Now you see it - now you don't

Near the center of our galaxy, an X-ray source
gobbles up part of companion star

Sept. 23, 1999: A known variable star apparently is acting up, feeding materials to a massive, compact companion that shows its appreciation with a brilliant X-ray belch.

Right: A computer model by Michael Owen of North Carolina State University depicts a high-mass X-ray binary system where a large star feeds a compact, massive companion, the sort of system that GM Sgr is believed to be. Links to 528x527-pixel, 77KB GIF. Credit: Theoretical Astrophysics Program North Carolina State University.

The star - GM Sgr - is in the constellation Sagittarius, in the direction of the center of our galaxy. GM Sgr is a visible star discovered in 1927. It's actually part of a binary system. Orbiting GM Sgr is a massive, compact body, most likely a neutron star or a black hole, that's causing all the fuss, starting with a modest announcement early this year.

"On Feb. 20, Beppo SAX and the Rossi X-ray Timing Explorer observed an X-ray source that was variable and noted that it was coincident with GM Sgr," said Dr. Mike McCollough, a high-energy astrophysicist working with the Universities Space Research Association at NASA's Marshall Space Flight Center. Beppo SAX is a Dutch-Italian X-ray astronomy satellite. Rossi X-ray Timing Explorer (RXTE) is operated by NASA's Goddard Space Flight Center.

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While observing the galactic center, the Beppo SAX team observed an X-ray transient, an object that became almost 1/10th as bright as the Crab, and then faded away. The neutron star at the heart of the Crab Nebula is so regular in its rotation and brightness than astronomers use it as a standard candle for calibrating their instruments and for measuring the brightness of other sources.

Both teams calculated that the source - named J1819.3-2525 by the Beppo SAX team and J1819-254 by the RXTE team, both using its location in the sky - was associated with GM Sgr. Then it faded and attention went elsewhere.

Fast forward to Sept. 15: Scientists using the Rossi X-ray Timing Explorer (RXTE) announced that the spacecraft's All-Sky Monitor had seen a strong X-ray transient apparently associated with GM Sgr.

"The All-Sky Monitor was seeing nothing and then, boom! it goes off, up to 12 Crab," McCollough explained. "It was pretty bright."

The brightness started at 1.6 to 3.7 Crab, then spiked to 10 and then 12.2 times the Crab before suddenly dropping off the scale.

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As soon as the RXTE team sent out a circular announcing the event, McCollough, Dr. Mark Finger, a high-energy astrophysicist with NASA/Marshall, and Pete Woods, a graduate student with the University of Alabama in Huntsville, started checking through the data from the Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma Ray Observatory.

BATSE comprises eight detectors that stare at the entire sky so they can record gamma-ray bursts from deep space. But it also sees every bright source in the 20,000 to 100,000 electron volt (20-100 keV) range, if the right computer codes are used to filter the data. "Now you see it, now you don't" actually becomes a way of seeing discrete sources.

This is done by making the Earth part of the instrument. If a compact body's location is known, then the BATSE team can watch for changes in the data as the source rises and sets as BATSE's orbits the Earth.

Left: BATSE can observe discrete sources like J1819-2525 by examining the data around the times that the source is known to rise and set, as illustrated here. In this case, not only did J1819-2525 show up well, it stood out with a distinctive spike shortly after rising. Breaks in the line were due to a telemetry drop (first gap) and a solar flare (second gap). Links to 600x598-pixel, 125KB JPG. Credit: NASA/Marshall Space Flight Center.

"We can take measurements from about two minutes before it sets, then as it drops below the horizon, and then get two minutes of background after it sets," McCollough explained. "You subtract the background from the observations before it set, and you know how bright the source was." The same is done for sources as they rise. Thus, a source can be checked every 45 minutes. (Gamma-ray bursts, however, last from a few seconds to a few minutes, at most, and never make a repeat performance.)

Checking through their data, McCollough and Finger found J1819-2525 acting up.

"In about 2 hours this source went from not being there for BATSE to being 5 times brighter than the Crab," McCollough said. "And an hour and a half later, it's gone."

He noted that "you always worry when you get hits like that" because an energetic cosmic ray can have the same effect in a detector. But the rise and fall in the data were seen by two detectors that differed only to the extent that one detector was pointed at a slightly different angle than the other, so it was not a cosmic ray event.

