分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Photoionization modeling of active galactic nuclei (AGN) predicts that diffuse continuum (DC) emission from the broad-line region makes a substantial contribution to the total continuum emission from ultraviolet through near-infrared wavelengths. Evidence for this DC component is present in the strong Balmer jump feature in AGN spectra, and possibly from reverberation measurements that find longer lags than expected from disk emission alone. However, the Balmer jump region contains numerous blended emission features, making it difficult to isolate the DC emission strength. In contrast, the Paschen jump region near 8200 \r{A} is relatively uncontaminated by other strong emission features. Here, we examine whether the Paschen jump can aid in constraining the DC contribution, using Hubble Space Telescope STIS spectra of six nearby Seyfert 1 nuclei. The spectra appear smooth across the Paschen edge, and we find no evidence of a Paschen spectral break or jump in total flux. We fit multi-component spectral models over the range $6800-9700$ \r{A} and find that the spectra can still be compatible with a significant DC contribution if the DC Paschen jump is offset by an opposite spectral break resulting from blended high-order Paschen emission lines. The fits imply DC contributions ranging from $\sim$10% to 50% at 8000 \r{A}, but the fitting results are highly dependent on assumptions made about other model components. These degeneracies can potentially be alleviated by carrying out fits over a broader wavelength range, provided that models can accurately represent the disk continuum shape, Fe II emission, high-order Balmer line emission, and other components.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We perform a systematic survey of active galactic nucleus (AGN) continuum lags using $\sim$3 day cadence $gri$-band light curves from the Zwicky Transient Facility. We select a sample of 94 type 1 AGN at $z<0.8$ with significant and consistent inter-band lags based on the interpolated cross-correlation function method and the Bayesian method JAVELIN. Within the framework of the lamp-post reprocessing model, our findings are: 1) The continuum emission (CE) sizes inferred from the data are larger than the disk sizes predicted by the standard thin disk model; 2) For a subset of the sample, the CE size exceeds the theoretical limit of the self-gravity radius (12 lt-days) for geometrically thin disks; 3) The CE size scales with continuum luminosity as $R_{\mathrm{CE}} \propto L^{0.48\pm0.04}$ with a scatter of 0.2 dex, analogous to the well-known radius-luminosity relation of broad H$\mathrm{\beta}$. These findings suggest a significant contribution of diffuse continuum emission from the broad-line region (BLR) to AGN continuum lags. We find that the $R_{\mathrm{CE}}-L$ relation can be explained by a photoionization model that assumes $\sim$23% of the total flux comes from the diffuse BLR emission. In addition, the ratio of the CE size and model-predicted disk size anti-correlates with the continuum luminosity, indicative of a potential non-disk BLR lag contribution evolving with luminosity. Finally, a robust positive correlation between CE size and black hole mass is detected.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The AGN STORM 2 collaboration targeted the Seyfert 1 galaxy Mrk 817 for a year-long multiwavelength, coordinated reverberation mapping campaign including HST, Swift, XMM-Newton, NICER, and ground-based observatories. Early observations with NICER and XMM revealed an X-ray state ten times fainter than historical observations, consistent with the presence of a new dust-free, ionized obscurer. The following analysis of NICER spectra attributes variability in the observed X-ray flux to changes in both the column density of the obscurer by at least one order of magnitude ($N_\mathrm{H}$ ranges from $2.85\substack{+0.48\\ -0.33} \times 10^{22}\text{ cm}^{-2}$ to $25.6\substack{+3.0\\ -3.5} \times 10^{22} \text{ cm}^{-2}$) and the intrinsic continuum brightness (the unobscured flux ranges from $10^{-11.8}$ to $10^{-10.5}$ erg s$^{-1}$ cm$^{-2}$ ). While the X-ray flux generally remains in a faint state, there is one large flare during which Mrk 817 returns to its historical mean flux. The obscuring gas is still present at lower column density during the flare but it also becomes highly ionized, increasing its transparency. Correlation between the column density of the X-ray obscurer and the strength of UV broad absorption lines suggests that the X-ray and UV continua are both affected by the same obscuration, consistent with a clumpy disk wind launched from the inner broad line region.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present the first results from a 100-day Swift, NICER and ground-based X-ray/UV/optical reverberation mapping campaign of the Narrow-Line Seyfert 1 Mrk 335, when it was in an unprecedented low X-ray flux state. Despite dramatic suppression of the X-ray variability, we still observe UV/optical lags as expected from disk reverberation. Moreover, the UV/optical lags are consistent with archival observations when the X-ray luminosity was >10 times higher. Interestingly, both low- and high-flux states reveal UV/optical lags that are 6-11 times longer than expected from a thin disk. These long lags are often interpreted as due to contamination from the broad line region, however the u band excess lag (containing the Balmer jump from the diffuse continuum) is less prevalent than in other AGN. The Swift campaign showed a low X-ray-to-optical correlation (similar to previous campaigns), but NICER and ground-based monitoring continued for another two weeks, during which the optical rose to the highest level of the campaign, followed ~10 days later by a sharp rise in X-rays. While the low X-ray countrate and relatively large systematic uncertainties in the NICER background make this measurement challenging, if the optical does lead X-rays in this flare, this indicates a departure from the zeroth-order reprocessing picture. If the optical flare is due to an increase in mass accretion rate, this occurs on much shorter than the viscous timescale. Alternatively, the optical could be responding to an intrinsic rise in X-rays that is initially hidden from our line-of-sight.