A Hubble Source Catalog (HSC) Use Case

Example #3: Using the Discovery Portal to search for Variable Objects in the HSC

(Time Variability in the dwarf irregular galaxy IC 1613)

GOAL: This tutorial shows you how to use the MAST Discovery Portal to search for variable objects in the HSC.

SCIENCE CASE: The science case is searching for variable objects (i.e. in the irregular galaxy IC 1613; Bernard et al. 2010, ApJ 712, 1259).

Step 1 - Go to the MAST Discovery Portal.

Use the pull down menu under Select Collection to choose the HSC (blue); you can also select whether you want MagAper2 or MagAuto values (purple). Enter (you can just cut and paste if you like) the coordinates and search radius (i.e. 01:04:28 2:09:36 r=0.5m) in the Search box (green); note that this is in the outskirts of IC 1613. Perform the search by just hitting a carriage return or by clicking on the (orange). The results are displayed in the List of Objects, while the AstroView window shows the objects against the DSS image. The left column is a series of Filters that can be used to refine the data selected.

The DSS is not helpful (the outskirts of the galaxy are very faint), so we need to load an HST image. Click on the Load Detailed Results icon (yellow in the previous figure) to bring up the HSC Detailed Form. Click on the Toggle Overlay Image icon (blue) to load the HST image. Note that you can click on the Preview image to bring up the HLA interactive viewer to get a better look at the HST data.

Step 2 - Go back to the search tab (blue), scroll down the Filters section (green) on the left to the Images block, and move the sliders to select sources with 20 to 24 images (orange). Note that fewer rows are now displayed (yellow), and that the image also shows fewer targets.

Step 3 - Make a plot by clicking the icon (blue). Under Configuration, select X = A_F475W and Y = A_F475W_Sigma (green). Click on the Update Series button (orange) to plot the ACS F475W magnitudes vs. the RMS scatter for each data point. The potential variable stars have large Sigmas (yellow).
Step 4 - Select a specific source by clicking on it (i.e. 23.4, 0.43) (blue). This will highlight the object in the AstroView window (green), and bring the source to the top of the List of Objects (i.e. matchID=87413475) (orange).
Step 5 - You can examine the 24 measurements and images that went into the MatchID=87413475 by clicking on the first icon under Actions (yellow in the previous figure) in the List of Objects. This brings up the HSC Detailed Form data. Note that there is a crosshair on the target (green).
The light curve can then be plotted using the icon (blue) again with X = Start Time and Y = MagAper2 (green).

Step 6 - While there is clear variability, the plot is confusing since there is both F475W and F814W data plotted. To look at only one magnitude at a time, go to the Filters section on the left and scroll to the Filter area and select one of the filters (blue).

Note that the plots (below) have fewer points, and that this object is clearly variable in the both filters. The fact that the shape of the light curves are similar (although the amplitude is different) indicates that the variability is real.

Step 7 - To make sure the variability is real (i.e. that there are no image artifacts "causing" the magnitude scatter), you should examine the images. Clicking on the image in the Preview column in the List of Measurements will bring up the HLA's Interactive display. Show the HSC controls (blue) and set Require NumImages to be greater than 20 (green); note that the Apply astrometric shift is automatically selected. Click in the HSC box (orange) and the HSC catalog is overlaid on the image. Find the star of interest (click on the object to verify the identification (yellow)) and verify that the object is free from artifacts. Ideally, you would do this for all the images.
You could also make an F475W - F814W (B-I) column (by using the Combine Columns option (blue)) and plot the color-magnitude diagram; see Use Case #5 for more on making a CMD. Note that 5 of the 6 variable candidates (green) fall in the instability strip, further suggesting that they really are variables. The sixth object (yellow) is not in the instability strip.
Looking at the details for that object (87418631), we see that the 12th image, which has the only discrepant magnitude (blue), is impacted by an image defect (green).