Burst and Transient Source Experiment (BATSE) finds most distant quasar yet seen in soft gamma rays

Contact: John Horack

john.horack@msfc.nasa.gov

256-544-1872

NASA/Marshall Space Flight Center–Space Sciences Laboratory

BATSE finds most distant quasar yet seen in soft gamma rays

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.