The Young Astronomers
Currently viewing the tag: "Blazar"
Nowadays, scientists have the ability to observe the universe across many wavelengths, including those that cannot be seen by unaided human eyes – such as infrared and gamma rays. However, there is still some “blind spots” for the astronomers in the electromagnetic spectrum, such as the high energy X-rays region.
In order to clarify our view of these cosmic X-rays, NASA has just launched the Nuclear Spectroscopic Telescope Array (NuSTAR), a sci-fi looking space observatory that will allow us to better understand the physics and chemistry behind the most extreme objects in the universe.
Artist's impression of NuSTAR on orbit. (credits: NASA/JPL-Caltech)
NuSTAR is going to observe the same kind of X-rays used in medicine – when the doctors need to take a look at our bones – and in airports – to see what’s inside our luggage. Although we have spacecraft able to take X-ray images, like NASA’s Chandra and ESA’s XMM-Newton, these facilities aren’t able to get sharp images of this energetic form of light.
Surveying the Extreme Universe
The NuSTAR’s primary mission – projected to last for at least two years – is intended to provide a detailed survey of both stellar and supermassive black holes. Although being known as objects capable of swallowing pretty much everything nearby – even light – the region around the black holes – called the accretion disk - emits vast amounts of X-ray radiation, produced by the friction between the pieces of debris orbiting the black hole. NuSTAR is equipped to detect these radiation bursts and locate black holes that would be otherwise invisible.
It’s also going to analyse the young-supernovae remnants, more specifically, the radioactive nuclei produced by the exploding stars. These nuclei are the key to understanding the conditions in which each explosions occurred, the nuclear ignition, structure and dynamics of the explosion, giving us clues as to how exactly the elements are formed in the hidden core of a star.
The brightest objects visible to NuSTAR are the jets emitted by supermassive black holes – that can be found in the core of most galaxies – in a specific kind of galaxy known as a blazar. These are a form of active galaxy that produce jets composed of particles travelling in very high speeds that happen to be pointing towards the Earth – therefore we see them much brighter than we would otherwise. By observing the variation of the light intensity of these beams, we’ll be able to figure out how the black hole’s accretion disk looks like, as well as the physical structure and composition of the jets.
And that’s not all: the probe will also peer at the Solar corona, the outer atmosphere of the Sun. This region is known for its extreme temperatures – as a matter of fact, even hotter than the Sun’s surface. By studying the corona, we can get a close-up look at the particle acceleration processes similar to those that take place in objects like supernova remnants and black hole jets.
This image comparison demonstrates NuSTAR's improved ability to focus high-energy X-ray light into sharp images. The image on the left, taken by ESA's INTEGRAL satellite, shows how we see these X-rays today. The image on the right is a simulation of what NuSTAR will see at comparable wavelengths. (Credits: ESA/NASA/JPL-Caltech)
Looking Further than Ever
What makes NuSTAR able to archive all these goals is its ingenious focusing mechanism: a pair of Wolter-I mirrors pointing at the same patch of sky. Moreover, in order to improve its reflection capacity, these mirrors are coated with structures known as “depth-graded layers”, made of a mix of low and high density materials.
The unprecedented sensibility of NuSTAR – along with the data from the other observatories – will allow us to study the universe as never before, from the surface of the Sun to the galaxies at the other edge of the universe, going through the most extreme objects known by humankind.
NuSTAR has successfully reached orbit and is preparing for the commencement of its observations.
The latest release from NASA’s WISE mission has shown that just over 200 previously unidentified high energy objects are likely to be blazars.
Artist's Impression of an active Blazar Credit:NASA/JPL-Caltech
A blazar is a form of active galactic nucleus (AGN) – a galaxy where the central black hole is ‘feeding’ on large amounts of material resulting in the release of huge amounts of radiation including two tight very bright jets.
The angle at which the AGN is situated relative to the Earth determines which form of AGN we observe even though all are tied to the same processes.
In the case of a blazar we are looking directly down the AGN’s jets you could even say right down the barrel of the gun!
AGN at various angles; Credit: Aurore Simonnet, SSU NASA E/PO.
As the AGN must be lined up almost exactly with Earth for a blazar to be observed they are understandably rare compared to the other forms of AGN which have a much larger range of possible viewing angles. That being said the WISE data has the potential to reveal several thousand more.
A team using the WISE data looked at 300 objects that had previously been detected as high energy gamma-ray sources by the Fermi Space Telescope, though up to now had remained unidentified.
Using WISE the team was able to observe these gamma ray hotspots in infra-red wavelengths and showed that just over half are most likely to be blazars. WISE had also observed 50 new blazars outside those Fermi oddities along with taking observations of more than 1000 previously identified blazar candidates.
One of the project leads, Francesco Massaro has explained that there may be several thousand more as of yet unknown blazars hidden within the WISE data that could be revealed using the techniques developed for this first sample.
