Many flavours of supernova

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from Nature 520, 411 (23 April 2015) doi:10.1038/520411d


Exploding stars grouped in one family because of their similarities actually form two distinct groups. This may have important cosmic implications because the explosions, called supernovae, are the primary evidence that the Universe’s expansion is accelerating.
Half of type Ia supernovae seem to have similar intrinsic brightnesses when seen in the visible spectrum. But when Peter Milne of the University of Arizona in Tucson and his team analysed data from the Hubble Space Telescope and NASA’s Swift satellite, they found that the supernovae fell into two subfamilies, each brighter than the other in a different part of the ultraviolet spectrum.
The relative abundances of the two subfamilies seem to have changed over the past several billion years, a fact that could complicate their use as markers of cosmic expansion, the authors say.

Neutrinos from a galaxy far away

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Nature 520, 266 (16 April 2015) doi:10.1038/520266b

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Two of the most energetic neutrinos detected by a telescope in the Antarctic may have come from the cores of distant galaxies.
Neutrinos are stable and can travel far in space, so they could shed light on distant astrophysical and galactic objects. The Antarctic telescope IceCube picked up signs of neutrinos in 2011 and 2012 that were the first ever measured with energies of 1 petaelectronvolt (1 × 1015 electronvolts), suggesting a powerful source such as a blazar — a type of high-energy galaxy.
A team led by Clancy James of the University of Erlangen and Matthias Kadler of the University of Würzburg, both in Germany, studied six years of data from the underwater ANTARES neutrino telescope off the coast of Toulon, France, scanning six blazars for further neutrinos. The two blazars considered to be the best candidates each yielded events that were consistent with the signature of a neutrino, suggesting that they could be the sources of the IceCube neutrinos.


ANTARES constrains a blazar origin of two IceCube PeV neutrino events, A&A 576, L8 (2015)

Secret ingredient exposed

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by Christopher M. Johns-Krull from Nature (2014) doi:10.1038/nature13932

Astronomers have suspected for some time that magnetic fields are a key ingredient in the accretion of material that surrounds young stars. New observations have just begun to reveal these fields in action.

Most stars are born surrounded by disks of gas and dust, and it is in these disks that planets form. However, the gas and dust that makes up these disks (Fig. 1) does not all go into forming planets. Indeed, much of the disk material slowly falls, or accretes, onto the newly formed star, setting its final mass. For many years, astronomers have studied such accretion disks and measured the rate at which the material accretes onto young stars, but exactly why such accretion occurs so efficiently has remained elusive. Over the past several years, researchers have started to zero in on a solution in theoretical studies for the evolution of an accretion disk that involves the action of magnetic fields. Unfortunately, until just this year there were no observations of the required magnetic fields in the disks. In a paper published on Nature’s website, Stephens et al.1 report how they have now clearly detected magnetic fields in one such disk.

These visible-light images show disks of gas and dust (dark rings) around four infant stars (central bright regions) in the Orion Nebula, a star-forming molecular cloud about 400 parsecs away from Earth. The dust in the disks, which are surrounded by hot gas from the nebula, makes them look dark at visible wavelengths. Magnetic fields threading these disks, now observed directly by Stephens et al.(1) in the disk of the young star HL Tau, are probably the main element causing much of the observed disk material to accrete onto the stars. Each square region shown is about 260 billion kilometres across.

These visible-light images show disks of gas and dust (dark rings) around four infant stars (central bright regions) in the Orion Nebula, a star-forming molecular cloud about 400 parsecs away from Earth. The dust in the disks, which are surrounded by hot gas from the nebula, makes them look dark at visible wavelengths. Magnetic fields threading these disks, now observed directly by Stephens et al.(1) in the disk of the young star HL Tau, are probably the main element causing much of the observed disk material to accrete onto the stars. Each square region shown is about 260 billion kilometres across.

