On the base of the analysis results, we propose that the episodic activity of the central engine in CTD 135 results in a blazar-like jet and the bubble-like lobes as the Fermi bubbles in the Galaxy. The strong γ-ray emission with obvious variability is from the jet radiations and the symmetric radio structure is attributed to the bubbles. The jet radiation power and disk luminosity in units of Eddington luminosity of CTD 135 follow the same relation as other young radio sources, indicating that its jet radiation may also be driven by the Eddington ratio.

Compact steep-spectrum sources (CSSs) likely represent a population of young radio-loud active galactic nuclei (AGNs) and have been identified as γ-ray-emitting sources. We present a comprehensive analysis of their γ-ray emission observed with Fermi/LAT and establish their broadband spectral energy distributions (SEDs). We derive their jet properties using SED fits with a two-zone leptonic model for radiation from the compact core and the large-scale extended region, and explore the possible signature of a unification picture of jet radiation among subclasses of AGNs. We show that the observed γ-rays of CSSs with significant variability are contributed by the radiation of their compact cores via the inverse-Compton process of the torus photons. The derived power-law distribution index of the radiating electrons is p 1 ∼ 1.5–1.8, magnetic field strength is B ∼ 0.15–0.6 G, and Doppler-boosting factor is δ ∼ 2.8–8.9. Assuming that the jet is composed of e ± pairs, the compact cores of CSSs are magnetized and have a high radiation efficiency, similar to that of flat-spectrum radio quasars. The γ-ray-emitting CSSs on average have higher Eddington ratio and black hole mass than those non-GeV-detected CSSs, and they follow the correlation between the jet power in units of Eddington luminosity ( ) and Eddington ratio (R Edd) with other subclasses of AGNs, , indicating that R Edd would be a key physical driver for the unification scheme of AGN jet radiation.

We have analyzed the radio light curves of PKS 1510-089 at 37 and 22 GHz from 1990 to 2005 taken from the database of the Metsähovi Radio Observatory, and find evidence of quasi-periodic outbursts. The light curves show great activity with very complicated non-sinusoidal variations. Using Jurkervich's method, the power spectrum method, and the discrete autocorrelation function to analyze these data, we have found two periods of p 1 = 0.92 ± 0.04 yr and p 2 = 1.82 ± 0.12 yr for the outbursts in PKS 1510-089. It is interesting to note that the results for two frequencies and three methods are almost the same and p 2 ≈ 2p 1 . In addition, these results are in good agreement with the periodic deep flux minima of 1.84 ± 0.1 yr (half period ∼ 0.92 ± 0.03 yr) observed by us and other authors in the optical band in 2002, 2004, and 2005.

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