Astroman

This is a great post! An excellent example of over-technical-izing the purchase of a telescope. It falls in the ralm of audiophiles searching for that extra 0.01% Total Harmonic Distortion they will never actually detect.

Conversely, for those of us that design and build telescopes, knowing we have a really fine piece of glass (of Strehl ration >0.8), it gives us something of a barometer for the success or failure of our design. If we can't see the Moon after hours of toil, we know we really screwed up something. If we've done due diligence in design and craftsmanship, we'll be assured of a great instrument.

My point I guess is the same as Gneiss'; don't be overwhelmed by minutae. Take in the whole picture.

Spotting scopes are an excellent starter! Wide field, good optics and ease of use are fortes. Not a great name when abreviated, though..."Mono's" gives an unpleasant overtone, especially to teens.

Now, theory on what causes these things is firming up in some areas, and not so much in others.

There appear to be two distinct types. When the energies of BATSE bursts are plotted according to energy and length, two groups appear. Short/hard bursts and long/soft bursts.

Short/hard bursts refer to the quick burst, under 10 seconds or so, and at energies in the mid to upper gamma ray spectrum-MeV.

Long/soft bursts last seconds to minutes and radiate mostly in the upper x-ray to mid gamma range-50Kev to 1MeV.

Both are “flavored” with variations within the light curve. They may have peaks of extremely high energies, while the rest is relatively low ultraviolet level.

The short/hard bursts seem to have a connection with supernova explosions. Two have been detected and have shown the connection. The GRB arrives first and is so bright it “drowns out” the light from the supernova. Spectra from the SN is detected later, sometimes weeks later and indicate a Type Ic which indicates a companion star of some kind. These GRB’s are extremely far away and are red shifted toward the x-ray or even UV band. It could be that these are the beginnings of black holes, neutron stars or more exotic objects. (Though how you get more exotic than a neutron star is beyond me…)

The actual mechanism for the creation of the burst is not well understood, beyond the apparent tie to supernova. There are SNe detected regularly, (in fact one was detected in M51 just a couple weeks ago), but only two have been definitely tied to a GRB, as I said. It could be that the SN connection is beamed straight at us, and although the burst is high energy, the actual event is of low total output, while the long/soft burst has more total output at lower energies. (That’s tough to get your head around I know, but take it slow and it’ll sink in.)

Other theories for producing GRB’s are: Convergence of two black holes, convergence of two neutron stars or combinations thereof, the collapse of a very rapidly spinning neutron star into a black hole, a “hypernova”-read SN on steroids.

Convergences of BH’s may satisfy the demands for energy release, but just barely. It depends on the total mass of both participants. Equally large BH’s, say 10 solar masses each may do the trick. Convergences of neutron stars and anything but a BH would not have enough mass to explode, much less create a GRB, unless the NS swallowed a BH quickly and with enough total mass to make trouble. Velocity and trajectory enter into this scenario, and the math is hellish, so it seems unlikely anyway.

According to Einstein’s general relativity, such collisions would also generate gravity waves of high order. There are some smaller gravitational wave detectors up today, but I don’t believe any are sensitive enough to detect these collisions, given their necessary distance. Maybe once LIGO and the orbital one, (name, anyone?), get up and running, they can, but none have been detected to date. So far, it looks like collisions are out.

Gamma ray bursts were discovered accidentally by satellites designed to detect nuclear test detonations by earthly cold-war opponents.  A satellite system called Vela was deployed in 1969 as a group of 4 to detect detonations anywhere on Earth in real time.  Detections were made on a daily basis, but locations were from outer space, not Earth.

And the mystery began.  Several smaller missions were launched to try to study these events.  Balloon-borne instrument packages were designed, built and flown by Dr. Jerry Fishman, now of Marshall Space Flight Center in Huntsville, Alabama.  Much was discovered, but the basic technology of HOW to detect and localize the bursts was proven in the balloon experiments.  GRB’s were the hot topic in the scientific community, so ASCA, Skylab, Solar Max and others did significant research too.  In fact, essentially every satellite launched since 1978 has had some form of gamma ray detector on board.  Most are tested and calibrated in flight by measuring known gamma ray sources and GRB’s.

NASA answered the call with the Compton Gamma Ray Observatory, launched by the Shuttle Columbia in 1991.  The second of “The Great Observatories”, (HST being the first), it held 4 instruments to detect, record and study GRB’s.  It was the first satellite dedicated to the study and at 17 tons, the largest payload up to that time.  During its lifetime, it detected 8000 events.  The instruments on board were the BATSE, (Burst And Transient Source Experiment), OSSE, (Oriented Scintillation Spectrometer Experiment) COMPTEL, (Imaging Compton Telescope) and EGRET, (Energetic Gamma Ray Experiment Telescope).

BATSE was the brainchild of Dr. Fishman, (whom I’ve met on several occasions and found to be a great guy, very passionate about his work and besides his obvious intelligence, a regular guy), from the same balloon experiments.  Placed on each corner of the spacecraft, these 8 instruments detected the bursts as they occurred and the direction, allowing a rough localization.  Its energy range was between 20KeV and 1200KeV.  (eV=Electron Volts)

OSSE held 4 aimable detectors.  This design allows the detector to compare the gamma ray source with background radiation for accurate energy measurements in the notoriously noisy range.  Energy sensitivity was 50KeV to 10 MeV.

COMPTEL is the main directional detector.  Based on the “Compton Effect”, for which the observatory’s namesake won a Nobel Prize, 2 layers of detectors are used to determine the energy and arriving direction of gamma rays.  Its range is from 1 MeV to 30 MeV.

EGRET is the highest energy detector on board.  It’s also the most sensitive detector at these energies.  Incoming gamma rays trigger a cascade of electron-positron pairs.  These are measured for direction and counted for energy level.  Its range is 20MeV to 30 BeV.

CGRO was deorbited in June, 2000, ostensibly for “safety reasons”.  It had a gyro that had failed and politicos pulled the plug.  (It was actually used as a pawn to persuade Russia to bring down the unsafe and dangerous MIR.  But don’t tell them I know, I’ll have to kill you.  Jerry Fishman still can't talk about it without getting snorked.)

Other GRB missions include HETE I and II, Chandra X-Ray Observatory, Multiple Mirror X-ray telescope, SWIFT and some others I forget at the moment.

HETE I and II were, ahem, less than spectacular successes.  Actually, HETE I crashed into the see at launch, and HETE II has decided that the Sun is a source of extra-galactic GRB’s.  It forgets that there are known sources of gamma rays, such as Cygnus X-1, the Crab Nebula and various other known soft gamma repeaters.  It does sometimes provide good localizations, but they are rare.

The SWIFT mission is the current bonanza operation.  I get localizations at least once per week, and sometimes in bunches.  It is the first mission to automatically slew the spacecraft toward an incoming GRB and take images, sometimes in real time.  Its band is in the ultraviolet and soft x-ray range, (10eV- 10 KeV).

Questions so far?