Sunday, August 31, 2014
What Are Two Alternative Energy Sources
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Dawn Of The Solar Peaking Plant
Whatever anyone thinks of the technology – and no one will really know until it is deployed and operating at a commercial scale – it is clear that the Solar Oasis consortium behind the 40MW “big dish” solar thermal plant planned for Whyalla are taking an innovative approach to financing and to the energy markets in general. And energy retailers and aspiring solar developers should probably take note.
RenewEconomy caught up with Alex Braisier, the managing director of Solar Oasis, by phone on Wednesday while he was in China, where he is negotiating supply and financing deals for the project. That was our first takeaway from the chat: the $230 million project will not be financed by Australian institutions, as had been envisaged originally, even though an in-principle arrangement had been made with ANZ. Braisier says Australian institutions are too inflexible and narrow in approach.
Instead, Braisier will source equity partners and debt finance mostly from China, possibly from suppliers and other interested parties. “Working with vending project financing arrangements in China is more imaginative than going through motions with an investment bank in Australia,” he says.
(This is not surprising. The fact is that Australian institutions have no experience with such technologies and no reference points. International banks are expected to carry much of the load, and the inspiration, for the solar flagships projects. As this article points out, however, the loan guarantee program in the US has helped financiers get comfortable with the technology and bring down the cost of financing. And check out this article on Forbes, about how investors are making big money from renewables. There are lessons here for the Clean Energy Finance Corp.)
The relationship with Chinese partners and suppliers has grown since Braisier first formed a partnership with Chinese-based solar PV manufacturer SunGen, which is linked with an energy retailer in Braisier’s stable, Sanctuary Energy, supplies its PV modules (at zero upfront cost) and has taken a 26 per cent stake.
Sanctuary is a specialist in green energy retailing, and has some 20,000 customers, mostly with rooftop PV of between 1.5kW to 10kW, and solar hot water systems, with an aggregate capacity of around 30MW. It is small and also nimble: Braisier and his partners are former energy traders, and have developed a solid business playing the market, hedging with caps and derivatives, and using the natural advantage of solar that produces energy in the shoulder and high peaking periods.
This leads us to the next interesting point: the proposed PPA for the Whyalla project, which will feature 330 “big dishes” first developed by ANU and then taken up by Wizard Power. (It is solar thermal with a design change, instead of parabolic troughs or flat mirrors, or solar towers, these dishes can generate temperatures of 2000°C – more than you need unless you are trying to crack hydrogen or turn coal into liquids. So around 600°C will do for Whyalla).
Anyway, Braisier sees the solar dishes as “peaking power,” and he will treat the plant as such. He shakes his head (I presume, it was over the phone) at the reported attempts by the Solar Dawn consortium to strike a PPA of around $200/MWh, in which they were unsuccessful. Why treat solar thermal like a coal-fired power station? he asks. It should be treated like a hydro or gas-fired plant, none of which are ever built with PPAs in hand. They simply play the market and sell into the peaks, when prices jump.
“It (the PPA) will be a derivative product – it’s a peaking plant,” Braisier says. “ “It will sells caps and derivative product to help retailers manage risk. That’s what solar thermal will do.” And given that South Australia has highly volatile prices, and its big peaks coincide with hot sunny days, Braisier can be certain that the Big Dish array will be producing energy when it is most needed, and most profitable. The Solar Oasis offtake agreement will be done through Sanctuary Energy.
“I think we have a different view of the market, we don’t have 100 per cent hedges, and we can take an innovative approach,” he says. “You don’t bank a hydro plant in Australia, or even a gas plant – they are not positioned in market as base-load plants. To suggest that a solar thermal behaves and looks like a coal-fired plant is ridiculous.”
Braisier says the consortium is not planning storage, but may consider adding a small gas-boosted generator to help in firming and dispatchability into the peaks, which he says would improve the project’s IRR. It is also talking, with SunGen, about the possibility of installing a 5MW solar PV installation.
Braisier’s company is currently working with SunGen to construct several solar PV installations of between 1MW and 10MW in the Philippines, replacing diesel, which is costing up to $600/MWh or more. He says PV plants of that size can be installed for around $200/MWh.
Braisier expects the nominated $230 million cost of the project will fall too. Since the tender was first accepted in 2009, the market has changed dramatically, and the price of power blocks, for instance, was down by around 30 per cent. “Vendors are falling over themselves to provide vendor financing,” he says. “We expect a significant reduction in costs.”
Now that the funding deed has been signed, Solar Oasis will work on final design, permitting, and arranging grid connection. Construction is expected to start in May next year, with the project completed before the end of 2015. It will be interesting to see which project provides electricity to the grid first – Solar Oasis, or the 250MW Solar Dawn project in Queensland.
FT Oil industry sums do not add up
Lewis notes this "should be a reality check for those now hyping a new age of global oil abundance".
Australian Geothermal industry pushes for more power
MIKE SEXTON, REPORTER: Every year thousands of punters head to Birdsville in Outback Queensland for the annual races. Perhaps few would be aware though their tinnies are being kept on ice in part thanks to electricity generated from scolding hot water coming from deep beneath the desert floor.
CHRIS SMITH, ERGON ENERGY: The water comes up from the Artesian Basin at 98 degrees Celsius. The water then passes through a gas field heat exchanger which heats the gas and pressurises it and then it goes through a turbine and produces electricity.
MIKE SEXTON: The engineerings relatively simple and the outcome is emission-free power 24 hours per day that doesnt rely on the wind blowing or the sun shining.
CHRIS SMITH: The plant at Birdsville was custom-made when it was done, so its done quite some time ago. But technologys changed now and theres - this sort of plant is readily available and is being used throughout the world.
MIKE SEXTON: This is just one form of whats known as geothermal energy where the heat stored in subterranean rock formations is harnessed to generate electricity. Although the Birdsville plant is tiny, the geothermal potential in Australia is huge.
