The article below is adapted from a little astronomy educational PR piece I wrote many years ago that is still on the BATSE website. BATSE stands for Burst And Transient Source Experiment and was an experiment on NASA's satellite observatory, the Compton Gamma Ray Observatory (CGRO), pictured above being deployed into orbit from the Space Shuttle. BATSE was an all-sky X-ray/Gamma-ray monitoring instrument composed of 8 detectors. Each detector was mounted on one of the corners of the the rectangular frame of the CGRO, so the entire sky (not blocked by the Earth below) could be seen all the time. The main body of the observatory housed 3 pointed telescopes (the two blue domes and the BBQ grill looking thing at the back in the picture above) which observed patches of the sky in the gamma-ray part of the electro-magnetic spectrum.

Among the myriad stars of our Galaxy, a handful are observed regularly emitting pulses of X-rays. The pulsing X-rays are detected with instruments on spacecraft orbiting above the Earth's atmosphere, such as BATSE on board the Compton Gamma Ray Observatory, since the atmosphere absorbs X-rays and prevent any from reaching the Earth's surface. These pulsing X-ray stars or X-ray pulsars actually consist of a pair of stars orbiting around each other. One member of the pair is a small, solid star commonly known as a neutron star while the other is a normal gaseous star like our Sun, but usually many times more massive. The neutron star is only a few dozen kilometers in size but has a mass roughly equal to the Sun.

The neutron star is the collapsed core of a giant star that ran out of nuclear fuel at an earlier time. Normal stars, like our Sun, are composed of hot gas that, like any unconfined gas will try and expand to fill up space. On the other hand, the gravity of the star tries to collapse the gas under its own weight. The balance between the forces of expansion and collapse produce an ordinary stable star. However, the balance can't last forever since stars are cooling down by emitting radiation into space; in other words they shine! Their gas remains hot by "burning'' hydrogen gas in their cores by the process of nuclear fusion. Eventually all stars use up their fuel supply, cool off and collapse. The collapsed remnant of stars within a certain mass range will be neutron stars while less massive stars become white dwarfs and more massive stars collapse into black holes.

The material in a relatively small body like the Earth is made of atoms. The atoms contain lightweight electrons orbiting around a small, heavy nucleus of protons and neutrons. But the pressures in a neutron star are so great that the atoms themselves collapse and the electrons are absorbed by the nuclei. The material in a neutron star is composed of atomic nuclei compressed tightly together and contains mostly neutrons; hence the name neutron star. The weight of the star attempts to further compress the neutrons together. But the neutrons respond to the increasing confinement not by squeezing closer together, but instead by moving faster, as prescribed by a fundamental quantum mechanical property known as the Heisenberg uncertainty principle. The neutrons thus act much like the particles in a hot gas and provide a counter pressure (technically known as "degeneracy pressure'') that resists further gravitational collapse. Since this pressure is only the result of confinement of neutrons and not due to heating, the neutron star can exist as a permanent object like the Earth.

Many neutron stars have magnetic fields that are more than one trillion times stronger than the familiar field on the surface of the Earth that moves compass needles. In this respect, these neutron stars are like giant rotating bar magnets. When in orbit with a normal stellar companion, gas from the companion can be pulled onto the neutron star. The hot gas falling onto the neutron star is channeled by the magnetic field onto the North and South magnetic poles. Thus two "hot spots" on the neutron star surface are formed akin to the two auroral zones on the Earth but far hotter. In fact, at these hotspots the infalling gas can reach half the speed of light before it impacts the surface. So much energy is released by the infalling gas that the hotspots, which are only a few hundred square meters in size, can be up to ten thousand times brighter than the Sun! Temperatures of millions of degrees are produced so the hotspots emit mostly X-rays. As the neutron star rotates, we observe pulses of X-rays.

The gas that supplies the X-ray pulsar can reach the neutron star by a variety of ways that depend on the size and shape of the neutron star's orbital path and the nature of the companion star. Some companion stars of X-ray pulsars are very massive young stars that emit a stellar wind from their surface. The neutron star is immersed in the wind and continuously captures gas that flows nearby. These are known as wind-fed high mass X-ray binary systems, the high mass referring the large mass of the companion star that can be 10-30 times that of our sun. Two such systems are Vela X-1 and 4U 1538-32.

In other systems, the neutron star orbits so closely to its companion that its strong gravitational force can pull material from the companion's atmosphere into an orbit around itself. This material forms a gaseous disk in which material spirals inwards to ultimately fall onto the neutron star. The companion star can be a massive star as in the case of LMC X-4, an intermediate mass star as in Hercules X-1 where the companion is twice as massive as our sun or low mass as in the system 4U 1626-67 which has a companion much smaller than our sun.

For still other types of X-ray pulsars, the companion star is rotating very rapidly and apparently shedding a disk of gas around its equator. These are known as Be X-ray binaries, the companion being a large B-type star that shows emission lines in its spectrum. The orbits of the neutron star with these companions are usually large and elliptical in shape. When the neutron star passes nearby or through the disk it will capture material and temporarily become an X-ray pulsar. The circumstellar disk expands and contracts for unknown reasons, so these temporary or transient X-ray pulsars are observed only intermittently often with months to years between episodes of X-ray pulsation.

