Tag: Fermi Gamma-ray Space Telescope

The search for a powerful natural particle accelerator

Earth is almost constantly beset by a stream of particles from space called cosmic rays. These particles consist of protons, bundles of two protons and two neutrons each (alpha particles), a small number of heavier atomic nuclei and a smaller fraction of anti-electrons and anti-protons. Cosmic rays often have high energy – typically up to half of 1 GeV. One GeV is almost the amount of energy that a single proton has at rest. The Large Hadron Collider (LHC) itself can accelerate protons up to 7,000 GeV.

But this doesn’t mean cosmic rays are feeble: historically, some detectors have recorded high-energy and very-high-energy cosmic rays. The most energetic cosmic ray – dubbed the “oh my god” particle – was a proton recorded over Utah in 1991 with an energy of around 3 x 1012 GeV, which is around three-billion-times higher than the energy to which the LHC can accelerate protons today. This proton was travelling at 99.9% the speed of light in vacuum. This is a phenomenal amount of energy – about as much kinetic energy as a baseball moving at 95 km/hr but concentrated into the volume of a proton, which has 1042-times less space in which to hold that energy.

Detectors have also spotted some cosmic-ray events with energies exceeding 1,000,000 GeV – or 1 PeV. They’re uncommon compared to all cosmic-ray events but relatively more common than the likes of the “oh my god” particle. Physicists are interested in them because they indicate the presence of a natural particle accelerator somewhere in the universe that’s injecting protons with ginormous amounts of energy and sending them blasting off into space. One term for such natural accelerators seemingly capable of accelerating protons to 0.1-1 PeV is ‘PeVatron’. And the question is: where can we find a PeVatron?

There are three broad sources of cosmic rays: from the Sun, from somewhere in the Milky Way galaxy and from somewhere beyond the galaxy. Most of the cosmic rays we have detected have been from the latter two sources. In fact, there’s a curious feature called the ‘knee’ that physicists believe could distinguish between these sources. If you plot the number of cosmic rays on the y-axis and the energies of the cosmic rays on the x-axis, you’ll find yourself looking at the famous Swordy plot:

The Swordy plot of cosmic-rays flux versus energy. The yellow zone accounts for solar cosmic rays, the blue zone for galactic cosmic rays and the pink zone for extragalactic cosmic rays. Credit: Sven Lafebre/Wikimedia Commons, CC BY-SA 3.0

As you can see, the plot shows a peculiar bump, an almost imperceptible change in slope, when transitioning from the blue to the pink zones – this is the ‘knee’. Physicists have interpreted the cosmic rays above the knee to be from within the Milky Way and those below to be from outside the galaxy, although why this is so isn’t clear.

Before cosmic rays interact with other particles in their way, they’re called primary cosmic rays. After their interaction, such as the atoms and molecules in Earth’s upper atmosphere, they produce a shower of secondary particles; these are the secondary cosmic rays. Physicists can get a tighter fit on the potential source of primary cosmic rays by analysing the direction at which they strike the atmosphere, the composition of the secondary cosmic rays, and the energies of both the primary and the secondary rays. This is why we suspect supernovae are one source of within-the-galaxy cosmic rays, with some possible mechanisms of action.

One, for example, is shockfront acceleration: a proton could get trapped between two shockwaves from the same supernova. As the outer wave slows and the inner wave charges in, the proton could bounce rapidly between the two shockfronts and emerge greatly energised out of a gap. However, we don’t know what fraction of cosmic rays, at different energies, supernovae can account for.

Potential extragalactic sources include active galactic nuclei – the centres of galaxies, including the neighbourhood of supermassive black holes – and the extremely powerful gamma-ray bursts. Physicists have associated them with cataclysmic events like neutron-star mergers and the formative events of black-holes.

However, exercises to triangulate the sources of high-energy cosmic rays are complicated by galactic magnetic fields (which curve the paths of charged particles). A proton accelerated by the shockfront mechanism could also bump into some other particle as it emerges, producing a flash of gamma rays that physicists can look for – but only if they have a way to isolate it from other sources of gamma rays in a supernova’s vicinity. This is difficult work.

Researchers from the US recently analysed gamma-ray data collected by the Fermi Gamma-ray Space Telescope (FGST), in low-Earth orbit, of the supernova remnant G106.3+2.7. Astrophysicists have suspected that this object could be a PeVatron for more than a decade, and the US research team used FGST data to check if they the suspicion could be true. The difficult bit? The data spanned 12 years.

In 2008, physicists recorded very high energy (100-100,000 GeV) gamma rays from G106.3+2.7, located around 800 parsec (2,600 lightyears) away. The US research team figured that they could have been produced in two ways. Let’s call them Mechanism A and Mechanism B. Physicists already know Mechanism A is associated with cosmic rays while Mechanism B is not. The US team members used 12 years of data to characterise gamma-ray, X-ray and radio emissions around the remnant so they could determine which mechanism could have been responsible for all of them the way they have been observed, with the gamma rays as secondary cosmic rays.