SUSAN JEANES, AUST. GEOTHERMAL ENERGY ASSN: The resource is vast. If we mined just one per cent of the national - the nations geothermal heat, in the top five kilometres of the crust we could make 26,000 times Australias annual energy supply. So theres no limitation on the resource.
MIKE SEXTON: Given the need for clean baseload power and the size of the resource, its no surprise that more than 50 geothermal licences have been issued in Australia. One of the more advanced is Petratherm, which has drilled shafts into hot rocks at Paralana in outback South Australia. The next step is to pump water down which converts to steam which is then used to drive turbines.
TERRY KALLIS, CEO, PETRATHERM: We estimate at Paralana alone we could produce 13,000 megawatts of power. Now thats about four times the power requirement of South Australia.
MIKE SEXTON: Excitement about the potential initially attracted investors prepared to take a risk on a new industry. But drilling wells hundreds of metres into granite in remote locations is a difficult and expensive business, and after years of promise, the industry has delivered only modest results. That, coupled with the GFC, has seen investors turning their backs on geothermal companies.
Saturday, August 30, 2014
Active galaxies and supermassive black hole jets
All black holes gravitationally attract any nearby matter, because of their high mass, which is generally ≥ 1 M⊙ even for comparatively tiny stellar mass black holes. Such matter does not necessarily fall directly into the black hole, but instead can go into orbit around the black hole. If there is enough matter close to the black hole, and if it is pulled in rapidly enough, the results can be a spectacular light show, such as one might see (if one could see simultaneously at all wavelengths from very high radio frequencies to X-rays) in the Centaurus A galaxy, about 13 million light years away:
This is a view of the whole galaxy – you can see that the central area, which contains the black hole, is unusually bright, and there are jets extending more than the radius of the galaxy itself in both directions perpendicular to the galactic plane. Centaurus A is an example of an "active galaxy", and it shows the impressive effects produced by the central black hole of such an object. (For more about Centaurus A and this image, see here, here. Another image: here.)
The innermost region of an active galaxy, which is the interesting part, is called an "active galactic nucleus" (AGN). This is a general term for a number of puzzling astronomical objects that were noticed at first on account of their unusually vigorous output of energy, but whose similarities were not immediately recognized. AGNs were eventually deduced to be (in almost all cases) just relatively ordinary galaxies with massive central black holes that appear to be responsible for liberating at least as much energy as all the stars in the remainder of the galaxy.
Although most galaxies seem to have a supermassive black hole in their center, behavior of AGNs is rather unusual, and AGNs are somewhat rare in the nearby universe, but more common at large distances – hence at an earlier time in the universe. AGN behavior requires not only a supermassive black hole, but also a substantial amount of surrounding interstellar gas that fuels their energy output. AGNs are rather profligate in using their fuel, so presumably most of the available interstellar gas close to a central black hole is consumed over a relatively short period of time (compared to the age of the universe). Consequently, in most nearby galaxies the fuel was used up long ago.
The Milky Way has a relatively small central black hole associated with the radio source named Sagittarius A* (Sgr A*). The black hole has a mass of ~4×106 M⊙. Sgr A* is not nearly luminous enough to be considered an AGN, so evidently either there is not now enough nearby interstellar gas, or perhaps the black hole is just too small to have ever attracted enough.
How luminous does a galactic nucleus need to be in order to qualify as an AGN? Its really a question of how bright the nucleus is compared to the rest of the galaxy. Messier 77, also known as NGC 1068, is the first galaxy now considered to have an AGN that came to special attention. In 1908 E. A. Fath obtained its spectrum and found it had unusually strong emission lines. (This was at a time when it was still assumed that nebulae were simply fuzzy objects inside our own galaxy.) V. N. Slipher later obtained a better spectrum and noted that the width of the lines implied high velocities – hundreds of kilometers per second. NGC 1068 is fairly nearby – 47 million light years away – and rather large, with a diameter of 170,000 light years (compared to the Milky Ways diameter of 100,000 light years). NGC 1068 is still under active study at this time – see here.
Finally in 1943 Carl Seyfert recognized that NGC 1068 was similar to a number of other galaxies that formed a distinct class, based on the nature of their spectra and because their innermost regions were as bright as the entire rest of the galaxy. This concentration of luminosity in the center was not only exceptional, but it was quite unlikely to be physically possible for a sufficient number of stars to be located in such a small volume of space. Naturally, galaxies of this sort became known as Seyfert galaxies.
Other peculiarities of Seyfert galaxies were eventually recognized as well. For example, their spectra contain broad, strong emission lines of hydrogen, helium, nitrogen, and oxygen. This in turn implied that the emitting material had to be in rapid motion in order to produce Doppler broadening of the emission lines. And this in turn implied that a large amount of mass needed to be concentrated in a small volume to account for such high velocities. The characteristics of high central luminosity and broad emission lines tended to occur together enough to justify recognizing Seyfert galaxies as a distinct class, which made up about 1% of nearby spiral galaxies.
About 15 years after Seyfert galaxies were discovered, another peculiar type of astrophysical object was noticed – quasars, or, as they were sometimes known, "quasi-stellar-objects". These came first to attention as strong sources of radio emission, in early radio telescope surveys, such as the original Third Cambridge Catalogue of Radio Sources. The strong radio signal was somewhat mysterious, since electromagnetic radiation at radio frequencies (up to 100 GHz at the high end) is normally emitted only by rather cold matter (under about 2 degrees above absolute zero).
Many of these sources ("radio galaxies") were eventually identified with optically visible objects, many of which had already been cataloged as unremarkable stars. But when they came under attention as radio sources and their optical spectra were obtained, their high redshift implied that they needed to have an astonishingly high intrinsic luminosity – too luminous to be explainable simply as very large galaxies containing even the brightest possible stars.
Phenomena that are seemingly impossible to explain in terms of familiar examples tend to draw a lot of attention – and this is true, in some respects, of quasars even today. A lot of the mystery is now explainable in terms of active galaxies that are powered by supermassive black holes, and well get more into that in a little bit. But there are also some AGN features, such as the jets that a few AGNs expel at relativistic velocities, which are still not well understood.
