Echo mapping of active galactic nuclei

Abstract

Active galactic nuclei (AGN) are the persistent high luminosity sources powered by accretion of matter onto super massive black holes (SMBHs) at their centres. They are known to show flux variation and this property can be used as an effective tool to map the extent of the broad line region (BLR) and the dusty torus that surrounds the central SMBH. Though the mass of the SMBH (MBH) can be directly measured using the dynamics of stars that are close to the SMBH, this method is limited to objects in the low redshift Universe (z < 0.1). On the other hand, the technique of reverberation mapping (RM) can provide MBH estimate over a range of redshifts. RM is based on the light travel time delayed response of the line emitting gas as well as the re-processed torus emission to changes in the continuum emission from the accretion disk. As of now, MBH measurements are available for more than 100 sources based on RM. The obtained BLR sizes (RBLR) are found to be correlated with the optical luminosity (L5100) at wavelength 5100 Å. This relationship is obtained based on measurements of AGN available over a limited range of luminosity. Moreover, many of these measurements also have larger error bars. Therefore, there is an urgent need to increase RM measurements with small errors on more AGN covering a wide range of redshifts to better constrain the RBLR − L5100 relation. We have carried out photometric reverberation observations on the AGN Mrk 590. Using both broad band (that overlaps with the continuum) and narrow band (that overlaps with the Hα emission line) observations, we found the BLR size to be 21.44+1.49 −2.11 days, which is equivalent to 0.018+0.001 −0.002 pc. Using this measured BLR size and the full width at half maximum of the Hα line measured from the newly acquired Subaru spectrum we found a black hole mass of 1.96+0.15 −0.21 ×108M , using virial relationship and adopting a scale factor of 1.12. Similar to BLR reverberation mapping, the extent of the dusty torus in AGN can also be measured using dust reverberation mapping (DRM) based on the delayed response of the near infrared (NIR) flux from the torus to the ionizing UV/optical continuum coming from the accretion disk. From these observations too, a strong correlation is known to exist between the torus size (Rdust) and the optical luminosity (LV). Based on the Rdust − LV relation, it is also possible to use AGN as standard candle to constrain the Hubble constant (H0) and other cosmological parameters. But for that, a large number of DRM observations are needed at different redshifts. As of today, very few NIR interferometric observations are available, but measurements of the torus size via such direct imaging observations are again limited to very near and bright AGN. Moreover, there is a large discrepancy in the size of the torus obtained via the direct imaging method through interferometry and that obtained from DRM. From long term monitoring in the optical and infra-red, we found the inner edge of the dust torus in H0507+164 and Z229−15 to lie at a distance of 0.029+0.010 −0.008 pc and 0.017+0.005 −0.005 pc, respectively, from the central optical continuum source. These two new measurements are also found to lie closely on the known Rdust − LV relation line. Also, by modeling the available BLR RM data in the literature, it is possible to constrain the size, structure and kinematics of the BLR using Bayesian approach. This approach is adopted several times to study the BLR morphology by different ways but is not applied to a large number of sources in an homogeneous manner. From an analysis of the RM data for a total of 57 sources following a Bayesian approach, we could constrain the structure of the BLR in them as well as derive other properties of BLR. Knowledge of the long term variability characteristics of AGN is very crucial in RM program. Ideal targets for such studies are AGN that are known to show flux variability. For this purpose a dedicated telescope is needed to monitor large number of AGN/quasars over long duration of time. The 4 m International Liquid Mirror Telescope (ILMT) is expected to be commissioned soon on the Aryabhatta Research Institute of Observational Sciences (ARIES) in Devasthal, India. The ILMT, which will observe in the Time Delayed Integration (TDI) mode, will be repeatedly scanning the sky within a narrow stripe of width ∼ 270 . In this thesis, we arrived at a catalogue of 6738 quasars that will be covered by ILMT. Observations from the ILMT in the future can enable one to select variable quasars from this new catalogue for RM programs. Also, the quasars presented in this catalogue can serve as astrometric calibrators to convert the ILMT observations in the pixel coordinate system to the world coordinate system (α, δ).