May 30, 2020

The Great Orbiting Observatory Series: Part III: Gamma-Ray Telescopes.

By far, the portion of the spectrum with the coolest Science fiction-friendly name is gamma-ray. The highest end of the spectrum, this range starts at energies above 100 keV and wavelengths of 10 pico-meters (that’s tiny…) or less. Gamma ray energy from space had been suspected since the mid-40’s, but it took the advent of the space age for gamma ray astronomy to really take off.  Studies of gamma rays have revealed an entirely new universe full of exotic beasties such as supernovae, gamma ray bursts, pulsars and black holes. Some gamma ray studies are conducted from high mountain peaks such as the VERITAS array in Arizona or the RAPTOR telescope in New Mexico which looks for optical GRB transients, as well as balloon borne observatories searching for soft cosmic rays aloft. These however can suffer degradation of the signal by energy interactions with oxygen and nitrogen molecules at altitude. Hence,  if you really want to get above our gamma-ray absorbing atmosphere, you’ve got to go to space to do it.

So you want to build a gamma ray telescope? These types of exotic instruments do not use telescope mirrors or an optical configuration in a traditional sense; instead, the employment of scintillors or photomultiplier tubes and sometimes diffracting masks or grates are used. The idea is that as a highly energetic cosmic ray hits the gas or crystals embedded within, a signature flash of Cerenkov radiation is emitted. If enough high speed photomultipliers can catch a particle in the act, a path and a direction of origin can be traced. Early detectors such as Explorer 1 were no more than simple cosmic ray counters; modern observatories such as Fermi can sweep the entire sky looking to pin-point gamma ray bursts.

One of the earliest mysteries of the space age was the source of space born high energy bursts. Satellites sent aloft to monitor nuclear weapons testing were also detecting gamma rays from cosmic sources… just what phenomena in the universe could produce such high energies?

As time and the field of astrophysics marched on, we began to realize that the universe, as J.B. Haldane once said, is “stranger than we can imagine.” Bizarre beasts such as quasars and gamma ray bursters joined our lexicon of exotic objects. A chief issue with observing gamma-ray bursts is their rapid onset and disappearance in the sky. If we were to study these sources, a telescope would need to not only be able to refine a field of view at gamma ray frequencies, but also be capable of quickly swinging into action once a burst had occurred.

The first true gamma ray telescope was NASA’s SAS-2 (for the Second Small Astronomy Satellite) launched in November 1972. Its mission lasted for about six months, and was the first gamma ray astronomy dedicated satellite. Its main discovery was the pulsar Geminga.

Some past orbital robotic greats in the field of gamma ray astronomy were…

Compton: Some great science came “straight outta Compton…” Launched April 1991 aboard the Space Shuttle Atlantis on STS-37, Compton was actually named after Nobel Prize winner Dr. Arthur Holly Compton. During its all too short life span, Compton provided some amazing breakthroughs, including the first gamma ray all sky survey above 100 MeV, including a thorough study of the galactic core at those energy levels. Compton also discovered the first four soft gamma ray repeaters. When GRB 990123 popped off, Compton was there, nailing down the fading afterglow of a galaxy over four giga-parsecs distant. One of the true crimes of our times was the untimely demise of this instrument; Compton was purposely de-orbited June 4th, 2000 after partial gyro-scope failure.

Granat: The International Astrophysical Observatory may have been one of the greatest orbiting projects that you’ve never heard of. A joint European Russian venture, Granat was placed into geocentric orbit on December 1st 1989 and carried three x-ray, three gamma ray instruments as well as an all sky detector. During its ten year life span, Granat provided an all sky survey in the 40 to 200 keV spectrum, as well as the discovery of several new black holes and pulsars. When you see a designator in a catalog marked “GRS” that stands for GRanat Source. Other unique feats include the ‘discovery of galactic micro quasars as well as a nearly two month long (!) exposure of our galactic center at high frequencies!

Cos-B: No, this wasn’t a mid-eighties sitcom…Cos-B was a European observatory conceived in the 60’s and launched by NASA in August 1975. This was back when the European Space Agency was still known as the European Space Research Organization… (Remember?) During its six year plus mission, COS-b advanced the wealth of gamma ray data and increased the catalog of known gamma ray emitting objects in what came to be known as the 2CG catalog. COS-B also conducted the first studies of the Cygnus X-3 pulsar.

But those pioneering observatories represented a mere beginning in the fledgling field of gamma ray astronomy. Among the orbiting telescopes currently in use are…

Fermi: the telescope formerly known as GLAST, the Fermi Gamma ray space telescope is now NASA’s pioneering high energy observatory in orbit.  Fermi sports two primary instruments; the LAT or Large Area Telescope, and the GBM or Gamma Ray Burst Monitor.  The LAT collects high energy positron-electron pairs generated as photons pass through special metal sheets and funnel them into a calorimeter stack. With a FOV spanning 20 degrees of the sky, the LAT can pinpoint a gamma ray source down to several arc minutes…pretty darn good in terms of high energy astronomy. The GBM is an all-sky instrument which is made of 14 scintillation detectors. Launched in June 2008, Fermi’s credits thus far include the detection of a pulsar that exclusively emits gamma ray energy (a first). It has but also been on hand to witness the most powerful GRB so far; GRB 080916C, a GRB with the force of an estimated 9,000 supernovae! Thankfully, this burst was over 12 billion light years distant.

HETE-2: The High Energy Transient Explorer, this was a sort of bridge between the Compton and Fermi timeline. HETE was launched in October 2000 and carried both X-ray and UV instruments; unfortunately its earlier version was to have gamma ray detection capability but was doomed by the failure of its payload separating explosive bolts on launch.

Swift: The Swift Gamma Ray Burst mission has been another high energy physics high performer since its launch in 2004. The goal of Swift is much as its name implies; detect events with its Burst Alert Telescope, pin point the source to within 1 to 4’ arc minutes within 15 seconds, then notify any ground based telescopes with a clear view to swing into action. Really, the kind of quick reaction demanded of gamma ray burst monitoring is that fast! Swift also comes equipped with X-ray, UV, and optical instruments of its own to assist in catching the afterglow. To date, Swift has performed admirably, passing the 500 GRB mark earlier this year on April the 13th, 2010.

Curiously, a quick search does not reveal a wealth of proposed gamma ray observatories on the books; we suspect this is because of the growth of modern balloon borne technology making it easier to place instrument payloads for high energy physics on atmospheric platforms. Of course, we would love to be proven wrong…

Digging around various astronomy forums yields only one proposed future scope; the Advanced Compton Telescope, a gamma-ray platform and successor to the original Compton, still very much in the proposal stage. Orbiting observatories such as Swift and Fermi should be destined for long lives, and provide an awesome science bang for the buck. And as noted, the balloon borne platforms used for cosmic ray studies are less than “stellar” when it comes to the gamma ray portion of the spectrum. Now that we’ve gotten a real taste for gamma ray astronomy, our suspicion is that astronomers will always want an orbiting workhorse on hand.

So there you have it, a quick peek into the exciting realm of orbital gamma ray astronomy. If your favorite GRB scope didn’t make the cut (y’know, the one you pinned your PhD on) do drop us a line, or follow us via the big, garish Twitter “Follow Me” button to the right… we love to talk astrophysics!