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 CGR

Long-duration gamma-ray bursts (GRBs) associated with supernovae (SNe) are believed to originate from massive star core-collapse events, whereas short-duration GRBs that are related to compact star mergers are expected to be accompanied by kilonovae. GRB 211227A, which lasted about 84 s, had an initial short/hard spike followed by a series of soft gamma-ray extended emission at redshift z = 0.228. We performed follow-up observations of the optical emission using BOOTES, LCOGT, and the Lijiang 2.4 m telescope, but we detected no associated supernova signature, even down to very stringent limits at such a low redshift. We observed the host galaxy within a large error circle and roughly estimated the physical offset of GRB 211227A as 20.47 ± 14.47 kpc from the galaxy center. These properties are similar to those of GRB 060614, and suggest that the progenitor of GRB 211227A is not favored to be associated with the death of massive stars. Hence, we propose that GRB 211227A originates from a compact star merger. Calculating pseudo-kilonova emission for this case by adopting the typical parameters, we find that any associated pseudo-kilonova is too faint to be detected. If this is the case, it explains naturally the characteristics of the prompt emission, the lack of SN and kilonova emission, and the large physical offset from the galaxy center.

We have monitored the quasar 3C 273 in the optical V, R, and I bands from 2005 to 2016. Intraday variability (IDV) is detected on seven nights. The variability amplitudes on most of the nights are less than 10%, and on four nights, more than 20%. When considering the nights with time spans >4 hr, the duty cycle (DC) is 14.17%. Over the 12 years, the overall magnitude and color index variabilities are , , , and , respectively. The largest clear IDV has an amplitude of 42% over just 5.8 minutes, and the weakest detected IDV is 5.4% over 175 minutes. The BWB (bluer when brighter) chromatic trend is dominant for 3C 273 and appears at different flux levels on intraday timescales. The BWB trend exists for short-term timescales and intermediate-term timescales but different timescales have different correlations. There is no BWB trend for our whole time-series data sets. A significant anticorrelation between the BWB trend and length of timescales is found. Combining with V-band data from previous works, we find a possible quasi-periodicity of P = 3918 ± 1112 days. The possible explanations for the observed variability, BWB chromatic trend, and periodicity are discussed.

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 (Δt_θ,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 ∝G^−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 E_0,p; (iv) when the rest-frame spectrum varies with time, the lag would become larger; (v) lag decreases with the increase of R_c; (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 θ_j < 0.6G⁻¹.

We analyze the quasiperiodic oscillation (QPO) of the historical light curve of flat-spectrum radio quasars PKS 0405-385 detected by the Fermi Large Area Telescope from 2008 August to 2021 November. To identify and determine the QPO signal of PKS 0405-385 in the γ-ray light curve, we use four time series analysis techniques based on frequency and time domains, i.e., the Lomb–Scargle periodogram (LSP), the weighted wavelet z-transform (WWZ), the REDFIT, and the epoch folding. The results show that PKS 0405-385 has a quasiperiodic behavior of ∼2.8 yr with the significance of ∼4.3σ in Fermi long-term monitoring. Remarkably, we also performed QPO analysis in the G-band light curve observed from 2014 October to 2021 October using LSP and WWZ technology, and the results (∼4σ of significance) are consistent with the periodic detection in γ-ray. This may imply that the optical emission is radiated by an electron population in the same way as the γ-ray emission. In discussing the possible mechanism of quasiperiodic behavior, either the helical motion within a jet or the supermassive black hole binary system provides a viable explanation for the QPO of 2.8 yr, and the relevant parameters have been estimated.

We have assembled the historical variability data of Mrk 421 at radio 15 GHz, X-ray, and γ-ray bands, spanning about 6.3, 10.3, and 7.5 yr, respectively. We analyzed the variability by using three methods. The results indicated that there is a period of 287.6 ± 4.4 days for 15 GHz, 309.5 ± 5.8 days for X-ray and 283.4 ± 4.7 days for γ-ray, respectively. This period can be reasonably explained by the nonballistic helical motion of the emitting material. The correlation analysis suggested that the variabilities of radio 15 GHz, X-ray, and γ-ray are remarkably correlated, and the emission of radio 15 GHz lags behind that of X-ray, and the X-ray flux lags behind the γ-ray. This suggests that the γ-ray is derived from inverse-Compton (IC) scattering of the synchrotron photons, supporting the synchrotron self-Compton (SSC) model. Moreover, the time delay between different wavebands could be explained by the shock-in-jet models, in which a moving emission region produces the radio to γ-ray activity, implying that the emission region of γ-ray is closer to the center than ones of X-ray and radio emission.