The key to understanding how an accretion disk works is to figure out what causes some of the material to spiral inward and eventually merge onto the star. Viscosity in the disk is one possible mechanism. At each point in the disk, the material slightly inside this position moves around the star just a bit faster than the material just outside the given point. This difference in speed results in viscosity, a type of friction, that slightly slows down the material on the inside, causing it to fall somewhat closer to the star. This viscosity, acting at all radii in the disk, leads to the accretion of some of the disk material onto the central star. The greater the viscosity, the more rapid the accretion of material. The trouble is, although astronomers can estimate the viscosity in disks on the basis of the temperature and density of the material there, the result fails by many orders of magnitude to produce the rate of accretion that is observed(2, 3).
Magnetic fields, through a process known as the magnetorotational instability(4), have been proposed as a possible means by which disks accrete material. If the material in the disk is threaded through by magnetic fields, then different regions of the disk will be connected to each other by these fields. Although the temperature of the disk is relatively low compared with that of the star, the material is warm enough for a few ions and free electrons to form. The radiation from the star can also cause ion formation. Ions are forced to move with magnetic fields — essentially, they are tied to the field lines. Two ions at slightly different radii in the disk, tied to the same field line, will exert forces on each other that are transmitted through the magnetic field. Effectively, the magnetic fields act as springs holding the ions together. The ion closer to the star should be moving faster in its orbit than the ion slightly farther out, but the field tying them together causes the inner particle to slow down a little and the outer particle to speed up, in exactly the same way as the viscosity described above. However, this magnetorotational-instability mechanism is much better at causing accretion than standard viscosity, and so seems to be a promising solution to the problem of how accretion disks work. But the question has been: are there magnetic fields in the disks around young stars?
Yes, according to Stephens and colleagues’ study. The authors detected the magnetic fields in the disk of a young star by using the Combined Array for Research in Millimeter-wave Astronomy (CARMA) to measure the polarization of the light (the preferred direction in which the light’s electric field oscillates) that is emitted by the dust in the disk. Dust particles in interstellar space and in the disks of young stars are not perfect spheres — they are somewhat oblong. They also spin, and usually have a small amount of electric charge, like the ions that are present in the gas. The result of their spinning oblong shape and electric charge is that the dust grains generally align their long axes perpendicular to the magnetic field(5). This then means that they will emit polarized light.
It is more likely that the light emitted by a dust grain will have its electric field aligned with the long axis of the grain. If the dust grains all have random alignment relative to each other, the resulting total emission of radiation will have no preferred orientation and no polarization. By contrast, if most or all of the dust grains are aligned with each other owing to a magnetic field, the total emission will be polarized. This is exactly what Stephens et al. saw in the disk surrounding the young star HL Tau. Furthermore, the authors were able to spatially map the disk around HL Tau and measure the polarization at several locations in the disk, making these observations unusually powerful for testing our understanding of accretion disks. Similar observations were also made in 2014 of an even younger star(6); however, the orientation of its disk and potential confusion from the surrounding environment did not allow as clear a study of the magnetic-field structure as that obtained by Stephens et al. in the disk of HL Tau.
This result is exciting because it strongly suggests that magnetorotational instability may be the long-sought answer to just how accretion disks work. However, this is far from the last word on this astrophysical puzzle. The theoretical models of this phenomenon generally predict that the magnetic fields in the disk should be wrapped around the star, basically following the motion of the disk material as it orbits the star. Unfortunately, this is not exactly what Stephens and colleagues observed. Their polarization measurements of HL Tau’s disk indicate that the magnetic fields are all more or less pointing in the same direction instead of wrapping around the star. This is a mystery with no good explanation at this time.
So what does the future hold for accretion-disk physics? A new, more sensitive radio-telescope array than the one used by Stephens and colleagues, the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, has begun operations. This facility will provide detailed views of the structure and magnetic fields of disks around young stars. We can hope that this will produce observational results that will also spur new theoretical investigations into the role of magnetic fields in accretion disks. Our understanding of this phenomenon is not complete yet, but it has just taken a dramatic step forward.


