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Pulsar acts a bit like a cheap clock

 

Sept. 15, 1999: It's not quite time for Halloween, but scientists may be discovering their 2-in-1 pulsar was really just masquerading and may be something else that they haven't quite figured out.

The pulsar, GRO J2058+42, was discovered in 1995 by Dr. Colleen Wilson-Hodge of NASA's Marshall Space Flight Center. The pulsar's signature was buried in data from the Burst and Transient Source Experiment (BATSE), which she was mining with the hope of finding discrete, regular sources.

Right: Dr. Colleen Wilson-Hodge with the prototype of the Burst and Transient Source Experiment (BATSE). The instrument is appropriately decorated with bats, the team's mascot. Credit: NASA/Marshall Space Flight Center.

Wilson-Hodge has found two so far, and now is reexamining her conclusions about her first, GRO J2058+42. She discusses her investigation this week at the 5th annual Compton Symposium on gamma-ray astronomy being held at the University of New Hampshire's Space Science Center in Portsmouth, N.H.

GRO J2058+42 was dubbed the 2-in-1 pulsar in 1998 because it appeared to burst twice on each orbit, instead of once like others of its kind, apparently as it orbited through an extended accretion disk of gas spun out from its companion star. It's an accretion-powered pulsar, one in which the neutron star's emissions come from matter, probably emitted by a visible-light companion, making a horrible gravitational plunge to the pulsar's surface. Where a freshly hammered nail will give a dull infrared glow, an accretion pulsar will shine in X-rays and gamma rays - but only when its surface experiences a rain of matter from space.

 

An artist's concept depicts the pulsar or neutron star passing twice each orbit through the circumstellar disk emitted by the probable Be companion star. Credit: NASA/Marshall

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"For the first nine outbursts we were seeing this odd-even pattern," Wilson-Hodge explained. The first outburst observed was about 10 times brighter than all subsequent bursts. After that, the pattern was that odd-numbered outbursts were about 1.5 to 2 times brighter than the even-numbered outbursts. That supported the theory that the pulsar was orbiting through an excretion disk, most likely spewed from the equator of a rapidly rotating type Be star into space (a Be star is a hot blue-white star with emission lines from glowing gas it has blown off. Because J2058+42's location has some uncertainty, the companion has not been identified). The strong odd-numbered bursts could be in close to the companion, where the disk would be thicker, and the even-numbered bursts would be farther out, where the disk is thinner.

 

Wilson made her discovery with the Burst and Transient Source Experiment on board the Compton Gamma Ray Observatory, and built on it with additional observations by the Rossi X-ray Timing Explorer. This is one of 12 known transient accreting X-ray pulsars with no visible companion.

 

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Since the April 1991 launch of BATSE, a NASA/Marshall instrument, astrophysicists have detected 22 of 70 known pulsars (as of February 1999) and discovered another 6 new X-ray pulsars. These accreting pulsars are not the same as the mysterious gamma-ray bursts that appear to be coming from the edge of the universe. They are energetic enough to be recorded by BATSE, but don't always get noticed right away.

So, as Wilson reviewed BATSE data in September 1995, she found a burst that registered 140 milliCrabs (or, 140/1,000ths the brightness of the Crab Nebula, which astrophysicists use as a standard candle). By using a computer to fold the data on itself, Wilson found that the source repeated every 198 seconds, an indication of a massive, compact object spinning at high speed. Further data established a regular strong-faint outburst pattern.

Then GRO J2058+42 started acting up.

"The outbursts have become more irregular and weaker," she explained, "and the pattern seems to have gone away or reversed itself. Now we're back to not being able to explain this pattern."

Part of the uncertainty stems from the fact that GRO J2058+42 is so faint that it's near the limit of what BATSE can detect. On a few cycles, BATSE saw only the odd-numbered (bright) outbursts and none of the even-numbered (faint) ones.

During a giant outburst period lasting the 46 days after the pulsar's discovery, the pulsar's rotation accelerated from 198 to 196 seconds per rotation. GRO J2058+42 was 2 seconds faster. That's a phenomenal increase, since pulsars have as much mass as our Sun packed into a ball about 20 km (12 mi) in diameter.