Right: Graph shows sharp increases - and equally sharp decreases - in the brightness of the J1819-2525 source. The line connecting the dots is provided as a visual aid and is not a true brightness curve since J1819-2525 can be seen in the BATSE data only every 45 minutes as it rises or sets. Links to 1373x1055-pixel, 158KB JPG. Credit: NASA/Marshall Space Flight Center.

"What we saw later makes it look more real," McCollough continued. Ten hours later, "nothing, and then a major jump about the same time that RXTE saw it. This things takes off, it goes to 8 times the Crab. Then it comes down very dramatically, but is intensely active for about a half of a day, then it drops off the scale."

The question on everyone's mind now is, What caused the outburst? The X-rays themselves are a result of material falling into the compact body which probably is a neutron star or a black hole.

"There are two ways to feed this beast," McCollough said. The first is a wind flowing out from the larger star and being partially captured by the compact companion.

The second is called Roche lobe overflow. The Roche lobe is a teardrop-shaped region of space where the gravitational pulls from the two stars balance each other. Periodically, gas from the larger star will overflow the lobe and squirt through the balance point between the two to feed an accretion disk of material swirling into the compact body.

Web Links

Information about GM Sgr's outburst from the Rossi X-ray Timing Explorer team at the Massachusetts Institute of Technology, with links to IAU circulars and Astronomer's Telegrams, and from the Variable Star Network at Kyoto University. The activities of interacting binary stars is depicted by the Theoretical Astrophysics Program at North Carolina State University. Related stories Is the 2-in-1 burster a masquerade? Sept. 15, 1999. Pulsar acts a bit like a cheap clock 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.

In either case, the X-ray outburst is powered by an incredibly fast fall down the steep gravitational field of the compact body. At the end, matter from the primary body either slams into a neutron star's tougher-than-diamond surface, or is superheated in an incredible traffic jam at the event horizon before it disappears into a black hole. The kinetic energy from the superhot matter is turned into X-rays.

"You don't want to be anywhere near this," McCollough cautioned. GM Sgr and J1819-2525 are estimated to be up to 10 times farther away than the Crab Nebula. Since brightness drops with the square of the distance, the source was as much as 100 x 12 or 1,200 times more luminous than the Crab at its peak.

"Just in X-rays, this is 100,000 times more luminous than the entire output of the sun at the object's peak," McCollough estimates. That peak output is estimated at about 10³⁸ ergs per second.

To put that in perspective, 1 erg is a mosquito bumping into your hand. The proverbial apple hitting Newton on the head had a total energy of 10⁷ ergs. The 10-megaton fusion device that obliterated part of the Bikini Atoll was 10²³ ergs. So, for a short period of time, J1819-2525 was equivalent 1 million billion high-yield nuclear bombs every second.

But what prompted GM Sgr to feed the compact object?

Left: Graph intensity of J1819-2525 in different "colors" - X-ray energies - as observed by BASTE. The brightness in the hard X-ray spectrum indicates that the source is not a white dwarf. The cross marks are the actual data and the yellow section shows a curve fitted to the data. Links to 1459x1091-pixel, 135K JPG. Credit: NASA/Marshall Space Flight Center.

That remains a mystery because the exact nature of the visible star remains uncertain, McCollough noted, even though several detailed measurements have been made. And right now is a difficult time to get a visible-light spectrum that would help identify the star because the compact body's X-ray burst would have energized the outer atmosphere of the visible star.

Still, measurements are being made that eventually help delineate both objects. The observatory at the University of Kyoto captured images that, when compared with images taken a few days earlier, show GM Sgr has brightened. And the Very Large Array of radiotelescopes has detected a "reasonably bright" radio source where none was detected in 1996.

While J1819-2525 has faded below detectable levels for now, the story is not over. The American Association of Variable Star Observers has put GM Sgr near the top of its watch list. Both the RXTE and BATSE teams are checking their data to see what J1819-2525 may do next.

And McCollough is going back in time.

"There's a chance that this guy has gone off before and we didn't catch it," he said. Buried in more than eight years of observations by BATSE may be other outbursts that may reveal clues to the true nature of J1819-2525.

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