At all times, the Burst and Transient Source Experiment (BATSE) observes the entire sky that is not hidden by the Earth. It was designed this way in order to be sure to detect all gamma-ray bursts, which occur at unpredictable times and in unpredictable locations in the sky. The experiment continuously sends back data about the state of the sky, telling us how bright the sky appears in each of the 8 BATSE detectors every second. This is done so that scientists on the ground know the nature of the background signal upon which the burst signal "rides".
In addition to their usefulness for burst studies, these background data are useful for an entirely different purpose: it gives us a tool to find new pulsars. Other, more sensitive telescopes would have to be pointed region-by-region in the sky to find them, and, since these pulsars only emit sporadically, the telescopes would have to continually revisit each region, perhaps monthly. This is not a good way to find bright, new transient sources!
Using BATSE, scientists can detect and monitor new pulsars no matter where they are in the sky, as long as the new (or re-appearing) pulsar is at least 1.5% as bright as the Crab Nebula, for several days.
Why it's important:Many pulsars are actually binary star systems, where two stellar objects are in orbit around each other. In order to understand the nature of these binary systems which produce the pulsations we see, it is very useful to simultaneously observe the source over a broad range of the spectrum (radio, optical, UV, xray, gamma-ray). By searching the BATSE data every day for new pulses, we can get word out to other observers within a few days, telling them that a source is active, so they can also observe it.
Since the beginning of the space program, only about 40 of these systems have been discovered. Just think how difficult it would be to understand how stars work if you only had 40 examples to study! Since the beginning of its mission in 1991, BATSE has discovered about 1 new pulsing object per year. The payoff can be great, depending on what niche in the pulsar "zoo" it occupies. A good example is the Bursting Pulsar.
The chances of correctly understanding (or modeling) a particular binary pulsar system greatly increase when many of the system's properties are studied continuously, and over significant changes in intensity - there may be many models which can correctly predict the source's spectrum and the shape of the pulses for one particular level of brightness, but only (hopefully!) one model which correctly predicts these features as the source intensity varies. BATSE provides an excellent database against which one can test these models for validity.
How We Do it:
- The data as we initially get them show large changes in the detected count rate as the satellite orbits around the earth.
- The data are passed through a high-pass filter, which improves sensitivity to pulses between ~2-200 s.
- Data from combinations of detectors are combined in groups of 2 and 4, as well as used individually, to obtain sensitivity to all locations in the sky. A Fast-Fourier transform is performed on the data, and the power density spectrum produced is searched for peaks that rise above the noise level.
We inspect these data every day for new sources. We also look closely at the periods of known objects (which may not have been observed since their original discovery as much as 20 years ago) using a different, more sensitive method that can be used when the period is known. Upon making a detection, we submit text for publication in an International Astronomical Union (IAU) Circular, which is provided within 1-3 days to astronomers all over the globe (and teams of other astrophysical space missions).
If the outburst , the period of time when the new pulsar is visible to BATSE, lasts for a significant fraction of that system's binary orbital period, the time required for the neutron star to make one complete revolution around the companion (normal) star, the size and shape of the orbit can be determined by analyzing Doppler shifts and pulse arrival delays in the pulsar signal.
As an outburst continues, BATSE obtains a continuous history of the daily source intensity and frequency.
The random scatter of points are times when the source is not detectable (the period with the largest signal is plotted, even if it's just due to noise). For the simplest source model, scientists would expect that the source brightens because more mass is falling on it. If each kilogram of material applied the same torque to the system, thereby changing the period of rotation, the slope of the frequency history would be greatest when the source is brightest. This is not observed.
Astronomers clearly have a great deal more to learn about the nature of these binary pulsar systems. With a lifetime well into the next century, BATSE can continually provide new data with which to hunt for and explore these interesting objects in the sky.
For more information on Hunting for New Pulsars, please contact
Dr. Robert B. Wilson
Huntsville, Alabama 35812