BATSE finds most distant quasar yet seen in soft gamma rays

November 23, 1999

Discovery will provide insight on formation of galaxies

Nov. 18, 1999: Once upon a time, in a galaxy far, far away.... OK, that one has been done, but it's appropriate for this story, and true, besides. About 11 billion years ago something went burp in a distant galaxy, causing it to flare up in virtually all of the electromagnetic spectrum. A few years ago the radiation from that event passed through our solar system and was recorded by a variety of instruments, including the Burst and Transient Source Experiment (BATSE).

Orbiting aboard the Compton Gamma Ray Observatory, BATSE was designed to detect flashes of radiation from gamma-ray bursts that have mystified scientists since the 1970s. Because it observes the entire sky, BATSE can't be pointed, in the conventional sense, to look at just one star or galaxy. But members of the BATSE team soon figured out how to do it by using the Earth as a giant occulting disk so (with a few mathematical tricks) they can extract the signal of a source as it rises and sets as seen by the satellite.

One of the results is that scientists using BATSE data have discovered that a distant quasar shines regularly in gamma rays and emits the occasional burst. The findings are reported by Angela Malizia, a doctoral candidate at the University of Southampton in Southampton, England, and several colleagues. Among them is Dr. Mike McCollough of the Universities Space Research Association. He's a member of the BATSE team at NASA's Marshall Space Flight Center. Also on the team were Drs. Loredana Bassani and J.B. Stephen of the Instituto TeSRE/CNR Bologna, Italy, and Drs. Bill Paciesas and Nan Zhang at The University of Alabama in Huntsville.

"We have now proven that we can detect quasars of such distance in gamma rays, the most energetic form of electromagnetic radiation," Malizia said. Her supervisor, Prof. Tony Dean, added: "This is exciting, for we have several years' worth of gamma-ray data to go through, and we can hope to extract more examples of these distant objects which formed when the universe was much younger."

"Angela was doing extragalactic studies with BATSE," McCollough explained, "with a huge database" comprising three years of BATSE observations. In effect, Malizia combined thousands of observations of the region with each other so the noise in the signal would cancel out and thus allow her to detect faint signals that normally escape BATSE's notice.

Malizia was studying parts of the sky at high galactic latitudes. That is, they are above and below the galactic plane -- the Milky Way -- of our galaxy. These parts of the sky afford a better view into deepest space because there are fewer strong gamma-ray sources as compared to the crowded Milky Way.

"Angela has now discovered this quasar in soft gamma rays," McCollough said. 4C 71.07 is its designation in the 4th Cambridge University catalog of radio sources. It is also known as QSO 0836+710, a quasar or quasi-stellar object that emits baffling amounts of radio energy. (The numbers actually designate the same place in the sky: 71.07 is its declination, and 0836+710 is right ascension and declination.)

Quasars, such as 4C 71.07, are also known as active galactic nuclei or AGNs. It has a red shift of z=2.17, putting it about 11 billion years away in a 12 to 15-billion year-old universe (using z=1 as 5 billion light years). "It's basically the nucleus of a galaxy that is showing extraordinary activity," McCollough said. It's believed to be a super massive black hole at the center of a galaxy that is forming.

"What BATSE has discovered is that it can be a soft gamma-ray source," McCollough said. This makes it the faintest and most distant object to be observed in soft gamma rays. 4C 71.07 has already been observed in gamma rays by the Energetic Gamma Ray Telescope (EGRET) also aboard the Compton Gamma Ray Observatory.

Black holes come in two varieties. "Ordinary" black holes have as much mass as a few suns compressed into a region just 10 to 20 kilometers across. It's a cosmological rabbit's hole where matter and light go in and only an ever-stronger gravitational pull comes out. Indeed, measuring the diameter is meaningless since even the fabric of space is stretched under these conditions. The other variety comprises supermassive black holes having a mass equivalent to millions of suns crammed into a volume about as wide as our solar system. In an AGN, the supermassive black hole comes from the enormous concentration of gas and newly forming stars at the center. Eventually the black hole consumes nearly everything nearby but is unable to reach stars that have formed in distant orbits where tidal forces don't rip them apart.

Astrophysicists think that observations by BATSE and other instruments in the same energy range "are even bigger news than EGRET because AGNs are so elusive at soft gamma-ray energies," noted Bassani, one of Malizia's early advisors at the University of Bologna in Italy. The quasar population peaks at about the same distance as 4C 71.07, so the ability to observe in soft gamma rays may carry additional information about their formation.

"Our own galaxy may have been an AGN at one time," McCollough said. But it eventually settled down, and the black hole at the center has to be content with the occasional snack that drifts close enough to be captured. Thus, an AGN represents what may have been normal youthful activity for staid galaxies like ours and the famous M31 spiral galaxy in Andromeda.

In the case of 4C 71.07, it's the brightest AGN seen above 20,000 electron volts (20 keV). Its average flux (the amount of radiation reaching our telescopes) is about 13 milliCrabs, or 13/1,000ths as much as the Crab Nebula, a standard candle in astrophysics.

But it also bursts, although not all at once across the entire spectrum. On Nov. 20, 1995, it reached its record optical brightness. Then, 55 days later, its gamma-ray emissions peaked, fading back to its average output three months later. This implies, Malizia wrote, that the source is only 100 billion km (about 60 billion mi) across, or about a third of a light year. (Since light, or particles moving nearly as fast, have to carry the signal to erupt, the duration of an event can translate into its maximum size.)

At the same time, the peak luminosity was around 2.6 x 1048 ergs/sec, a mere 1/10,000th that of a gamma ray burst, but continuous for two months. That makes for a total output of 1055 ergs, 1,000 to 10,000 times greater than the output of a gamma-ray burst.

But if nothing gets out, what is broadcasting gamma rays, X-rays, light, radio, and so on? It's the death scream of material about to be swallowed by the black hole as the gases and dust swirl around the drain, cramming against each other. It's so crowded that some materials escape (just before the event horizon) by overflowing into jets firing along the black hole's rotational axis. "You're dealing with things that have much more inertia than X-ray binary stars," McCollough said. "These are large flows of gas that take many orbits to spiral in, and on a much longer time scale. So, there's more energy to be released."

Radio astronomers have observed what are known as superluminal jets, jets headed away from each other faster than light. There's no real paradox: it is a purely geometrical effect due to the jet being nearly head-on to the Earth.

The radio signal is caused by electrons spiraling along the magnetic fields trapped in the jets. The electrons can also interact with visible light emitted by the disk around the black hole, "and that pumps them into the X-ray and gamma-ray regime," McCollough explained.

The cause of the bursts remains unknown for now. It may have been a star that was shredded and finally was partially eaten by the black hole and partially burped along the superluminal jets.

To determine the cause, 4C 71.07 will be under closer scrutiny to see what it does next.
-end-


NASA/Marshall Space Flight Center--Space Sciences Laboratory

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