Although there was still controversy at the time whether redshift could be used as a reliable gauge of distance, if the conventional redshift interpretation (Hubble expansion) was assumed, then quasars would need to be (what was considered at the time, ca. 1960) extremely distant. One of the earliest-recognized quasars, 3C 273, which is the quasar with the largest apparent magnitude and is visible through amateur telescopes, has z=0.158, corresponding to an optical distance of ~2.4 billion light years. 3C 273 was therefore intrinsically far brighter than any star, about 100 times as bright as an entire spiral galaxy. 3C 273 is sometimes considered to be the nearest unambiguous quasar. So bright quasars are absent from the local universe, although there are ambiguous cases as close as ~800 million light years (z=0.06).
Its now pretty clear that all quasars are extremely luminous active galaxies, but when 3C 273 and similar objects were first discovered they were point-like objects without the visible appearance of a galaxy. After all, the rest of the galaxy of which they were a part was only 1/100 as bright as the nucleus. So the objects were referred to as "quasi-stellar objects" (QSOs), or quasars for short. (Sometimes the term QSO was reserved for the minority of such objects that did not have appreciable radio emissions, while "quasar" meant a QSO with a strong radio signal.)
Its also clear now that the distinction between Seyfert galaxies and quasars is rather arbitrary – the brightest Seyferts have characteristics much like the least bright quasars. Further, there are a small number of characteristics which may or may not be present in either class. Some of the "optional" features that may be present include X-ray emissions, narrow (optical or ultraviolet) emission lines in the spectrum, broad (optical or ultraviolet) emission lines, strong radio emissions, and evidence of relativistic jets of matter (such as seen in Centaurus A, above).
The last two of these features – radio emissions and jets – almost always are either present or absent together. Naturally, the jets are expected to account for the radio emissions, and there are very reasonable models that explain the connection. However, only about 10% of all AGNs, when Seyfert galaxies are included, have the jets and strong radio emissions, so whatever physical process is responsible for AGN behavior doesnt automatically produce relativistic jets as well.
Indeed, the actual physical process responsible for jets is not well understood, and its one of the more mysterious phenomena in astrophysics. The research to be discussed here is a contribution towards elucidation of the mystery.
Physical models
But first, lets quickly review the physical model that now has consensus support to account for AGNs in general. The main characteristic, which was at first the most puzzling, is the enormous quantity of energy emitted in a very small volume of space. How small a volume? One indication is that the energy output can vary over periods of just a few days. Consequently, the diameter of the source must be only a few light days – about 10 times the diameter of Plutos orbit. And yet the source may produce energy ranging from 1 to 100 times as much as an entire good-size galaxy containing hundreds of billions of stars. A little breath-taking, when you think about it.
So where does all this energy comes from? Its not thermonuclear energy thats produced by fusion the way that stars do. The material around even the largest black hole is not dense enough. Thats basically because of whats known as the Eddington limit. Any time a sufficiently large mass of gas is collapsing under the force of gravity, the potential energy of the gas is converted to electromagnetic radiation. The energy released as radiation increases to the point where the outward radiation pressure equals the pressure due to gravity, stopping the collapse. This is what prevents very large stars – more than about 100 M⊙ – forming out of a large mass of gas. The same process also limits the rate at which gas can collapse around a black hole.
However, in the case of a supermassive black hole it is exactly this conversion of potential energy into radiation that supplies the enormous output of energy found in AGNs. Lets look at a simple calculation to show just how effectively a very large compact gravitating object can release energy.
Consider a supermassive black hole whose mass MBH is 109 M⊙. Since M⊙=1.99×1030 kg, we have MBH=1.99×1039 kg. The black hole is surrounded by an event horizon – the boundary from the inside of which neither matter nor radiation can escape. In the simple case of a non-rotating black hole the event horizon is a sphere whose radius is the Schwarzschild radius, which is rs=2GMBH/c2, where G=6.67×10-11 m3 kg-1 sec-2 is the gravitational constant and c=3×108 m/sec is the speed of light. Plugging things into the formula gives rs=2.96×1012 m. Thats very close to the radius of the orbit of Uranus.
Next lets ask how fast an object or particle in orbit around a supermassive black hole might be moving. There is a very simple formula for orbital velocity: v≅(GM/r)½, where M is the mass of the central object, and r is the radius of the orbit. Thats an approximation, since it makes some assumptions – the orbit is nearly circular, and the mass of the orbiting object is much less than M – reasonable for the sake of discussion. Squaring both sides and rearranging: r=GM/v2. We could plug in various values and see what we get, but suppose we want to know r in some reasonable units, such as the black hole Schwarzschild radius rs=2GMBH/c2. Then using plausible values for v, say v=c/10. Thats "small" enough that relativistic effects are minor: the Lorentz factor γ=(1-(v/c)2)-½≅1.005. The result is that r/rs=100GMBH/2GMBH=50.
So an object or particle in orbit around a black hole at a distance of 50 rs is moving at 1/10th of the speed of light. Notice that this isnt dependent on the black holes mass – its true for any black hole. (It doesnt apply to objects that arent black holes, since their Schwarzschild radius is very small, much smaller than the size of the object.) This calculation isnt necessarily realistic physically, since it neglects a number of other considerations, for example gas viscosity and turbulence. But it shows that black holes are nothing to be trifled with – they can have rather sizable physical effects.
In fact, although c/10 is not quite a relativistic velocity, its still rather sprightly. For instance, at that rate one could get from the Sun to the Earth in an hour and 23 minutes – faster than the commute into a big city in bad traffic. Its also a velocity that gives even something as small as a proton quite a bit of kinetic energy. Lets compute it. The proton mass mp≅1.67×10-27 kg. Kinetic energy E=mv2/2 = (1.67×10-27)(3×108/10)2/2 ≅ 7.5×10-13 kg m2 sec-2 = 7.5×10-6 ergs. Since one erg is 6.2415×1011 eV (electron volts), the kinetic energy of a proton moving at 1/10th the speed of light is about 4.68×106 eV = 4.68 MeV.
