| Abstract
| - Context. The growth of supermassive black holes (SMBHs) through merging has long been predicted but its detection remains elusive. However, a promising target has been discovered in the Seyfert-1 galaxy J1430+2303, where two SMBHs may be about to merge. Aims. If a binary system truly lies at the center of J1430+2303, the usual symmetry expected from pole-on views in active galactic nuclei (AGNs) responsible for the observed low (≤1%) optical linear polarization in the continuum of these objects is expected to be broken. This should lead to higher-than-usual polarization degrees, together with time-dependent variations of the polarization signal. Methods. We used the specialized photopolarimeters RoboPol mounted on the 1.3 m telescope at the Skinakas Observatory and the Alhambra Faint Object Spectrograph and Camera (ALFOSC) mounted on the 2.56 m Nordic Optical Telescope (NOT) at the “Roque de los Muchachos” Observatory to measure the B-, V-, R-, and I-band polarization of J1430+2303. Observations were complemented using the FORS2 spectropolarimeter mounted on the VLT to acquire 3500−8650 Å polarized spectra. We compared our set of observations to Monte Carlo radiative-transfer predictions to look for the presence of a SMBH binary. Results. The observed linear continuum polarization of J1430+2303 in the V and R bands is ∼0.4% with an associated polarization angle of slightly larger than 0°. We detected no significant changes in polarization or photometry between May, June, and July of 2022. In addition, there is no significant difference between the polarization of H α and the polarization of the continuum. A single SMBH at the center of an AGN model is able to reproduce the observed spectrum and polarization, while the binary hypothesis is rejected with a probability of ∼85%. Conclusions. The low degree of continuum polarization, the lack of variability in photometry and polarization over three months, and the absence of H α polarization different than that of the continuum tend to indicate that J1430+2303 is a standard Seyfert-1 AGN whose nuclear inclination is 24−31° according to our model.
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