The team’s analysis found that the theory of Mechanism A almost exactly accounted for the energies of the gamma rays from the remnant while also accommodating the other radiation – whereas the theory of Mechanism B couldn’t explain the gamma rays and the remnant’s X-ray emissions together. In effect, the team had a way to justify the idea that G106.3+2.7 could be a PeVatron.

Mechanism B is inverse Compton scattering by relativistic electrons. Inverse Compton scattering is when high-energy electrons collide with low-energy photons and the photons gain energy (in regular Compton scattering, the electrons gain energy). When this model couldn’t account for the gamma-ray emissions, the team invoked a modified version involving two sets of electrons, with each set accelerated to different energies by different mechanisms. But the team found that the FGST data continued to disfavour the involvement of leptons, and instead preferred the involvement of hadrons. Leptons – like electrons – are particles that don’t interact with other particles through the strong nuclear force. Hadrons, on the other hand, do, and they were implicated in Mechanism A: the decay of neutral pions.

Pions are the lightest known hadrons and come in three types: π+, π0 and π–. Neutral pions are π0. They have a very short lifetime, around 85 attoseconds – that’s 0.000000000000000085 seconds. And when they decay, they decay into gamma rays, i.e. high-energy photons.

Some 380,000 years after the Big Bang, a series of events in the universe left behind some radiation that survives to this day. This relic radiation is called the cosmic microwave background, a sea of photons in the microwave frequency pervading the cosmos. When a cosmic-ray proton collides with one of these photons, a delta-plus baryon is formed that then decays into a proton and a neutral pion. The neutral pion then decays to gamma rays, which are detectable as secondary cosmic rays.

Source: Wikipedia/’Greisen–Zatsepin–Kuzmin limit’

Knowing the energy of the gamma rays allows physicists to work back to the energy of the cosmic ray. And according to the team’s calculations, the 2009 gamma-ray emission indicates G106.3+2.7 could be a PeVatron. As the team’s preprint paper concluded,

“… only a handful, out of hundreds of radio-emitting supernova remnants, have been observed to emit very high energy radiation with a hard spectrum. The scarcity of PeVatron candidates and the rareness of remnants with very high energy emission make … G106.3+2.7 a unique source. Our study provides strong evidence for proton acceleration in this nearby remnant, and by extension, supports a potential role for G106.3+2.7-like supernova remnants in meeting the challenge of accounting for the observed cosmic-ray knee using galactic sources”.

Featured image: An artist’s impression of supernova 1993J. Credit: NASA, ESA and G. Bacon (STScI).

Why this ASTROSAT instrument could be a game-changer for high-energy astrophysics

On November 17, NASA announced its Swift satellite had recorded its thousandth gamma-ray burst (GRB), an important milestone that indicates how many of these high-energy explosions, sometimes followed by the creation of blackholes, happen in the observable universe and in what ways.

Some five weeks before the announcement, Swift had observed a less symbolically significant GRB called 151006A. Its physical characteristics as logged and analysed by the satellite were quickly available, too, on a University of Leicester webpage.

On the same day as this observation, on October 6, the 50-kg CZTI instrument onboard India’s ASTROSAT space-borne satellite had come online. Like Swift, CZTI is tuned to observe and study high-energy phenomena like GRBs. And like every instrument that has just opened its eyes to the cosmos, ISRO’s scientists were eager to do something with it to check if it worked according to expectations. The Swift-spotted GRB 151006A provided just the opportunity.

CZTI stands for Cadmium-Zinc-Telluride Imager – a compound of these three metals (the third is tellurium) being a known industrial radiation detector. And nothing releases radiation as explosively as a GRB, which have been known to outshine the light of whole galaxies in the few seconds that they last. The ISRO scientists pointed the CZTI at 151006A and recorded observations that they’d later compare against Swift records and see if they matched up. A good match would be validation and a definite sign that the CZTI was working normally.

It was working normally, and how.

