Stars Have Found A New Way To Die

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Gamma-Ray Bursts (GRBs) are brief and intense flashes of gamma-ray radiation that can occur randomly from any direction of the sky. They are so powerful that a single such event can be as luminous as all the visible stars in the sky, but just for a few seconds.

They were discovered first during the cold war, when the USA were busy verifying the Nuclear Ban Treaty, since for those primitive detectors, GRBs had properties akin to those of the atmospheric atomic explosions. Fortunately, our atmosphere is opaque to gamma rays, which has allowed preserving life on the Earth. But this remarkable property has as a counterpart that GRBs can only be detected by instruments on board of spacecrafts such as NASAs Swift satellite.

Swift localizes GRBs and distributes their coordinates to astronomers all over the world, who can follow up these explosive events using ground-based telescopes. These observations have shown that GRBs are accompanied by fading emissions from ultraviolet to radio wavelengths, the so-called “afterglow”. This afterglow emission is usually produced by synchrotron radiation emitted by charged particles (mostly electrons and positrons) moving in magnetic fields at ultra-relativistic speeds: velocities above 99.9999% of the speed of light (note that differently from the fractious neutrinos from Gran Sasso, this particles are subluminal).

On Christmas Day 2010 a very peculiar GRB occurred, designated GRB101225A according to the date of its discovery, also nicknamed “the Christmas Burst”. It lasted more than half an hour and, in addition to its extraordinary duration (typically GRBs last a few seconds), it promptly attracted scientists’ attention the fact that the spectrum displayed a thermal contribution (like a classical black body) unusually powerful. Indeed, the thermal component was so powerful that dominates completely the X-rays-to-ultraviolet emission of this object, challenging the long-standing paradigm that GRB afterglows are produced non-thermal emission processes (like synchrotron).

Artist’s impression of the Christmas Burst. The merging of the helium and neutron star produces a broad torus, plus two jets aligned with the rotation axis of the system. The jets interact with the previously ejected torus causing the observed black body spectrum.

Credit: Asociación RUVID

An international group of researchers led by Christina Thöne and Antonio de Ugarte Postigo from the Instituto de Astrofisíca de Andalucía (Granada), in collaboration with Miguel Ángel Aloy and Petar Mimica from the Universitat de València have found a plausible explanation to the connandrum posed by the Christmas Burst, which has been published in the prestigious journal Nature. “This burst was really a puzzle to us and it had a lot of bizzare properties, such that we were investigating a wide range of possible explanations even including a Galactic origin” explains C. Thöne.

 “The fact that in the same GRB we see no classical afterglow, a hot thermal component, an unprecedentedly dim supernova explosion, and extremely long activity in gamma-rays alerted us about the very peculiar nature of this GRB” adds de Ugarte. In fact, on the basis of the large set of space and ground-based observations, “we were almost forced to propose a new scenario to explain this exotic explosive event” claims C. Thöne: GRB101225A is the result of a neutron star merging with the helium core of an evolved giant star, at a distance from Earth of about 5.500 million light-years (redshift z ~ 0.3).

This exotic binary system underwent a phase in which the neutron star penetrated the atmosphere of the giant star, producing such a disturbance that the giant star expelled most of its external envelop. Miguel A. Aloy points “the journey of the neutron star trough the stellar interior ended dramatically, when it merged with the massive stellar core. The result of such a fusion is a gigantic explosion inside of the star, therefore initially invisible from the Earth”. The tremendous amount of energy released by the explosion is channeled away from the stellar center by means of a pair of plasma jets, moving in opposite directions at almost the speed of light, which need a few minutes before they break out of the previously ejected envelope, unveiling them for our detectors.

Along the way, the jets are thermalized, giving rise to the observed black body spectrum. “The idea that it can all be explained as if an extremely energetic jet had to find its way through a narrow hole in the debris ejected by the initial interaction of the neutron star with the Helium core was triggered by our previous experience modeling relativistic jets” explains Aloy, “though we had to discard many other alternatives and choose what initially seamed less plausible” emphasizes Mimica. A careful checking of the afterglow spectrum revealed, according to C. Thöne, “that the initially very hot jet material was expanding quickly until it reached a size similar to the Saturn orbit around the Sun and, simultaneously, it was cooling down from 1 million K immediately after the GRB down to only 5000 K two weeks later”.

Finally, about 10 days after the GRB, a faint supernova explosion started to emerge, reaching its maximum luminosity 40 days after the GRB and dominating the fading blackbody radiation. ”It was reassuring for us that the late component we were able to observe fit optimally with our templates for supernova explosions of Type Ic if the supernova had taken place right at the distance we had previously obtained by fitting the properties of the fading blackbody” states de Ugarte. The fact that the proposed helium-neutron star scenario predicts the production of a modest amount of radioactive nickel that would lead to a weak supernova component, “makes even more consistent our model with the observations” declares M. A. Aloy.

Even after many years of research GRBs still hold new surprises waiting for us. Similar to the increasing diversification of supernova classes, the classification of GRBs might have to be revisited. Stars seem to find many different ways to die.

Contacts and sources:
Asociación RUVID