arXiv subjects

S. K. Li

Publications and source records attributed to S. K. Li.

6 recordsLinked to original sources

arXiv2019

Search for Intra-day Optical Variability in $γ$-ray--loud Blazars S5 0716+714 and 3C 273

We present the photometric observations of blazars S5 0716+714 and 3C 273 with high temporal resolution (30--60s) in the $I$ or $R$ bands. The observations were performed with a 1.02 m optical telescope from 2007 March 07 to 2012 May 16. The $F$-test, one-way analysis of variance (ANOVA) test, and $z$-transformed discrete correlation function (ZDCF) cross-correlation zero lag test are used to search for intra-day variability (IDV). Four and five reliable IDVs survive three tests for S5 0716+714 and 3C 273, respectively. IDVs are found for S5 0716+714 and 3C 273. A flare on 2008 May 08 has $ΔI \approx$ 0.06$\pm$0.01 mag in a duration of 0.54 hr for S5 0716+714. A flare on 2011 May 10 shows $ΔR \approx$ 0.05$\pm$0.01 mag in a duration of 0.40 hr for 3C 273. Sharp dips appear on 2011 May 9 for 3C 273, and show $ΔR \approx$ 0.05$\pm$0.01 mag. Under the assumptions that the IDV is tightly connected to black hole mass, $M_{\bullet}$, and that the flare durations are representative of the minimum characteristic timescales, we can estimate upper bounds to $M_{\bullet}$. In the case of the Kerr black holes, $M_{\bullet} \la 10^{8.91} M_{\odot}$ and $M_{\bullet} \la 10^{9.02} M_{\odot}$ are given for S5 0716+714 and 3C 273, respectively. These mass measurements are consistent with those measurements reported in the literatures. Also, we discuss the origins of optical variations found in this work.

astro-ph.GA↗

arXiv2018

Direct observation of magnetic domain evolution in the vicinity of Verwey transition in Fe3O4 thin films

We report a direct observation of magnetic domain evolution near the Verwey transition (TV) in Fe3O4 films. We found the stripe domains in the Fe3O4/Mg2TiO4 film while the irregular domains in the Fe3O4/MgO film and the similar characters of magnetic domains in the vicinity of TV for both samples: the bigger domain size and the higher contrast of the phase signal below TV and the more disordered domain images at TV. Remarkably, the magnetic behaviors can be well understood and the domain-wall energy and the demagnetizing energy can be calculated from the magnetic domains near TV in the Fe3O4/Mg2TiO4 film. Our work presents a demonstration of the low temperature magnetic domains and gives a new perspective to understand the Verwey transition in Fe3O4 thin films.

cond-mat.mtrl-sci↗

arXiv2018

The flat-spectrum radio quasar 3C 345 from the high to the low emission state

We report simultaneous observations at different energy bands in radio, optical, UV, X-rays and $γ$ rays of the flat-spectrum radio-quasar 3C 345. We built the light curve of the source at different frequencies from 2008, the beginning of the \textit{Fermi} all-sky survey, to 2016, using new data and public archives. In particular we obtained several optical spectra, to study the behavior of emission lines and the continuum in different activity states and to derive the black hole mass. 3C 345 showed two flaring episodes in 2009, which occurred simultaneously in $γ$ ray, optical/UV and X-rays, and were later followed in radio. The source shows an inverse Compton dominated spectral energy distribution, which moved from higher to lower frequencies from the high to the low state. The reverberation of emission lines during one outburst event allowed us to constrain the location of production of $γ$ rays very close to the broad-line region, and possibly in the jet-base. We report the observation of an increased accretion after the outburst, possibly induced by the decrease of magnetic field intensity with respect to the low state.

astro-ph.HE↗

arXiv2017

Search for intra-day optical variability in Mrk 501

We present our observations of the optical intra-day variability (IDV) in $γ$-ray BL Lac object Mrk 501. The observations were run with the 1.02 m and 2.4 m optical telescopes at Yunnan Observatories from 2005 April to 2012 May. The light curve at the $R$ band on 2010 May 15 passes both variability tests (the $F$ test and the ANOVA test). A flare within the light curve on 2010 May 15 has a magnitude change $Δm = 0.03 \pm 0.005_{\rm{stat}} \pm 0.007_{\rm{sys}}$ mag, \textbf{a darkening timescale of $τ_{\rm{d}}=$ 26.7 minutes}, and an amplitude of IDV $Amp=2.9\% \pm0.7\%$. A decline \textbf{described by 11 consecutive flux measurements} within the flare can be fitted linearly with a Pearson's correlation coefficient $r = 0.945$ at the confidence level of $> 99.99\%$. Under the assumptions that the IDV is tightly connected to the mass of the black hole, \textbf{and that the flare duration, being two times $τ_{\rm{d}}$, is representative of the minimum characteristic timescale, we can derive upper bounds to the mass of the black hole}. In the case of the Kerr black hole, the timescale of $Δt_{\rm{min}}^{\rm{ob}}=$ 0.89 hours gives $M_{\bullet}\la 10^{9.20} M_{\odot}$, which is consistent with measurements reported in the literature. This agreement indicates that the hypothesis about $M_{\bullet}$ and $Δt_{\rm{min}}^{\rm{ob}}$ is consistent with the measurements/data.

