Copyright 2011 Rick Boozer
In my earlier article about the experience of earning my Masters degree, I mentioned that, “I learned so many fascinating new things that I never suspected, nor had I ever seen them mentioned in any popular astronomy publication. For instance, in certain unusual cases, there is a way to use radio signals from quasars to give image resolutions of less than a microarcsecond. That’s much finer than the Hubble Space Telescope’s best resolution and even better than from the widest baseline radio interferometer with dishes on opposite sides of the world!”
After some reflection, I thought it might be a good idea for me to share some of the cutting edge information about quasars with fellow astronomy enthusiasts. Indeed, as shall be shown, astronomical discoveries may have unexpected practical applications. Thus I wrote this article. However, before I cover the new stuff, some background information may be in order.
In astronomy it is not unusual that a strange new discovery is considered too far away to have practical uses, but later reveals a useful application beyond anything that came to mind when it was first detected. For instance, helium was discovered from its absorption lines in the Sun’s spectrum long before it was physically identified on Earth. This observation sparked a worldwide search until it was found in certain oil wells in the U.S. Of course, this discovery brought the myriad industrial and scientific applications for which helium is now used. No wonder the name of this gas is derived from the Greek word Helios meaning Sun.
In this article I present information about a very strange and incredibly remote type of object that at first glance may appear to have little significance to our understanding of local phenomena or our everyday lives. However, these objects offer surprising new investigatory directions into the understanding of interstellar conditions within our own small section of the Milky Way galaxy, while also yielding at least one “down to Earth” practical application and a possible utility that future interstellar travelers may want to use.
Quasi-stellar Objects or QSOs are some of the brightest known objects in the universe seen in gamma rays, X-rays, ultraviolet and visible light. It is not unusual for a QSO to shine at a level millions of times greater than the entire radiant output of our galaxy! QSOs appear extremely faint because of their cosmically extreme lookback [1] distances of anywhere between nearly a billion to around 13 billion light-years. Though they are not stars, they are usually seen as a star-like point of light and so are called quasi-stellar. Of course, even members of the general public have heard the name for a special type of QSO that emits very strongly in radio frequencies: quasar for quasi-stellar radio object. Somewhat confusingly, it is not uncommon nowadays for scientists to call a QSO a “quasar” even when referring to a QSO that does not appear as a strong radio emitter.
The Nature of the Beast
Residing at the very center of a host galaxy, the source of the QSO’s immense radiant power is an Active Galactic Nucleus AKA an AGN. An AGN consists of an enormous black hole massing the equivalent of hundreds of millions or even billions of Suns along with a surrounding accretion disk of swirling matter that is pulled inward by the black hole’s gravitational field.
It is the accretion disk that is the source of the intense radiation. Heat induced by the compression of continually in-falling gases piling up within the accretion disk brings the disk to incandescent brilliance.
The intense magnetic field produced by the rotating black hole attracts some of the accretion disk’s matter away from the disk. Matter siphoned off in this manner then gets shot out in two opposing continuous jets of plasma near the black hole’s north and south magnetic poles. In some instances, the strength of the black hole’s magnetic field is so strong that the plasma is ejected at speeds approaching that of light! This plasma may emit magnetically induced radio emission that is highly polarized, known as synchrotron radio waves. A classical quasar’s strong radio signature is the result of this synchrotron radiation.
The tiny star-like appearance of a QSO is not only due to its enormous distance, but also its relatively compact physical size. Shortly after their initial discovery in the 1960’s, it was observed that the optical brightness of an entire QSO would sometimes vary drastically over time periods as small as several days. Because nothing can travel faster than light, these short pulses in increased brightness would imply that the AGN could be no bigger than the distance light could travel from one side of the QSO to the opposite side during the fluctuation period. Thus it was deduced that the size of the emitting part of a QSO could be no more than mere light-days to light-months across: a distance much smaller than the typical separation of stars in the tightest packed part of a normal galaxy. Indeed, the radiation equivalent of billions of Sun’s can be confined to a region within the AGN that is roughly on the scale of our own Solar System’s diameter! As the reader will see, sophisticated observational techniques developed in later decades confirmed the relatively small size for a QSO’s AGN and even allowed a highly accurate direct measurement of its diameter.
