What’s Hiding Amongst The Pulses?: Using Phase Modulation In The Light Curves Of RR Lyrae Variables To Search For Black Holes

The Light Travel Time Effect (LTTE) is a powerful tool to identify variable stars in binary systems. The phase change in the light curve of a variable star due to the star’s orbital motion can be used to find the mass of its companion. We developed a phase detector, which fits temporal subsets of data to determine phase changes over time in order to identify occurrences of LTTE. The phase variation of two candidate RR Lyrae were analyzed: EL Hya, which was identified from Asteroid Terrestrial-impact Last Alert System (ATLAS) data, and AR Ser, which was the dimmest candidate binary identified by proper motion anomaly in Kervella et al. (2019). Using data from ATLAS, All-Sky Automated Survey for Supernovae (ASAS-SN), the Catalina Sky Survey, and the Siding Spring Survey, we found that for both EL Hya and AR Ser the amplitude and period of their phase variation changes as a function of time. Hence, it is likely that both stars are impacted by the Blazhko effect, an internal phase variation in RR Lyrae, and it is unlikely that the phase change is caused by LTTE. If the variations are caused by binarity, EL Hya would be orbiting an object of at least 4.33 M_☉ and AR Ser would be orbiting one of at least 34.1 M_☉, which would likely be a black hole. Thus, LTTE offers the potential to discover variable stars in binaries with black holes.