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<jats:title>Abstract</jats:title> <jats:p>Reverberation mapping (RM) is one of the most efficient ways to investigate the broad-line region around the central supermassive black holes of active galactic nuclei (AGNs). A common way of performing the RM is to perform a long term spectroscopic monitoring of AGNs, but the spectroscopic monitoring campaign of a large number of AGNs requires an extensive amount of observing time of medium to large size telescopes. As an alternative way, we present the results of photometric RM with medium-band photometry. As the widths of medium-band filters match well with the widths of AGN broad emission lines, the medium-band observation with small telescopes can be a cost-effective way to perform RM. We monitored five nearby AGNs with available spectroscopic RM results showing days to weeks scale variability. Observations were performed for ∼3 months with an average of 3 days cadence using three medium-band filters on a 0.43 m telescope. The time lags between the continuum and the H<jats:italic>α</jats:italic> emission line light curves are calculated using the JAVELIN software and the discrete correlation function. We find time lags of 1.5–15.9 days for these AGNs, which are consistent with the time lags derived from previous spectroscopic RM measurements. This result demonstrates that even a 0.5 m class telescope can perform RM with medium-bands. Furthermore, we show that RM for tens of thousands AGNs is possible with a dedicated 1 m class telescope.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The characteristics of the spectral evolution of the prompt emission of gamma-ray bursts (GRBs), which are closely related to the radiation mechanism (synchrotron or photosphere), are still an unsolved subject. Here, by performing the detailed time-resolved spectral fitting of GRB 131231A, which has a very bright and well-defined single pulse, some interesting spectral evolution features have been found. (i) Both the low-energy spectral index <jats:italic>α</jats:italic> and the peak energy <jats:italic>E</jats:italic> <jats:sub>p</jats:sub> exhibit the “flux-tracking” pattern (“double-tracking” characteristics). (ii) The parameter relations, i.e., <jats:italic>F</jats:italic> (the energy flux)-<jats:italic>α</jats:italic>, <jats:italic>F</jats:italic>–<jats:italic>E</jats:italic> <jats:sub>p</jats:sub>, and <jats:italic>E</jats:italic> <jats:sub>p</jats:sub>–<jats:italic>α</jats:italic>, along with the analogous Yonetoku <jats:italic>E</jats:italic> <jats:sub>p</jats:sub>–<jats:italic>L</jats:italic> <jats:sub> <jats:italic>γ</jats:italic>,iso</jats:sub> relation for the different time-resolved spectra, show strong monotonous (positive) correlations, both in the rising and the decaying phases. (iii) The values of <jats:italic>α</jats:italic> do not exceed the synchrotron limit (<jats:italic>α</jats:italic> = −2/3) in all slices across the pulse, favoring the synchrotron origin. We argue that the one-zone synchrotron emission model with the emitter streaming away at a large distance from the central engine can explain all of these special spectral evolution characteristics.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Recently, very high-energy photons above 100 GeV were reported to be detected from GRB 190114C and GRB 180720B at, respectively, 100–1000 s and 10 hr after the burst. We model the available broadband data of both GRBs with the synchrotron plus synchrotron self-Compton (SSC) emission of the afterglow shocks. We find that the sub-TeV emission of GRB 180720B can be interpreted as the SSC emission from afterglow shocks expanding in a constant-density circumburst medium. The SSC emission of GRB 190114C dominates over the synchrotron component from GeV energies at ∼100 s, which can explain the possible hard spectrum of the GeV emission at this time. The extrapolated flux of this SSC component to sub-TeV energies can explain the high-significance detection of GRB 190114C by the MAGIC telescope. The parameter values (such as the circumburst density and shock microphysical parameters) in the modeling are not unusual for both gamma-ray bursts, implying that the detection of sub-TeV photons from these two bursts should be attributed to their large burst energies and low redshifts.</jats:p>