Unusual explanation for peculiar stellar explosion

The so called ”Christmas Burst“, a very energetic stellar explosion that took place a year ago, might have a very unusual origin: an international team of scientists propose a new model for this gamma-ray burst (GRB). This event could have been caused by the merger of a neutron star with the helium core of an evolved giant star, which would not only lead to a GRB-like jet, but also a faint supernova component.

Fig. 1: Artist's impression of the Christmas Burst and the physics involved.
Credit: A. Simonnet, NASA, E/PO, Sonoma State University

Fig. 2: Colour image of the field of GRB 101225A observed 40 days after the explosion with the afterglow marked by a box linkPfeilExtern.gif(movie).
(from Thöne et al, Nature, 480, 72-74)

Gamma-Ray Bursts (GRBs) are brief and intense flashes of gamma-ray radiation that can occur randomly from any direction of the sky. Their durations range from a few milliseconds up to over half an hour. They are so energetic that we can detect them even at distances of thousands of millions of light years. Since our atmosphere is opaque to gamma-ray photons, GRBs are detected by gamma-ray detectors on board spacecraft such as NASAs Swift satellite. Follow-up observations using ground-based telescopes 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 moving in magnetic fields at ultra-relativistic speeds (velocities above 99% of the speed of light).

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, much longer than most GRBs detected so far. Its low-energy emission (i.e., all radiation measured below the gamma-ray regime) was dominated by a hot thermal component - a classical blackbody spectrum, challenging the long-standing paradigm that GRB afterglows are produced by synchrotron radiation.

An international group of researchers, led by Christina Thöne and Antonio de Ugarte Postigo from the Instituto de Astrofisica de Andalucia (IAA — CSIC, Granada, Spain) recently published an article in Nature on the Christmas Burst. The collaboration of researchers also included scientists at the Max Planck Institute for Astrophysics, who contributed to the theoretical interpretation of the data and its explanation by a feasible model. Based on a large set of space and ground-based observations, the team proposes a new scenario to explain this exotic explosive event. The current standard models to explain the two broad types of GRBs that have been observed are the "Compact Binary Merger" model, for short duration (<2 sec) GRBs and the "Collapsar" model, for long duration (>2 sec) GRBs. However, according to Thöne et al, the peculiar properties of GRB101225A require a different model altogether.

They propose that GRB101225A is the result of a neutron star merging with the helium core of an evolved giant star. This somewhat exotic binary system underwent a common envelope phase when the neutron star entered the atmosphere of the giant star, during which the giant star expelled most of its hydrogen envelope. The final explosion created a GRB-like jet, which became thermalized by its interaction with the dense, previously ejected envelope, giving rise to the observed black body spectrum. This ejected material was cooling down progressively from 1 million K immediately after the burst, to ~ 5,000K at 20 days after the event.

Finally, about 10 days after the explosion a faint supernova component started to emerge, reaching its maximum 40 days after the GRB and dominating the fading blackbody radiation. The best fit to this supernova component is a faint broadline Type Ic-like supernova at a distance of 5.5 billion light-years (redshift z~0.3).

The high velocities and the high density of the material make it difficult to observe such an event the large distances, where GRBs are normally seen. This could explain why such an event has been seen only recently.

Original publication:

Thöne et al., "The unusual gamma-ray burst GRB 101225A from a helium star/neutron star merger at redshift 0.33", linkPfeilExtern.gifNature, 480, (issue 7375), 72-74 (2011)


Dr. Hannelore Hämmerle
Press officer
Max Planck Institute for Astrophysics
Tel.: +49 89 30000-3980
Email: prmpa-garching.mpg.de

Dr. Christina C. Thöne
Tel.: +34 958 230 612
Email: cthoeneiaa.es