Generating a sky map

What you will learn

You will learn how to use the ctskymap tool to generate a sky map from the selected event data.

Now let’s have a look into the selected data by generating a sky map. You do this with the ctskymap tool as follows:

$ ctskymap
Input event list or observation definition XML file [events.fits] selected_events.fits
Coordinate system (CEL - celestial, GAL - galactic) (CEL|GAL) [CEL]
Projection method (AIT|AZP|CAR|GLS|MER|MOL|SFL|SIN|STG|TAN) [CAR]
First coordinate of image center in degrees (RA or galactic l) (0-360) [83.63]
Second coordinate of image center in degrees (DEC or galactic b) (-90-90) [22.01]
Image scale (in degrees/pixel) [0.02]
Size of the X axis in pixels [200]
Size of the Y axis in pixels [200]
Lower energy limit (TeV) [0.1]
Upper energy limit (TeV) [100.0]
Background subtraction method (NONE|IRF|RING) [NONE]
Output skymap file [skymap.fits]

The tool produces the file skymap.fits which contains a sky map of the events in FITS format. The sky map is centred on the location of the Crab nebula (Right Ascension 83.63 deg, Declination 22.01 deg) and consists of 200 x 200 spatial pixels of 0.02 x 0.02 degrees in size, covering an area of 4 deg x 4 deg.

Below an image of the sky map, displayed using ds9 in logarithmic color scale with a 3 pixel Gaussian kernel smoothing applied:

Sky map of the selected events

The sky map shows the Crab nebula on top of a wide-spread distribution of events that originates from the instrumental background. The instrument response functions contain templates that describe the spatial and spectral distribution of the background, and ctskymap can make use of these templates to subtract the background contribution from the sky map. You enable the background subtraction by running the ctskymap tool with the background subtraction method set to IRF, as shown in the following example:

$ ctskymap
Input event list or observation definition XML file [selected_events.fits]
Coordinate system (CEL - celestial, GAL - galactic) (CEL|GAL) [CEL]
Projection method (AIT|AZP|CAR|GLS|MER|MOL|SFL|SIN|STG|TAN) [CAR]
First coordinate of image center in degrees (RA or galactic l) (0-360) [83.63]
Second coordinate of image center in degrees (DEC or galactic b) (-90-90) [22.01]
Image scale (in degrees/pixel) [0.02]
Size of the X axis in pixels [200]
Size of the Y axis in pixels [200]
Lower energy limit (TeV) [0.1]
Upper energy limit (TeV) [100.0]
Background subtraction method (NONE|IRF|RING) [NONE] IRF
Calibration database [prod2]
Instrument response function [South_0.5h]
Output skymap file [skymap.fits] skymap_subtracted.fits

The resulting sky map, displayed using ds9 in logarithmic color scale with a 3 pixel Gaussian kernel smoothing applied is shown below:

Sky map of the selected events with background subtraction from the IRFs

If a reliable model of the instrumental background is not available, you can estimate it from the data by running the ctskymap tool with the background subtraction method set to RING. For each position in the map the number of background counts is estimated from a ring, scaled according to the background model in the instrument response functions, and subtracted from a circular region centred on the trial source region. Regions with with significant gamma-ray emission need to be excluded from the ring background estimate. You can do this by iteratively determining an exclusion map that contains all pixels above a given significance threshold. We chose here 3 iterations and a threshold of 5 sigma.

$ ctskymap
Input event list or observation definition XML file [selected_events.fits]
Coordinate system (CEL - celestial, GAL - galactic) (CEL|GAL) [CEL]
Projection method (AIT|AZP|CAR|GLS|MER|MOL|SFL|SIN|STG|TAN) [CAR]
First coordinate of image center in degrees (RA or galactic l) (0-360) [83.63]
Second coordinate of image center in degrees (DEC or galactic b) (-90-90) [22.01]
Image scale (in degrees/pixel) [0.02]
Size of the X axis in pixels [200]
Size of the Y axis in pixels [200]
Lower energy limit (TeV) [0.1]
Upper energy limit (TeV) [100.0]
Background subtraction method (NONE|IRF|RING) [IRF] RING
Source region radius for estimating on-counts (degrees) [0.1] 0.05
Inner background ring radius (degrees) [0.6]
Outer background ring radius (degrees) [0.8]
Number of iterations for exclusion regions computation (0-100) [0] 3
Significance threshold for exclusion regions computation [5.0]
Calibration database [prod2]
Instrument response function [South_0.5h]
Output skymap file [skymap_subtracted.fits] skymap_ring.fits

The resulting sky map, displayed using ds9 in logarithmic color scale is shown below.

Sky map of the selected events with background subtraction from the ring method