Interstellar burp leads to discovery of new pulsar

Astrophysicist makes her second pulsar find

Dec. 11, 1998: Few people get to discover a new star. Colleen Wilson-Hodge, an astrophysicist at NASA's Marshall Space Flight Center, now has two with her second discovery of an astrophysical oddity - a pulsar orbiting a massive star.

Pulsars - rotating neutron stars - are among the most intriguing objects in the sky. They were found in 1965 when radio astronomers discovered several objects that emitted radio waves with clock-like precision.

Left: A sky map showing the approximate location of this discovery (J1946+274). The pulsar and its companion are located near the familiar constellation of Cygnus. At this time of year, Cygnus is only briefly visible to Northern observers just after sunset. With an apparent visual magnitude of +14.4, J1946+274 is not visible to the naked eye or to small telescopes.

Right, above: A diagram shows a pulsar (small white object) orbiting a more massive companion. more details below.

The sources were identified as rapidly rotating neutron stars with intense magnetic fields. Another type of pulsar, called an accretion powered X-ray pulsar, was first detected in 1971 by an X-ray satellite called Uhuru. Whereas radio pulsars have the regularity of a Swiss watch, accretion pulsars are like cheap alarm clocks that often gain and lose time - and go off when you least expect it.

The massive companion, a type B[e] star, is a superhot blue-white star (type B) about 8 to 15 times as massive as our sun, with a distinctive signature - emission lines caused by glowing hydrogen and oxygen blown off the star.

Wilson-Hodge discovered her first pulsar in 1995 with the Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma Ray Observatory, and studied it with additional observations by the Rossi X-ray Timing Explorer (RXTE). Besides being the 12th known transient accreting X-ray pulsar with no visible companion, it went off twice each orbit, rather than once, because its lopsided orbit took it through the B[e] star's excretion disk twice each orbit. GRO J2058+42, as it is called, appeared to be orbiting a B[e] star.

Wilson-Hodge's second pulsar discovery came on Sept. 7, 1998. At the same time, astronomers operating the RXTE's All-Sky Monitor noticed the same burst pattern in their data.

"They got a better location and we got a better period," said Wilson-Hodge, illustrating the value of having more than one set of eyes studying the sky.

An earlier set of eyes - Canada's Ariel satellite - may have seen the pulsar in 1976. The position for 3A 1942+274 lies within the RXTE and BATSE error boxes, but there's enough uncertainty that no one can be sure.

Above: A simplified diagram shows the pulsar's inclined orbit through the B[e] star's circumstellar disk. As the pulsar passes through the and absorbs matter from the excretion disk, created when the B[e] star ejects matter, the pulsar bursts in brightness. (Note that this diagram does not show the distortion in the disk that would be caused by the massive gravitational field of the pulsar.)

This pulsar has two complex names, XTE J1946+274, and GRO J1944+26, acknowledging its dual discoverers. The numbers give the position in the sky - right ascension (or, hour angle: 19h 46m) and declination (angle above or below the equator: +27.4 degrees) - for the center points of the Rossi XTE and BATSE estimates of the position. Other observations have refined the position to 19h 45m 34s+27deg 30.0 min, and provided a strong hint that the visible companion indeed is a type B[e] star about 13,000 light years away.

Wilson-Hodge found a 15.8-second period in the data, clearly making the object a pulsar. Nothing rotates that fast unless it is very compact, and that has to be a neutron star or black hole (for various reasons, it's not the latter).

The burst pattern makes it the 33rd member of a small, growing clan of transient, accretion-powered pulsars. Accretion means that they are gobbling up material thrown off by a companion. Transient means they turn on and off without warning.

"Transient sources begin and end periods of activity seemingly at random," Wilson-Hodge explained, "but while they are active, they can have very regularly spaced outbursts.

"About half of these sources are known to orbit type B[e] stars. We think the rest are similar. Visible companions haven't been found for all of them because some are too far away and others are hidden by dust between us and the star.""Several of these pulsars that we've seen with BATSE have started out with a giant outburst and have followed with a series of regularly spaced smaller outbursts," she continued. Scientists suspect the giant outburst is caused by a massive ejection of material from the B[e] star into an excretion disk surrounding the B[e] star.

The pulsar then orbits through it and causes the big outburst. The smaller outbursts occur as the pulsar orbits through again and again, gradually sweeping the area clean and possibly dragging some material in its trail.

"What causes these ejections by the B[e] star is not known," she said. "These B[e] stars seem to be rotating very rapidly, but not quite at breakup speed," she noted. As they spin, they cast out large quantities of gas in an expanding disk which the neutron star passes through during its orbit.

"We get a giant outburst," she said, "from matter being dumped onto the surface of the neutron star."

With the giant outburst having announced the presence of the star, Wilson-Hodge and other scientists now are awaiting the "normal" bursts to see if they can determine the orbital period and other secrets of this oddity. The giant outburst is still going on, but it has faded to about 25% as bright as its peak of 35 milliCrabs (3.5% the brightness of the Crab Nebula).

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