An image of one of the new WISE identified blazars Credit:NASA/JPL-Caltech/Kavli
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3C273 Credit: SDSS
The central object looks just like any other star in the image above, but it’s as far from a star as you can get. It is in fact a Quasar (the less catchy name is Quasi-Stellar Radio Source), which are some of the most distant and most luminous objects seen in the observable universe. This one however isn’t as distant as many of its type, at just a redshift (Z) of 0.15 it’s just 1.88 billion (yes, just) light years away [1].
Some of the most distant Quasars have a Z of 6, placing them around 12 billion light years away, or 24 billion light years away in commoving distance. One such example is SDSS J1148+5251 which has a Z of 6.41. It was until recently the most distant quasar found, but this record has been replaced by CFHQS J232908-030158 at a Z of 6.43, placing it 12.8 billion light years away [2].
Most of these objects appear as point-like objects, which is why they look exactly like stars in images. Most galaxies at high redshift are hard to see, so why are Quasars so luminous that they can be seen at such great distances; and what are they anyway?
Quasars are a type of Active Galactic Nuclei (AGN). If you could see in detail the nucleus of any galaxy that is classified as ‘active’ you’ll see a doughnut shaped torus of matter surrounding a more compact, thinner accretion disk which in turn surrounds the galaxy’s super massive black hole. And all this could fit snugly inside a solar system!
The black hole at the centre of this powers the AGN. The huge gravitational pull from the black hole drags matter into its vicinity, surrounding itself with an accretion disk in the process. As the matter travels round the disk, it releases energy due to the friction caused by the material in the disk interacting with each other as it races round at thousands of kilometres per hour. The radiation emitted ranges from gamma rays to radio waves, and the sheer amount of radiation emitted can cause an AGN to completely outshine its host galaxy by hundreds of times!
The powerful magnetic fields caused by the AGN can also cause material to collimate into jets of plasma. This plasma races out of the galactic nucleus along the black hole’s spin axis at relativistic speeds, stretching out for thousands upon thousands of light years. The creation of these jets is still under a lot of debate, but currently it is thought that the material in the accretion disk escapes via the hole in the disk – which compared to the rest of the disk, is relatively dust free. The material escapes in this direction simply because there’s less resistance.
There are different types of AGN, from radio galaxies to Blazars. The AGN are basically the same thing, just viewed at different angles:
AGN at various angles; Credit: Aurore Simonnet, SSU NASA E/PO.
A Seyfert 1 for instance is where the AGN is viewed at a 30 degree angle and a Seyfert 2/ Radio galaxy is viewed at a 90 degree angle and so on. A Quasar is viewed at a 30 degree angle, so is it not just a Seyfert galaxy? What differentiates a Quasar from a Seyfert is the luminosity; an AGN with a luminosity of over 1011 L?is classed as a Quasar [3].
Back to the Quasars and 3C 273…
3C273 was the first Quasar to be indentified in 1963, when Maarten Schmidt published a paper in Nature, after the star-like object was associated with a radio source already documented in the Third Cambridge Catalogue of Radio Source [4]. Schmidt’s paper showed that 3C273 has a high redshift, placing it billions of light years outside our own galaxy.
This particular Quasar has a jet that stretches out for 60 kilo parsecs in length, making it almost twice the diameter of our galaxy [5]! You can see the jet in figure one; it’s the faint streak just below the quasar on the bottom right. This jet makes 3C273 one of just 10% of Quasars that have large scale jets as big as 3C273’s [6]. The other 90% have less powerful jets that are just parsecs in length.
Quasars aren’t seen (to the best of our knowledge) after a redshift of 0.06; they become more common the higher the redshift. Why are there more Quasars in the early universe? The universe was full with young galaxies absolutely brimming with new stars and therefore plenty of gas for a super massive black hole to consume. Eventually the fuel runs out, either by the AGN exceeding the Eddington luminosity, where the AGN becomes so bright the photons it emits buffets the gas out of the way and prevents it from falling in, or simply by the AGN having consumed all the available material.
List of references:
[1] SDSS DR7 ObjID: 587726014535237707
[2] Willot C. J et al (2007) “Four quasars above redshift 6 discovered by the Canada-France High-z Quasar Survey” Astro-ph:arXiv:0706.0914v2.
[3] Sparke, L.S. and Gallagher, J.S. (2000) ‘’Galaxies in the Universe: An Introduction’’ 2nd ed. New York: Cambridge University Press.
[4]Schmidt. M. (1963) “3C273: A Star-Like Object with Large Red-Shift”. Nature 197: 1040–1040. doi:10.1038/1971040a0.
[5] Uchiyama. Y. et al (2006) “Shedding New Light on the 3C 273 Jet with the Spitzer Space Telescope” Astrophys. J. 648 910 doi: 10.1086/505964
[6] “UT Austin scientists find evidence that all radio-loud quasars may be blazars’’. The University of Texas (14/2001)http://www.utexas.edu/news/2001/06/14/nr_blazers/
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