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Register a new userExtending Virial Black Hole Mass Estimates to Low-Luminosity or Obscured AGN: the cases of NGC 4395 and MCG -01-24-012
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Authors
Fabio La Franca
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In the last decade, using single epoch (SE) virial based spectroscopic optical observations, it has been possible to measure the black hole (BH) mass on large type 1 Active Galactic Nuclei (AGN) samples. However this kind of measurements can not be applied on those obscured type 2 and/or low luminosity AGN where the nuclear component does not dominate in the optical. We have derived new SE relationships, based on the FWHM and luminosity of the broad line region component of the Pabeta emission line and/or the hard X-ray luminosity in the 14-195 keV band, which have the prospect of better working with low luminosity or obscured AGN. The SE relationships have been calibrated in the 10^5-10^9 M_sol mass range, using a sample of AGN whose BH masses have been previously measured using reverberation mapping techniques. Our tightest relationship between the reverberation-based BH mass and the SE virial product has an intrinsic spread of 0.20 dex. Thanks to these SE relations, in agreement with previous estimates, we have measured a BH mass of M_BH =1.7^+1.3_-0.7 X 10^5 M_sol for the low luminosity, type 1, AGN NGC 4395 (one of the smallest active galactic BH known). We also measured, for the first time, a BH mass of M_BH = 1.5^+1.1_-0.6 X 10^7 M_sol for the Seyfert 2 galaxy MCG -01-24-012.
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We present the variability study of the lowest-luminosity Seyfert 1 galaxy NGC 4395 based on the photometric monitoring campaigns in 2017 and 2018. Using 22 ground-based and space telescopes, we monitored NGC 4395 with a $sim$5 minute cadence during a period of 10 days and obtained light curves in the UV, V, J, H, and K/Ks bands as well as the H$alpha$ narrow-band. The RMS variability is $sim$0.13 mag on emph{Swift}-UVM2 and V filter light curves, decreasing down to $sim$0.01 mag on K filter. After correcting for continuum contribution to the H$alpha$ narrow-band, we measured the time lag of the H$alpha$ emission line with respect to the V-band continuum as ${55}^{+27}_{-31}$ to ${122}^{+33}_{-67}$ min. in 2017 and ${49}^{+15}_{-14}$ to ${83}^{+13}_{-14}$ min. in 2018, depending on the assumption on the continuum variability amplitude in the H$alpha$ narrow-band. We obtained no reliable measurements for the continuum-to-continuum lag between UV and V bands and among near-IR bands, due to the large flux uncertainty of UV observations and the limited time baseline. We determined the AGN monochromatic luminosity at 5100AA $lambda L_lambda = left(5.75pm0.40right)times 10^{39},mathrm{erg,s^{-1}}$, after subtracting the contribution of the nuclear star cluster. While the optical luminosity of NGC 4395 is two orders of magnitude lower than that of other reverberation-mapped AGNs, NGC 4395 follows the size-luminosity relation, albeit with an offset of 0.48 dex ($geq$2.5$sigma$) from the previous best-fit relation of Bentz et al. (2013).
At low Eddington ratio (mdot), two effects make it harder to detect AGN given some selection criteria. First, even with fixed accretion physics, AGN are diluted/less luminous relative to their hosts; the magnitude of this depends on host properties and so on luminosity and redshift. Second, they may transition to a radiatively inefficient state, changing SED shape and dramatically decreasing in optical/IR luminosity. These effects lead to differences in observed AGN samples, even at fixed bolometric luminosity and after correction for obscuration. The true Eddington ratio distribution may depend strongly on luminosity, but this will be seen only in surveys robust to dilution and radiative inefficiency (X-ray or narrow-line samples); selection effects imply that AGN in optical samples will have uniformly high mdot. This also implies that different selection methods yield systems with different hosts: the clustering of faint optical/IR sources will be weaker than that of X-ray sources, and optical/IR Seyferts will reside in more disk-dominated galaxies while X-ray selected Seyferts will preferentially occupy early-type systems. If observed mdot distributions are correct, a large fraction of low-luminosity AGN currently classified as obscured are in fact diluted and/or radiatively inefficient, not obscured by gas or dust. This is equally true if X-ray hardness is used as a proxy for obscuration, since radiatively inefficient SEDs near mdot~0.01 are X-ray hard. These effects can explain most of the claimed luminosity/redshift dependence in the obscured AGN population, with the true obscured fraction as low as 20%.
