# Licensed under a 3-clause BSD style license - see LICENSE.rst
from astropy.table import Table, Column
import astropy.units as u
from ..stats import excess_matching_significance_on_off
from .models import PowerLaw
from .utils import SpectrumEvaluator
__all__ = ["SensitivityEstimator"]
[docs]class SensitivityEstimator:
"""Estimate differential sensitivity.
Uses a 1D spectral analysis and on / off measurement.
Parameters
----------
arf : `~gammapy.irf.EffectiveAreaTable`
1D effective area
rmf : `~gammapy.irf.EnergyDispersion`
energy dispersion table
bkg : `~gammapy.spectrum.CountsSpectrum`
the background array
livetime : `~astropy.units.Quantity`
Livetime (object with the units of time), e.g. 5*u.h
index : float, optional
Index of the spectral shape (Power-law), should be positive (>0)
alpha : float, optional
On/OFF normalisation
sigma : float, optional
Minimum significance
gamma_min : float, optional
Minimum number of gamma-rays
bkg_sys : float, optional
Fraction of Background systematics relative to the number of ON counts
An example can be found in :gp-notebook:`cta_sensitivity` .
Notes
-----
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 ``sigma``, and being larger than ``gamma_min`` and ``bkg_sys`` percent larger than the
number of background events in the ON region.
"""
def __init__(
self,
arf,
rmf,
bkg,
livetime,
index=2.0,
alpha=0.2,
sigma=5.0,
gamma_min=10.0,
bkg_sys=0.05,
):
self.arf = arf
self.rmf = rmf
self.bkg = bkg
self.livetime = u.Quantity(livetime).to("s")
self.index = index
self.alpha = alpha
self.sigma = sigma
self.gamma_min = gamma_min
self.bkg_sys = bkg_sys
self._results_table = None
@property
def results_table(self):
"""Results table (`~astropy.table.Table`)."""
return self._results_table
[docs] def run(self):
"""Run the computation."""
# TODO: let the user decide on energy binning
# then integrate bkg model and gamma over those energy bins.
energy = self.rmf.e_reco.center
bkg_counts = (self.bkg.data.data.to("1/s") * self.livetime).value
excess_counts = excess_matching_significance_on_off(
n_off=bkg_counts / self.alpha, alpha=self.alpha, significance=self.sigma
)
is_gamma_limited = excess_counts < self.gamma_min
excess_counts[is_gamma_limited] = self.gamma_min
model = PowerLaw(
index=self.index, amplitude="1 cm-2 s-1 TeV-1", reference="1 TeV"
)
# TODO: simplify the following computation
predictor = SpectrumEvaluator(
model, aeff=self.arf, edisp=self.rmf, livetime=self.livetime
)
counts = predictor.compute_npred().data.data.value
phi_0 = excess_counts / counts
dnde_model = model(energy=energy)
diff_flux = (phi_0 * dnde_model * energy ** 2).to("erg / (cm2 s)")
# TODO: take self.bkg_sys into account
# and add a criterion 'bkg sys'
criterion = []
for idx in range(len(energy)):
if is_gamma_limited[idx]:
c = "gamma"
else:
c = "significance"
criterion.append(c)
table = Table(
[
Column(
data=energy,
name="energy",
format="5g",
description="Reconstructed Energy",
),
Column(
data=diff_flux,
name="e2dnde",
format="5g",
description="Energy squared times differential flux",
),
Column(
data=excess_counts,
name="excess",
format="5g",
description="Number of excess counts in the bin",
),
Column(
data=bkg_counts,
name="background",
format="5g",
description="Number of background counts in the bin",
),
Column(
data=criterion,
name="criterion",
description="Sensitivity-limiting criterion",
),
]
)
self._results_table = table
return table