Thats not chicken feed – its well within the gamma ray range (100 keV to several 10s of GeV). What this means is that in any collision between protons moving this fast, its no sweat at all to give off gamma-ray photons, or photons of any other form of electromagnetic energy. And this is how black holes of any size, from stellar mass up to the supermassive kind, can convert a substantial fraction of the mass-energy of matter that falls in sufficiently close to electromagnetic radiation.
Given all this, the questions that occupy astrophysicists interested in supermassive black holes, AGNs, quasars, and the like include: Whats the exact physical configuration in which the energy is released? What processes bring about the energy release? How do these physical details explain observable effects, such as total energy output, emission lines, relativistic jets, and so forth?
Astrophysicists have been working on these questions for at least 50 years, since the first quasars were discovers, and a consensus has emerged about many of the physical details.
The main feature that all AGNs have (at least in the standard model) is a substantial accretion disk of matter orbiting around them. In many cases that have been studied in detail, theres a lot of evidence for such disks, besides the powerful emission of electromagnetic energy at frequencies from far infrared to far ultraviolet. As the name implies, the disks are flat and relatively thin. The inner and outer radii of the disks vary from case to case, but since there are minor fluctuations of output over periods of days, the inner radius must be on the order of at most a few light-days, around 1014 m, or 1000 times the size of the Earths orbit.
Detailed analysis of the physics indicates that the innermost part of the disk should be the hottest, with the temperature gradually tapering off toward the outside. Since the emission is thermal ("black body"), the relation between temperature and wavelength is given by Wiens law: T=b/λmax, where b≅2.9×10-3 m-K. λmax is the wavelength at which intensity per unit wavelength is maximized. Thus peak temperatures may range from 300,000 K when λmax=10-8 m (ultraviolet) down to 300K when λmax=10-5 m (infrared) – possibly even more at the high end. Higher energies and temperatures actually occur with stellar mass black holes instead of the much larger supermassive ones, because the maximum rate at which matter can accrete (the Eddington limit) is higher for smaller black holes.
As explained above, the energy to heat disk material to such temperatures comes ultimately from gravitational potential energy as matter falls inward and gains kinetic energy, which manifests as heat and ultimately electromagnetic radiation. The efficiency of this conversion of matter into EM energy can actually reach about 10%, which is a lot higher than nuclear fusion, for which the efficiency is only about 0.7%.
The net result is that a certain fraction of matter (mostly diffuse hydrogen and helium gas) in the vicinity of a black hole is converted to electromagnetic energy. This process can go on for a long time (perhaps hundreds of millions of years) until the matter is mostly used up or falls into the black hole itself. Calculations have verified that this process is entirely adequate to account for the observed luminosity of AGNs.
Eventually there is not enough matter sufficiently close to the black hole to be sucked in, and the process stops. This is why most quasars are observed only at great distances – more than a billion light years – because they no longer have the means to sustain the extremely high luminosity. It could be that all or most galaxies go through a quasar/Seyfert/AGN phase. One can even make a rough estimate of how long this phase lasts. Only about 1% of galaxies are Seyfert/AGN, so any given galaxy ought to be in that phase for only about 1% of the age of the universe, i. e. perhaps 130 million years.
As noted above, there are various prominent characteristics that may or may not accompany the high luminosity of an AGN, including broad and narrow emission lines in the spectrum, strong radio emissions, and relativistic jets.
The broad emission lines are thought to originate in clouds of colder gas (under ~100 K) orbiting outside the accretion disk. Although such clouds emit little EM energy, they consist of atomic hydrogen, helium, and traces of heavier elements. Intense radiation coming from the disk will put these atoms in an energetically excited state. But when electrons drop back from higher energy levels, spectral lines at frequencies characteristic of each atomic species are emitted. Because the clouds are in rapid motion, the emission lines are broadened due to varying amounts of Doppler shifting of the spectral lines.
One would expect that such clouds should be present around all supermassive black holes. However, there are many AGNs, both quasars and Seyfert galaxies, in which broad emission lines are not observed. Seyferts were originally placed into one of two classes, according as broad lines were either present or absent. But now intermediate cases are known with only weaker broad lines, so intermediate types are recognized according to the prominence of broad line features.
The thinking now is that there is no actual difference between AGNs with and without broad lines. Instead, the full or partial absence of broad emission lines is ascribed to the degree by which the broad-line clouds are hidden within a thicker torus-shaped ring of even colder gas and dust that surrounds both the accretion disk and the inner clouds. Because of the thick toroidal shape, if our line of sight to the object is mostly face-on, the inner disk and the clouds will be visible. But if we see the object mostly edge-on, those features will be partially or fully hidden.
Narrow emission lines are seen in the spectra of most AGNs. The fact that the lines are narrower indicates that the gas they come from is not moving as rapidly as the gas responsible for broad lines. The type of lines and other evidence indicates that the source of the narrow line emissions is a large but diffuse corona of very hot, ionized gas, which can extend for many light years, surrounding all other parts of the AGN. In AGN that are close enough, the corona is large enough that its actual size can be measured directly. Much of the emissions from the corona is at far ultraviolet and X-ray wavelengths, showing that temperatures in the corona must be quite high.
Approximately 10% of AGNs, both Seyfert galaxies and quasars, are "radio loud" – that is, a source of strong radio frequency emissions. In fact, it was strong radio emissions from the first quasars to be recognized that sharply distinguished them from the normal stars they appeared to be at visible wavelengths. Now that many quasars can be recognized by the high redshift of their spectra – indicating very distant and hence very luminous sources – it turns out that only about 10% of quasars are radio loud.
Long baseline radio interferometry makes it possible to "see" the source of the radio emissions in some detail. The source is not spherically symmetrical, but instead takes the form of very long, narrow "jets", as seen in Centaurus A. Such jets can be hundreds of thousands of light years long. The evidence is that these jets consist of plasmas in which electrons near the central black hole can have relativistic velocities – with Lorentz factors of 104 or more. Electromagnetic emissions from jets often run all the way from radio up to X-rays.
