From CfA: “Shocks in a Distant Gamma-Ray Burst”

Smithsonian Astrophysical Observatory
Smithsonian Astrophysical Observatory

September 11, 2015
No Writer Credit

Gamma ray bursts (GRBs)–flashes of high-energy light occur about once a day, randomly, from around the sky–are the brightest events in the known universe. While a burst is underway, it is many millions of times brighter than an entire galaxy. Astronomers are anxious to decipher their nature not only because of their dramatic energetics, but also because their tremendous brightness enables them to be seen across cosmological distances and times, providing windows into the young universe.

This all-sky map shows the locations of Swift’s 500 gamma-ray bursts, color coded by the year in which they occurred. In the background, an infrared image shows the location of our galaxy and its largest satellites. Credit: NASA/Swift/Francis Reddy

NASA SWIFT Telescope

There appear to be two general types of GRBs: those associated with the deaths of massive stars, and ones believed to originate from the coalescence of two extreme objects (neutron stars or black holes) that had been orbiting each other in a binary system. In general the two types can be distinguished by the lengths of their bursts, the former lasting longer than a few seconds, while the latter are briefer. Astronomers think that, despite the differences, both kinds of GRBs have hot discs accreting material leading to the production of bipolar jets of charged particles moving at relativistic speeds. In the standard model, shocks internal to the fireball produce the gamma-rays in the first (longer duration) case, while shocks from the jets’ interactions with the external medium produce the initial burst of gamma-rays in the second case. Many details are similar in both scenarios, however, while some others vary according to the type, and astronomers have been trying to constrain these various parameters so that they can trace the origin of each GRB more precisely.

CfA astronomer Raffaella Margutti and her colleagues used several ground-based telescopes to follow-up a GRB event that went off in June of 2014, examining the afterglow from about three days after the detection to about one hundred and twenty days later. They conclude that the burst is associated with a massive star’s death (a supernova), but find that some of its emission apparently results from shocks external to the fireball as are seen in the less luminous class of GRBs. The results are consistent with the predictions of supernova modeling, but the fact that this object spans both classes highlights the complexity of the sometimes-overlapping physical processes at work and the importance of observations at multiple wavelengths.

“GRB 140606B/iPTF14bfu: Detection of Shock-Breakout Emission from a Cosmological γ-Ray Burst,” Zach Cano, A. de Ugarte Postigo, D. Perley, T. Kruhler, R. Margutti, M. Friis, D. Malesani, P. Jakobsson, J. P. U. Fynbo, J. Gorosabel, J. Hjorth, R. Sanchez-Ramırez, S. Schulze, N. R. Tanvir, C. C. Thone, and D. Xu, MNRAS 452, 1535, 2015.

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About CfA

The Center for Astrophysics combines the resources and research facilities of the Harvard College Observatory and the Smithsonian Astrophysical Observatory under a single director to pursue studies of those basic physical processes that determine the nature and evolution of the universe. The Smithsonian Astrophysical Observatory (SAO) is a bureau of the Smithsonian Institution, founded in 1890. The Harvard College Observatory (HCO), founded in 1839, is a research institution of the Faculty of Arts and Sciences, Harvard University, and provides facilities and substantial other support for teaching activities of the Department of Astronomy. The long relationship between the two organizations, which began when the SAO moved its headquarters to Cambridge in 1955, was formalized by the establishment of a joint center in 1973. The CfA’s history of accomplishments in astronomy and astrophysics is reflected in a wide range of awards and prizes received by individual CfA scientists.

Today, some 300 Smithsonian and Harvard scientists cooperate in broad programs of astrophysical research supported by Federal appropriations and University funds as well as contracts and grants from government agencies. These scientific investigations, touching on almost all major topics in astronomy, are organized into the following divisions, scientific departments and service groups.