The short-term optical variation is normally the result of processes occurring within the QSO itself. A description of what is known of these processes would fill a long article all by itself; therefore, coverage of this topic is not appropriate for this short written piece and would also deflect our attention from other interesting features of QSOs. From this point on, our focus will be on observed variations in a QSO’s radio emission and some surprising discoveries associated with these observations.
The Mystery of Rapid Radio Signal Variations
One perplexing problem presented itself when radio intensity variations measured in hours were seen. It was calculated that for these fluctuations to be in the QSO itself, that the QSO’s temperature would have to be at least 1019 Kelvin! This temperature seemed absurdly high. Quantum mechanics dictates that any body with a temperature greater than about 1012 K should emit copious amounts of a special type gamma ray known as inverse Compton radiation. (Savolainen and Koralev 2008) It then seemed unlikely that the QSO was actually causing the radio fluctuations when no inverse Compton radiation was detected. (Tsang and Kirk 2006) Clearly some mechanism external to the QSO must be in play.
Observed radiation fluctuations, whether they are in visible light, radio or any other part of the electromagnetic spectrum are called scintillation. All of us have seen scintillation of visible light with our own eyes as the twinkling of stars in the night sky and an explanation of this twinkling may give some insight into its radio counterpart.
In the case of visible star scintillation, turbulence in the upper atmosphere causes changes in the optical properties of a high level layer of air to make the starlight seen by an observer appear to either vary rapidly in brightness or cause equally fast apparent shifts in the star’s position. Astute sky gazers may notice that even when the stars twinkle noticeably, any planets that are visible at the same time will shine with an unvaryingly steady light.
Why is there a marked difference in the perception of these two types of objects when light from each is passing through turbulent cells of air? The farther away from an observer that an object of a given physical size is, the smaller it appears to be. Expressed differently, the object’s apparent size measured as an angle will be smaller with increasing distance. Turbulent air cells generally measure but a fraction of an arcsecond across. Though a star’s physical size is much greater than a planet’s physical size and much greater still than any air cell’s physical size, the immense distance of the star is so great that its apparent angular size is much smaller than the apparent angular size of the invisible cells of air turbulence that are causing the twinkling; therefore, any changes in the foreground turbulence will greatly affect the appearance of the star. Though the apparent angular diameter of a planet may be so small that an earthbound observer will perceive it as a point, it is still proportionately much closer to the observer than would be any star. In other words, the ratio of a planet’s physical diameter to its distance is enormously larger than the ratio of a star’s physical diameter to its distance. Since a planet’s apparent angular diameter is also usually much larger than the apparent angular diameter of any cell of air turbulence, the light from the planet will appear not to vary. The following illustration depicts this principle and is, of course, not drawn to scale.
The observer’s location is marked with X and both turbulent air cells are of identical absolute physical size and of equal absolute distance from the observer. Though the star is physically much larger than the planet, the planet is much closer. Thus the apparent angular diameter, α, of the star is smaller than the apparent angular diameter, β, of the planet. The cell in front of the star has a wider apparent angular diameter than the star; therefore, the star twinkles. The same size cell in front of the planet has a smaller apparent angular diameter than the apparent angular diameter of the planet, so that the planet does not twinkle.
