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• Source files: cta_sensitivity.ipynb | cta_sensitivity.py

Estimation of the CTA point source sensitivity

Introduction

This notebook explains how to estimate the CTA sensitivity for a point-like IRF at a fixed zenith angle and fixed offset using the full containement IRFs distributed for the CTA 1DC. The significativity is computed for a 1D analysis (On-OFF regions) and the LiMa formula.

We use here an approximate approach with an energy dependent integration radius to take into account the variation of the PSF. We will first determine the 1D IRFs including a containment correction.

We will be using the following Gammapy class:

• gammapy.estimators.SensitivityEstimator

Setup

As usual, we’ll start with some setup …

%matplotlib inline
import matplotlib.pyplot as plt
import numpy as np

import astropy.units as u
from astropy.coordinates import Angle, SkyCoord
from regions import CircleSkyRegion

from gammapy.irf import load_cta_irfs
from gammapy.makers import SpectrumDatasetMaker
from gammapy.data import Observation
from gammapy.estimators import SensitivityEstimator
from gammapy.datasets import SpectrumDataset, SpectrumDatasetOnOff
from gammapy.maps import MapAxis

Define analysis region and energy binning

Here we assume a source at 0.5 degree from pointing position. We perform a simple energy independent extraction for now with a radius of 0.1 degree.

center = SkyCoord("0 deg", "0.5 deg")
region = CircleSkyRegion(center=center, radius=0.1 * u.deg)

e_reco = MapAxis.from_energy_bounds("0.03 TeV", "30 TeV", nbin=20)
e_true = MapAxis.from_energy_bounds(
    "0.01 TeV", "100 TeV", nbin=100, name="energy_true"
)

empty_dataset = SpectrumDataset.create(
    e_reco=e_reco, e_true=e_true, region=region
)

Load IRFs and prepare dataset

We extract the 1D IRFs from the full 3D IRFs provided by CTA.

irfs = load_cta_irfs(
    "$GAMMAPY_DATA/cta-1dc/caldb/data/cta/1dc/bcf/South_z20_50h/irf_file.fits"
)

pointing = SkyCoord("0 deg", "0 deg")
obs = Observation.create(pointing=pointing, irfs=irfs, livetime="5 h")

Invalid unit found in background table! Assuming (s-1 MeV-1 sr-1)

spectrum_maker = SpectrumDatasetMaker(
    selection=["exposure", "edisp", "background"]
)
dataset = spectrum_maker.run(empty_dataset, obs)

Now we correct for the energy dependent region size:

containment = 0.68

# correct exposure
dataset.exposure *= containment

# correct background estimation
on_radii = obs.psf.containment_radius(
    energy=e_reco.center, theta=0.5 * u.deg, fraction=containment
)[0]
factor = (1 - np.cos(on_radii)) / (1 - np.cos(region.radius))
dataset.background *= factor.value.reshape((-1, 1, 1))

And finally define a SpectrumDatasetOnOff with an alpha of 0.2. The off counts are created from the background model:

dataset_on_off = SpectrumDatasetOnOff.from_spectrum_dataset(
    dataset=dataset, acceptance=1, acceptance_off=5
)

Compute sensitivity

We impose a minimal number of expected signal counts of 5 per bin and a minimal significance of 3 per bin. We assume an alpha of 0.2 (ratio between ON and OFF area). We then run the sensitivity estimator.

sensitivity_estimator = SensitivityEstimator(
    gamma_min=5, n_sigma=3, bkg_syst_fraction=0.10
)
sensitivity_table = sensitivity_estimator.run(dataset_on_off)

Results

The results are given as an Astropy table. A column criterion allows to distinguish bins where the significance is limited by the signal statistical significance from bins where the sensitivity is limited by the number of signal counts. This is visible in the plot below.

# Show the results table
sensitivity_table

Table length=20

energy	e2dnde	excess	background	criterion
TeV	erg / (cm2 s)
float64	float64	float64	float64	bytes12
0.0356551	1.63575e-11	332.671	3326.71	bkg
0.0503641	8.9751e-12	170.92	1709.2	bkg
0.0711412	3.30491e-12	93.4135	772.154	significance
0.10049	1.79794e-12	65.8876	376.846	significance
0.141945	1.31428e-12	46.4073	181.867	significance
0.200503	9.38912e-13	31.2766	78.9115	significance
0.283218	6.43339e-13	21.5383	35.068	significance
0.400056	5.17919e-13	15.004	15.4897	significance
0.565095	3.79871e-13	11.0668	7.50542	significance
0.798218	3.32974e-13	8.24132	3.53956	significance
1.12751	2.887e-13	6.51802	1.84222	significance
1.59265	2.41231e-13	5.67388	1.20949	significance
2.24968	2.16291e-13	5	0.75267	gamma
3.17776	2.29643e-13	5	0.473836	gamma
4.48871	2.73642e-13	5	0.321889	gamma
6.34047	3.57511e-13	5	0.188679	gamma
8.95615	4.65419e-13	5	0.0862868	gamma
12.6509	6.65067e-13	5	0.0456195	gamma
17.8699	7.96271e-13	5	0.0266682	gamma
25.2419	1.12581e-12	5	0.0124775	gamma

# Save it to file (could use e.g. format of CSV or ECSV or FITS)
# sensitivity_table.write('sensitivity.ecsv', format='ascii.ecsv')

# Plot the sensitivity curve
t = sensitivity_table

is_s = t["criterion"] == "significance"
plt.plot(
    t["energy"][is_s],
    t["e2dnde"][is_s],
    "s-",
    color="red",
    label="significance",
)

is_g = t["criterion"] == "gamma"
plt.plot(
    t["energy"][is_g], t["e2dnde"][is_g], "*-", color="blue", label="gamma"
)
is_bkg_syst = t["criterion"] == "bkg"
plt.plot(
    t["energy"][is_bkg_syst],
    t["e2dnde"][is_bkg_syst],
    "v-",
    color="green",
    label="bkg syst",
)

plt.loglog()
plt.xlabel(f"Energy ({t['energy'].unit})")
plt.ylabel(f"Sensitivity ({t['e2dnde'].unit})")
plt.legend();

We add some control plots showing the expected number of background counts per bin and the ON region size cut (here the 68% containment radius of the PSF).

# Plot expected number of counts for signal and background
fig, ax1 = plt.subplots()
# ax1.plot( t["energy"], t["excess"],"o-", color="red", label="signal")
ax1.plot(
    t["energy"], t["background"], "o-", color="black", label="blackground"
)

ax1.loglog()
ax1.set_xlabel(f"Energy ({t['energy'].unit})")
ax1.set_ylabel("Expected number of bkg counts")

ax2 = ax1.twinx()
ax2.set_ylabel(f"ON region radius ({on_radii.unit})", color="red")
ax2.semilogy(t["energy"], on_radii, color="red", label="PSF68")
ax2.tick_params(axis="y", labelcolor="red")
ax2.set_ylim(0.01, 0.5)

(0.01, 0.5)

Exercises

• Also compute the sensitivity for a 20 hour observation

• Compare how the sensitivity differs between 5 and 20 hours by plotting the ratio as a function of energy.