Studying the Dust Distribution Around Accreting Black Holes with Reverberation Mapping Using PRIMA

TL;DR

This work argues that dust reverberation mapping can be extended from near-IR to mid-IR (25–30 μm) to probe the bulk dust distribution around accreting black holes, including a polar outflow component. It presents the PRIMA mission concept, a cryogenic 1.8 m telescope with PRIMAger and FIRESS, designed to monitor AGN at 25–30 μm with high cadence, enabling regular RM across a wide luminosity range. The authors illustrate the approach with a time-domain dust model from Zw229-015, showing how light-curve delays encode spatial structure and highlighting the importance of cadence and sensitivity. They discuss instrument sensitivity, target visibility, and synergies with optical surveys to enable a large, multiwavelength RM program, aiming to connect dust geometry to AGN luminosity and potentially use the dust response as a distance indicator.

Abstract

Variability studies are a powerful tool for studying the structures of unresolved sources. One such type of variability study, called reverberation mapping (RM), established that the dominant source of infrared radiation from an active galactic nucleus (AGN) was from dust absorption and re-emission, which demonstrated that the optical brightening and fading of light from the accretion disk (AD) around a supermassive black hole was followed by a corresponding (delayed) variation at infrared (IR) wavelengths from the surrounding dust distribution. Since that time a great deal more has been learned about the dust distribution around ADs, both near the AD in the form of a potential torus, as well as extended emission in the form of polar dust outflows. Understanding the dust distribution is vital to understanding how AGNs affect their host galaxy as well as the overall energetics of AGNs as ~50% of energy from an AGN comes out in the IR. Dust RM has been done exclusively in the near-IR (1-5 microns) which traces the inner edge of the dust near the dust sublimation radius. Hence extending RM to the mid-IR, especially to the peak of the dust emission between 25 and 30 microns, allows for an examination of the dust distribution around ADs and potentially traces the source of the polar outflow. RM with the proposed PRobe far-Infrared Mission for Astrophysics (PRIMA) can focus on variability monitoring of a sample of low- to high-luminosity AGNs to trace the 25-30 microns emission that is reverberated from the UV/optical AD emission which will be monitored by other space and ground-based observatories and made available to PRIMA users. Then, by detailed modeling of the response of the dust emission to shorter wavelengths, the distribution of the dust around an AGN will be revealed and can be linked to the accretion disk luminosities.

Figures (6)

• Figure 1: A schematic (not to scale) representation of reverberation mapping. The differing arrival times of differing wavelengths (UV/optical vs IR) to an observer from the top correspond to the physical separation at a length of c$\Delta$t between those regions where $\Delta$t is the difference in arrival times of differing wavelengths. To guarantee that the dust does not block the UV/optical emitting region, only Type I AGN are selected for RM where the presence of broad lines in the AGN spectrum guarantee an unobstructed view of the BLR and hence the accretion disk. Adapted from Ref[CackettBentzKara2021]
• Figure 2: Normalized mean SEDs for strong FIR quasars (left, top curve) and weak FIR quasars (right, top curve)NetzerEtal07 The red SED curves show "intrinsic" AGN SEDs obtained by the subtraction of the scaled mean starburst (ULIRG) spectrum (shown in black) from the mean SEDs. The red AGN curves indicates how IR emission ($>$1 $\mu$m) carries a substantial portion of the AGN’s energy and that the peak AGN emission is between 20 and 30 $\mu$m. In the PRIMA 25–30 $\mu$m wavelength region, the variability will be dominated by the AGN, regardless of the more constant mid-IR light from the host galaxy, allowing for RM to provide critical insight into the dust distribution around the AGN
• Figure 3: Kepler (dark blue points) and Spitzer (dark red points) observed light curves for the 2010- 2011 observing season for the AGN Zw229-015, plotted with the interpolated optical (light blue) and interpolated infrared (light red dashes) that corresponded to the highest posterior distribution.
• Figure 4: Distribution of 10,000 dust clouds surrounding the Zw229-015 accretion diskGuise.etal.22 These distributions reproduce the delayed infrared light curves and are derived from the mean parameters of the MCMC modeling of the delayed IR light curves vs the optical light curves shown in Figure \ref{['fig:Zw229-light-curve']}. Images a , b , and c show the modeled distribution of clouds from multiple angles. Image d shows the modeled delay map of arrival times from the near side and far side of the dust distribution while deriving an inclination of 49$^{+3}_{-13}$ degrees. Finally image e shows the derived illumination map of the dust distribution
• Figure 5: Presenting the observed optical light curve for NGC 6418 with interpolation using a damped random-walk model (gray points and gray solid line) and then simulating the IR response light curves (solid = 3.6 $\mu$m and dashed–dotted = 30 $\mu$m lines). The figure is a slightly modified version from the original publicationAlmeyda.etal.2017. The peak-to-peak variation at 30 $\mu$m is 20% noted in the gray shaded region on the plot and on the right from which we derive our exposure times. The red line is a simulation of an isotropically illuminated torus where cloud orientations have not been taken into account. The blue line is the same simulation but with the orientation of the clouds taken into account. The torus model is made of 50,000 clouds and is viewed face-on with the ratio of 10 between the inner radius to the outer radius. Finally the power law index of the radial cloud distribution is 0, with the an angular size of 45$^{\circ}$ for the torusNenkova.etal.2008.