2016ApJ...826..132S

We report the discovery of a "triple-double radio galaxy," J1216+0709, detected in deep low-frequency Giant Metrewave Radio Telescope (GMRT) observations. J1216+0709 is only the third radio galaxy, after B0925+420 and Speca, with three pairs of lobes resulting from three different episodes of active galactic nucleus (AGN) jet activity. The 610 MHz GMRT image clearly displays an inner pair of lobes, a nearly coaxial middle pair of lobes, and a pair of outer lobes that is bent with respect to the axis of the inner pair of lobes. The total end-to-end projected sizes of the inner, middle, and outer lobes are 40'' (∼95 kpc), 1.'65 (∼235 kpc), and 5.'7 (∼814 kpc), respectively. Unlike the outer pair of lobes, both the inner and middle pairs of lobes exhibit asymmetries in arm lengths and flux densities, but in the opposite sense, i.e., the eastern sides are farther and also brighter than the western sides, thus, suggesting the possibility of the jet being intrinsically asymmetric rather than due to a relativistic beaming effect. The host galaxy is a bright elliptical (m_r ∼ 16.56) with M_SMBH ∼ 3.9 x 10⁹ M_☉ and a star formation rate of ∼ {4.66}_{-1.61}^{+4.65} M_☉/yr. The host galaxy resides in a small group of three galaxies (m_r <= 17.77) and is possibly going through an interaction with faint dwarf galaxies in the neighborhood, which may have triggered the recent episodes of AGN activity.