Getting back to radio variations of QSOs, the question being asked was, “Were the observed rapid fluctuations being induced into the signal as the QSO’s radio waves traveled through some turbulent cell of material on their way toward Earth?” The first clue that this situation might be the case came in 1998 when two radio telescopes located extremely far apart (one in Australia and one in New Mexico) made simultaneous observations of a QSO designated PKS 0405-385. When a particular intensity fluctuation pattern appeared at the Australian radio telescope, the same variation would show up approximately two minutes later at the New Mexico instrument. This situation was very strange because if the variation was intrinsic to the QSO, the same variation should have shown up on the New Mexico instrument only milliseconds later, that is, after the time it takes light to travel the distance between the two instruments. (Jauncey et al. 2002; Bignall et al. 2007; Savolainen and Koralev 2008) Soon, simultaneous observations of other QSOs revealed delay times that were often much longer.
The extremely tenuous gas and dust spread throughout the space between the stars within our galaxy is called the Interstellar Medium or ISM. Most of it contains only a few hydrogen atoms per cubic meter and is thus a better vacuum than the best that science has ever achieved, though randomly interspersed throughout the ISM are occasional denser clouds of dust and gas. As all amateur astronomers know, some of these nebulae can be seen in visible light. In other words, the nebulae may shine by reflecting the light of nearby stars or their constituent atoms may absorb ultraviolet radiation from local stars and re-emit the absorbed energy as visible light. Conversely, a cloud may be dense enough that the dust within it absorbs the light of the stars behind it, producing what appears to be a black void within the sky that is commonly referred to as a coal sack. Another alternative may also occur where such a cloud is so extremely thin that it may be transparent enough as to be nearly or completely optically invisible.
After the observations were made by the Australians and Americans, astrophysicists were strongly suspecting that QSO radio scintillation could be the result of turbulent cells as the radio waves from the QSO pass through the last type of cloud described in the immediately preceding paragraph. That would explain the excessively long difference in arrival times seen at the widely separated radio receivers. In other words, a particular turbulent cell might induce a characteristic fluctuation pattern at the Australian radio telescope, but that same cell might have to travel a few minutes before it was in a position to cause the same fluctuation to appear at the American radio telescope. Other corroborating evidence was needed to clinch this conclusion, but that confirmation was not long in coming.
Something very strange began to be seen in very short-term radio intensity variations in QSOs that went up then down over time spans ranging from minutes to several days. A gradual orderly change in these short-term scintillation patterns showed up over the course of a year and the same cycle of change repeated again on following years. (Jauncey et al. 2002; Linsky et. al 2007; Savolainen and Kovalev 2008) As any scientifically literate person knows, a year is the time it takes the Earth to complete one orbit around the Sun. It was soon realized that this was the clincher as far as proving that scintillation was being induced by material in the ISM relatively close to us. The speed of the Earth’s orbit around the Sun is around 30 km s-1; however, the speed of the material in the local ISM is also close to 30 km s-1. (Jauncey et al. 2002) When the motion of the Earth is approximately parallel to the velocity of the ISM, they have a low relative speed and the variation of the scintillation pattern is slow. But six months later, when their motions are in opposite directions, they have a high relative speed and the variations are observed to be much faster. Thus, we are given conclusive proof of two facts: 1) that the variations are turbulence induced scintillation and 2) that the Earth indeed orbits around the Sun a la Copernicus! (Jauncey et al. 2002; Savolainen and Kovalev 2008) After this conclusive evidence was obtained, short-term variations of QSO radio intensity were christened Interstellar Scintillation or ISS for short. Any turbulent interstellar cloud inducing radio variation is called a screen.
But the evidence got even better. When computer models were constructed using the fluctuation times as input data, the theoretical predicted distance and position for each screen was almost a perfect match for a known “local” thin interstellar cloud that was at least barely detectable in either visible light or ultraviolet light! (Linsky et al. 2007) All of the evidence put together was about as close to a smoking gun as one ever gets in science.
But here is the exciting part. Those same radio variations can be used to reveal fine details of the structure of a QSO in far greater resolution than any ground-based or orbiting telescope is capable of accomplishing! For decades the finest resolutions astronomers attained were achieved using a technique called Very Long Baseline Interferometry or VLBI. VLBI involves multiple radio telescopes observing the same object at the same time but separated by thousands of kilometers to give them the same resolution as a single stupendous radio telescope with a dish as wide as the distance between the two most widely separated radio telescopes. However, the scintillation technique even out-performs VLBI. The previously introduced analogy of visible atmospheric scintillation when stars twinkle may be extended to illustrate how such incredibly fine resolution is obtained.
