Education Activities To Accompany Chandra Data Analysis Software
Cen X-3 & Clocks in the Sky
Light Curves, Power Spectra, & Periodicities
Timing is everything. Our ancestors' very survival depended upon timing: knowing when to plant crops, when to move their living quarters to lower elevations, or when animals they hunted began the migration to different feeding grounds. When the earliest humans realized that certain recurring star patterns at night presaged the return of spring and the planting season or the onset of fall and the harvest season, they had discovered, in effect, that the earth makes a trip around its parent sun once every 365 or so sunrises.
Many cultures divided this "year" into smaller units based on the recycling of the phases of the moon and into still smaller ones, such as our presently-used weeks and hours. Galileo and his contemporaries learned to use pendulums and water clocks to fine tune timing to units of seconds. Their successors in today's modern laboratories have created atomic clocks that can pin down time intervals that are smaller than a nanosecond (0.000000001 sec).
And just as there are clocks in the sky that have long periods [like the sun, earth, and moon motions that govern the year, the day, and the month], there are other celestial clocks which have much smaller periods. These clocks were only discovered by astronomers after the construction and perfection of short-period clocks on earth. Some stars pulsate (expand and contract) regularly with periods that are only minutes long; others take years to complete one cycle. All stars rotate, just as the earth does, with periods that range from milliseconds to months. Binary systems contain two stars that mutually orbit in regular periods of days to years. Clocks, indeed, can be found in almost every variety throughout the sky.
One of the most astonishing discoveries of the 20th century occurred in the late 1960’s, when Jocelyn Bell, then a graduate student in Cambridge, England, discovered a source of radio waves in the sky seemed to be changing its brightness every 1.337 seconds. Furthermore, the period of the brightness variation was precisely repetitive to better than 1 part in 10 million. Such a precise celestial clock was unheard of, and the discoverers (jokingly?) referred to the new signals as originating from Little Green Men. However, soon thereafter, many such sources were discovered, and the LGMs seemed to be begging for another explanation. Renamed “pulsars”, they are among the most intriguing cosmic sources of radiation we know. They have extremely well defined periods, making exceptionally accurate clocks. For example, the period of PSR 1937+214 has been measured to be 0.00155780644887275 seconds, a measurement that challenges the accuracy of the best (atomic) clocks we have here on Earth.
How can something change its brightness almost 1000 times each second?? It turns out these objects are not “pulsating” at all, but are incredibly dense neutron stars that ROTATE 1000 times each second. These stars are so compact that one thimbleful of material from their surface would weigh as much as 6 million full sized African elephants! Their extremely large gravitational fields prevents them from breaking apart and their light variations are due to beacons similar to those of lighthouses that beam radiation in a searchlight fashion as they rotate.
Because these compact objects are small and have intense gravitational fields, they can accelerate material to very high speeds. When this material collides with some neighboring gas, the object can heat up to millions of degrees. This leads to emission of X-rays, and indeed, some of the most exciting discoveries concerning the nature of white dwarfs, neutron stars, and black holes have been made by looking at x-radiation using satellites such as Chandra.
Cen X-3, discovered more than 30 years ago, beautifully illustrates the process of astronomical discovery. Using the x-ray data, we can reconstruct the contents, properties, and behavior of the entire system. We can determine that Cen X-3 is a binary system, that it contains a neutron star and a companion much larger and more massive. We can find the rotation period of the neutron star, the orbit size of the neutron star, the size and mass of the companion star, the luminosity of the source (about 10,000 times brighter than the Sun!), and much more. Not only can we tell the size of objects using the clocks, sometimes we can also deduce their ages. These objects are like huge flywheels, storing vast quantities of rotational energy. As they radiate, their energy stores get depleted, and they slow down. The more slowly rotating pulsars tend to be older.
Let’s look in detail at Cen X-3, and see how we can piece together the observations to understand this fascinating system…. This set of activities will walk you through the process of analyzing an object that varies in brightness with time. You will learn how to discover if the light varies with a regular period, how to determine the value of that period accurately, and how to determine something about the size of the objects involved or the scale of the system.