Creating Light Curves

 

Example 1: Create an effective area and dead time corrected light curve from a point-like source (PSPC data, see note 1):

Midas 001> SET/PROJECTION CORRECT
Midas 002> SEL/RING center=(150,-2300) radius=300 BIN TIME 10 *events *light
Midas 003> SET/PROJECTION NOCORRECT

Example 1a: Create an effective area and dead time corrected light curve from a point-like source (HRI data, see note 1a):

Midas 001> SEL/RING center=(150,-2300) radius=300 BIN TIME 10 *events *light
Midas 002> CREATE/PARFIL corr hcor
Midas 003> WRIT/PAR hcor EFFECTIVE_AREA_CORRECTION_TABLE EXSAS_CAL:effarea_hri
Midas 004> CORRECT/LIGHT hcor DATA=light
Midas 005> COMPUTE/TABLE light :corr = #2*#3

        Example 2: Same as example 1, but selecting a circle via the cursor on a reference image (PSPC data only, see note 2):

Midas 001> SET/PROJECTION CORRECT
Midas 002> SELECT/RING cursor BIN TIME 10 i=events o=light
Midas 003> SET/PROJECTION NOCORRECT

              Example 3: Same as example 2, but the result is written to table column :BRIGHT (PSPC data only, see note 3). Also, cursor dimensions are set in advance.

Midas 001> SET/PROJECTION CORRECT
Midas 002> PRJ\_RAD(1) = 200
Midas 003> PRJ\_RAD(2) = 700
Midas 004> SELECT/RING cursor BIN TIME 10 id=BRIGHT in=events out=light
Midas 005> SET/PROJECTION NOCORRECT

            Example 4: Create a light curve from the intersection of a circle with a sector on the sky (specified via cursor input) (see note 4):

Midas 001> SEL/RING cursor SEL/SECTOR cursor BIN TIME 10 in=events out=out

              Example 5: Create a light curve from specified amplitude channels (available for PSPC data only), time intervals, and a circular portion of the sky selected via the cursor (see note 5):

Midas 001> SELECT AMPL 8,40-
           SELECT TIME 20000,30000-
           SELECT/RING cursor -
           BIN TIME 1 *events *light2

                      Example 6: Create three light curves from three specified amplitude intervals (available for PSPC data only, see note 6):

Midas 006> SELECT AMPL [8,40][41,100][>100] BIN TIME 1 *events *lights

              Example 7: Create a corrected and background subtracted light curve from a point-like source (see note 7):

Midas 001> SET/PROJECTION CORRECT
Midas 002> SELECT/RING [0,0,300][0,0,400,600] BIN TIME 10 *events *lights
Midas 003> PREPARE/LIGHT lights :RATE

Example 7a: Create a corrected and background subtracted light curve from a point-like source (HRI data, see note 7a):

Midas 001> SELECT/RING [0,0,300][0,0,400,600] BIN TIME 10 *events *lights
Midas 002> SELECT/RING 0,0,400,600 *events *back
Midas 003> CREATE/PARFIL corr hcor
Midas 004> WRIT/PAR hcor EFFECTIVE_AREA_CORRECTION_TABLE EXSAS_CAL:effarea_hri
Midas 005> CORRECT/LIGHT hcor DATA=lights,:counts_1
Midas 006> CORRECT/LIGHT hcor DATA=lights,:counts_2 mode=photon att=back
Midas 007> PREPARE/LIGHT lights :RATE ? C_COUNTS_1,C_COUNTS_2

                Example 8: Create a light curve from a circle having radius 5 arcmin centered at given RA and DEC (see note 8):

Midas 001> SEL/RIN 2:32:45.2, -23:32:01.3, 5 min BIN TIM 10 *events *rate

               

Note 1

In the example it is assumed that a point-like source is positioned at the sky pixel (150,-2300). A selection centered on the source is specified, followed by a BIN TIME command. The time bin, here set to 10, is a real number expressed in seconds (spacecraft clock). The * symbol is a short form for indicating the input and the output filenames. The correction is meant to the instrument optical axis (i.e. produces a light curve free of effects due to different instrument response relative to the instrument optical axis). The output light curve is written to the table light.tbl at the columns :COUNTS_1 (non corrected light curve), and :CORR_1 (corrected light curve). The column :ERR_1 contains the statistical errors referred to the corrected light curve. In case Projection were not set in correction mode, only the column :COUNTS_1 is created. Data correction is a delicate matter, so read carefully section 4.3.4! The default correction is not always the best you can choose (it could be not appropriate for your data). Warning: the Projection correction mode is not available for WFC and HRI data. For these instruments the correction must be applied separately, see section 4.3.4 on page for this case, and the next example 1a.