Remember that if an object located behind a turbulent cell of air (from the point of view of the observer) has a smaller apparent angular diameter than the cell, the object will appear to scintillate. But if the object has a bigger apparent angular diameter than the cell, no twinkling is seen.
What if an observer was able to somehow detect a turbulent air cell and measure its apparent angular diameter? During the course of a night, a number of different turbulent air cells of varying diameters might come between the observer and an observed object. The observer would then be able to notice the maximum apparent angular diameter of a cell that caused twinkling and a minimum apparent angular diameter for a cell that did not cause twinkling. He/she would then know that the apparent angular diameter of the observed object would have to be an angle with a size between the diameters of the former and the latter.
As mentioned before, turbulent cells within an interstellar screen cause the radio scintillations that are equivalent to atmospheric twinkling. The method described in the immediately preceding paragraph has been used to measure the extremely tiny angular diameter of various QSOs. In fact, the resolution obtained is so fine, that astronomers have even resolved structures within the AGNs of some of the closer QSOs!
Angular resolutions on the order of 1 micro-arcsecond can be achieved. In comparison, this resolution is around 1000 times finer than that of the Hubble Space Telescope at its shortest usable wavelength! (Jauncey et al. 2002) And since the distance to a QSO can be determined from the amount of cosmological redshift observed in its emitted light [2] (grist for another entire article), the actual physical size of the QSO can be calculated from its apparent angular diameter. In this case, even assuming a QSO is halfway across the observable universe at a lookback distance of about 6.8 billion light-years, a structure of a mere three light-months in physical diameter can be measured. (Jauncey et al. 2002)
But just as radio scintillation can be used to gather information about a QSO, it can also be employed to investigate the properties of interstellar space in our neighborhood. This convenient situation is the result of the fact that the screening clouds have to be relatively close to our solar system. How do we know this? There is a maximum distance away from us that a turbulent cell of a particular physical size can be and still induce scintillation in a QSO. This distance is where the apparent angular diameter of the cell equals the apparent angular diameter of the QSO. Any farther away would lead to a situation in which the angular diameter of the QSO would be greater than the angular diameter of the turbulent cell and thus no scintillation would occur. (Bignall et al. 2007)
So the fraction of material capable of producing fast variability is restricted to the ISM in the Sun’s vicinity. Furthermore, the scarcity of detected screens relative to the overall number of QSOs observed indicates that such clouds of scattering material are few and far between in our immediate section of the galaxy. (Bignall et al. 2007) ISS observations indicate that there are on average 1.7 screens along any line of sight, with a typical line of sight usually having between only 1 and 3 screens (Linsky et. al 2007)
One may wonder what produces the turbulence in the interstellar cloud material. It is thought that areas of the highest scintillation-causing turbulence occur at places where the outer edges of two or more of these clouds come in contact with their different speeds of motion and travel direction. The slightly different velocities of the two clouds produce turbulence where they interact. (Linsky et. al 2007) Because they are on the outside of the cloud, these border edges lack shielding from ionizing radiation put out by one giant blue-white star that is relatively near our Solar System and several local white dwarf stars. The result is a much larger than normal number of fast freely moving electrons that increase turbulence to an even higher level, making these interacting areas hot beds for the production of scintillation. (Linsky et. al 2007)
Finding Our Way Around the Earth with QSOs
Everyone nowadays is familiar with the Global Positioning System that employs a fleet of special navigational satellites in Earth orbit. GPS has pretty much totally supplanted celestial navigation for ship and airplane travel. Even more immediate to people’s everyday lives is the fact that the technology has trickled down to the individual level in the form of automobile navigation systems and emergency location in life or death situations.
