Observing Bursts from an X-ray Burster
by James Humphreys
Certain X-ray bright objects in the sky at times emit sudden large bursts of X-rays, many times the normal number being received from these objects. Some of these objects seem to release these bursts in a periodic fashion. The objects GRO J1744-28 and 4U 1820-30 are examples of bursting sources.
The mechanism behind these surges of energy involves a neutron star and its accretion disk. The material falling onto a neutron star from the accretion disk builds up on the star's surface. This leads to tremendous gravitational pressures within this added material. Finally the pressure is great enough to allow the atomic nuclei of the gaseous material to combine and release enormous amounts of energy in a process similar to that in the sun and in hydrogen bombs. This results in an X-ray burst, a sudden, intense flash of X-rays that lasts from seconds to hundreds of seconds.
Another suggested mechanism to account for these observations is that there are instabilities in the gas as the accreting material penetrates deeply into the magnetic field of the neutron star.
Gathering very short time resolution data on the changes in the light curves and spectra of these objects will help determine the correct description of the process.
In this segment the student is introduced to the use of X-ray data to make size and energy estimates of the source and the processes occurring there.
Using XTE observations of the object GRO 1744-28 the student will be able to:
- Determine whether bursts occur periodically.
- Use the duration of the burst to estimate a maximum diameter
for the object.
- Make a qualitative comparison of the spectrum of this object with that of the Crab nebula's pulsar to determine the presence of additional activity due to complex interaction with the companion star.
- Grade level: 11th or 12th
- Prerequisites: Algebra, physics, chemistry or physical science (some formal introduction to energy processes and their measurement)
- Preparation: The teacher must download and distribute copies of the following plots (or be able to instruct the student to get these):
- Light Curve of GRO 1744-28 (10 hour span)
- Light Curve of GRO 1744-28 (1 hour span)
- Light Curve of GRO 1744-28 (36 second span)
- Materials (per lab group): One copy of the light curve graphs, calculator, graph paper, pencil.
- Setup: Be sure to know how the student groups will obtain the necessary graphs.
- Estimated class time: Once the graphs are obtained, approximately 20 minutes.
This lesson depends largely on the student's understanding of energy conversion in a nuclear reaction, and of the fundamental implications of the speed of light in space.
How long would it take for a photon of light to cross a distance equal to the diameter of the Sun?
Distribute the graphs (Light curves of GRO 1744-28) with the following directions/questions (Note that gaps in the light curves are simply times when GRO 1744-28 was not visible to XTE):
- Estimate the average number of counts per second received
by the detector from this source. Estimate the average increase in the
number of counts per second recorded during bursts.
- Estimate the average duration of the bursts.
- Do the bursts appear to be periodic? If so, can you estimate
the period? Is there additional data you would like to have, in order to
defend your statement about the periodicity of this phenomenon?
- The X-rays travel through space at the same speed as visible
light. Use the duration of the bursts to give an upper limit to the
diameter of the source. (HINT: See the warm-up.)
At the end of the lab portion of the class time, groups are called upon to display their results and answer questions from classmates about their methods and findings. The teacher may then lead a discussion of the suggested observations, the methods of investigation used by the groups, a statistical analysis of the reported results, or other aspects of this lab.
Neutron stars have masses up to about 3 solar masses and diameters of about 20 km. Assuming this mass and size, answer the following questions (The sun's mass is 1.99 x 1030 kg.):
- What is the density of the neutron star?
- What would be the gravitational acceleration at its surface
- What is the escape velocity at the surface?
Note that the diameter you obtained from the duration of the burst (step 4 in the Activity) is likely very much larger than 20 km. Why might this be so ?
- Zeilik, Michael. Astronomy: The Evolving Universe.
John Wiley & Sons, Inc., New York (1991, 6th ed.)
- Arny, Thomas. Explorations: an Introduction to
Astronomy. Mosby, St. Louis (1994) pp.401-402
- Graham, Sir Francis. "The Binary and
Millisecond Pulsars." Contemporary Physics 33
(May/June 1992): 165
- Lewin, Walter H. G.. "The Sources of Celestial X-ray
Bursts." Scientific American 244 (May 1981): 72
- Trimble, Virginia. "Neutron Stars and Black Holes in
Binary Systems." Contemporary Physics 32
(March/April 1991): 103