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Register a new userSimulations of Torus Reverberation Mapping Experiments with SPHEREx
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Reverberation mapping (RM) is an efficient method to investigate the physical sizes of the broad line region (BLR) and dusty torus in an active galactic nucleus (AGN). The Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer (SPHEREx) mission will provide multi-epoch spectroscopic data at optical and near-infrared wavelengths. These data can be used for RM experiments for bright AGNs. We present results of a feasibility test using SPHEREx data in the SPHEREx deep regions for the torus RM measurements. We investigate the physical properties of bright AGNs in the SPHEREx deep field. Based on this information, we compute the efficiency of detecting torus time lags in simulated light curves. We demonstrate that, in combination with the complementary optical data with a depth of $sim20$ mag in $B-$band, lags of $le 750$ days for tori can be measured for more than $sim200$ bright AGNs. If high signal-to-noise ratio photometric data with a depth of $sim21-22$ mag are available, RM measurements can be applied for up to $sim$900 objects. When complemented by well-designed early optical observations, SPHEREx can provide a unique dataset for studies of the physical properties of dusty tori in bright AGNs.
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The central engines of Active Galactic Nuclei (AGNs) are powered by accreting supermassive black holes, and while AGNs are known to play an important role in galaxy evolution, the key physical processes occur on scales that are too small to be resolved spatially (aside from a few exceptional cases). Reverberation mapping is a powerful technique that overcomes this limitation by using echoes of light to determine the geometry and kinematics of the central regions. Variable ionizing radiation from close to the black hole drives correlated variability in surrounding gas/dust, but with a time delay due to the light travel time between the regions, allowing reverberation mapping to effectively replace spatial resolution with time resolution. Reverberation mapping is used to measure black hole masses and to probe the innermost X-ray emitting region, the UV/optical accretion disk, the broad emission line region and the dusty torus. In this article we provide an overview of the technique and its varied applications.
At the Universitaetssternwarte Bochum near Cerro Armazones we have monitored the Seyfert-1 galaxy 3C 120 between September 2014 and March 2015 in BVRI and a narrow band filter covering the redshifted H alpha line; in addition we obtained a single con-temporary spectrum with FAST at Mt. Hopkins. Compared to earlier epochs 3C 120 is about a factor of three brighter, allowing us to study the shape of the broad line region (BLR) and the dust torus in a high luminosity phase. The analysis of the light curves yields that the dust echo is rather sharp and symmetric in contrast to the more complex broad H alpha BLR echo. We investigate how far this supports an optically thick bowl-shaped BLR and dust torus geometry as proposed by Kawaguchi & Mori (2010) and Goad et al. (2012). The comparison with several parameterizations of these models supports the following geometry: the BLR clouds lie inside the bowl closely above the bowl rim, up to a half covering angle 0 deg < theta < 40 deg (measured against the equatorial plane). Then the BLR is spread over many isodelay surfaces, yielding a smeared and structured echo as observed. Furthermore, if the BLR clouds shield the bottom of the bowl rim against radiation from the nucleus, the hot dust emission comes essentially from the top edge of the bowl (40 deg < theta < 45 deg). Then, for small inclinations as for 3C120, the top dust edge forms a ring which largely coincides with a narrow range of isodelay surfaces, yielding the observed sharp dust echo. The scale height of the BLR increases with radial distance from the black hole. This leads to luminosity dependent foreshortening effects of the lag. We discuss implications and possible corrections of the foreshortening for the black hole mass determination and consequences for the lag (size) - luminosity relationships and the difference to interferometric torus sizes.
The determination of the size and geometry of the broad line region (BLR) in active galactic nuclei is one of the major ingredients for determining the mass of the accreting black hole. This can be done by determining the delay between the optical continuum and the flux reprocessed by the BLR, in particular via the emission lines. We propose here that the delay between polarized and unpolarized light can also be used in much the same way to constrain the size of the BLR; we check that meaningful results can be expected from observations using this technique. We use our code STOKES for performing polarized radiative transfer simulations. We determine the response of the central source environment (broad line region, dust torus, polar wind) to fluctuations of the central source that are randomly generated; we then calculate the cross correlation between the simulated polarized flux and the total flux to estimate the time delay that would be provided by observations using the same method. We find that the broad line region is the main contributor to the delay between the polarized flux and the total flux; this delay is independent on the observation wavelength. This validates the use of polarized radiation in the optical/UV band to estimate the geometrical properties of the broad line region in type I AGNs, in which the viewing angle is close to pole-on and the BLR is not obscured by the dust torus.
By using standard broad-band VRI photometry we were able to discriminate the variations of the broad hydrogen alpha line from the continuum variations for the active galaxy Mkn 279. Cross-correlating both light curves enabled us to determine the time lag of the broad line variations behind the continuum and thus to determine the BLR size (about 8 light days). Our preliminary results are rather consistent with the spectroscopic reverberation mapping results (about 12/17 days). This study is a part of an ambitious program to perform photometric reverberation mapping and determine BLR sizes (respectively - the central black hole masses) for more that 100 nearby AGN.
We present the first results from the Las Cumbres Observatory Global Telescope (LCOGT) Networks Active Galactic Nuclei Key Project, a large program devoted to using the robotic resources of LCOGT to perform time domain studies of active galaxies. We monitored the Seyfert 1 galaxy Arp~151 (Mrk~40) for $sim$200 days with robotic imagers and with the FLOYDS robotic spectrograph at Faulkes Telescope North. Arp~151 was highly variable during this campaign, with $V$-band light curve variations of $sim$0.3 mag and H$beta$ flux changing by a factor of $sim$3. We measure robust time lags between the $V$-band continuum and the H$alpha$, H$beta$ and H$gamma$ emission lines, with $tau_mathrm{cen} = 13.89^{+1.39}_{-1.41}$, 7.52$^{+1.43}_{-1.06}$ and 7.40$^{+1.50}_{-1.32}$ days, respectively. The lag for the ion{He}{2} $lambda4686$ emission line is unresolved. We measure a velocity-resolved lag for the H$beta$ line, which is clearly asymmetric with higher lags on the blue wing of the line which decline to the red, possibly indicative of radial inflow, and is similar in morphology to past observations of the H$beta$ transfer function shape. Assuming a virialization factor of $f$=5.5, we estimate a black hole mass of $M_mathrm{BH}=6.2^{+1.4}_{-1.2}times$10$^{6}$~$M_{odot}$, also consistent with past measurements for this object. These results represent the first step to demonstrate the powerful robotic capabilities of LCOGT for long-term, AGN time domain campaigns that human intensive programs cannot easily accomplish. Arp 151 is now one of just a few AGN where the virial product is known to remain constant against substantial changes in H$beta$ lag and luminosity.
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