Note 1a

In the case of HRI data Projection cannot be run in correction mode. The alternative (valid also for PSPC data, at any rate) is outlined in this example. The light curve is created in the usual way, and written to the column COUNTS_1 of the table light.tbl; next, a correction parameter file (in this case named hcor.par) is created and filled in with the right parameter values (in this case the HRI effective area table name must be specified explicitly, while all the rest is defaulted: correction in attitude mode -- assuming that the attitude table is present in the working directory -- and other details described in section 4.3.4, on page ). The appropriate correction command is then run, creating a new column C_COUNTS_1 containing a list of correction factors for the light curve. The corrected light curve can be derived by simply multiplying the light curve with the correction factor, writing the result to a new table column.

Note 2

If a reference image is loaded into the image display, the ring selection can be defined via the circular cursor.

Note 3

The output column label for the non corrected light curve is set to :BRIGHT, the corrected light curve is written to :C_BRIGHT and the statistical errors to :E_BRIGHT. Note that the inner and outer radii of the circular cursor are set in advance using the auxiliary keyword PRJ_RAD, therefore only cursor positioning will be performed interactively).

Note 4

Here the light curve is not corrected.

Note 5

In creating a light curve the photons might be generally filtered according to any arrival times, energies, and positions on the detector face or on the sky. In this example the light curve is not corrected. Binsize is set to 1 second.

Note 6

Up to 255 light curves can be created and output simultaneously to the same table. Here is an example of the production of 3 light curves in different energy bands. The squared brackets are required in order to process the three selections in amplitude independently. To remove them would turn out in a single selection of all photons having amplitudes greater than 7 (the two commands SEL AMPL 8,40 41,100 >100 and SEL AMPL >7 are equivalent). The 3 light curves are written to the columns :COUNTS_1, :COUNTS_2 and :COUNTS_3 of the table lights.tbl. If Projection were set in correction mode the corrected light curves are written to the columns :CORR_1, :CORR_2 and :CORR_3, while the statistical errors are written to the columns :ERR_1, :ERR_2 and :ERR_3

Note 7

The point source is assumed to lay at the sky coordinates (0,0). The background light curve is extracted from an annulus around the source. The two light curves are both corrected to the instrument optical axis (see note 1). The background subtracted light curve is written to the table column :RATE and the statistical errors to the column :ERR_RATE.

Midas 001> SELECT/RING [0,0,300][0,0,400,600] BIN TIME 10 *events *lights
Midas 002> SELECT/RING 0,0,400,600 *events *back
Midas 003> CREATE/PARFIL corr hcor
Midas 004> WRIT/PAR hcor EFFECTIVE_AREA_CORRECTION_TABLE EXSAS_CAL:effarea_hri
Midas 005> CORRECT/LIGHT hcor DATA=lights,:counts_1
Midas 006> CORRECT/LIGHT hcor DATA=lights,:counts_2 mode=photon att=back
Midas 007> PREPARE/LIGHT lights :RATE ? C_COUNTS_1,C_COUNTS_2

Note 7a

As in the PSPC example (7) the source and the background light curves are created. Also an explicit list of the photons contributing to the background is stored in table back.tbl for later use. Next, the correction parameter file is created and upgraded in the appropriate way (and left in attitude mode by default). The source light curve, at column COUNTS_1 of table lights.tbl, is then corrected, followed by the correction of the background light curve (in photon mode, using table back.tbl as reference). The two resulting correction vectors are stored in columns C_COUNTS_1 and C_COUNTS_2. The last command, PREPARE/LIGHT_CURVE, is used to perform the background subtraction: the corrected light curves for source and background are stored at columns COUNTS_1_CORR and COUNTS_2_CORR, with the statistical errors in columns COUNTS_1_ERR and COUNTS_2_ERR, while the result of the background subtraction is written to column CORR with the statistical errors in CORR_ERR.

Note 8

Spatial positions in the SELECT command can be specified in equatorial coordinates only; spatial intervals can be specified in units of arcsecond, arcminute, or degree.

For more details about light curve production, see sections 4.2.4 and 4.2.6. In order to get familiar with the Projection command syntax it is strongly suggested to read thoroughly section 4.3 also.