Scientists have discovered the closest black hole yet, a mere 1,600 light years from Earth. Its discovery was heralded by four of the most dramatic rapid X-ray intensity changes ever seen from one star.
Astronomers from the Massachusetts Institute of Technology (MIT) and the National Science Foundation’s National Radio Astronomy Observatory (NRAO) announced their findings at the American Astronomical Society’s meeting in Atlanta.
The black hole in the constellation Sagittarius, along with a normal star dubbed V4641 Sgr, form a violent system that briefly flooded part of our Milky Way Galaxy with X-rays and ejected subatomic particles moving at nearly the speed of light one day last September. At the peak of its X-ray output, V4641 Sgr was the brightest X-ray emitter in the sky.
Astronomers call this type of system an X-ray nova because it suddenly becomes a bright source of X-rays, but this object shows characteristics never seen in an X-ray nova.
“V4641 Sgr turns on and off so fast that it seems to represent a new subclass of X-ray novae,” said Donald A. Smith, postdoctoral associate in MIT’s Center for Space Research. Smith worked on data from this object with MIT principal research scientist Ronald Remillard and NRAO astronomer Robert Hjellming. “In X-rays, the intensity rose by a factor of more than 1,000 in seven hours, then dropped by a factor of 100 in two hours,” Remillard said.
The radio emission was seen as an image of an expanding “jet” of particles shooting out from the binary system. After reaching a maximum, the radio intensity dropped by a factor of nearly 40 within two days. “Radio telescopes give us a quick glimpse of something moving at a fantastically high velocity,” Hjellming said.
Black holes harbor enormous gravitational force that can literally rip the gas away from a nearby star. This transfer of gas is visible in many forms of radiation. Both orbiting X-ray telescopes and ground-based radio and optical telescopes saw the outburst of V4641 Sgr.
The radio observations revealed the presence of a jet escaping from the system at mind-boggling speeds. Only three other galactic X-ray stellar systems have been found to eject material at such speeds. They have been dubbed “microquasars” because, on a smaller scale, they resemble quasars, which lie at the hearts of distant galaxies and also spew out high-velocity jets of particles. In galaxy-core quasars, the black holes are millions of times more massive than the Sun; in the more nearby microquasars the black holes are roughly three to twenty times more massive than the Sun.
The extremely high velocity of the jets suggests that their origin lies close to the event horizon of a black hole. Microquasar activity is thought to arise when the black hole in the binary system draws material away from its companion star. The material surrounding the black hole forms a rapidly spinning disk called an accretion disk. This disk is heated by friction to millions of degrees, causing it to emit X-rays.
As spiralling gas moves into the gravity well of the black hole, it moves faster and faster. Magnetic fields in the disk are believed to expel the charged subatomic particles at speeds close to that of light. As the charged particles interact with the magnetic fields, they emit radio waves.
If some of the material escapes by being magnetically expelled into space, the matter may continue moving at the tremendous speed it had attained near the black hole. After their ejection, the jets of particles expand and cool, fading from astronomers’ view.
V4641 Sgr excites astronomers because it is close and because it acted so differently from other microquasars. In other microquasars, outbursts have dimmed more slowly over weeks or months rather than hours.
“There’s something fundamentally different about this one; it’s more extreme than any other example,” Hjellming said. “And because this system happens to be so close to us, `it is very likely that there are more objects like V4641 Sgr waiting to be discovered,” said Smith. “The rapidly flaring systems in our galaxy may have been too faint and too fast for us to notice them,” added Smith.
What makes it so different? Astronomers aren’t sure, but Remillard speculated that, “in V4641 Sgr, either the matter can flow into the black hole without forming a large accretion disk, or the black hole itself is significantly different in its mass, spin or charge.”
“Theory is lagging far behind the observations in terms of explaining what’s going on in this system,” Hjellming said.
The drama of V4641 Sgr began Sept. 15, 1999, when Australian amateur astronomer Ron Stubbings noticed that the “star” was more than six times brighter than it had been the night before. He sent an e-mail message around the world. One recipient, Japanese astronomer Taichi Kato, recalled that this object had been associated with variable X-ray emission by scientists working with the Dutch-Italian BeppoSAX spacecraft. Kato forwarded the message to Smith, a member of the All-Sky Monitor (ASM) team using the Rossi X-ray Timing Explorer (RXTE) satellite.
The ASM surveys the entire sky about once every two hours, and Smith found that the most recent observation of V4641 showed it as a bright X-ray emitter. Subsequent observations showed the rapid rise and fall of the object’s X-ray brightness. A few hours later, it flared again.
Within 24 hours, the National Science Foundation’s Very Large Array (VLA) radio telescope in New Mexico was observing V4641 Sgr. “We could immediately see that it had structure — it was big,” Hjellming said. The first VLA observation showed an object three times longer than the distance from the Sun to Pluto. “What we were seeing was the jets, and we could tell they were moving so fast that they already had expanded to a considerable size,” he said.
The VLA observations showed that the object’s jet was moving at nine-tenths the speed of light. Other radio telescopes observing the object were NRAO’s Green Bank Interferometer in West Virginia; the Australia Telescope Compact Array; the Molonglo Observatory Synthesis Telescope, also in Australia; the MERLIN array in Britain; the Ratan 600-meter radio telescope in Russia; and radio telescopes at the Owens Valley Radio Observatory in California. The radio observations also provided the distance measurement for the binary system.
The dramatic X-ray flare on Sept. 15 was not the only time V4641 Sgr exploded into activity. Further examination of ASM data revealed a bright flare (about one-third as intense as the brightest flare) on Sept. 14th that lasted between three minutes and three hours.
In response to the ASM team’s alert, Michael McCollough and Peter Woods, members of the BATSE team at Marshall Space Flight Center, scoured their data for evidence of V4641 Sgr. In addition to the flares seen by the ASM, they found a third rapid flare that peaked two hours after the brightest flare, reaching a peak intensity about half that of the brightest flare.
The RXTE Proportional Counter Array (PCA), a very large X-ray telescope, was rapidly reoriented to observe V4641 Sgr about 4.5 hours after the brightest flare. A fourth event, lasting 20 minutes, was recorded by the PCA to reach an intensity of one-sixth that of the brightest flare.
The PCA data reveal complex substructure, with luminosity changes by a factor of four within one second, and by a factor of 500 within minutes. No further high-energy emission from V4641 Sgr has been observed with any satellite since the end of the flare seen by the PCA. “Combining the data from all three instruments, we saw four of the most dramatic rapid X-ray intensity changes ever seen from one star,” Smith said. “This behavior is new. We’ve never see anything like it.”
The proximity of the object “gives us an unusual close-up look at this phenomenon,” Hjellming sai
d. If future searches for brief X-ray flares reveal that there are more objects like V4641 Sgr, “we will have a whole new source of information that can help us decipher just how jets in X-ray binaries work,” Remillard said.
The VLA is an instrument of the National Radio Astronomy Observatory, a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The RXTE is a NASA explorer mission consisting of X-ray instruments built by teams at Goddard Space Flight Center, MIT and the University of California at San Diego.
Dave Finley, NRAO
Deborah Halber, MIT