Search for: All records

ABSTRACT Near IR spectroscopic reverberation of Active Galactic Nuclei (AGN) potentially allows the infrared (IR) broad line region (BLR) to be reverberated alongside the disc and dust continua, while the spectra can also reveal details of dust astro-chemistry. Here, we describe results of a short pilot study (17 near-IR spectra over a 183 d period) for Mrk 509. The spectra give a luminosity-weighted dust radius of 〈Rd,lum〉 = 186 ± 4 light-days for blackbody (large grain dust), consistent with previous (photometric) reverberation campaigns, whereas carbon and silicate dust give much larger radii. We develop a method of calibrating spectral data in objects where the narrow lines are extended beyond the slit width. We demonstrate this by showing our resultant photometric band light curves are consistent with previous results, with a hot dust lag at >40 d in the K band, clearly different from the accretion disc response at <20 d in the z band. We place this limit of 40 d by demonstrating clearly that the modest variability that we do detect in the H and K band does not reverberate on time-scales of less than 40 d. We also extract the Pa β line light curve, and find a lag which is consistent with the optical BLR H β line of ∼70–90 d. This is important as direct imaging of the near-IR BLR is now possible in a few objects, so we need to understand its relation to the better studied optical BLR. 

Abstract We have recently initiated the first spectroscopic dust reverberation programme on active galactic nuclei in the near-infrared. Spectroscopy enables measurement of dust properties, such as flux, temperature, and covering factor, with higher precision than photometry. In particular, it enables measurement of both luminosity-based dust radii and dust response times. Here we report results from a 1 yr campaign on NGC 5548. The hot dust responds to changes in the irradiating flux with a lag time of ∼70 light-days, similar to what was previously found in photometric reverberation campaigns. The mean and rms spectra are similar, implying that the same dust component dominates both the emission and the variations. The dust lag time is consistent with the luminosity-based dust radius only if we assume a wavelength-independent dust emissivity law, i.e. a blackbody, which is appropriate for grains of large sizes (of a few μm). For such grains the dust temperature is ∼1450 K. Therefore, silicate grains have most likely evaporated and carbon is the main chemical component. But the hot dust is not close to its sublimation temperature, contrary to popular belief. This is further supported by our observation of temperature variations largely consistent with a heating/cooling process. Therefore, the inner dust-free region is enlarged and the dusty torus rather a ‘dusty wall’, whose inner radius is expected to be luminosity-invariant. The dust-destruction mechanism that enlarges the dust-free region seems to also partly affect the dusty region. We observe a cyclical decrease in dust mass with implied dust reformation times of ∼5–6 months.