 

"Don't set your clock by this pulsar," said Dr. Mark Finger, a co-investigator at NASA/Marshall. "Most pulsars are more regular than any clock in the world, but not this one."

Left: What looks like an outline of twin skyscrapers is a trace, from BATSE, of J2058+42's pulse profile. Links to

. Credit: NASA/Marshall Space Flight Center.

 

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. The sources soon were identified as rapidly rotating neutron stars with intense magnetic fields. Where radio pulsars have the regularity of a atomic watch, accretion pulsars are like cheap alarm clocks that easily gain and lose time - and go off when you least expect it.

The rotational increase may be due to GRO J2058+42 swallowing more of the matter streaming off of its companion star. Or it could be an observational effect from the pulsar's motion. A larger question is why it has such a long rotational period when the great majority of pulsars have periods ranging from less than 10 seconds to as fast as a few milliseconds. Radio pulsars typically have short periods, less than 10 seconds, and accretion-powered pulsars have longer periods, up to 10,000 seconds.

 

Web Links

Scientist finds 2-in-1 burster; March 25, 1998. Original story on Wilson-Hodge's discovery. Other news from the 5th Compton Symposium includes a new gamma-ray catalog of the universe and a method of weighing black holes. The 5th biennial Huntsville Gamma-Ray Burst Symposium is coming up, Oct. 19-22. Things that go bump in the night. Jan. 21, 1998. Team finds that pulsars get wound up - and down. Astronomers discover bursting pulsar. May 23, 1996. Return of the bursting pulsar - June 1996. NASA-/Goddard web page about GRO J1744-28; with links to 1.7MB AVI and 1.6MB Quicktime animation. The one-man band of astrophysics - Dec. 1, 1998. An unusual x-ray pulsar bursts, pulses, and puzzles astronomers. BATSE pulsar stories. Data on J1744-28 and other pulsars observed by BATSE. The Astrophysical Journal on-line.

"The change in the period during the first outburst was most likely due to matter being dumped from the companion star," Wilson-Hodge explained. "In the weaker outbursts, where the period change is smaller, the observed period decrease may be, produced by the neutron star moving toward us in its orbit or it may be due to matter being dumped from the companion."

A leading theory holds that all pulsars start out fast, but accreting pulsars go through a blackout period when they are surrounded by material that blocks them from view. In the "propeller effect," the pulsar's magnetosphere bats the material away while the material drags on the magnetic field, slowing the star's rotation. Eventually, the star slows to the point that the magnetosphere can no longer bat the material away, and the star accretes enough to clear the field of view. We now see an older, slower pulsar.

What the outbursts actually show is a hot spot, probably a magnetic pole, offset from the pulsar's north or south geographic pole. Stellar material is funneled along the pulsar's strong magnetic field lines, somewhat like Earth's own magnetic field directing materials to the polar cusps and forming the aurora borealis.

The pulsar's rotation shines the hot spot around space like a lighthouse, so it appears to blink off and on every 196 seconds.

The change in the outbursts, including the flip in the odd-even pattern, is leading Wilson-Hodge to reconsider her initial belief that the pulsar has a 110-day orbital period and instead has a 55-day period.

 

Right: Graphs depict the odd (blue) and even (red) bursts from J2058-42 as seen by BATSE (the light green dots are outbursts seen by the Rossi X-ray Timing Explorer). Intensity is measured in thousandths of a Crab (milliCrabs), which use the Crab Nebula's X-ray pulsar as a "standard candle." Links to , or click here for a full-page, 42K Acrobat PDF. Credit: NASA/Marshall Space Flight Center.

Then why would the Be companion or the excretion disk, neither with a link to the pulsar, act in apparent concert with the pulsar?

"That's a very good question," Wilson-Hodge replied. "It may be in a disk with an instability, or in the material coming off the Be companion. Why it should seem to be two times the orbital period is really hard to explain."

For now, Wilson-Hodge just has questions.

"Maybe it will have the courtesy to brighten up again," she said. "That would help a lot."

 

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Author: Dave Dooling Curator: Linda Porter NASA Official: M. Frank Rose