NASA has two satellites adept at measuring high-energy radiation coming from different sources in the observable universe – Swift and the Fermi Gamma-ray Space Telescope (FGST). Swift is good at detecting incoming particles that have an energy of up to 150 keV, but not so good at determining the peak energy of hard-spectrum emissions. In astrophysics, spectral hardness is defined as the position of the peak – in power emitted per decade in energy – in the emission spectrum of the GRB. This spectrum is essentially a histogram of the number of particles with some values of a property that strike a detector, so a hard-spectrum emission has a well-defined peak in that histogram. An example:

The plot of argon dense plasma emission is a type of histogram – where the intensity of photons is binned according to the energies at which they were observed. Credit: Wikimedia Commons
The plot of argon dense plasma emission is a type of histogram – where the intensity of photons is binned according to the energies at which they were observed. Credit: Wikimedia Commons

FGST, on the other hand, is better equipped to detect emissions higher than 150 keV but not so much at quickly figuring out where in the sky the emissions are coming from. The quickness is important because GRBs typically last for a few seconds, while a subcategory of them lasts for a few thousandths of a second, and then fade into a much duller afterglow of X-rays and other lower-energy emissions. So it’s important to find where in the sky GRBs could be when the brighter flash occurs so that other telescopes around the world can better home in on the afterglow.

This blindspot between Swift and FGST is easily bridged by CZTI, according to ISRO. In fact, per a deceptively innocuous calibration notice put out by the organisation on October 17, CZTI boasts the “best spectral [capabilities] ever” for GRB studies in the 80-250 keV range. This means it can provide better spectral studies of long GRBs (which are usually soft) and better localisation for short, harder GRBs. And together, they make up a strong suite of simultaneous spectral and timing observations of high-energy phenomena for the ASTROSAT.

There’s more.

Enter Compton scattering

The X-rays and gamma rays emanating from a GRB are simply photons that have a very low wavelength (or, very high frequency). Apart from these characteristics, they also have a property called polarisation, which describes the plane along which the electromagnetic waves of the radiation are vibrating. Polarisation is very important when studying directions along long distances in the universe and how the alignment of intervening matter affects the path of the radiation.

All these properties can be visualised according to the wave nature of radiation.

But in 1922, the British physicist Arthur Compton found that when high-frequency X-rays collided with free electrons, their frequency dropped by a bit (because some energy was transferred to the electrons). This discovery – celebrated for proving that electromagnetic radiation could behave like particles – also yielded an equation that let physicists calculate the angle at which the radiation was scattered off based on the change in its frequency. As a result, instruments sensitive to Compton scattering are also able to measure polarisation.

Observed count profile of Compton events during GRB 151006A. Source: IUCAA
Observed count profile of Compton events during GRB 151006A. Source: IUCAA

This plot shows the number of Compton scattering events logged by CZTI based on observing GRB 151006A; zero-time is the time at which the GRB triggered the attention of Swift. That CZTI was able to generate this plot was evidence that it could make simultaneous observations of timing, spectra and polarisation of high-energy events (especially in X-rays, up to 250 keV), lessening the burden on ISRO to depend on multiple satellites for different observations at different energies.

The ISRO note did clarify that no polarisation measurement was made in this case because about 500 Compton events were logged against the 2,000 needed for the calculation.

But that a GRB had been observed and studied by CZTI was broadcast on the Gamma-ray Coordinates Network:

V. Bhalerao (IUCAA), D. Bhattacharya (IUCAA), A.R. Rao (TIFR), S. Vadawale (PRL) report on behalf of the Astronaut CZTI collaboration:

Analysis of Astronaut commissioning data showed the presence of GRB 151006A (Kocevski et al. 2015, GCN 18398) in the Cadmium Zinc Telluride Imager. The source was located 60.7 degrees away from the pointing direction and was detected at energies above 60 keV. Modelling the profile as a fast rise and exponential decay, we measure T90 of 65s, 775s and 50s in 60-80 keV, 80-100 keV and 100-250 keV bands respectively.

In addition, the GRB is clearly detected in a light curve created from double events satisfying Compton scattering criteria (Vadawale et al, 2015, A&A, 578, 73). This demonstrates the feasibility of measuring polarisation for brighter GRBs with CZTI.

That CZTI is a top-notch instrument doesn’t come as a big surprise: most of ASTROSAT’s instruments boast unique capabilities and in some contexts are the best on Earth in space. For example, the LAXPC (Large Area X-ray Proportional Counter) instrument as well as NASA’s uniquely designed NuSTAR space telescope both log radiation in the 6-79 keV range coming from around blackholes. While NuSTAR’s spectral abilities are superior, LAXPC’s radiation-collecting area is 10x as much.

On October 7-8, ISRO also used CZTI to observe the famous Cygnus X-1 X-ray source (believed to be a blackhole) in the constellation Cygnus. The observation was made coincidental to NuSTAR’s study of the same object in the same period, allowing ISRO to calibrate CZTI’s functioning in the 0-80 (approx.) keV range and signalling the readiness of four of the six instruments onboard ASTROSAT.

The two remaining instruments: the Ultraviolet Imaging Telescope will switch on on December 10 and the Soft X-ray Telescope, on December 13. And from late December to September 2016, ISRO will use the satellite to make a series of observations before it becomes available to third-parties, and finally to foreign teams in 2018.

The Wire
November 21, 2015