BATSE continuously monitors the entire sky except for that portion blocked by the Earth and so is well-suited to observe X-ray pulsars. Both persistent and transient X-ray pulsations are observed. Typically about seven X-ray pulsars are being observed at any one time with pulsation periods ranging from 0.5 seconds to 681 seconds. Most X-ray satellites must point their detectors at a source in the sky to obtain observations and so only get quite limited coverage in time. Thus many transient X-ray outbursts or unusual events are potentially missed with pointed detectors. With BATSE, five new transient X-ray pulsars have been discovered since the launch of the Compton Gamma Ray Observatory in April 1991. BATSE is also providing an unprecedented record of the pulse frequency histories for many X-ray pulsars. The same material that produces the X-ray pulsations also cause the neutron stars to spin faster or slower so much can be learned about the details of the accretion process or the neutron star by monitoring its changes in pulse frequency, which measures the rotation rate. Monitoring by BATSE also allows detection of new outbursts by known transient X-ray pulsars and alerts other satellites, generally with more sensitive pointed instruments, of what's happening in the sky.

First of all, in addition to the 1.7 day orbital period and 1.24 second neutron spin period, this binary system has long been known to display an unusual 35-day long cycle of High and Low X-ray flux states. Within a single 35-day cycle are found a Main High and Short High X-ray flux state lasting roughly ten and five days each respectively and separated by ten day long Low states. X-ray pulsations are detected during the High states but cease during the intervening Low states. The High states are punctuated by deep X-ray eclipses every 1.7 days indicating a line-of-sight close to the binary plane. The source of the 35-day long cycle is believed, on very strong observational evidence, to be caused by periodic obscuration of the neutron star by an accretion disk that is tilted out of the binary plane and precessing with a 35-day period. As the disk precesses, it obscures the neutron star twice in each cycle. The disk is also twisted like a spiral staircase, so that as one moves from the outer disk rings to the inmost rings, the line-of-nodes of each radial ring gradually changes through an angle of roughly 100 degrees. In most binary systems, the accretion disk lies flat in the binary plane as in the illustration above. The exact cause of the unusual disk is still uncertain and is a big mystery in science.

The system has other unusual properties as well. The companion star HZ Her is a 2 solar mass star. All other neutron star binaries have either giant companions of many solar masses or small stars much less massive than the Sun. So Her X-1/HZ Her is the only intermediate mass X-ray binary system known. The system also lies many thousand of light-years above the plane of the Milky Way galaxy and is relatively isolated. All other X-ray systems in the Milky Way lie well within the plane of the galaxy where most of the stars lie.

The neutron star is so close to HZ Her that the X-rays it emits instensely heat the near side of HZ Her, raising the surface temperature by 10,000 degrees above the temperature of the far side. The gravity of the neutron star also distorts the shape of HZ Her so it gets pulled into a cone with the point facing the neutron star. As we see the system orbit, the brightness of HZ Her changes with a 1.7 day period because of this heating effect. In addition the brightness of HZ Her changes with a 35-day period due to the changing shadow cast by the accretion disk as it precesses.

There are also a set of periodic X-ray absorption dips seen in the X-ray lightcurve that occur just before the neutron star is eclipsed by HZ Her. These absorption dips systematically take place at an earlier and earlier time with each orbit of the neutron star during a High state. The same pattern repeats during each X-ray High state when the disk does not block the neutron star. There is another related set of absorption dips in the light curve that take place just after the neutron star passes closest to the observer in its orbit. The source of these dips is uncertain, but is believed to have something to do with the mass transfer from HZ Her which occurs in blobs rather than a continuous stream due to the changing X-ray heating of the point of HZ Her's atmosphere that is closest to the neutron star.

Another effect that I studied in my thesis is the changing pulse shape of Her X-1. The X-ray pulse shape show two different pulse shapes, one in the ten day long bright High state and another in the shorter five day long High state which is one-third as bright. The pulse shape also changes in a systematic and different way during each High state. In most other X-ray pulsars the observed X-ray pulse shape is relatively constant because the mass inflow pattern onto the neutron star is stable. After much thought and exploration of many different possible causes I was able to show that this effect is almost certainly due to the accretion disk sweeping across the neutron star and successively blocking out different portions of the emitting regions. Since the accretion disk is tilted and precessing, it passes over the neutron star in both an upwards and downwards fashion with each precession cycle. You can easily see that if you take a plate, tilt it and rotate it around while looking edge-on. From the two different changes in the pulse shape patterns, we were able to model the pulse emission regions and so see details of how the matter flows onto the neutron star and emits X-rays in a way that cannot be done with any other X-ray system or any other star system for that matter. It is somewhat amazing to see that we can resolve features near the neutron star, which is only ten kilometers in radius, and in its surrounding magnetic field, which is a few hundred miles in size, from a distance of many thousands of light years away!

Here are a few web pages devoted to Her X-1/HZ Her by others who have also studied this incredible star system. It is too bad that we can't get close up pictures to see this system in all its unusual glory. It must be a amazing sight, one of the wonders of our galaxy.