# Licensed under a 3-clause BSD style license - see LICENSE.rst
import numpy as np
from astropy.table import Column, Table
from gammapy.maps import Map
from gammapy.modeling.models import PowerLawSpectralModel, SkyModel
from gammapy.stats import WStatCountsStatistic
from ..core import Estimator
from gammapy.utils.deprecation import deprecated_renamed_argument
__all__ = ["SensitivityEstimator"]
[docs]
class SensitivityEstimator(Estimator):
"""Estimate sensitivity.
This class allows to determine for each reconstructed energy bin the flux
associated to the number of gamma-ray events for which the significance is
``n_sigma``, and being larger than ``gamma_min`` and ``bkg_sys`` percent
larger than the number of background events in the ON region.
Parameters
----------
spectral_model : `~gammapy.modeling.models.SpectralModel`, optional
Spectral model assumption. Default is power-law with spectral index of 2.
n_sigma : float, optional
Minimum significance. Default is 5.
gamma_min : float, optional
Minimum number of gamma-rays. Default is 10.
bkg_syst_fraction : float, optional
Fraction of background counts above which the number of gamma-rays is. Default is 0.05.
Examples
--------
For a usage example see :doc:`/tutorials/analysis-1d/cta_sensitivity` tutorial.
"""
tag = "SensitivityEstimator"
@deprecated_renamed_argument("spectrum", "spectral_model", "v1.3")
def __init__(
self,
spectral_model=None,
n_sigma=5.0,
gamma_min=10,
bkg_syst_fraction=0.05,
):
if spectral_model is None:
spectral_model = PowerLawSpectralModel(
index=2, amplitude="1 cm-2 s-1 TeV-1"
)
self.spectral_model = spectral_model
self.n_sigma = n_sigma
self.gamma_min = gamma_min
self.bkg_syst_fraction = bkg_syst_fraction
[docs]
def estimate_min_excess(self, dataset):
"""Estimate minimum excess to reach the given significance.
Parameters
----------
dataset : `SpectrumDataset`
Spectrum dataset.
Returns
-------
excess : `~gammapy.maps.RegionNDMap`
Minimal excess.
"""
n_off = dataset.counts_off.data
stat = WStatCountsStatistic(
n_on=dataset.alpha.data * n_off, n_off=n_off, alpha=dataset.alpha.data
)
excess_counts = stat.n_sig_matching_significance(self.n_sigma)
is_gamma_limited = excess_counts < self.gamma_min
excess_counts[is_gamma_limited] = self.gamma_min
bkg_syst_limited = (
excess_counts < self.bkg_syst_fraction * dataset.background.data
)
excess_counts[bkg_syst_limited] = (
self.bkg_syst_fraction * dataset.background.data[bkg_syst_limited]
)
excess = Map.from_geom(geom=dataset._geom, data=excess_counts)
return excess
[docs]
def estimate_min_e2dnde(self, excess, dataset):
"""Estimate dnde from a given minimum excess.
Parameters
----------
excess : `~gammapy.maps.RegionNDMap`
Minimal excess.
dataset : `~gammapy.datasets.SpectrumDataset`
Spectrum dataset.
Returns
-------
e2dnde : `~astropy.units.Quantity`
Minimal differential flux.
"""
energy = dataset._geom.axes["energy"].center
dataset.models = SkyModel(spectral_model=self.spectral_model)
npred = dataset.npred_signal()
phi_0 = excess / npred
dnde_model = self.spectral_model(energy=energy)
dnde = phi_0.data[:, 0, 0] * dnde_model * energy**2
return dnde.to("erg / (cm2 s)")
def _get_criterion(self, excess, bkg):
is_gamma_limited = excess == self.gamma_min
is_bkg_syst_limited = excess == bkg * self.bkg_syst_fraction
criterion = np.chararray(excess.shape, itemsize=12)
criterion[is_gamma_limited] = "gamma"
criterion[is_bkg_syst_limited] = "bkg"
criterion[~np.logical_or(is_gamma_limited, is_bkg_syst_limited)] = (
"significance"
)
return criterion
[docs]
def run(self, dataset):
"""Run the sensitivity estimation.
Parameters
----------
dataset : `SpectrumDatasetOnOff`
Dataset to compute sensitivity for.
Returns
-------
sensitivity : `~astropy.table.Table`
Sensitivity table.
"""
energy = dataset._geom.axes["energy"].center
excess = self.estimate_min_excess(dataset)
e2dnde = self.estimate_min_e2dnde(excess, dataset)
criterion = self._get_criterion(
excess.data.squeeze(), dataset.background.data.squeeze()
)
return Table(
[
Column(
data=energy,
name="e_ref",
format="5g",
description="Energy center",
),
Column(
data=dataset._geom.axes["energy"].edges_min,
name="e_min",
format="5g",
description="Energy edge low",
),
Column(
data=dataset._geom.axes["energy"].edges_max,
name="e_max",
format="5g",
description="Energy edge high",
),
Column(
data=e2dnde,
name="e2dnde",
format="5g",
description="Energy squared times differential flux",
),
Column(
data=np.atleast_1d(excess.data.squeeze()),
name="excess",
format="5g",
description="Number of excess counts in the bin",
),
Column(
data=np.atleast_1d(dataset.background.data.squeeze()),
name="background",
format="5g",
description="Number of background counts in the bin",
),
Column(
data=np.atleast_1d(criterion),
name="criterion",
description="Sensitivity-limiting criterion",
),
]
)
