Abstract
Abstract The advent of time domain astronomy is revolutionizing our understanding of the universe. Programs such as the Catalina Real-time Transient Survey (CRTS) or the Palomar Transient Factory (PTF) surveyed millions of objects for several years, allowing variability studies on large statistical samples. The inspection of ≈250 k quasars in CRTS resulted in a catalog of 111 potentially periodic sources, put forward as supermassive black hole binary (SMBHB) candidates. A similar investigation on PTF data yielded 33 candidates from a sample of ≈35 k quasars. Working under the SMBHB hypothesis, we compute the implied SMBHB merger rate and we use it to construct the expected gravitational wave background (GWB) at nano-Hz frequencies, probed by pulsar timing arrays (PTAs). After correcting for incompleteness and assuming virial mass estimates, we find that the GWB implied by the CRTS sample exceeds the current most stringent PTA upper limits by almost an order of magnitude. After further correcting for the implicit bias in virial mass measurements, the implied GWB drops significantly but is still in tension with the most stringent PTA upper limits. Similar results hold for the PTF sample. Bayesian model selection shows that the null hypothesis (whereby the candidates are false positives) is preferred over the binary hypothesis at about 2.3σ and 3.6σ for the CRTS and PTF samples respectively. Although not decisive, our analysis highlights the potential of PTAs as astrophysical probes of individual SMBHB candidates and indicates that the CRTS and PTF samples are likely contaminated by several false positives.
Highlights
In the past two decades, it has been realized that supermassive black holes (SMBHs) are a fundamental ingredient of galaxy formation and evolution, and it is well established that possibly all massive galaxies host an SMBH at their center
This is true when virial mass estimates of the candidates are taken at face value; in this case, both the Catalina Real-time Transient Survey (CRTS) and the Palomar Transient Factory (PTF) sample are severely inconsistent with pulsar timing arrays (PTAs) upper limits
Model_True: Lower Signal Normalization Taking virial mass measurements at face value, the CRTS sample is inconsistent with PTA limits, implying that for the majority of the candidates, inferred variability cannot be linked to the presence of an supermassive black hole binary (SMBHB)
Summary
In the past two decades, it has been realized that supermassive black holes (SMBHs) are a fundamental ingredient of galaxy formation and evolution (see, e.g., Kauffmann & Haehnelt 2000; Croton et al 2006), and it is well established that possibly all massive galaxies host an SMBH at their center (see Kormendy & Ho 2013 and references therein). We show that, when properly converted into an SMBHB merger rate and extrapolated to lower frequencies, the CRTS and PTF samples are in tension with current PTA measurements This is true when virial mass estimates of the candidates are taken at face value; in this case, both the CRTS and the PTF sample are severely inconsistent with PTA upper limits. The approach in the “True” models further assumes that the scatter observed in the virial mass estimates, measured in practice at fixed luminosity (Lλ) and line width (FWHM of broad lines), represents the scatter at a fixed true mass This is the most common interpretation, which leads to Equation (3) (see, e.g., Shen et al 2008).
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