Tag: gamma-ray bursts

A shorter article about short gamma ray bursts lights up little

  • Identify a simple and well-defined question
  • Describe the question and answer it
  • Get the fuck out

Writing with these three rules in mind makes for a good science article. You stick to the point, you know what details to include and what to leave out and, most importantly, you set straightforward expectations and meet them. The overall effect is for the reader to walk away feeling not disappointed. That’s always a happy ending.

Sadly, not everyone writes like this – rather, more broadly, not all news publishers think of science articles this way. For example, The Hindu regularly publishes science articles so packed with information – about the study as much as its authors – that you’re left confused about what you just read. Was it a profile or was it an explainer? It doesn’t matter because it failed either way.

The latest example of this kind of writing is an article about short gamma ray bursts. The binary neutron star merger known by the gravitational-wave event designation GW170817 was expected by astrophysicists to have unleashed a short gamma ray burst at the moment of collision – but data obtained of the event shows no signs of the expected radio signature. A group of scientists led by Kunal Mooley from Oxford University suggested this could be because GW170817 released a new kind of gamma ray burst.

BusinessLine (a business newspaper with the same publisher and top management as The Hindu) carried an article attempting to discuss all this. Sample the opening para, a mulch of facts and inaccuracies:

Screen Shot 2017-12-23 at 09.18.40

“The one located on the outskirts of Pune”? Sounds like everyone must know about it even if they don’t. “First-ever detection of gravitation waves”? Not really: gravitation waves, a.k.a. gravity waves and unlike gravitational waves, can be observed in Earth’s atmosphere. Also, the first-ever detection of gravitational waves came last year; what came in August was the first-ever detection of a neutron star merger. The three US scientists won the award for building LIGO, not detecting GW170817.

The rest of the article tries to simultaneously explain Mooley and co.’s interpretation of the data and also provide a glimpse of his educational trajectory. Why would I want to know he studied in Pune and Mumbai? Unless this is because the author wanted to drive home the India connection – which is all the more troubling because it plays up an aspect of the researcher’s identity that is irrelevant to their professional accomplishment. I’ve noticed many publications succumbing to this kind of thinking: if researcher is Indian, cover the paper/study/whatever irrespective of the legitimacy, strength and/or novelty of what they’re saying.

The science ought to take precedent, not the researcher’s identity. But when it doesn’t, you typically end up writing something that’s definitely not news and likely trash. You end up wrapping your national pride around a core of stupidity. I recommend the pages of Scoopwhoop, The Quint, The Better India, DailyO, The Times of India and The New Indian Express, among others, for examples. It’s also possible that the author was conscious about providing an India connection so readers in India took the article more seriously. I’ve made noise about such behaviour many times before, such as here: science shouldn’t be assessed, or enjoyed, solely according to what it can do for humankind.

Finally, it’s possible that the newspaper itself wanted to establish all details on record for posterity – but AFAIK, the BusinessLine is not a newspaper of record. This of course is a minor point.

By ditching the extraneous details, the author and the editor could’ve had the space to focus on the science more, using better language and without the painful economy of words it’s currently striving to. They could even have devoted some words to discussing whether other astronomers have disputed Mooley’s interpretation (they have), an exercise that would’ve made the article more reliable than it is. And to those who’re saying the article was probably kept short because there might not have been space in the newspaper, I’ve a bigger complaint: why wasn’t a short version published in print and a longer version online?

In all, I don’t think BusinessLine is taking its science journalism seriously. The time is past when they could’ve gotten ahead simply by being one of the few publications in the country to write articles about short gamma ray bursts. But given the complacency with which the article seems to have been composed and edited, maybe that time shouldn’t have existed in the first place. It surely doesn’t now.

Featured image: An artist’s illustration of a bright gamma-ray burst. Caption and credit: NASA/Wikimedia Commons.

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