Transit light-curve models

PyORBIT provides two main families of transit models. The models described in this page use one orbital ephemeris for each planet: the period P and one reference time of inferior conjunction Tc define all the predicted transits as Tc + N * P.

Use these models when the transit times are assumed to follow a linear ephemeris, or when the timing deviations are not part of the fit. If each transit must have its own fitted mid-time, use the TTV models documented in Transit models for TTV measurements.

Planet setup

For a standard transit fit, define the planet as a transiting planet by enabling the time of inferior conjunction:

planet_b:
  common: planets
  orbit: circular
  parametrization: Eastman2013
  use_time_inferior_conjunction: True

With this option the planet common object exposes Tc as the epoch parameter. If it is not enabled, PyORBIT uses the orbital longitude parametrization and derives the inferior-conjunction time internally.

The transit shape is controlled by the usual planet, stellar and limb-darkening parameters:

Parameter	Scope	Meaning
P	planet common	Orbital period.
Tc	planet common	Reference time of inferior conjunction.
R_Rs	planet common	Planet-to-star radius ratio.
b or i	planet common	Impact parameter or inclination, depending on the planet setup.
a_Rs or stellar density	planet/star common	Scaled semi-major axis, or the stellar density used to compute it.
e, omega	planet common	Eccentricity and argument of periastron, or the selected eccentricity parametrization.
limb-darkening coefficients	limb-darkening common	Coefficients used by the selected limb-darkening law.

Available models

Model name	Backend	Use case
batman_transit	batman	Standard transit model with one linear ephemeris per planet.
pytransit_transit	PyTransit	Standard transit model using the PyTransit backend. It uses the RoadRunner model by default when available.
batman_transit_rprs_subset	batman	Same linear ephemeris, but with a different R_Rs value for each dataset subset.
pytransit_dynamical	PyTransit	Dynamical transit model. Transit times are predicted by the dynamical model, not fitted as independent TTV parameters.

The alias subset_batman_transit_rprs is accepted for the subset radius-ratio model. The transit, secondary-eclipse and phase-curve model is documented in Secondary eclipse and phase curve.

Shared model keywords

Keyword	Models	Meaning
planets	all	List of planet common objects included in the light-curve model.
limb_darkening	transit models	Limb-darkening common object used by the transit backend.
supersample_factor	all	Number of sub-exposures used to integrate long-cadence observations.
exposure_time	all	Exposure time used together with supersample_factor.
nthreads	batman models	Number of threads passed to the batman backend.
use_roadrunner	pytransit_transit	Use the PyTransit RoadRunner implementation when possible.

Minimal examples

A standard batman transit model:

lc_model:
  model: batman_transit
  planets: [b]
  limb_darkening: ld_quadratic
  supersample_factor: 5
  exposure_time: 0.02043365

The equivalent PyTransit setup:

lc_model:
  model: pytransit_transit
  planets: [b]
  limb_darkening: ld_quadratic
  use_roadrunner: True

The dataset using the model can then be declared in the usual way:

input:
  LCdata:
    file: lightcurve.dat
    kind: Phot
    models:
      - lc_model

Radius-ratio subsets

Use batman_transit_rprs_subset when all subsets share the same ephemeris and orbital shape, but each subset needs its own radius ratio. The input dataset must include a subset column, and PyORBIT creates parameters such as R_Rs_0, R_Rs_1, and so on for the active subset identifiers.

lc_model:
  model: batman_transit_rprs_subset
  planets: [b]
  limb_darkening: ld_quadratic

This is useful for multi-band or multi-instrument light curves where the transit depth can change, but the timing is still described by a single P and Tc.

Choose the TTV models instead when the goal is to measure an independent mid-transit time for each observed transit.