The innermost regions in active galactic nuclei (AGNs) were not being spatially resolved so far but spectropolarimetry can provide us insight about their hidden physics and the geometry. From spectropolarimetric observations in broad emission lines and assuming equatorial scattering as a dominant polarization mechanism, it is possible to estimate the mass of supermassive black holes (SMBHs). We explore the possibilities and limits and to put constraints on the usage of the method for determining SMBH masses using polarization in broad emission lines by providing more in-depth theoretical modeling. Methods. We use the Monte Carlo radiative transfer code STOKES for exploring polarization of Type 1 AGNs. We model equatorial scattering using flared-disk geometry for a set of different SMBH masses assuming Thomson scattering. In addition to the Keplerian motion in the BLR, we also consider cases of additional radial inflows and vertical outflows. We model the profiles of polarization plane position angle, degree of polarization and total unpolarized line for different BLR geometries and different SMBH masses. Our modeling confirms that the method can be widely used for Type-1 AGNs when viewing inclinations are between 25 and 45 degrees. We show that the distance between the BLR and scattering region (SR) has a significant impact on the mass estimates and the best mass estimates are when the SR is situated at the distance 1.5-2.5 times larger than the outer BLR radius. Our models show that if Keplerian motion can be traced through the polarized line profile, then the direct estimation of the mass of the SMBH can be performed. When radial inflows or vertical outflows are present in the BLR, this method can be applied if velocities of the inflow/outflow are less than 500 km/s. We find that models for NGC4051, NGC4151, 3C273 and PG0844+349 are in good agreements with observations.
We present an analysis of broad emission lines observed in moderate-luminosity active galactic nuclei (AGNs), typical of those found in X-ray surveys of deep fields, with the aim to test the validity of single-epoch virial black hole mass estimates. We have acquired near-infrared (NIR) spectra of AGNs up to z ~ 1.8 in the COSMOS and Extended Chandra Deep Field-South Survey, with the Fiber Multi-Object Spectrograph (FMOS) mounted on the Subaru Telescope. These low-resolution NIR spectra provide a significant detection of the broad Halpha line that has been shown to be a reliable probe of black hole mass at low redshift. Our sample has existing optical spectroscopy which provides a detection of MgII, a broad emission line typically used for black hole mass estimation at z > 1. We carry out a spectral-line fitting procedure using both Halpha and MgII to determine the virial velocity of gas in the broad line region, the monochromatic continuum luminosity at 3000 A, and the total Halpha line luminosity. With a sample of 43 AGNs spanning a range of two decades in luminosity (i.e., L ~ 10^44-46 ergs/s), we find a tight correlation between the continuum and line luminosity with a distribution characterized by <log(L_3000/L_Halpha)> = 1.52 and a dispersion sigma = 0.16. There is also a close one-to-one relationship between the FWHM of Halpha and of MgII up to 10000 km/s with a dispersion of 0.14 in the distribution of the logarithm of their ratios. Both of these then lead to there being very good agreement between Halpha- and MgII-based masses over a wide range in black hole mass (i.e., M_BH ~ 10^7-9 M_sun). We do find a small offset in MgII-based masses, relative to those based on Halpha, of +0.17 dex and a dispersion sigma = 0.32. In general, these results demonstrate that local scaling relations, using MgII or Halpha, are applicable for AGN at moderate luminosities and up to z ~ 2.
We present a detailed X-ray spectral analysis of the nearby Seyfert 2 galaxy MCG-01-24-12 based on a multi-epoch data set. Data have been taken with different X-ray satellites, namely XMM-Newton, NuSTAR, Swift and Chandra and cover different time intervals, from years down to a few days. From 2006 to 2013 the source had a 2-10 keV flux of $sim$1.5$times$10$^{-11}$ erg cm$^{-2}$ s$^{-1}$, consistent with archival observations based on textit{HEAO} and textit{BeppoSAX} data, though a 2019 textit{Chandra} snapshot caught the source in an extreme low flux state, a factor of $sim$10 fainter than its historical one. Based on phenomenological and physically motivated models, we find the X-ray spectrum of MCG-01-24-12 to be best modelled by a power-law continuum emission with $Gamma$=1.76$pm$0.09 with a high energy cut-off at E$_{rm c}=70^{+21}_{-14}$ keV that is absorbed by a fairly constant column density of N$_{rm H}$=(6.3$pm$0.5)$times10^{22}$ cm$^{-2}$. These quantities allowed us to estimate the properties of the hot corona in MCG-01-24-12 for the cases of a spherical or slab-like hot Comptonising plasma to be kT$_{rm e}$=27$^{+8}_{-4}$ keV, $tau_{rm e}$=5.5$pm$1.3 and kT$_{rm e}$=28$^{+7}_{-5}$ keV, $tau$=3.2$pm$0.8, respectively. Finally, despite the short duration of the exposures, possible evidence of the presence of outflows is discussed.
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