The more the merrier

Imagine two celestial objects – black holes – colliding and merging to form an even more massive black hole somewhere in deep space. Collisions and mergers such as these would be rather ubiquitous given the billions of black holes that probably dot our vast, perhaps somewhat cavernous Universe if you step just outside of Earth. But of course, with our limited technological prowess, probing the Universe’s mysteries remains out of reach – till early 2016, we had no way of directly detecting such a binary black hole merger.

Then on 11th February, 2016, the Advanced Laser Interferometer Gravitational-Wave Observatory (ALIGO) announced GW 150914, a binary black hole merger occurring on 14th September 2015 that resulted in the verification of General Relativity’s last remaining prediction – the existence of gravitational waves, powerful enough to distort our familiar four-dimensional space-time fabric. The masses involved were immense, the resultant black hole super-massive, and the energy radiated was more than the combined power of all light radiated by all stars in the thus-far observed Universe.

Physicists will tell you though that such a violent astrophysical event can hardly ever have only one tell-tale signature. 0.4 seconds after GW 150914, the Fermi Gamma-ray Burst Monitor (GBM) mounted on the Fermi Gamma-ray Space Telescope (FGST), had recorded a short gamma-ray burst (GRB).

Short GRBs are sources of extremely large amounts of energy concentrated within a very short interval of time and energetic enough to emit as much energy as our Sun does in 10 billion years. GRBs arise typically from a supernova explosion by way of which a high-mass star collapses to form a neutron star or a black hole, or perhaps from two colliding neutron stars.

Gamma-ray bursts consist of gamma-rays which are a form of electromagnetic radiation similar to X-rays, visible light or radio waves, the only difference being their underlying wavelength and the energy they bear. Along with gamma-rays though, GRBs dispense energy also in the form of highly accelerated protons. And where there are accelerated protons, can neutrinos be very far behind?

Neutrinos are uncharged, almost massless, almost non-interacting elementary particles that are everywhere around us. Forget your wonder at the dust particles you see floating in that sunbeam. A trillion neutrinos from the Sun pass through our eyes every second! Being almost non-interacting, they pass unhindered through normal matter. You need a billion kilometers of lead to detect a neutrino! But latest detectors such as the IceCube Neutrino Observatory in South Pole with its thousands of ultra-sensitive sensors buried under a cubic kilometer of dense Antarctic ice are catching up with these elusive particles.

GRBs may produce copious amounts of high-energy neutrinos (HENs). So what information do the HENs possibly associated with the GRB that was recorded 0.4s after GW150914 within the localization uncertainty limits of the GBM give us? That was precisely the question that a team of researchers from Raman Research Institute, Bengaluru, University of Johannesburg and Pennsylvania State University asked. They first theoretically modelled the GBM observation to find the amount of HENs that IceCube should detect from this short GRB and could find none. But this non-result placed an upper limit on the total energy released by this short GRB, which, incidentally turned out to be very small compared to that emitted in gravitational waves by GW150914.

No matter though, because combined HEN, GW and electromagnetic radiation studies of the Universe are here to stay – the more the merrier as the saying goes.

Dr. Nayantara Gupta, RRI says “We look forward to more instances of coinciding detections, because by combining such observations and theoretical modelling, we can learn not only from what we detect, but even what we do not.”

About the author and work:

Dr. Nayantara Gupta is an Associate Professor at the Department of Astronomy and Astrophysics at Raman Research Institute, Bengaluru. Contact: <nayan@rri.res.in>. For the publication see https://arxiv.org/pdf/1602.08436.pdf

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