astro-ph.GA↗

arXiv2015

Revisiting Correlations Between Broad-Line and Jet Emission Variations for AGNs: 3C 120 and 3C 273

We restudy the issue of cross-correlations between broad-line and jet emission variations, and aim to locate the position of radio (and gamma-ray) emitting region in jet of active galactic nuclei (AGNs). Considering the radial profiles of the radius and number density of clouds in a spherical broad-line region (BLR), we derive new formulae connecting jet emitting position $R_{\rm{jet}}$ to time lag $τ_{\rm{ob}}$ between broad-line and jet emission variations, and BLR radius. Also, formulae are derived for a disk-like BLR and a spherical shell BLR. The model-independent FR/RSS method is used to estimate $τ_{\rm{ob}}$. For 3C 120, positive lags of about 0.3 yr are found between the 15 GHz emission and the H$β$, H$γ$ and He II $λ4686$ lines, including broad-line data in a newly published paper, indicating the line variations lead the 15 GHz ones. Each of the broad-line light curves corresponds to a radio outburst. $R_{\rm{jet}}=$1.1--1.5 parsec (pc) are obtained for 3C 120. For 3C 273, a common feature of negative time lags is found in the cross-correlation functions between light curves of radio emission and the Balmer lines, and as well Ly$α\/\ λ1216$ and C~{\sc iv} $λ1549$ lines. $R_{\rm{jet}}=$ 1.0--2.6 pc are obtained for 3C 273. The estimated $R_{\rm{jet}}$ are comparable for 3C 120 and 3C 273, and the gamma-ray emitting positions will be within $\sim$ 1--3 pc from the central engines. Comparisons show that the cloud number density and radius radial distributions and the BLR structures only have negligible effects on $R_{\rm{jet}}$.

astro-ph.GA↗

arXiv2011

Constraining broad-line regions from time lags of broad emission lines relative to radio emission

In this paper, a new method is proposed to estimate the broad-line region sizes of UV lines $R^{\rm{uv}}_{\rm{BLR}}$. It is applied to 3C 273. First, we derive the time lags of radio emission relative to broad emission lines Ly$α$ and C IV by the ZDCF method. The broad lines lag the 5, 8, 15, 22 and 37 GHz emission. The measured lags $τ^{\rm{uv}}_{\rm{ob}}$ are of the order of years. For a given line, $τ^{\rm{uv}}_{\rm{ob}}$ decreases as the radio frequency increases. This trend results from the radiative cooling of relativistic electrons. Both UV lines have a lag of $τ^{\rm{uv}}_{\rm{ob}}=-2.74^{+0.06}_{-0.25}$ yr relative to the 37 GHz emission. These results are consistent with those derived from the Balmer lines in Paper I. Second, we derive the time lags of the lines Ly$α$, CIV, H$γ$, H$β$ and H$α$ relative to the 37 GHz emission by the FR/RSS Monte Carlo method. The measured lags are $τ_{\rm{ob}}=-3.40^{+0.31}_{-0.05}$, $-3.40^{+0.41}_{-0.14}$, $-2.06^{+0.36}_{-0.92}$, $-3.40^{+1.15}_{-0.20}$ and $-3.56^{+0.35}_{-0.18}$ yr for the lines Ly$α$, CIV, H$γ$, H$β$ and H$α$, respectively. These estimated lags are consistent with those derived by the ZDCF method within the uncertainties. Based on the new method, we derive $R^{\rm{uv}}_{\rm{BLR}}=2.54^{+0.71}_{-0.35}$--$4.01^{+0.90}_{-1.16}$ and $2.54^{+0.80}_{-0.43}$--$4.01^{+0.98}_{-1.24}$ light-years for the Ly$α$ and CIV lines, respectively. Considering the uncertainties, these estimated sizes are consistent with those obtained in the classical reverberation mapping for the UV lines and the Balmer lines. This indicates that their emitting regions are not separated so large as in the classical mapping of the UV and optical lines. These results seem to depart from the stratified ionization structures obtained in the classical mapping.

astro-ph.CO↗