Imagine the Universe!
Imagine Home  |   Teachers' Corner | HERA Overview  |  

Vela X-1

Vela X-1 is a massive X-ray binary in which the compact object is an X-ray pulsar (neutron star), and the primary is an OB Supergiant.

The data set that is provided with the Hera module is taken from the All Sky Monitor of the RXTE satellite. You can follow the steps used to find the period for the source GX301-2, but with the data set selected to find the best period for Vela X-1.

To find the best period, run File Plot to get a general sense of the data. You can then refine your plot using Plot Light Curve to get a first guesstimate of the period. You will be able to estimate the period better if you only view a small section of the data (try 400 rows, and also try looking at different segments of the data, not necessarily the first 400 rows...).

The period of this source is known very accurately (down to 0.01 days). In order to find the period, it may be necessary to use an iterative approach. To do this, you will need to run Search With Fold several times, narrowing in on the best period and refining the resolution of the periods to be searched. To start, you will want to use a wise choice of parameters . If you do not get a strong peak (high chi-squared value), it is possible that your search did not include the actual period. Try changing the initial guess slightly, and use a fairly large value for the Number of Periods to Search parameter.

Once you have gotten a best period using Search with Fold, try generating a folded light curve with the period using Epoch Fold to verify that you have the right period. If you see only a weak period (no clear strong peak in the folded curve - it may look like several smaller peaks, rather than one big one), you may not have the right period. Try going back and running Search With Fold with slightly different starting parameters.

Exercise V1

    What is the best period for this source? Print out a copy of the output for Search with Fold showing a best period with a strong peak. Convert the outputted best period from seconds to days.

Once you have found the period for Vela X-1, you can generate a folded light curve using Epoch Fold. You can vary the value for Phasebins/Period (the size of the bins, as a fraction of the entered period on which the data is folded), which may give a clearer plot. Remember, to connect the points, type in "line step" and "p" at the PLT> prompt in the efold Output window (see Plotting Basics for more ways to manipulate the plotted output). The Y-axis is a measure of the normalized intensity of the source (the higher the value, the brighter the source).

Exercise V2

Print out a folded light curve for this source and answer the following questions:

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

The Imagine Team
Acting Project Leader: Dr. Barbara Mattson
All material on this site has been created and updated between 1997-2012.

DVD Table of Contents
Educator's Index