arXiv subjects

Wei-Jian Guo

Publications and source records attributed to Wei-Jian Guo.

43 records · Page 3Linked to original sources

arXiv2023

Searching for quasar candidates with periodic variations from the Zwicky Transient Facility: results and implications

We conduct a systematic search for quasars with periodic variations from the archival photometric data of the Zwicky Transient Facility by cross-matching with the quasar catalogs of the Sloan Digital Sky Survey and V{é}ron-Cetty \& V{é}ron. We first select out 184 primitive periodic candidates using the generalized Lomb-Scargle periodogram and auto-correlation function and then estimate their statistical significance of periodicity based on two red-noise models, i.e., damped random walk (DRW) and single power-law (SPL) models. As such, we finally identify 106 (DRW) and 86 (SPL) candidates with the most significant periodic variations out of 143,700 quasars. We further compare DRW and SPL models using Bayes factors, which indicate a relative preference of the SPL model for our primitive sample. We thus adopt the candidates identified with SPL as the final sample and summarize its basic properties. We extend the light curves of the selected candidates by supplying other archival survey data to verify their periodicity. However, only three candidates (with 6-8 cycles of periods) meet the selection criteria. This result clearly implies that, instead of being strictly periodic, the variability must be quasi-periodic or caused by stochastic red-noise. This exerts a challenge to the existing search approaches and calls for developing new effective methods.

astro-ph.GA↗

arXiv2023

Testing Super-Eddington Accretion onto a Supermassive Black Hole: Reverberation Mapping of PG 1119+120

We measure the black hole mass and investigate the accretion flow around the local ($z=0.0502$) quasar PG 1119+120. Spectroscopic monitoring with Calar Alto provides H$β$ lags and linewidths from which we estimate a black hole mass of $\log \left(M_{\bullet}/\mathrm{M}_{\odot} \right) = 7.0$, uncertain by $\sim0.4$ dex. High cadence photometric monitoring over two years with the Las Cumbres Observatory provides lightcurves in 7 optical bands suitable for intensive continuum reverberation mapping. We identify variability on two timescales. Slower variations on a 100-day timescale exhibit excess flux and increased lag in the $u'$ band and are thus attributable to diffuse bound-free continuum emission from the broad line region. Faster variations that we attribute to accretion disc reprocessing lack a $u'$-band excess and have flux and delay spectra consistent with either $τ\propto λ^{4/3}$, as expected for a temperature structure of $T(R) \propto R^{-3/4}$ for a thin accretion disc, or $τ\propto λ^{2}$ expected for a slim disc. Decomposing the flux into variable (disc) and constant (host galaxy) components, we find the disc SED to be flatter than expected with $f_ν \sim \rm{const}$. Modelling the SED predicts an Eddington ratio of $λ_{\rm Edd} > 1$, where the flat spectrum can be reproduced by a slim disc with little dust extinction or a thin disc which requires more dust extinction. While this accretion is super-Eddington, the geometry is still unclear, however a slim disc is expected due to the high radiation pressure at these accretion rates, and is entirely consistent with our observations.

astro-ph.GA↗

arXiv2022

Monitoring AGNs with H$β$ Asymmetry. III. Long-term Reverberation Mapping Results of 15 Palomar-Green Quasars

In this third paper of the series reporting on the reverberation mapping (RM) campaign of active galactic nuclei with asymmetric H$β$ emission-line profiles, we present results for 15 Palomar-Green (PG) quasars using spectra obtained between the end of 2016 to May 2021. This campaign combines long time spans with relatively high cadence. For 8 objects, both the time lags obtained from the entire light curves and the measurements from individual observing seasons are provided. Reverberation mapping of 9 of our targets has been attempted for the first time, while the results for 6 others can be compared with previous campaigns. We measure the H$β$ time lags over periods of years and estimate their black hole masses. The long duration of the campaign enables us to investigate their broad line region (BLR) geometry and kinematics for different years by using velocity-resolved lags, which demonstrate signatures of diverse BLR geometry and kinematics. The BLR geometry and kinematics of individual objects are discussed. In this sample, the BLR kinematics of Keplerian/virialized motion and inflow is more common than outflow.

astro-ph.GA↗

arXiv2022

Accretion Disk Size Measurements of Active Galactic Nuclei Monitored by the Zwicky Transient Facility

We compile a sample of 92 active galactic nuclei (AGNs) at z<0.75 with $gri$ photometric light curves from the archival data of the Zwicky Transient Facility and measure the accretion disk sizes via continuum reverberation mapping. We employ Monte Carlo simulation tests to assess the influences of data sampling and broad emission lines and select out the sample with adequately high sampling cadences (3 days apart in average) and minimum contaminations of broad emission lines. The inter-band time delays of individual AGNs are calculated using the interpolated cross-correlation function and then these delays are fitted with a generalized accretion disk model, in which inter-band time delays are a power function of wavelength, black hole mass, and luminosity. A Markov-chain Monte Carlo method is adopted to determine the best parameter values. Overall the inter-band time delays can be fitted with the $τ\ \propto λ^{4/3}$ relation as predicted from a steady-state, optically thick, geometrically thin accretion disk, however, the yielded disk size is systematically larger than expected, although the ratio of the measured to theoretical disk sizes depend on using the emissivity -- or responsivity -- weighted disk radius. These results are broadly consistent with previous studies, all together raising a puzzle about the "standard" accretion disk model.