The only plausible physical model for the jets requires very strong magnetic fields. These fields collimate the matter into narrow jets – which emanate in opposite directions from the central black hole – and accelerate the plasmas charged particles to extreme velocities. Relativistic electrons moving in a helical pattern around the jet axes are responsible for the radio emissions via synchrotron radiation.
Many of the details presented so far are based largely on theoretical models, even though astronomers have known of AGNs for over 50 years. Observational studies of active galaxies – quasars in particular – are difficult, since most of the objects are quite distant, and much of the action occurs in a volume of only ~100 cubic light years – impossible to resolve with existing technology. But observational evidence for some of the details is slowly accumulating. The research were now ready to discuss is an example.
The Hard X-Ray View of Reflection, Absorption, and the Disk-Jet Connection in the Radio-Loud AGN 3C 33
We present results from Suzaku and Swift observations of the nearby radio galaxy 3C 33, and investigate the nature of absorption, reflection, and jet production in this source. We model the 0.5-100 keV nuclear continuum with a power law that is transmitted either through one or more layers of pc-scale neutral material, or through a modestly ionized pc-scale obscurer. The standard signatures of reflection from a neutral accretion disk are absent in 3C 33: there is no evidence of a relativistically blurred Fe Kα emission line, and no Compton reflection hump above 10 keV. We find the upper limit to the neutral reflection fraction is R < 0.41 for an e-folding energy of 1 GeV. We observe a narrow, neutral Fe Kα line, which is likely to originate at least 2000 Rs from the black hole. We show that the weakness of reflection features in 3C 33 is consistent with two interpretations: either the inner accretion flow is highly ionized, or the black-hole spin configuration is retrograde with respect to the accreting material.
3C 33 (which means it is object number 33 in the Third Cambridge Catalogue of Radio Sources) has a redshift z=0.0597, which equates to a distance of about 800 million light years. 3C 33 is one of the brightest narrow-line radio galaxies (NLRGs). An NLRG is a radio-loud AGN in which the spectrum contains narrow width emission lines but there is little or no evidence of broadened spectral lines.
At visible and infrared wavelengths 3C 33 is nothing special to look at, but radio images show structures typical of radio galaxies, with pronounced lobes on both sides of the central object. (See here and here for images at various wavelengths.)
For an explanation of why 3C 33 is an interesting object of study, we need to go into a little more detail about what makes up the "hard" X-ray part of the spectrum of an AGN. This involves photons having energies from 1 keV to 120 keV.
The temperature of the plasma that makes up the corona is much higher than the temperature of the gas in the accretion disk, which is mostly un-ionized. Even the hottest parts of the accretion disk have their intensity peaks in the ultraviolet, with wavelengths of at most 10 nm, which implies temperatures of about 300,000 K by Wiens law. Hard X-rays with 12 keV photons are two orders of magnitude smaller in wavelength, implying temperatures around 30 million K. Quite a difference. So its not unreasonable to regard the gas in the accretion disk as "cold" – compared to the gas of the corona.
Its somewhat messy to describe what happens with very energetic radiation from the corona interacting with the less energetic radiation from the hottest (innermost) parts of the accretion disk. However, various simulation studies have investigated models where a hard X-ray spectrum is "reflected" from an opaque slab of relatively "cold" gas that mostly emits in the ultraviolet. High-energy photons can reflect off of lower energy electrons in the process of Compton scattering. The high-energy photons lose some of their energy in the process, while the lower-energy particles gain energy. The reverse can also happen: low-energy photons from the accretion disk can scatter off higher energy electrons and photons in the corona. In this process (inverse Compton scattering) the lower-energy photons gain energy.
The starting assumption is that the X-ray spectrum of the corona is approximated by a power law in which the distribution of number of photons of given energy is a power -α (with α>0) of the energy, i. e. N(E) ∝ E-α. Models with typical assumptions about the configuration of the accretion disk suggest that there should be a slight enhancement of number of photons at energies above 10 keV in the corona X-ray spectrum. This enhancement is referred to as the "Compton reflection bump".
The observational evidence is that this Compton bump is usually found in the X-ray spectra of radio-quiet AGNs. But radio-loud AGNs tend not to have this feature in the X-ray spectra, or have it only weakly. This suggests that there may be something different about the accretion disks of radio-loud AGNs – that is, AGN that also have a jet structure responsible for their radio emissions.
There is one other common feature in AGN X-ray spectra – an emission line from fluorescing iron (Fe) atoms around 6.4 keV. This is called the Fe Kα line. It is normally observed to be relativistically broadened, indicating that it arises from accretion disk reflection. Again, the Fe Kα line is commonly found in radio-quiet AGNs and not in radio-loud AGNs. This is a further indication of something different about the accretion disks of radio-loud AGNs.
3C 33 is a radio-loud AGN, so its a good candidate for closer investigation. However, theres an additional complication in all this in the radio-loud case. The jets radiate over the full EM spectrum, not just at radio frequencies. In particular, theres an X-ray component to the spectrum, and its especially strong at the base of the jets. If this part of the jets adds its contribution to the X-ray spectrum the shape of the spectrum will be changed so that the Compton bump (if any) is harder to distinguish.
As it happens, most of the radio galaxies previously studied have been of the broad line sort. Recall that this means we are seeing the galaxy more or less along the axis of the jet, so that the base of the jet and the surrounding accretion disk are not obscured by the outer torus of cold gas and dust. 3C 33, however, is a narrow line radio galaxy (NLRG). That means that the common axis of the jets and the accretion disk is at a large angle (more than ~60°) to our line of sight. Consequently we cant see the accretion disk or the base of the jets directly, due to the obscuring dust, and so broad emission lines arent visible.
Thats actually good, because it means unobscured X-ray emissions from the jets are relatively minor and would not make it difficult to detect a Compton bump and Fe Kα lines – if they were present. If there were a Compton bump, it would be due to photons reflected from the accretion disk and scattered to higher energies in the corona. Since the corona may be hundreds of light years in radius, it is not obscured.