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We report the results of near-infrared spectroscopic observations of 37 quasars in the redshift range $6.36.5$, forming the largest quasar near-infrared spectral sample at this redshift. The spectra, taken with Keck, Gemini, VLT, and Magellan, allow investigations of central black hole mass and quasar rest-frame ultraviolet spectral properties. The black hole masses derived from the MgII emission lines are in the range $(0.3-3.6)\times10^{9}\,M_{\odot}$, which requires massive seed black holes with masses $\gtrsim10^{3-4}\,M_{\odot}$, assuming Eddington accretion since $z=30$. The Eddington ratio distribution peaks at $\lambda_{\rm Edd}\sim0.8$ and has a mean of 1.08, suggesting high accretion rates for these quasars. The CIV - MgII emission line velocity differences in our sample show an increase of CIV blueshift towards higher redshift, but the evolutionary trend observed from this sample is weaker than the previous results from smaller samples at similar redshift. The FeII/MgII flux ratios derived for these quasars up to $z=7.6$, compared with previous measurements at different redshifts, do not show any evidence of strong redshift evolution, suggesting metal-enriched environments in these quasars. Using this quasar sample, we create a quasar composite spectrum for $z>6.5$ quasars and find no significant redshift evolution of quasar broad emission lines and continuum slope, except for a blueshift of the CIV line. Our sample yields a strong broad absorption line quasar fraction of $\sim$24%, higher than the fractions in lower redshift quasar samples, although this could be affected by small sample statistics and selection effects.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We perform a systematic survey of active galactic nucleus (AGN) continuum lags using $\sim$3 day cadence $gri$-band light curves from the Zwicky Transient Facility. We select a sample of 94 type 1 AGN at $z<0.8$ with significant and consistent inter-band lags based on the interpolated cross-correlation function method and the Bayesian method JAVELIN. Within the framework of the lamp-post reprocessing model, our findings are: 1) The continuum emission (CE) sizes inferred from the data are larger than the disk sizes predicted by the standard thin disk model; 2) For a subset of the sample, the CE size exceeds the theoretical limit of the self-gravity radius (12 lt-days) for geometrically thin disks; 3) The CE size scales with continuum luminosity as $R_{\mathrm{CE}} \propto L^{0.48\pm0.04}$ with a scatter of 0.2 dex, analogous to the well-known radius-luminosity relation of broad H$\mathrm{\beta}$. These findings suggest a significant contribution of diffuse continuum emission from the broad-line region (BLR) to AGN continuum lags. We find that the $R_{\mathrm{CE}}-L$ relation can be explained by a photoionization model that assumes $\sim$23% of the total flux comes from the diffuse BLR emission. In addition, the ratio of the CE size and model-predicted disk size anti-correlates with the continuum luminosity, indicative of a potential non-disk BLR lag contribution evolving with luminosity. Finally, a robust positive correlation between CE size and black hole mass is detected.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present reverberation mapping measurements for the prominent ultraviolet broad emission lines of the active galactic nucleus Mrk817 using 165 spectra obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope. Our ultraviolet observations are accompanied by X-ray, optical, and near-infrared observations as part of the AGN Space Telescope and Optical Reverberation Mapping Program 2 (AGN STORM 2). Using the cross-correlation lag analysis method, we find significant correlated variations in the continuum and emission-line light curves. We measure rest-frame delayed responses between the far-ultraviolet continuum at 1180 A and Ly$\alpha$ $\lambda1215$ A ($10.4_{-1.4}^{+1.6}$ days), N V $\lambda1240$ A ($15.5_{-4.8}^{+1.0}$days), SiIV + OIV] $\lambda1397$ A ($8.2_{-1.4}^{+1.4}$ days), CIV $\lambda1549$ A ($11.8_{-2.8}^{+3.0}$ days), and HeII $\lambda1640$ A ($9.0_{-1.9}^{+4.5}$ days) using segments of the emission-line profile that are unaffected by absorption and blending, which results in sampling different velocity ranges for each line. However, we find that the emission-line responses to continuum variations are more complex than a simple smoothed, shifted, and scaled version of the continuum light curve. We also measure velocity-resolved lags for the Ly$\alpha$, and CIV emission lines. The lag profile in the blue wing of Ly$\alpha$ is consistent with virial motion, with longer lags dominating at lower velocities, and shorter lags at higher velocities. The CIV lag profile shows the signature of a thick rotating disk, with the shortest lags in the wings, local peaks at $\pm$ 1500 $\rm km\,s^{-1}$, and a local minimum at line center. The other emission lines are dominated by broad absorption lines and blending with adjacent emission lines. These require detailed models, and will be presented in future work.