分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: As Setti & Woltjer noted back in 1973, quasars could be used to construct the Hubble diagram but the actual application was not that straightforward. It took years to implement the idea successfully. Most of the ways to use quasars for cosmology now require an advanced understanding of their structure, step by step. We briefly review this progress, with unavoidable personal bias, and concentrate on bright unobscured sources. We will mention the problem of the gas flow character close to the innermost stable circular orbit close to the black hole, discussed 50 years ago, which later led to the development of the slim disk scenario, but was recently revived in the context of Magnetically Arrested Disks (MAD) and Standard and Normal Evolution (SANE) disk models. We also discuss the hot/warm corona issue, which is still under discussion and complicates the analysis of the X-ray reflection. We present the scenario of the formation of the low ionization part of the Broad Line Region as a mostly failed wind powered by radiation pressure acting on dust (FRADO - Failed Radiatively Driven Dusty Outflow model). Next, we discuss the cosmological constraints that are currently achievable with quasars, mostly concentrating on light echo methods (continuum time delays and spectral line time delays with respect to the continuum) which are (or should be) incorporating the progress mentioned above. Finally, we briefly mention future prospects in this direction.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Inconsistent conclusions are obtained from recent active galactic nuclei (AGNs) accretion disk inter-band time-lag measurements. While some works show that the measured time lags are significantly larger (by a factor of $\sim 3$) than the theoretical predictions of the Shakura \& Sunyaev disk (SSD) model, others find that the time-lag measurements are consistent with (or only slightly larger than) that of the SSD model. These conflicting observational results might be symptoms of our poor understanding of AGN accretion physics. Here we show that sources with larger-than-expected time lags tend to be less-luminous AGNs. Such a dependence is unexpected if the inter-band time lags are attributed to the light-travel-time delay of the illuminating variable X-ray photons to the static SSD. If, instead, the measured inter-band lags are related not only to the static SSD but also to the outer broad emission-line regions (BLRs; e.g., the blended broad emission lines and/or diffuse continua), our result indicates that the contribution of the non-disk BLR to the observed UV/optical continuum decreases with increasing luminosity ($L$), i.e., an anti-correlation resembling the well-known Baldwin effect. Alternatively, we argue that the observed dependence might be a result of coherent disk thermal fluctuations as the relevant thermal timescale, $\tau_{\mathrm{TH}}\propto L^{0.5}$. With future accurate measurements of inter-band time lags, the above two scenarios can be distinguished by inspecting the dependence of inter-band time lags upon either the BLR components in the variable spectra or the timescales.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The correlation between the broad line region radius and continuum luminosity ($R-L$ relation) of active galactic nuclei (AGN) is critical for single-epoch mass estimates of supermassive black holes (SMBHs). At $z \sim 1-2$, where AGN activity peaks, the $R-L$ relation is constrained by the reverberation mapping (RM) lags of the Mg II line. We present 25 Mg II lags from the Australian Dark Energy Survey (OzDES) RM project based on six years of monitoring. We define quantitative criteria to select good lag measurements and verify their reliability with simulations based on both the damped random walk stochastic model and the re-scaled, re-sampled versions of the observed lightcurves of local, well-measured AGN. Our sample significantly increases the number of Mg II lags and extends the $R-L$ relation to higher redshifts and luminosities. The relative iron line strength $\mathcal{R}_{\rm Fe}$ has little impact on the $R-L$ relation. The best-fit Mg II $R-L$ relation has a slope $\alpha = 0.39 \pm 0.08$ with an intrinsic scatter $\sigma_{\rm rl} = 0.15^{+0.03}_{-0.02}$. The slope is consistent with previous measurements and shallower than the H$\beta$ $R-L$ relation. The intrinsic scatter of the new $R-L$ relation is substantially smaller than previous studies and comparable to the intrinsic scatter of the H$\beta$ $R-L$ relation. Our new $R-L$ relation will enable more precise single-epoch mass estimates and SMBH demographic studies at cosmic noon.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Reverberation mapping is a robust method to measure the masses of supermassive black holes (SMBHs) outside of the local Universe. Measurements of the radius -- luminosity ($R-L$) relation using the Mg II emission line are critical for determining these masses near the peak of quasar activity at $z \approx 1 - 2$, and for calibrating secondary mass estimators based on Mg II that can be applied to large samples with only single-epoch spectroscopy. We present the first nine Mg II lags from our five-year Australian Dark Energy Survey (OzDES) reverberation mapping program, which substantially improves the number and quality of Mg II lag measurements. As the Mg II feature is somewhat blended with iron emission, we model and subtract both the continuum and iron contamination from the multi-epoch spectra before analyzing the Mg II line. We also develop a new method of quantifying correlated spectroscopic calibration errors based on our numerous, contemporaneous observations of F-stars. The lag measurements for seven of our nine sources are consistent with both the H$\beta$ and Mg II $R-L$ relations reported by previous studies. Our simulations verify the lag reliability of our nine measurements, and we estimate that the median false positive rate of the lag measurements is $4\%$.