Nevertheless, what this research has shown is that a Compton bump and significant Fe Kα fluorescence are not present in 3C 33. Therefore theres probably something different from the norm of AGNs about the accretion disk of 3C 33. And the most natural assumption is that difference is related to the jets.
What could the difference be? Another research team that has considered the issue of lack of Compton bump in radio-loud AGNs hypothesized that the hottest inner part of the accretion disk could be partially ionized. Therefore it would be semitransparent and not reflect photons strongly. Calculations showed that this was a viable hypothesis.
The team responsible for the present research has a different hypothesis: the strong magnetic fields that create the jets also force the inner part of the accretion disk farther away from the black hole – provided that the black hole itself is spinning in the opposite direction ("retrograde") from the accretion disk. So the research team suggests that the lack of Compton bump is possible evidence for opposing spins of black hole and accretion disk.
Even without effects due to the magnetic field, a retrograde spin of the black hole would cause the radius of the smallest stable circular orbit outside the black hole to be larger than in the prograde case. In other words, the material that would otherwise orbit closer to the black hole isnt there since it has to fall into the black hole. Since magnetic field lines cannot be anchored in a black hole, they must be attached to the accretion disk, and thus assume a different shape than they would if disk and black hole were spinning in the same direction.
At this point, there is no direct evidence for retrograde spin. The competing hypothesis of a semitransparent inner accretion disk isnt ruled out. Further study will be required to distinguish between the two hypotheses.
 | Evans, D., Reeves, J., Hardcastle, M., Kraft, R., Lee, J., & Virani, S. (2010). THE HARD X-RAY VIEW OF REFLECTION, ABSORPTION, AND THE DISK-JET CONNECTION IN THE RADIO-LOUD AGN 3C 33 The Astrophysical Journal, 710 (1), 859-868 DOI: 10.1088/0004-637X/710/1/859 |
Further reading:
Black hole spin may create jets that control galaxy (2/11/10)
Backward Black Holes Control Fate of Galaxies (2/12/10)
The Hard X-Ray View of Reflection, Absorption, and the Disk-Jet Connection in the Radio-Loud AGN 3C 33 – arXiv copy of research paper
Related articles:
Winds of Change: How Black Holes May Shape Galaxies (4/19/10)
Galactic black holes may be more massive than thought (6/8/09)
Black hole outflows from Centaurus A (2/6/09)
Evidence that quasars are powered by black holes (10/21/06)
The wind from a black hole (7/8/06)
Other resources
Black Hole Models for Active Galactic Nuclei – excellent technical introduction by Martin Rees
3CRR Atlas Home Page
NASA/IPAC Extragalactic Databse: NED
Midland Energy Resources
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Marcotts Climate Reconstruction For the past 11 000 Years
Back in 1999 Penn State climate scientist Michael Mann released the climate change movements most potent symbol: The "hockey stick," a line graph of global temperature over the last 1,500 years that shows an unmistakable, massive uptick in the twentieth century when humans began to dump large amounts of greenhouse gases into the atmosphere. Its among the most compelling bits of proof out there that human beings are behind global warming, and as such has become a target on Manns back for climate denialists looking to draw a bead on scientists.Now its gotten a makeover: A study published in Science reconstructs global temperatures further back than ever before -- a full 11,300 years. The new analysis finds that the only problem with Manns hockey stick was that its handle was about 9,000 years too short.
Friday, August 29, 2014
Pesticides Make Bees Lose Their Way
Scientists have discovered ways in which even low doses of widely used pesticides can harm bumblebees and honeybees, interfering with their homing abilities and making them lose their way.
In two studies published in the journal Science on Thursday, British and French researchers looked at bees and neonicotinoid insecticides – a class introduced in the 1990s now among the most commonly used crop pesticides in the world. …
In the first of the Science studies, a University of Stirling team exposed developing colonies of bumblebees to low levels of a neonicotinoid called imidacloprid, and then placed the colonies in an enclosed field site where the bees could fly around collecting pollen under natural conditions for six weeks.
At the beginning and end of the experiment, the researchers weighed each of the bumblebee nests – which included the bees, wax, honey, bee grubs and pollen – to see how much the colony had grown.
Compared to control colonies not exposed to imidacloprid, the researchers found the treated colonies gained less weight, suggesting less food was coming in.
The treated colonies were on average eight to 12 percent smaller than the control colonies at the end of the experiment, and also produced about 85 percent fewer queens – a finding that is key because queens produce the next generation of bees.
Ocean Energy Resources
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New anti cancer role for p53
However, the complexity is to be expected, because evolution doesnt "design" cellular mechanisms to work in a straightforward way. The mechanisms are simply the result of about a billion years of trial and error. Being pretty and elegant was not a criterion for success.
Nature is "hairy", knowing nothing of Occams Razor, and caring even less. Simplicity is for wimps.
But one thing is clear: p53 plays a large role in preventing, or at least suppressing, the development of cancer. In many types of cancer, p53 is found to have mutations more than 50% of the time. Even if p53 isnt mutated, cancer cells generally have other p53 abnormalities, such as low levels of the protein or the presence of various factors that interfere with its activity.
Until the latest research, there have been two principal ways known in which p53 works against cancer, and several additional minor ways. The two main ways p53 has been known to act are binding to DNA as a transcription factor, and binding directly to certain proteins. And each of these mechanisms can lead to either of two main types of tumor suppression: apoptosis (cell death) and temporary or permanent suspension of the cell cycle, which is the process a cell goes through in order to divide and proliferate.
P53 is primarily a transcription factor. In this role it is found in a cell nucleus and binds to various specific DNA gene promoter regions, in order to direct transcription of the associated gene – the first step in production of proteins from a gene.
The proteins that are expressed as a result of this p53 activity can play a part in either apopotosis or cell cycle control (as well as other functions not directly related to cancer – see here, here, here). Which function is invoked depends on the type of signal that activates the p53. Among the possible conditions that may be signaled are detection of correctable or uncorrectable damage to DNA and detection of chromosome telomeres that are too short.
