November 19, 2017

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...]

Review: What Are Gamma-ray Bursts? by Joshua S. Bloom.

Out from Princeton Press!

In 1888, astronomer Simon Newcomb made the now infamous quip that “we are probably nearing the limit of all we can know about astronomy…” One has to wonder what these 19th century scientists would make of the wonderful cosmological menagerie of black holes, energetic galactic nuclei, and the topic of today’s review. [Read more...]

23.06.10- Swift Spies Black Holes Feeding on Galaxy Mergers.

Anatomy of an AGN. (Credit: NASA/Swift/EPO/Sonoma State/Aurore Simonnet).

Anatomy of an AGN. (Credit: NASA/Swift/EPO/Sonoma State/Aurore Simonnet).

 

   NASA’s orbiting Swift telescope is in the news again, this time providing a key link between energetic nuclei and active galaxy mergers. The findings come after a survey conducted since 2004 by Swifts’ Burst Alert Telescope (BAT) of active galactic nuclei. A small percentage of these (less than 1 %) are extremely active, emitting 10 billion times the equivalent solar output. While theories have long posited that galaxy mergers feed and create galactic mass black holes, the observations carried out by Swift catch these monsters switching on in their energetic youth, and thus provide insight into their evolution. Only instruments such as BAT can penetrate the thick layers of gas and dust masking these massive black holes, which emit copious amounts of radiation in the hard x-ray spectrum. In fact, Swift has built the first ever comprehensive all sky survey in hard x-rays, with sensitivity to active galactic nuclei (AGN) 650 million light years distant. In the process, Swift has also uncovered numerous unknown AGN. The picture emerging will no doubt force scientists to rethink galaxy evolution; about 25% of the galaxies that BAT sees are potential close mergers, and 60% of those are destined to merge in the next 1 billion years or so. As we fill in the galaxy “family scrapbook,” key information will be deduced about how common (or rare) our own Milky Way galaxy is. And yes, our galaxy does harbor a galactic mass black hole of its own! And we’re also due for a collision of our own with the Andromeda galaxy in about 3 billion years, with the resulting merger tentatively dubbed Milkomeda… will whatever we evolve into, (or get replaced by) be blogging then? Imagine the views as the Andromeda closes in!

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

Compton is placed into orbit by the Space Shuttle Atlantis. (Credit: NASA/Art Explosion).
Compton is placed into orbit by the Space Shuttle Atlantis. (Credit: NASA/Art Explosion).

   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.

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Granat ready for launch. (Credit: NASA/Roskosmos).

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…

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A breakdown of Fermi/GLAST. (Credit: NASA/ E/PO, Sonoma State University). 

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.

GRB061121_small

BAT energy curve of GRB 061121. (Credit: NASA/Jay Norris)

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!

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 The Moon as seen in Gamma Rays by Compton’s EGRET detector! (Credit: NASA/GSFC/USRA). 

The Great Orbiting Observatories II: The Ultraviolet.

Galaxy M81 blazes with star birth in the ultraviolet. (Creidt: GALEX/NASA).

Galaxy M81 blazes with star birth in the ultraviolet. (Credit: GALEX/NASA).

 

   When we last left our installment of this saga, we covered the observatories that target the visible edge of our spectrum. This is a narrow slice; a tiny sliver of what we call the electromagnetic spectrum. This week, we move into the ultraviolet, a span of the spectrum at roughly between 10 to 320 nanometers. UV from space is almost entirely absorbed by our atmosphere, and thus, if you want to observe the universe or do UV astronomy, you have to go into space to do it. [Read more...]

11.05.10: Ancient Galaxy Mergers.

Hickson Compact Group 31. (Credit: NASA/HST/ESA/S. Gallagher/J. English).

Hickson Compact Group 31. (Credit: NASA/HST/ESA/S. Gallagher/J. English).

