New Constraint on Scalar Gauss-Bonnet Gravity and a Possible Explanation for the Excess of the Orbital Decay Rate in a Low-Mass X-ray Binary

TL;DR

This paper constrains scalar Gauss-Bonnet gravity using black hole low-mass X-ray binaries and explores whether scalar radiation can explain the observed excess in orbital decay rate in XTE J1118+480, proposing future tests with gravitational wave detectors.

Contribution

It provides the strongest current bounds on sGB gravity from BH-LMXBs and suggests a possible astrophysical explanation for the orbital decay excess.

Findings

A0620-00 bounds are six orders of magnitude stronger than solar system bounds.

Scalar radiation in sGB theory can potentially explain the excess in XTE J1118+480.

Future gravitational wave observations could improve constraints by three orders of magnitude.

Abstract

It was recently shown that a black hole (BH) is the only compact object that can acquire a scalar charge in scalar Gauss-Bonnet (sGB) theory under the small coupling approximation. This leads to the fact that scalar radiation is emitted from a binary containing at least one BH. In this letter, we find the constraints on this theory from BH low-mass X-ray binaries (BH-LMXBs). The main result of this letter is that from the orbital decay rate of A0620-00, we obtained a conservative bound that is six orders of magnitude stronger than the solar system bound. In addition to this, we look at XTE J1118+480, whose orbital decay rate has been recently measured with an excess compared to the theoretical prediction in GR due to the radiation reaction. The cause of this excess is currently unknown. Although it is likely that the cause is of astrophysical origin, here we investigate the possibility of explaining this excess with the additional scalar radiation in sGB theory. We find that there still remains a parameter range where the excess can be explained while also satisfying the constraint obtained from A0620-00. The interesting point is that for most of other alternative theories of gravity, it seems difficult to explain this excess with the additional radiation. This is because it would be difficult to evade the constraints from binary pulsars or they have already been constrained rather strongly from other observations such as solar system experiments. We propose several ways to determine whether the excess is caused by the scalar radiation in sGB gravity including future gravitational wave observations with space-borne interferometers, which can give a constraint three orders of magnitude stronger than that from A0620-00.