A tentative 114-minute orbital period challenges the ultra-compact nature of the X-ray binary 4U 1812-12

TL;DR

This study presents a 114-minute modulation in 4U 1812-12, challenging its classification as an ultra-compact X-ray binary and suggesting it may be a progenitor of such systems.

Contribution

It provides the first detailed photometric analysis indicating a longer orbital period, questioning previous classifications of the system as ultra-compact.

Findings

The 114-minute modulation is similar to a superhump.

The system's properties suggest it is not an ultra-compact X-ray binary.

A progenitor scenario can reconcile all observational data.

Abstract

We present a detailed time-resolved photometric study of the ultra-compact X-ray binary candidate 4U 1812-12. The multicolor light curves obtained with HiPERCAM on the 10.4-m Gran Telescopio Canarias show an aprox 114 min modulation similar to a superhump. Under this interpretation, this period should lie very close to the orbital period of the system. Contrary to what its other observational properties suggest (namely, persistent dim luminosity, low optical-to-X-ray flux ratio and lack of hydrogen features in the optical spectrum), this implies that 4U1812-12 is most likely not an ultra-compact X-ray binary, which are usually defined as systems with orbital periods lower than 80 min. We discuss the nature of the system, showing that a scenario in which 4U 1812-12 is the progenitor of an ultra-compact X-ray binary may reconcile all the observables.