May 31, 2020

31.05.11: Cosmic Distance Record Broken?

GRB 090429B as seen by Swift. (Credit: NASA/Swift/Stefan Immler).

Last week, a new possible record smasher was announced in the realm of cosmology. It seems that every few months, we get another “largest, biggest, farthest” in the world of gamma-ray bursters. This one, designated GRB 090429B was discovered by NASAs Swift satellite and recent photometric calculations place its redshift at z=9.4, which would make it about 13.14 billion light years distant. [Read more...]

07.04.11: Catching a Black Hole in the Act.

NASA’s swift spacecraft caught something interesting on the night of March 28th, 2011. Launched in 2004, the spacecraft is designed to detect extragalactic x-ray and gamma-ray flashes. And what a flash they caught in GRB 110328A; a burst four billion light years distant that peaked at a brightness one trillion times that of our own Sun. But what’s truly interesting was that the power curve seen by astronomers was consistent with a galactic mass black hole devouring a star. Word on the astro-street from the Bad Astronomer, Phil Plait is that a yet to be released set of Hubble follow up images of the region seem consistent with the burst occurring near the core of a distant galaxy. In addition, NASA’s Fermi satellite, which also watches for gamma-ray bursts, has detected no past activity from the galaxy in question; this was an individual event without precedent. Did astronomers witness a “death by black hole” of a star? Perhaps such an event could occur if a nearby passage of another star put the body on a doomsday orbit. And interesting side note; astronomers established a thread to track GRBs in another pair of science/astronomy blogs that you might have heard of, the Bad Astronomy/Universe Today bulletin board. Much of the initial discovery and follow-up action occurred here, a forum worth following. And they say, “What good is blogging…”


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!


01.06.10: Do Primordial Magnetic Fields Roam the Cosmos?

NASA’s Fermi Gamma-ray Space Telescope may have uncovered a new high energy mystery. Like our own Earth, galaxies and even large scale galaxy clusters have magnetic fields shrouding them. The fields of mature galaxies such as our own Milky Way should have been seeded during the early formation of their weaker ancient counterparts. Perhaps these got their start from early supernovae, which spewed charged particles forth into the cosmos. These galactic magnetic fields may even help control the modern era rates of star formation, as well as regulate interstellar gas and guide cosmic rays.

[Read more...]

04.04.10-Fermi: Einstein Still Rules.

We just can’t seem to get enough of NASA’s Fermi Gamma-Ray Space Telescope. The successor to the late Compton observatory that was de-orbited in 2002, Fermi has already pinpointed over 1,000 discrete gamma-ray sources, five times more than previously known. Now Fermi has also provided a rare test of Einstein’s theories of relativity. Relativity says that all electromagnetic waves (including highly energetic gamma-rays) travel through space at the same cosmic speed of 186,282 miles per second. Being a classical theory, however, what Einstein doesn’t do is meld gravity satisfactorily with the other three fundamental forces; electromagnetism and the strong and weak nuclear forces. Gravity stubbornly refuses to be unified, and such a goal has been the holy grail of physics for over the last half century. An alternative model of gravity at the microscopic scale would say that the nature of space-time is “frothy,” and a predicted effect should be a measureable drag induced on high energy photons. Recently, Fermi had a chance to put this to the test; on May 10th of last year, GRB 090510, a short gamma-ray burst 7.3 billion light years distant, was measured by Fermi’s Large Area Telescope. The verdict; gamma-ray photons varying a by a factor of a million times in energy arrived just nine-tenths of a second apart, far below what would be predicted by “frothy” space… that’s round one for Einstein!

30.03.10- Fermi: On the Hunt for Dark Matter.

