Generating X-ray transit profiles with batman

TL;DR

This paper adapts the batman exoplanet transit code to generate X-ray transit profiles, accounting for coronal emission and providing tools to analyze how planetary and stellar parameters affect transit detectability.

Contribution

It introduces a modified batman code for X-ray transits with a simple coronal model and derives scaling laws for transit detectability based on key parameters.

Findings

Transit detectability scales linearly with planet cross-sectional area in X-rays.

Coronal temperature affects detectability with a power-law relation of slope -1/4.

Impact parameter has minimal effect on detectability until it nears unity.

Abstract

We present an adaptation of the exoplanet transit model code batman, in order to permit the generation of X-ray transits. Our underlying extended coronal model assumes an isothermal plasma that is radially symmetric. While this ignores the effect of bright, active regions, observations of transits in X-rays will require averaging across multiple epochs of data for the foreseeable future, significantly reducing the importance of more complex modelling. Our publicly available code successfully generates the predicted W-shaped transit profile in X-rays due to the optically thin nature of the emission, which concentrates the expected observational emission around the limb of the photospheric stellar disc. We provide some examples based on the best known X-ray transit target, HD 189733b, and examine the effect of varying the planet size, coronal temperature, and impact parameter on the resulting transit profile. We also derived scaling relationships for how the overall transit detectability is affected by changing these parameters. Over most of the parameter space, we find that the detectability scales linearly with the cross-sectional area of the planet in X-rays. The relationship with increasing coronal temperature is less fixed, but averages out to a power law with slope $-1/4$ except when the impact parameter is high. Indeed, varying impact parameter has little effect on detectability at all until it approaches unity.