When exoplanets pass in front of their stars, they imprint a transit signature on the stellar light curve which to date has been assumed to be symmetric in time, owing to the planet being modelled as a circular area occulting the stellar surface. However, this signature might be asymmetric due to different temperature/pressure and/or chemical compositions in the different terminator regions of the transiting planet. If we could be able to model these asymmetric signatures directly from transit light curves it could give us an unprecedented glimpse into planetary 3-D structure, helping constrain models of atmospheric evolution, structure and composition.

Now, Kathryn Jones and Nestor Espinoza proposed a Python package called “catwoman”, which allows us to model these asymmetric transit lightcurves and calculate them (lightcurves) for any radially symmetric stellar limb darkening law. Their study appeared in Journal of Open Source Software.

In order to obtain the desired light curves, catwoman first calculates many models, with varying widths and geometrically searches for a width that produces an error less than 1% away (and always less than) the specified level. The model then uses this width value to calculate the light curves. A lower specified error, and therefore thinner iso-intensity bands, produces more accurate light curves.

In catwoman, we can model planets as two semi-circles, of different radii, using the integration algorithm. It also allows for φ, the angle of rotation of the semi-circles, to vary as a free parameter, which is something no other model has tried to implement, accounting for the possibility of spin-orbit misalignments of the planet.

It was designed to be used by astronomical researchers. For a realistic light curve with 100 in-transit data points, catwoman takes around 340 seconds to produce 1 million quadratic-limb-darkened light curves on a single 1.3 GHz Intel Core i5 processor. It is used in Espinoza & Jones (in prep.)

It is fast and efficient and open source with full documentation available to view at

https://catwoman.readthedocs.io .

**Featured image:*** Diagram of the geometric configuration during transit of two stacked semi-circles (one of radius Rp,1, and another of radius Rp,2) that model the different limbs of an exoplanet transiting in front of a star. The area of the star has been divided in different sections of radius xi (dashed circles) — between each subsequent section, the star is assumed to have a radially symmetric intensity profile (e.g., blue band between x_i–1 and xi above). In order to obtain the light curve, the challenge is to calculate the sum of the intersectional areas between a given iso-intensity band and the semi-circles, ∆A (blue band with dashed grey lines). Note the stacked semi-circles are inclined by an angle φ with respect to the planetary orbital motion. © Jones et al. *

**Reference***: Jones et al., (2020). catwoman: A transit modelling Python package for asymmetric light curves. Journal of Open Source Software, 5(55), 2382, https://doi.org/10.21105/joss.02382*

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