Water megamaser emission in hard X-ray selected AGN

Abstract

Context. Water megamaser emission at 22 GHz has proven to be a powerful tool for astrophysical studies of active galactic nuclei (AGN) because it allows an accurate determination of the mass of the central black hole and of the accretion disc geometry and dynamics. However, after searches among thousands of galaxies, only about 200 of them have shown such spectroscopic features, most of them of uncertain classification. In addition, the physical and geometrical conditions under which a maser activates are still unknown. Aims. We characterize the occurrence of water maser emission in an unbiased sample of AGN by investigating the relation with the X-ray properties and the possible favourable geometry that is required to detect water maser. Methods. We searched for 22 GHz maser emission in a hard X-ray selected sample of AGN, taken from the INTEGRAL/IBIS survey above 20 keV. Only half of the 380 sources in the sample have water maser data. We also considered a volume-limited sub-sample of 87 sources, for which we obtained new observations with the Green Bank and Effelsberg telescopes (for 35 sources). We detected one new maser and increased its radio coverage to 75%. Results. The detection rate of water maser emission in the total sample is 15 ± 3%. This fraction increases to 19 ± 5% for the complete sub-sample, especially when we consider type 2 (22 ± 5% and 31 ± 10% for the total and complete samples, respectively) and Compton-thick AGN (56 ± 18% and 50 ± 35% for the total and complete samples, respectively). No correlation is found between water maser and X-ray luminosity. We note that all types of masers (disc and jet) are associated with hard X-ray selected AGN. Conclusions. These results demonstrate that the hard X–ray selection may significantly enhance the maser detection efficiency over comparably large optical or infrared surveys. A possible decline in detection fraction with increasing luminosity might suggest that an extremely luminous nuclear environment does not favour maser emission. The large fraction of CT AGN with water maser emission could be explained in terms of geometrical effects. The maser medium would then be the very edge-on portion of the obscuring medium.

ALMA Spatially Resolved Dense Molecular Gas Survey of Nearby Ultraluminous Infrared Galaxies

“…The detection rate of very luminous AGN-origin megamaser 183 GHz H 2 O emission is low (1/8 = 12.5%). The detection rates of 22 GHz H 2 O 6 1,6 -5 2,3 megamaser emission in previously conducted ex- tensive surveys are also low ( 10%) for active galaxies (e.g., Braatz et al 1997;Hagiwara et al 2002;Greenhill et al 2003;Braatz et al 2004;Henkel et al 2005), but can increase (10-50%) if only highly obscured AGNs are observed (e.g., Kondratko et al 2006;Yamauchi et al 2017;Castangia et al 2019;Kuo et al 2020;Panessa et al 2020). Observations of a larger number of nearby (U)LIRGs that are diagnosed to host luminous obscured AGNs, are needed to better constrain how the detection rates of very luminous 183 GHz H 2 O megamaser emission differ from those of 22 GHz H 2 O magamaser emission.…”

Searching Water Megamasers By Using Mid-infrared Spectroscopy (I): Possible Mid-infrared Indicators

“…Moreover, the maser detection rate could increase significantly in a criterion exploiting MIR and X-ray sample (Castangia et al 2016(Castangia et al , 2019. Panessa et al (2020) found the maser detection rate tend to be much higher in type 2 and Compton-thick AGNs (Further discussion in §5.4). In this studies, we focus on improving maser detection rate using MIR spectra.…”

“…We adopted the total maser sample of 161 targets from Kuo et al (2018) with classification. In order to get a complete maser type classification, we also cross-matched maser catalogue from Panessa et al (2020) and replenish the additional maser types (e.g. jet/outflow).…”

“…Furthermore, Castangia et al (2016Castangia et al ( , 2019 found the maser detection rate increased significantly (up to 50%) in a criterion exploiting MIR and X-ray sample. Panessa et al (2020) studied an unbiased AGN sample, for which they found the detection rate significantly increased when considering type 2 and Compton-thick AGNs. The detection rate reached to 56± 18% in Compton-thick AGN sample.…”

“…Besides, Castangia et al (2016) and Castangia et al (2019), based on a selection criterion presented in Severgnini et al (2012), suggested that the combination of mid-infrared (MIR) and X-ray luminosity might be able to increase the detection rate of masers. Panessa et al (2020) suggested the maser detection rate was significantly increased in Compton-thick AGNs and hard X-ray luminosity. Moreover, there are a few characteristic spectral features of maser galaxies which are different from other galaxies, and these may be helpful to increase the detection rate.…”