In addition to binding to DNA as a transcription factor, p53 is also capable of binding directly to other proteins in order to control their behavior. Mainly these proteins are involved with apoptosis, such as members of the Bcl2 family.
P53 itself is actually a family of proteins – there are at least 9 different RNA transcripts that can be derived from the TP53 gene. But one thing that each of these family members have in common is a segment, called the DNA binding domain. It is this part of the p53 that is capable of binding to either DNA or other proteins. (In general, a protein domain is a more-or-less self-sufficient component of a protein. Often the same domain appears in different members of a family of proteins.)
One indication of the importance of this p53 domain is the fact that point mutations (errors involving only a single nucleotide pair) in the part of TP53 that code for the binding domain are the only type of point mutations of p53 that are commonly found in tumors. Errors that affect portions of p53 outside of the binding domain are not associated with cancer.
Theres one more thing to note about p53s role as a transcription factor. Namely, the RNA that is transcribed under the direction of p53 is not always messenger RNA (mRNA) that will eventually code for the production of a protein. P53 can also initiate the transcription of genes that code for microRNA (miRNA), which is a single-stranded RNA molecule thats normally only 21 to 23 nucleotides in length. Over 500 different types of miRNA have been found in human cells.
MicroRNA is never translated into a protein. Instead, miRNA molecules regulate the translation of messenger RNA for many different proteins (by binding with the mRNA to prevent translation). It has been known for some time that p53 acts as a transcription factor for the miRNA family known as miR-34. It has also been learned that among the proteins regulated by miR-34 are some found in pathways that lead to apoptosis or cell cycle arrest. The net effect is that miR-34 has tumor-suppressing properties, so this is another way that p53, as a transcription factor, helps suppress tumors.
Many other miRNA molecules, on the other hand, are found at high levels in cancer cells. Such miRNAs most likely inhibit expression of tumor suppressing genes, whose proteins might otherwise control cell proliferation or migration. Weve discussed a number of miRNAs associated with cancer, mostly of the sort that promote cancer, here and here.
Nevertheless, there are miRNAs besides miR-34 that have anti-cancer effects. Three in particular are miR-16-1, miR-143, and miR-145. It has been observed that these miRNAs, and several others, are found at higher levels in cells where p53 has been activated as a result of DNA damage. (Normally, p53 formed in non-cancer cells is either quickly degraded or else inhibited by certain proteins, especially MDM2, so as not to unnecessarily promote apoptosis or cell cycle arrest. The presence of DNA damage results in the removal of these inhibitions on p53.)
It therefore appears that p53 is doing something to help produce a number of miRNAs, some of which are tumor suppressors. The curious thing, though, is that it can be shown that p53 is not a transcription factor for the genes that encode these miRNAs.
So what is it that p53 is doing instead to help produce these miRNAs? New research published in the July 23, 2009 issue of Nature answers this question – and it uncovers an entirely new mechanism through which p53 (and its binding domain, in particular) acts as a tumor suppressor. Heres the research abstract:
Modulation of microRNA processing by p53
MicroRNAs (miRNAs) have emerged as key post-transcriptional regulators of gene expression, involved in diverse physiological and pathological processes. Although miRNAs can function as both tumour suppressors and oncogenes in tumour development, a widespread downregulation of miRNAs is commonly observed in human cancers and promotes cellular transformation and tumorigenesis. This indicates an inherent significance of small RNAs in tumour suppression. However, the connection between tumour suppressor networks and miRNA biogenesis machineries has not been investigated in depth. Here we show that a central tumour suppressor, p53, enhances the post-transcriptional maturation of several miRNAs with growth-suppressive function, including miR-16-1, miR-143 and miR-145, in response to DNA damage. ... These findings suggest that transcription-independent modulation of miRNA biogenesis is intrinsically embedded in a tumour suppressive program governed by p53. Our study reveals a previously unrecognized function of p53 in miRNA processing, which may underlie key aspects of cancer biology.
To understand whats going on, its necessary to explain a few things about how miRNAs are produced. Its not a simple 1-step process of transcribing an miRNA gene into the final short piece of RNA.
There are, instead, three steps. The first step is transcription, done just as is done for any other gene. The RNA produced in this step is many nucleotides long, and is called the "primary transcript" or pri-miRNA. This pri-miRNA is then cut into smaller pieces having a hairpin shape, called pre-miRNA. The pre-miRNA, in turn, is further processed to produce the final "mature" miRNA.
The intermediate step that converts pri-miRNA to pre-miRNA is performed by a protein complex known as the "microprocessor complex" (having nothing to do with computers, of course). One of the key proteins in this complex is an enzyme called Drosha. The final step, which is performed by another enzyme called Dicer, splits the pre-miRNA apart to yield the mature miRNA.
The main contribution of p53 in this process is to facilitate the action of Drosha. It seems that, although Drosha can do the job by itself (since miRNAs are needed even if p53 isnt active), p53 helps by binding (via its binding domain) with parts of the microprocessor complex. This is indicated by the observation that mutations in the binding domain disable p53 binding to the complex, resulting in lower levels of miRNA production.
So there you have it: an essentially novel way that p53 acts as a tumor suppressor, by facilitating production, non-transcriptionally, of tumor-suppressing miRNAs.
| Suzuki, H., Yamagata, K., Sugimoto, K., Iwamoto, T., Kato, S., & Miyazono, K. (2009). Modulation of microRNA processing by p53 Nature, 460 (7254), 529-533 DOI: 10.1038/nature08199 |
Further reading:
Protein plays three cancer-fighting roles (7/22/09) – Science News article on the research
Link between p53 and miRNA – editors summary in Nature of the research
Cancer: Three birds with one stone (7/23/09) – Nature news article on the research
Tags: p53, microRNA, cancer
Thursday, August 28, 2014
The Nuclear Breakthrough That Wasnt
The New York Times ran a story about Johnsons speech on page one under the headline, "Johnson Reports a Breakthrough in Atomic Power." They followed up with a series of stories, as did the other major newspapers. Word of a breakthrough in the cost of nuclear power was big news because everyone had been waiting for economically feasible nuclear power for a decade. After the heavy promotion of the early nuclear power days--exemplified by Walt Disneys classic nuclear cartoon, Our Friend the Atom--nuclear power had stalled out with just a few demonstration plants in operation. The coal lobby smelled blood. In March of 1964 the coal industry assailed nuclear power, saying Congress needed to remove "the sheltering umbrella of Government subsidies."