But to figure locations anywhere on the face of the Earth within an accuracy of mere meters requires exacting determination of satellite positions at ultra-precise times. Constantly occurring variations in the tilt of Earth’s axis have to be continually taken into account for the system to function with pinpoint accuracy. The tilt variations are detected by referencing the locations of QSOs because their distances are so immense that their motion is not detectable as a change in the object’s position and thus they “stay put” in their apparent relative places all over the sky. VLBI measurements have been used to obtain precise positions of a number of QSOs and have been compiled into a catalog to serve as base navigational references. The catalog is called the International Celestial Reference Frame abbreviated ICRF. (Ma et al. 1998)
Beyond Terrestrial Navigation
Finally, it would stand to reason that QSOs might eventually be used as a natural “galactic” GPS in the event that humanity ever achieves the capability to travel multiple light-year distances. Extremely miniscule changes in observed relative positions of QSOs in relation to each other would be attributable to a traveler’s change in position within the galaxy and thus could be used for navigation purposes. Assuming measurement capabilities continue to progress as they have heretofore, it is not unreasonable to expect that equipment to measure such incredibly minute deviations may be achievable by any future civilization technically advanced enough for interstellar travel.
Who knows what other uses we’ll find for these exotic objects as time goes on?
For that matter, what as-yet-to-be-conceived applications may follow once we know more about the nature of what we now call dark matter and dark energy? After all, those two vaguely descriptive names were chosen because we couldn’t choose better ones since we don’t really know what those properties physically represent!
In short, judging by the past history of scientific discovery, it would seem unwise for anyone to say that any particular realm of scientific knowledge will always only be of purely academic interest.
References
Bignall, H.E, D. L. Jauncey, J. E. J. Lovell, A. K. Tzioumi, J-P. Macquart, and L. Kedziora-Chudczer “Observations of Intrahour Variable Quasars: Scattering in our Galactic Neighbourhood” Astronomical and Astrophysical Transactions, 26 (2007) 567 - 573
Jauncey, David, Hayley Bignall, Jim Lovell, Tasso Tzioumis, Lucyna Kedziora-Chudczer, J-P Macquart, Steven Tingay, Dave Rayner and Roger Clay, “Interstellar Scintillation and PKS 1257-326” ATNF News (October 2002)
Linsky, Jeffrey L., Barney J. Rickett, and Seth Redfield “The Origin of Radio Scintillation In the Local Interstellar Medium”, The Astrophysical Journal, 675 (2008) 413-419
Ma, C, E. F. Arias, T. M. Eubanks, A. L. Fey, A.-M. Gontier, C. S. Jacobs, O. J. Sovers, B. A. Archinal and P. Charlot, “The International Celestial Reference Frame as Realized by Very Long Baseline Interferometry”, The Astronomical Journal, 116 (1998) 516-546
Savolainen,T. and Y. Y. Kovalev. “Serendipitous VLBI Detection of Rapid, Large-amplitude, Intraday Variability in QSO 1156+295” Astronomy and Astrophysics 489 (2008) L33-L36
Tsang, O. and J. G. Kirk “The Inverse Compton Catastrophe and High Brightness Temperature Radio Sources” Astronomy and Astrophysics 463 (2007) 145-152
Footnotes
[1] The lookback distance is how far the light traveled from an object to reach the Earth. In the case of the farthest detectable QSOs, this distance is about half of the true present-day distance between the Earth and the QSO – called the comoving distance. The reason why is that the universe was continually expanding while the light was en route, causing the Earth and the QSO to become further and further apart during the transit time as the space between them was stretched wider by the expansion.
[2] A wave of light is lengthened (i.e., cosmologically redshifted) because the space it is traveling through is stretched by the continual expansion of the Universe; which in turn, stretches the wave of light. Some inaccurately term it as a cosmological Doppler shift. But the relative motion of a light emitting object, not the Universe’s expansion, causes a true Doppler shift!