The Tantalizing Case of the Quasar J0950+5128. I. Presentation of the Data and Detailed Exploration of the Binary Supermassive Black Hole Scenario

Abstract Spectroscopic observations of the quasar J0950+5128 spanning 22 yr reveal monotonic radial velocity variations in its broad H β emission line. Moreover, the line profile becomes broader over time, necessitating careful measurements. We present robust H β velocity shift measurements obtained via cross correlation, applied to both the full spectra and to isolated broad H β components derived from spectral decomposition. We also examine the light curves for variability consistent with the spectroscopic trends. Using Lomb–Scargle periodogram analysis, we find no significant periodic signal. We consider several interpretations for the observed changes, including a binary supermassive black hole, dust-cloud obscuration, outflows, a recoiling black hole, and a single perturbed, disk-like broad-line region (BLR). We deem the binary and perturbed BLR scenarios to be physically plausible. The binary interpretation is the only one for which we can immediately compare a physical model to the available data. Thus, we incorporate radial velocity “jitter” to emulate typical quasar variability and fit the radial velocity curve with a Keplerian model to examine whether it can reproduce the observations. In this context, the available observations trace only a segment of the putative orbit. The fit yields a period of 33 yr (observed frame) and an eccentricity of 0.65, with lower limits on the semimajor axis and black hole mass of 10 −2 pc and 10 7 M ⊙ , respectively. Thus, J0950+5128 is a binary candidate deserving further study. The single, perturbed BLR interpretation remains viable but requires additional observations and modeling for further evaluation. Continued monitoring is, therefore, essential.