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VLBI observations of the water gigamaser in TXS 2226-184

Abstract: Outside the Milky Way the most luminous water masers at 22 GHz, called megamasers because of their extreme luminosity with respect to the Galactic and extragalactic water masers associated with star formation, are mainly detected in active galactic nuclei. In the case of the water maser detected in the nuclear region of the galaxy TXS 2226-184, the term gigamaser was used for the first time. However, the origin of this very luminous water maser emission has never been investigated in detail.
We studied the nature of the water gigamaser in TXS 2226-184 by measuring its absolute position for the first time at milliarcsecond resolution, by comparing the morphology and characteristics of the maser emission on VLBI scales after about 20 years, and by trying to detect its polarized emission.
We observed the water maser emission toward TXS 2226-184 three times: the very first time with the NRAO VLBA (epoch 2017.45) and the next two times with the EVN (epochs 2017.83 and 2018.44). The first two epochs (2017.45 and 2017.83) were observed in phase-reference mode, while the last epoch (2018.44) was observed in full-polarization mode, but not in phase-reference mode to increase the on-source integration time. We also retrieved and analyzed the VLBA archival data at 22 GHz of TXS 2226-184 observed in epoch 1998.40.
In my talk I will show the results that we obtained from our VLBI observations and I will also present possible scenarios to determine the nature of the gigamaser.
Brief CV of Dr. Gabriele Surcis
Gabriele Surcis was born in Chiavari (Genova) and, at the age of seven years, he moved with his family in Sardinia, at Cagliari. He graduated in Physics at the University of Cagliari in 2007 with a thesis on extragalactic water maser. In 2008 he started his PhD at the Argelander-Institut fuer Astronomie in Bonn, Germany. Three years later he successfully defended his PhD Thesis based on the measurement of magnetic fields around massive young stellar object by studying the polarized emission of masers. Then he moved in Dwingeloo (the Netherlands) to work at the Joint Institute for VLBI ERIC (JIVE) as support scientist of the European VLBI Network. There he continued his polarization studies of masers. In 2016 he moved back to Cagliari where he became a research staff member of the INAF-Osservatorio Astronomico di Cagliari. Here, he is at the moment a member of the Operational Team of the Sardinia Radio Telescope (SRT) with the duty of VLBI friend of SRT within the EVN.