(1) Stephens, I. W. et al. Nature (2014).
(2) Valenti, J. A., Basri, G. & Johns, C. M. Astron. J. 106, 2024–2050 (1993).
(3) Gullbring, E., Hartmann, L., Briceno, C. & Calvet, N. Astrophys. J. 492, 323–341 (1998).
(4) Balbus, S. A. & Hawley, J. F. Rev. Mod. Phys. 70, 1–53 (1998).
(5) Lazarian, A. J. J. Quant. Spectrosc. Radiat. Transf. 106, 225–256 (2007).
(6) Rao, R., Girart, J. M., Lai, S.-P. & Marrone, D. P. Astrophys. J. 780, L6 (2014).

The age of the quasars

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by Daniel Mortlock from Nature 514, 43–44 (02 October 2014) doi:10.1038/514043a

An infrared census of accreting supermassive black holes across a wide range of cosmic times indicates that the canonical understanding of how these luminous objects form and evolve may need to be adjusted.

Ask an astronomer when quasars were at their peak and they will probably tell you it was about 10 billion years ago, when the Universe was about one-third of its current size(1, 2). Before then, the quasar population was still growing along with other large structures in the young Universe; there has since been a steady decrease in quasar numbers. However, in a paper published in The Astrophysical Journal, Vardanyan et al.(3) present results suggesting that this widely accepted picture may not be correct – or at least that it does not tell the whole story.
That story started in 1963 with the discovery(4, 5) of a new type of astronomical object, referred to variously as quasi-stellar objects or quasars, the name that is generically used today. Their physical nature was initially unknown, but it was gradually deduced(6) that a quasar is a glowing disk of hot, dense material that can form around the supermassive black hole at the centre of a large galaxy, often the result of a collision with a second galaxy. Although such accretion disks are ‘only’ about the size of the Solar System, they can outshine all the stars in the host galaxy by a factor of a thousand or so. Quasars can hence be seen comparatively easily at great distances, which makes it possible to trace their evolution back to the first billion years after the Big Bang.
More than a million quasars have been catalogued in the 50 years since their discovery. Although this is more than enough for most demographic studies of astronomical objects, it is difficult to obtain a representative sample of quasars that spans a wide range of distances from Earth, and hence cosmic look-back times. It is also challenging to properly account for all the energy output of a quasar, because some of the ultraviolet light that is emitted from the accretion disk is absorbed by dust in the host galaxy and re-radiated at much longer, infrared wavelengths. Most surveys of the quasar population have been undertaken using observations made at optical or near-infrared wavelengths (between about 0.2 and 2 micrometres), and it is these types of measurement that have provided the strongest evidence that quasar numbers peaked fairly sharply 10 billion years ago.
Vardanyan and colleagues studied a comparatively small sample of 10,000 quasars that were initially identified using optical data from the Sloan Digital Sky Survey. But, crucially, the authors had access to longer wavelength measurements (at about 8 μm) of the same objects from the Wide-Field Infrared Survey Explorer (WISE) satellite. They were thus able to get a more complete census of the quasars’ energy output and, after correcting for the various complicated observational selection effects that inevitably make such studies so difficult, found some striking results. They confirmed the steady decrease in the quasar population over the past 10 billion years but, rather than the expected drop at cosmic times before 3 billion years, they found a ‘plateau’ in the quasars’ energy output back to a little over a billion years after the Big Bang (Fig. 1). The authors were unable to probe any earlier than this, and one of their conclusions was that extending these sorts of measurements to earlier times is the best way to explore this issue further.

In the standard picture of quasar evolution, from the Big Bang to the present day, the total energy output of quasars increases to a peak value some 3 billion years (Gyr) after the Big Bang as galaxies form, collide and trigger the activation of quasars. This output then declines steadily as the accelerating expansion of the Universe results in a decrease in the number of galaxy collisions. Vardanyan et al.(3) found a surprising 'plateau' (dashed line) from about 1 billion to 3 billion years in the quasars' energy output.

