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File Plot
Now you know how to find all the information in the header, what was observed and when, the mean, minimum and maximum observed values, what a data row contains and how to perform algebraic computations on the data in a column. What if you want to plot a lightcurve from the raw data, to actually take a look at what the source is doing? Click on the data set gx301-2.lc to select it. Click on File Plot and a parameter box will appear (look at the description box to see what File Plot does...). Type in the names of the X and Y axes (the independent and dependent variables, respectively, obtained from Column List) that you want to plot (TIME, the time of the observation, and RATE, the number of photons per second collected by the satellite - this is an indication of how bright the source is, as the bighter the source, the more photons per second we receive from it). The Lists of Rows parameter allows you to select only a subsection of the entire data set to plot. This is useful for data sets that are very large (like this one). If you try to plot too many data points at once the plot will be blank! Try selecting several hundred rows to plot, as a first stab. Click "run". An output box will appear with the specified plot of the raw data. If the plot is too small to read on the screen you can re-size it by clicking and dragging at the corner. You can also print or save the outputted plot to your local machine by clicking on the "save/print" tab and following the directions. Important - In order to make sure that your plot takes up as much of the page as possible, you must expand the POW window on your computer screen to its maximum. Then, when printing, click on the "stretch to fit page" radio button. If you are using Windows 98 and are having printing problems, click here.
Plotting note: To change the appearance of your plot, look at the Plotting Basics help.
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Write down your initial impressions of this plot. Do you see any repeating pattern in the data? Now take a closer look at the plot. What time interval does your plot represent? You may need to estimate how much time each tick on the X-axis represents. What explanation can you give for the appearance of this plot?
Try running File Plot again, but change the number of rows you plot.
- What do you see when you plot 50 rows? 1000 rows? 2000 rows? 3000 rows?
- Print out plots using several different values for the rows included and explain similarities and differences in each.
- How does changing the number of data rows viewed change what you can learn from the data? Which plots make it easier for you to estimate the time between peaks or troughs of brightness? Is it possible to plot too many rows? Too few? What happens in each of these cases?
- Compare plots with 3000 and 17000 rows. Can you identify the peaks of the intensity from the 3000 rows plot on the 17000 rows plot?
- Can you find a period for this source? Try to estimate the time between peaks in the plot.
- How accurately can you determine the period?
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The RXTE All Sky Monitor records a new measurement every 90 seconds,
then moves on to a different patch of sky and records another 90
seconds of data. It covers the entire sky in about 90 minutes (this
varies, as the time to move between observing positions varies
slightly). The data set you have starts in 1996 and contains more than
23,000 data points. You can see how the raw data quickly become cumbersome to plot (it would be almost impossible to plot by hand!!). What about if we wanted to plot the data over a long time, but with fewer actual points to graph? Is there a way to "bin" or average the data and make a lightcurve? Of course, there is a tool to help!
You have now successfully used the File Utilities tools to take an initial look at the data, and you are ready to try out the next suite of analysis tools.
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