Rubin Observatory. Astronomers estimate that the optical sky survey will detect signs of up to 100 binary massive black holes in galaxies due to their light fluctuations. Credit: Vera C. Rubin Observatory
A "massive black hole" (MBH), is one with a mass greater than one hundred thousand solar masses. MBHs are located at the center of most galaxies. They emit electromagnetic radiation from their hot disk environments and emit gas and dust across the electromagnetic spectrum.
Many galaxies have been involved with a merger with another star during their lives. These interactions are an important phase of galaxy evolution. When two MBHs merge, they can create a binary system that orbits one another. These binaries are very rare. However, theoretical estimates suggest that one in every thousand active galaxies will have a binary MBH.
Astronomers still believe that binary MBHs are possible. Binary MBHs, which are promising sources for gravitational waves, should be visible as they travel past pulsars. This is because they distort the timings of pulsed radio emission. Binary MBHs made from accrete material will emit weak electromagnetic signals across the spectrum. However, neither of these effects have been detected and no binary MBHs are confirmed.
Rosanne DiStefano, CfA Astronomer, and her coworkers have discovered a third way to spot binary BMHs. This method was first proposed by Di Stefano and colleagues in 2018. They suggest looking for variations in the optical signal caused by gravitational lensing by the MBHs of light emitted from accreting material within their surrounding disk.
Variable light curves can be used to detect gravitational lensesing from exoplanets. They distort light from background stars and have been successful in this regard. To estimate the frequency, orientation, and other properties for binary MBHs, the scientists use the Illustris computer simulations of galaxy evolution. Based on the capabilities of Rubin Observatory Legacy Survey of Space and Time, the astronomers predicted that between ten and one hundred gravitationally self-lensed binaries MBHs could potentially be positively detected. This is even after taking into account many complicating factors such as dust obscuration and intrinsic AGN variability. These results will be important, not only in proving the existence of binary MBHs, but also for studying the orbital parameters and accretion activity of these objects.
The research was published by Monthly Notices of the Royal Astronomical Society.
More information: Luke Zoltan Kelley and colleagues, Gravitational Self-Lensing in Populations of Massive Black Hole Binaries, Monthly Notices of the Royal Astronomical Society (2021). Information for the Journal: Monthly Notices from the Royal Astronomical Society Luke Zoltan Kelley and others, Gravitational Self-Lensing in Populations of Massive Black Hole Binaries, (2021). DOI: 10.1093/mnras/stab2776
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