Differential light curve (DR1)

APPLAUSE DR1 provides a source catalogue with uncalibrated magnitudes that can be used for constructing differential light curves. In this example we query the APPLAUSE_DR1 database for photometry on the classical cepheid KN Cen and its comparison star. Then we download the results and construct a light curve with TOPCAT.

For convenience, there is the raw_lightcurve table in the APPLAUSE_DR1 database. The table combines raw magnitudes from the source table, sky coordinates, Tycho-2 and UCAC4 designations from the source_calib table, and observation time from the exposure table. A query form called "Raw light curve by star ID" simplifies the data acquisition. We just need to enter the Tycho-2 or UCAC4 designation of KN Cen in the identifier field. Here we use the UCAC4 identifier, 128-096874, and we also specify the name for the result table:

In the Download tab we choose VOTable XML file and save the file on disk:

Now we need to choose a suitable comparison star for KN Cen. This task can be accomplished with Aladin Sky Atlas. Let's choose the star with the UCAC4 identifier of 128-097146, submit the raw light curve query, as we did for KN Cen, and download the result.

Next, we open the two downloaded XML files in TOPCAT and create a new table by matching rows in the two tables. The matching can be done based on the ut_start columns in both tables. The resulting table will have columns from both tables, with _1 appended to column names of the first table and _2 appended to column names from the second table.

It is time to create plots in TOPCAT. By choosing the Julian date of the mid-exposure, jd_mid_1, for X-axis and the differential magnitude, mag_auto_1-mag_auto_2, for Y-axis, we get the following plot:

Let's select points only from the second season:

Taking the period of KN Cen from J/A+A/504/959, 34.048 days, we can calculate the pulsation phase and plot a phase diagram: