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A lower limit to the accretion disc radius in the active galactic nucleus NGC 1052

Author/s: L. Reb, J. A. Fernández-Ontiveros, M. A. Prieto, K. Dolag

Reference: 2018 MNRAS 478 L122 | Link

Spectral energy distribution for the nucleus of NGC 1052. Black dots represent the sub-arcsec resolution measurements (< 0.4 arcsec), grey spikes are low-angular resolution measurements (> 1 arcsec). The interpolation of the measured values (light blue line) is used to derive the continuum luminosity of the nucleus. The power law (dark green dotted line) imposes flux constraints for the maximum contribution of the standard disc (orange dotted line) and for the truncated disc (red line). A hot standard disc (magenta dashed line) of equal accretion power as the truncated disc violates these constraints. In addition, the Seyfert 2 sub-arcsec resolution template of Prieto et al. (2010) is overlaid (grey line), which is dominated by thermal emission from dust in these nuclei.
Spectral energy distribution for the nucleus of NGC 1052. Black dots represent the sub-arcsec resolution measurements (< 0.4 arcsec), grey spikes are low-angular resolution measurements (> 1 arcsec). The interpolation of the measured values (light blue line) is used to derive the continuum luminosity of the nucleus. The power law (dark green dotted line) imposes flux constraints for the maximum contribution of the standard disc (orange dotted line) and for the truncated disc (red line). A hot standard disc (magenta dashed line) of equal accretion power as the truncated disc violates these constraints. In addition, the Seyfert 2 sub-arcsec resolution template of Prieto et al. (2010) is overlaid (grey line), which is dominated by thermal emission from dust in these nuclei.

Low-luminosity active galactic nuclei (LLAGNs) are found in about 1/3 of all the galaxies in the Local Universe, establishing the most numerous class of AGNs. At low accretion rates, LLAGNs are expected to develop major changes in the structure of the accretion disc when compared to their bright counterparts, Seyfert galaxies and Quasars. Here we present high-angular resolution data (~13 pc) for the LLAGN in the nucleus of NGC 1052, covering 10 orders of magnitude in frequency from radio to X-rays. The flux distribution of the nucleus is well described by a broken power law plus an inverse Compton component, indicative of a dominant synchrotron emission from a jet. This is in contrast with the dust thermal continuum that dominates the infrared range in Seyfert 2 galaxies. A further analysis of the power balance in NGC 1052 revealed that a standard accretion disc, similar to that observed in Quasars and type 1 Seyferts, cannot provide enough power to support the continuum luminosity of this source. To reconcile the inferred accretion power with the observed luminosity, we introduced a truncated accretion disc model. When applied to sub-arcsec resolution data, this results in a lower limit to the truncated accretion disc radius of rtr > 26 Rg (where Rg corresponds to a gravitational radius), which is consistent with the inner radius derived from previous observations of the iron Kα line in the X-ray spectrum of this nucleus. This is the first estimate for a lower limit on the truncation radius of the accretion disc based on high angular resolution data only.

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