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arXiv2021

Reverberation Mapping of Two Luminous Quasars: the Broad-line Region Structure and Black Hole Mass

We report the results of a multi-year spectroscopic and photometric monitoring campaign of two luminous quasars, PG~0923+201 and PG~1001+291, both located at the high-luminosity end of the broad-line region (BLR) size-luminosity relation with optical luminosities above $10^{45}~{\rm erg~s^{-1}}$. PG~0923+201 is for the first time monitored, and PG~1001+291 was previously monitored but our campaign has a much longer temporal baseline. We detect time lags of variations of the broad H$β$, H$γ$, Fe {\sc ii} lines with respect to those of the 5100~Å continuum. The velocity-resolved delay map of H$β$ in PG~0923+201 indicates a complicated structure with a mix of Keplerian disk-like motion and outflow, and the map of H$β$ in PG~1001+291 shows a signature of Keplerian disk-like motion. Assuming a virial factor of $f_{\rm BLR}=1$ and FWHM line widths, we measure the black hole mass to be $118_{-16}^{+11}\times 10^7 M_{\odot}$ for PG~0923+201 and $3.33_{-0.54}^{+0.62}\times 10^7 M_{\odot}$ for PG~1001+291. Their respective accretion rates are estimated to be $0.21_{-0.07}^{+0.06} \times L_{\rm Edd}\,c^{-2}$ and $679_{-227}^{+259}\times L_{\rm Edd}\,c^{-2}$, indicating that PG~0923+201 is a sub-Eddington accretor and PG~1001+291 is a super-Eddington accretor. While the H$β$ time lag of PG~0923+201 agrees with the size-luminosity relation, the time lag of PG~1001+291 shows a significant deviation, confirming that in high-luminosity AGN the BLR size depends on both luminosity and Eddington ratio. Black hole mass estimates from single AGN spectra will be over-estimated at high luminosities and redshifts if this effect is not taken into account.

astro-ph.GA↗

arXiv2021

AGN STORM 2: I. First results: A Change in the Weather of Mrk 817

We present the first results from the ongoing, intensive, multi-wavelength monitoring program of the luminous Seyfert 1 galaxy Mrk 817. While this AGN was, in part, selected for its historically unobscured nature, we discovered that the X-ray spectrum is highly absorbed, and there are new blueshifted, broad and narrow UV absorption lines, which suggest that a dust-free, ionized obscurer located at the inner broad line region partially covers the central source. Despite the obscuration, we measure UV and optical continuum reverberation lags consistent with a centrally illuminated Shakura-Sunyaev thin accretion disk, and measure reverberation lags associated with the optical broad line region, as expected. However, in the first 55 days of the campaign, when the obscuration was becoming most extreme, we observe a de-coupling of the UV continuum and the UV broad emission line variability. The correlation recovers in the next 42 days of the campaign, as Mrk 817 enters a less obscured state. The short CIV and Ly alpha lags suggest that the accretion disk extends beyond the UV broad line region.

astro-ph.HE↗

arXiv2020

Evidence for Two Distinct Broad-Line Regions from Reverberation Mapping of PG 0026+129

We report on the results of a new spectroscopic monitoring campaign of the quasar PG 0026+129 at the Calar Alto Observatory 2.2m telescope from July 2017 to February 2020. Significant variations in the fluxes of the continuum and broad-emission lines, including H$β$ and He II, were observed in the first and third years, and clear time lags between them are measured. The broad H$β$ line profile consists of two Gaussian components: an intermediate-width H$β_{\rm IC}$ with a full width at half-maximum (FWHM) of 1964$\pm$18 $\rm km~s^{-1}$ and another very broad H$β_{\rm VBC}$ with a FWHM of 7570$\pm$83 $\rm km~s^{-1}$. H$β_{\rm IC}$ has long time lags of $\sim$40--60 days in the rest frame, while H$β_{\rm VBC}$ shows nearly zero time delay with respect to the optical continuum at 5100 Å. The velocity-resolved delays show consistent results: lags of $\sim$30--50 days at the core of the broad H$β$ line and roughly zero lags at the wings. H$β_{\rm IC}$ has a redshift of $\sim$400 $\rm km~s^{-1}$ which seems to be stable for nearly 30 years by comparing with archived spectra, and may originate from an infall. The root mean square (rms) spectrum of H$β_{\rm VBC}$ shows a double-peaked profile with brighter blue peak and extended red wing in the first year, which matches the signature of a thin disk. Both the double-peaked profile and the near-zero lag suggest that H$β_{\rm VBC}$ comes from a region associated with the part of the accretion disc that emits the optical continuum. Adopting the FWHM (in the rms spectrum) and the time lag measured for the total H$β$ line, and a virial factor of 1.5, we obtain a virial mass of $2.89_{-0.69}^{+0.60} \times10^7 M_{\odot}$ for the central black hole in this quasar.

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