In the standard picture of quasar evolution, from the Big Bang to the present day, the total energy output of quasars increases to a peak value some 3 billion years (Gyr) after the Big Bang as galaxies form, collide and trigger the activation of quasars. This output then declines steadily as the accelerating expansion of the Universe results in a decrease in the number of galaxy collisions. Vardanyan et al.(3) found a surprising ‘plateau’ (dashed line) from about 1 billion to 3 billion years in the quasars’ energy output.

These results are not unprecedented – there have been several similar previous claims(7, 8) that the canonical understanding of the quasar population from optical data was incomplete. However, the scale and quality of the WISE data are superior to any previously available. The findings demand serious attention, both in terms of subjecting them to further scrutiny and exploring their implications for quasar formation if the simplest interpretation – that large numbers of high-luminosity quasars were in place just a billion years after the Big Bang – is indeed correct.
The most exciting potential implication of Vardanyan and colleagues’ study is that we need to adjust our understanding of the quasar population, especially how the early quasars formed. Most current models are based on the idea that galaxy collisions trigger quasar activation, so the number of quasars should rise sharply as galaxies form, grow and collide in the early Universe. The authors’ results suggest that this link is not so strong, and that the most luminous quasars in particular form more rapidly than astronomers might suspect using simple models of black-hole accretion and galaxy collisions.
The word ‘suspect’ is appropriate here, because this sort of science really is like detective work, in which indirect clues must be combined with inspired deduction to reach any interesting conclusions. It is remarkable that it is possible to make any kind of inference about black holes that are billions of light years away and have long since ceased to exist as quasars. One ambiguity is that the infrared light being used to assess the quasars’ energy output could come from other sources, because any mechanism that heated whatever dust was present in the host galaxy would contribute to this signal. Also problematic is that various corrections to the inferred output of the quasars have to account for the expansion of the Universe: the light seen at any given wavelength here and now has, since its emission, been redshifted by an amount that depends on how distant the source is, and hence how far back in time astronomers are seeing it. Perhaps the most uncertain aspect of all attempts to measure the evolution of the quasar population is deciding how best to account for this effect and how to test whether it has been done correctly. The approach taken by Vardanyan et al. is reasonable, but it is easy to imagine future data that would allow these corrections to be improved.
‘More data’ is something of a mantra in astronomy. Technological developments such as WISE have been one of the main drivers of discovery for the past century, and probably will continue to be in the future. We already have exciting projects such as the Large Synoptic Survey Telescope and the Square Kilometre Array just a few tantalizing years away, and both should tell us a great deal more about the age of the quasars.


(1) Fan, X. et al. Astron. J. 122, 2833 (2001).
(2) Richards, G. T. et al. Astron. J. 131, 2766 (2006).
(3) Vardanyan, V., Weedman, D. & Sargsyan, L. Astrophys. J. 790, 88 (2014).
(4) Hazard, C., Mackey, M. B. & Shimmins, A. J. Nature 197, 1037–1039 (1963).
(5) Schmidt, M. Nature 197, 1040 (1963).
(6) Rees, M. J. Ann. Rev. Astron. Astrophys. 22, 471–506 (1984).
(7) Casey, C. M. et al. Astrophys. J. 761, 139 (2012).
(8) Carilli, C. L. et al. Astrophys. J. 763, 120 (2013).

Space ripples could pump up stars

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Just a bit observation: to read the little news published on Nature you must pay, but the scientific paper is… free!


Gravitational waves could energize and brighten stars — possibly providing indirect evidence for the weak ripples in space time that are thought to be emitted by high-energy events such as exploding stars.
Barry McKernan at the City University of New York and his colleagues calculated the effect that gravitational waves would have on a star if the waves have frequencies matching those of the star’s natural vibrations. They found that the star absorbs those waves, and if close to a powerful source such as merging black holes, it could heat up and brighten.
The study suggests that gravitational waves, which are difficult to detect, could interact more strongly with matter than previously thought.

McKernan B., B. Kocsis & Z. Haiman (2014). Stars as resonant absorbers of gravitational waves, Monthly Notices of the Royal Astronomical Society: Letters, 445 (1) L74-L78. DOI: http://dx.doi.org/10.1093/mnrasl/slu136

from Nature