A new star in the Christmas sky

Posted by fi17 at Dec 01, 2011 12:13 PM |
University of Leicester scientists involved in ‘Christmas Burst’ discovery
A new star in the Christmas sky

Credit: A. Simonnet, NASA, E/PO, Sonoma State University

Issued by University of Leicester Press Office, Thursday 1 December 2011

A large, hi-res version of the accompanying image is available. Please email pressoffice@le.ac.uk

The image shows an 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 blackbody spectrum.

A Christmas Day explosion investigated by scientists at the University of Leicester has led to the advance of a new theory on how it occurred - a merger of two stars in a cosmic embrace. 

A Gamma Ray Burst (GRB)- the most powerful phenomena in the Universe- was investigated by a team of scientists after it was detected by satellites on Christmas Day 2010.  Writing in the international science journal, Nature, they propose that the GRB is the result of a neutron star merging with the helium core of an evolved giant star. 

An international group of researchers, led by Christina Thöne from the Instituto de Astrofisica de Andalucia and including Kim Page from the Department of Physics and Astronomy, University of Leicester, investigated the GRB which has been called ‘The Christmas Burst.” 

Appropriately enough, Kim was watching the Christmas special of Dr Who when the alert came of the out-of-this world phenomenon. 

Dr Page said: “The explosion 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.” 

Based on a large set of space (specifically Swift) and ground-based observations, the researchers propose 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 GRB 101225A require a different model altogether. 

They propose that GRB 101225A 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 thermalised by its interaction with the dense envelope which had been previously ejected, giving rise to the observed blackbody spectrum. This observed blackbody cooled down progressively from 1 million K immediately after the burst to ~ 5000K 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 was discovered and dominating the fading blackbody radiation. 

As part of the Swift-XRT team, Kim Page led the X-ray analysis, finding that the X-ray data showed evidence for a blackbody component at early times. She said, “I was on call as the X-ray Burst Scientist over Christmas last year. The gamma-ray skies had been quiet all day, and I was happily watching the Dr Who Christmas special, when my phone beeped to alert me to the detection of a burst. GRB 101225A turned out to be so interesting that I didn't mind being distracted from the end of Dr Who!” 

Swift has now been in orbit for just over 7 years now, and has detected more than 600 GRBs. However, even after all these years, GRBs are still able to surprise 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. 





For interviews contact Dr Kim Page, University of Leicester, on 0116 223 1706

Email: kpa@star.le.ac.uk

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