 

   Astronomers may have found a cosmological missing link in the realm of galactic evolution. The early universe was a crowded place; galaxy mergers must have been much more common in the primeval universe than they are today. But studying those early collisions has been problematic; the immense distances involved over time and space mean that resolving clusters and individual stars are out of the question. Now, a team from the University of Western Ontario led by Sara Gallagher has published a study of an object which may serve as a “living fossil” of those early times; Hickson Compact Group 31. A cluster of irregular galaxies “only” 166 million light years away in the constellation Eridanus, this merger has somehow escaped coalescence over 10 billion years of cosmic history to just begin merging. “Because HCG 31 is so nearby,” Gallagher notes, “we can indentify individual star clusters.” In fact, two main components of HCG 31 approach visual magnitude +13 and have been snared by amateur instruments. HCG 31 is approximately 75,000 light years in diameter, and will probably one day form one huge elliptical galaxy. To conduct this study, Gallagher utilized time and instruments that spanned the spectrum, from Hubble in visible light to Spitzer in infrared to Galex and Swift in the ultraviolet. It is amazing that astronomers now have such capabilities in their bag of tricks at their ready disposal!

Space Telescopes, Part I: Optical.

 
(Credit: NASA/ESA/S. Gallagher/J. English).
(Credit: NASA/ESA/S. Gallagher/J. English).
 

 Hickson Group 31 of galaxies as imaged by Hubble.

   This weeks’ expose will kick off our four part series on orbiting space telescopes. For starters, we’ll begin with the most familiar; the optical wavelength. True, we as humans are biased towards this narrow band of the spectrum; we love to see pretty pictures that we can relate to.  But beyond this, telescopes that operate in the visual wavelengths have no less than revolutionized astronomy, as well as laid promise for perhaps giving us images of exo-Earths in our lifetimes. What follows is a rapid fire list of what was, is, and what to look for up and coming in the realm of optical astronomy in space: [Read more...]

20.03.10: Spying a Black Hole Welterweight.

An Artist's conception of NGC 5408 X-1. (Credit: NASA).

An Artist's conception of NGC 5408 X-1. (Credit: NASA).

  

   Astronomers now have observational evidence for a missing class of black hole. Stellar mass black holes, those up to about 10 solar masses, are well known as the remnants of supernovae. Likewise for supermassive black holes of 10,000 solar masses or greater known to reside in the hearts of galaxies like our own. The “missing link” in astrophysics has been intermediate mass black holes, or those between 100 and 10,000 solar masses. Now, scientists at the Goddard Space Flight Center in Greenbelt Maryland have used the XMM-Newton and Swift X-ray satellites to pinpoint a likely candidate; NGC 5408 X-1, a black hole with about 1,000 to 9,000 solar masses in a galaxy about 15.8 million light years away in the constellation Centaurus. This would include an event horizon about 3,800 to 34,000 miles across. An X-ray flux occurs once every 115.5 days, strongly suggesting that NGC 5408 X-1 has a stellar companion accreting donor material. This star would be 3-5 times the Sun’s mass.   “Astronomers have been studying NGC 5408 X-1… because it’s one of the best candidates for an intermediate mass black hole.” States Philip Kaaret of the University of Iowa. The contributing companion also gives astronomers the unique opportunity to probe the near-space environment as well as study this intermediate class of enigmatic objects.

4.10.9:A Gamma-Ray Burst for the Record Books.

A Gamma-ray burst from the primordial universe sent astronomers reeling earlier this year with the most distant sighting yet. The burst was picked up by NASA’s Swift spacecraft on April 23, 2009 at 3:55 EDT. E-mails and instant messages flew to observatories around the globe as astronomers raced to pin-point the fading afterglow. Dubbed GRB 090423, (get the year/month/day thing?) This burst measures in at a redshift of 8.2, or a distance of 13.035 billion light years. This hails from a time when the universe was a tender young age of only 630 years old, young, compared to our circa 14 billion year current age. The old record was a red shift of 6.7 set in September 2008. the current “holy grail” in cosmology is to break the “redshift 10″ barrier, which may well happen in the coming year. A gamma-ray burst occurs when a super massive star collapses into a black hole, briefly creating a “hyper-nova” in the process. Such events are the most luminous in the universe and are thought to have been common amoung first generation stars. Backup observations were provided by Italy’s Galileo national telescope in the Canary Islands and the ESO’s Very Large Telescope in Chile.