The one-man band of astrophysics

An unusual x-ray pulsar bursts, pulses, and puzzles astronomers

Dec. 1, 1998: The original astrophysical one-man band has sounded off again, this time for a encore that wasn't quite as long or loud as its debut.

The band is a pulsar rotating around a low mass star, in a binary system called GRO J1774-28. (Such binaries are also called low-mass X-ray binaries.)

"It was originally discovered by the Burst and Transient Source Experiment (BATSE) in December 1995," said Peter Woods, a graduate student working at NASA's Marshall Space Flight Center. Woods is using data from BATSE which rides aboard the Compton Gamma Ray Observatory (CGRO). "It burst and pulsed for five months during which the source released about 40 bursts per day. Then it became inactive again for seven months. In December 1996, it started up for the second time, lasting 4 months but only becoming half as bright as before. During these two stints of activity, the source released about 10,000 bursts!" It's been quiet since then.

Right: Seven frames from a NASA animation depict GRO J1744-28 tuning up and sounding off as it is orbited by its low-mass companion (in the distance). Click to get a 240x1260-pixel, 40KB copy.

"What was really interesting is that nothing like it had been observed in more than 25 years of X-ray astronomy," Woods said, "and then we get two burst periods in the space of two years."

Woods and several colleagues will present the results of a year-long study of the one-man band in an upcoming issue of The Astrophysical Journal. J1744-28 doesn't play Mozart - or even Motorhead - but its beat appeals to scientists.

Left: A sky map showing the approximate location of GRO J1744-28. The pulsar is located between the familiar constellations Sagittarius and Scorpius. Sagittarius is just visible above the south-western horizon as the sun sets in early December. The yellow dot shows the location of Venus on December 1st, 1998. GRO J1744-28 is invisible to the naked eye.

"What's unique about this object is that it does so many different things all at once," said Dr. Fred Lamb, an astrophysicist at the University of Illinois at Urbana-Champaign. He gave J1744-28 its nickname in 1996. "We've seen some sources that play the drums, some that crash cymbals, and a few that play the trumpet, but this source is a one-man band."

Pulsars are rotating neutron stars with strong magnetic fields (about 10¹¹ to 10¹² Gauss, similar to magnetars, but with weaker magnetic fields), formed when a large star goes supernova and compresses its core as it blows off its outer layers, or when a white dwarf accretes enough material to force gravitational collapse. The pulsating nature comes from the rapid rotation of the neutron star whose magnetic field axis is not aligned with its spin axis. As material is funneled onto the magnetic poles of the star, energy from this accreted matter is released in the form of X-rays. The concentration of emitted X-rays from the magnetic poles appears to a distant observer as a periodic change in the intensity of the source, like a lighthouse.

Above, right: Neutron stars are usually formed in violent supernova explosions, but GRO J1744-28 may have been born in the collapse of a white dwarf. For more information about neutron stars and pulsars see our pulsar tutorial (java required).

In addition to pulsations, they can emit occasional flashes of X-rays or bursts in two different classes.

Type I bursts occur when accreted material accumulates on the star and becomes hot enough to trigger a thermonuclear instability.

"Basically it's a big fusion bomb on the surface," Woods said. (These are not to be confused with more energetic and enigmatic gamma ray bursts from deeper in the universe.)

Right: A typical tune - a burst profile - from GRO J1744-28 as seen by the BATSE. It rises from count of near zero to a peak of almost 1,900 counts per second, then fades almost to zero again. Through it all, the period is clearly visible as a series of spikes. Click here for a 785x605-pixel, 48KB JPG.

Type II bursts occur when the mass accretion rate is too high to allow this instability to arise. It is believed that GRO J1744-28 emits Type II bursts which are due to an uneven flow of material onto the star.

"Instead of having a constant stream of matter, you have a big blob that falls in and causes the burst," he explained. The model was first proposed by Dr. John Cannizzo of NASA's Goddard Space Flight Center.

How did the bursting pulsar form?

Neutron stars can be formed in at least two ways: from the collapse of an initially massive normal star (greater than 10 solar masses), called Type II supernova, or by the accretion-induced collapse of a white dwarf. Dr. Jan van Paradijs of the University of Alabama in Huntsville and the University of Amsterdam believes the latter applies to GRO J1744-28.

Scientists know from the companion star which is donating material that this binary is old and a lot of material has been transferred between the two stars already.

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