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$+$5128 spanning 22 years reveal monotonic radial velocity variations in its broad H$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. We disfavor all but the binary and perturbed broad-line region scenarios. 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 years (observed frame) and an eccentricity of 0.65, with lower limits on the semi-major axis and black hole mass of $10^{-2}\;$pc and $10^7\;{\rm M}_\odot$, respectively. Thus, J0950$+$5128 is a compelling supermassive binary candidate. The single, perturbed broad-line region interpretation remains viable but requires additional observations and modeling for further evaluation. Continued monitoring is, therefore, essential.

Figures (16)

• Figure 1: The full observed spectra of J0950 spanning approximately 22 years, shown after all the standard calibrations and corrections described in Section \ref{['subsec:available_spectra']} have been applied. For visual clarity, each spectrum was linearly scaled to match the [O iii] flux and continuum of the 2002 spectrum, then vertically offset by a constant amount from the last spectrum. The sharp feature seen at a rest wavelength of 4600 Å is the residual from imperfect correction of the [O i] $\lambda$5577 telluric emission line. The absorption lines near rest wavelengths of 5650 and 6260 Å are also telluric (due to molecular Oxygen transitions).
• Figure 2: The light curves of J0950 from the Zwicky Transient Facility's (ZTF) g-, r-, and i-band, the Catalina Real-Time Transient Survey (CRTS), and the Near-Earth Object Wide-Field Infrared Survey Explorer's (NEOWISE) W1 and W2 filters.
• Figure 3: Example spectral decomposition of J0950's 2010 spectrum performed using PyQSOfitguo2018pyqsofit. The observed spectrum after subtracting the host galaxy component (cyan), is shown in grey, with the best-fitting model overplotted in blue. The residuals are presented as a light-grey dotted line in the lower panel. The separate components of the model, including the continuum, Fe ii lines, broad H$\beta$, and narrow H$\beta$ and [O iii] lines, are also shown.
• Figure 4: The spectra of J0950 obtained over 22 years, identical to those in Figure \ref{['fig:complete_spectra']} but shown over a narrower wavelength range and on a velocity scale. The broad H$\beta$ line is Doppler shifted to bluer wavelengths at first, and becomes redshifted in the later spectra. The line shape also displays variability over time. The rest wavelength of H$\beta$ (vertical black dashed line) is set by the redshift measured from the [O iii] $\lambda\lambda$4959,5007 lines emitted from larger scales in the host galaxy. The vertical grey dotted line at 4853 Å ($-597\,\mathrm{km\,s^{-1}}$) marks the location of the wavelength of the Na I D emission line from the night sky. This line is not always corrected properly; therefore, the residual from Na I D subtraction in some spectra has been masked out in our later analysis.
• Figure 5: The broad H$\beta$ emission of J0950 isolated by spectral decomposition (the continuum, host, Fe ii lines, broad & narrow He ii, narrow H$\beta$, and [O iii] doublet have been subtracted out, as described in Section \ref{['subsec:crosscor_data']}). The lines are vertically offset by a constant amount. See Figure \ref{['fig:data']} caption for further details.