General Electric and Westinghouse, who had helped build Americas military and civilian nuclear program, were getting antsy that their knowledge would go to waste. "Our people understood this was a game of massive stakes, and that if we didnt force the utility industry to put those stations on line, wed end up with nothing," as John Gitterick, a GE vice president, later told Fortune. It was this corporate desire to capture rents on a technology that only a few companies could provide that generated the "economic breakthrough" of Johnsons speech.
As soon as the words left Johnsons mouth, scientists at national laboratories around the country knew what he was talking about, even though he was a few months late with the announcement. When a Chicago Tribune reporter called Stephen Lawrowski, associate director of Argonne National Laboratory, the scientist told him that the president must have been talking about the guaranteed price that General Electric had offered Jersey Central Light and Power for the Oyster Creek plant. That announcement had "caused a flurry" in scientific circles because the price GE was charging for the plant--$68 million for the 515-megawatt plant--made the plant economically competitive with fossil fuels. [Editors note: Oyster Creek was a boiling water reactor with the same basic design and containment vessel as the Fukushima reactor in Japan.]
Yet the scientists knew from the available evidence that nuclear power was far from economically competitive in mid-1964. However, instead of setting the Tribune reporter straight, Lawrowski simply punted, saying "The New Jersey plant is a significant milestone in nuclear power progress because it has affected thinking not only in America but also in Europe."
The price was a door-buster, a loss-leader, an advertisement for a nuclear age that had not actually yet arrived. The so-called "turnkey" plants, as they later became known, probably cost Westinghouse and General Electric over $1 billion combine, though they did not say that at the time.
Coal officials told the Wall Street Journal that GE had "priced the Oyster Creek plant at less than cost." A GE executive denied that, claiming the company would "make a slight profit unless we run into some unforeseen difficulties." British and Russian engineers also called the estimates into question--and French officials unsuccessfully tried to get details out of GE. But American news accounts, though they reported those foreign doubts, always made sure to note the bias that national competition could introduce into other countries expert opinion. None questioned the U.S. expert corps own Cold War sympathies.
Newspaper reporters, with the help of sources within the nuclear industries, came up with stories to explain how prices could have fallen so far, so fast. But like a trend piece about raising chickens in Manhattan, they were little more than anecdotes strung together by plausibility and the publics desire to believe. Although they reported doubts about the breakthrough, they were often run deep inside the paper whereas the optimistic pieces led the sections of the paper. Even the most skeptical piece, a September 1964 article by Washington Post reporter Howard Simons, noting that "not all experts accept General Electrics figures," only questioned the figures within 12 percent. In reality, nuclear power would end up costing not $104 or $1,040 per kilowatt of capacity but more than $3,750 per kilowatt by the mid-1980s.
Perhaps Lewis Strauss, then-chairman of the AEC, overstated the case when he told a crowd of science writers in 1954 that "Our children will enjoy in their homes electrical energy too cheap to meter," but his optimism was obviously widely shared within the nuclear establishment. The countrys political leaders were more than willing to believe and promote these technical promises. It was a wonderfully convenient solution to an America battling Communist agitation across the world.
And besides, nuclear proponents said energy usage would soar and they had nice graphs to back it up. Their vision was expansive, expensive, and rather brilliant. Technical reports came out purporting to show energy "needs" for Americans in the future that were spectacularly high. In 1960 the AEC, which had as its mandate to promote the commercialization of nuclear power, projected that Americans would use 170 quadrillion BTUs in 2000. In reality, that year Americans used about 99 million quads of energy. And we still do. Imagine adding 70 percent more power plants, cars, and buildings to our current energy infrastructure. Its nearly unthinkable.
Yet from the early 1950s until the energy crises of the 1970s, politicians accepted as gospel truth nuclear proponents overblown visions of Americas energy needs emanating from the nations national laboratories and the AEC. Legislators continually delivered high-levels of steady funding to nuclear research.
Of course, the political relationship ran both ways. The AEC knew what the government needed and the government knew what the AEC needed. In both cases, the answer was: Dont stop believing!
Despite the occasional call for the free market to work, the opposite happened. For example, nuclear power plant operators are indemnified by the U.S. government for catastrophic disasters (the Price-Anderson Act), thereby lowering their insurance rates. They were given preferential access to markets for borrowing money. There was plenty of informal and regulatory help to go with the R&D and commercialization boosts. In effect, the government socially engineered the cost structure of the industry so nuclear could compete with coal, which got to dump all its extra costs, such as air and water pollution, into the environment.
But even then, convincing utilities that they needed to go nuclear wasnt easy until General Electric hit on the genius idea of guaranteeing a fixed price to risk-averse utilities, effectively subsidizing the cost of the construction. And Oyster Creek was born. If they could just build a ton of plants, they could learn and scale and standardize: Costs would drop. Westinghouse matched GEs pricing, and what came to be known as the "turnkey" plants were built. In the bandwagon market that followed until 1973, utilities ordered more than two hundred nuclear reactors. Nuclear power had arrived.
But the turnkey plant prices did not reflect the actual costs of building a nuclear power plant. As the years wore on, that nuclear power was not as cheap as coal and other fossil fuels became increasingly clear: The prestige of the nuclear authorities began to fall; nuclear whistleblowers came forward; environmental risks were reassessed, perhaps too stringently; the protest movements of the 1960s turned their attention to nuclear power and all the centralization of power it represented. It turned out that Americans were ready to extend democracy to technocratic decision making, and they did not like what they saw from the nuclear industry.