One of the major astrophysical mysteries of our time may be on the verge of being solved. Namely, where is 85% of our universe? That’s the amount that is predicted to be composed of enigmatic dark matter. Now, scientists using NASA’s Fermi Gamma-ray Space Telescope (formerly known as GLAST) have found tantalizing clues at the core of or galaxy; an electron haze thought to be the signature of dark matter annihilations. Fermi passed a milestone of 100 billion detection events with its Large Area Telescope (LAT) last month; such unprecedented sensitivity is giving scientists a new window on the gamma-ray universe. The key is to isolate dark matter sources from other, more “mundane” cosmic events. The tale started back in 2004, when the Wilkinson Microwave Anisotropy Probe (WMAP) began detecting a microwave “haze” centered on the center of our galaxy. Then, just over a year ago, Europe’s Payload for Antimatter Matter Exploration & Light-nuclei Astrophysics (PAMELA) and NASA’s balloon based Advanced Thin Ionization Calorimeter (ATIC) both independently detected high energy positrons and electrons that seemed to emanate from the vicinity of our solar system. This could be explained either by dark matter annihilation or a hidden local dark body source, either conclusion equally bizarre. A good candidate for the Fermi emissions are the annihilation of dark matter neutralinos, which serve as their own anti-particle. The predicted number of neutralino events, however, do not match the quantity of gamma-ray emissions that Fermi sees. Other Earth-bound dark matter detectors are entering the fray, such as the XENON100, and Large Underground Xenon (LUX) dark matter experiment.  Could the puzzle of dark matter be on the verge of an answer soon? Stay tuned…

23.10.09:Fermi Pegs Gamma-Ray Pulsars.

NASA’s Large Area Telescope aboard the orbiting Fermi gamma-ray observatory continues to turn out some amazing science, picking up where Compton left off in 2002 as it surveys the gamma-ray sky. Of particular interest are gamma-rays emitted from pulsars. Pulsars are the swiftly rotating remnants of massive stars that have gone supernova, leaving a superdense core in their wake. These are sometimes called “neutron stars” because the matter comprising them is packed so tightly the individual nuclei are literally stacked end to end, making a spoonful weigh as much as a mountain! After all, most ordinary matter is made of….nothing. A neutron star can be thought of as a large, singular atomic nucleus, again weird stuff. Most of the 1,800 pulsars thus detected are because of their copious radio emissions beaming from their poles. Thus, we have to be in the line of sight before we see their blinking radio pulsations. Enter Fermi, which has thus far spotted 16 new pulsars via their gamma-ray emissions alone. This promises to aid in identifying pulsars whose poles aren’t tipped to our line of sight, which are probably in the majority. But even the gamma-ray sky is relatively dim; for example, the Vela pulsar is one of the brightest in the sky, and it emits a mere 1 gamma-ray photon every 2 minutes! Initially dubbed “Little Green Men” (LGMs!) during their discovery in the 1960′s, pulsars were soon naturally explained, but still continue to amaze. Watch this space and the Fermi mission for news from the high energy end of the spectrum!

11.10.09: Zooming in on Blazars.

Astronomers have recently utilized an enormous radio telescope to examine some of the most exotic objects in the universe; active galactic nuclei. Sometimes called “Blazars”, these distant galaxies are spewing huge jets of particles at amazing relativistic speeds. These emit immense energy across the electromagnetic spectrum. NASA’s Fermi Gamma Ray Space Telescope has identified and monitored these sources since its launch in 2008 and now scientists at the Max Planck Institute for Radio Astronomy have used the National Science Foundation’s Very Long Baseline Array (VLBA) to map these jets with unprecedented accuracy. The VLBA is a series of 10 interlinked radio telescopes spanning an area from the Virgin Islands to Hawaii that utilize interferometry to produce an effective baseline of 5,300 miles and can resolve details less than 100 light years across at a distance of 7 billion light years. Fermi, the predecessor to the Compton Gamma Ray Observatory that was de-orbited in 2000, scans the entire sky once every three hours looking for gamma-ray bursts. First spotted in the early 70′s during global monitoring of nuclear weapons tests, pinning down gamma-ray bursts has been the name of the game in astrophysics over the past decades. The backup study proves the link between the gamma-ray emissions seen by Fermi and the energetic radio jets pinpointed by the VLBA… expect more high resolution radio maps to come!

March 2009 News & Notes.

Student exoplanet confirmed! Three students of Leiden University of the Netherlands are now the proud parents of a recently discovered exoplanet. And what a weird world it is; OGLE-Tr-L9b (OK, I know…we need better names!) orbits its fast spinning parent star in 2.5 Earth days at only 0.03 AU.

[Read more...]