HEAPOW: Burst of Knowledge (2013 Nov 25)

[bystander]

HEAPOW: Burst of Knowledge (2013 Nov 25)

A definition of luck is preparation meeting opportunity. Last April 27, astronomers were very lucky when one of the brightest gamma-ray bursts occurred at a time when a full complement of gamma-ray and X-ray observatories, including the Fermi Gamma-ray Space Telescope, Swift, and NASA's newest high-energy observatory, NuSTAR, were able to observe it. This capability allowed astronomers to measure the burst's brightness over a range of one million in energy, and enabled astronomers to pinpoint its location using the Large Area Telescope (LAT) on Fermi (shown above), along with the imaging capabilities of Swift (at soft X-ray energies) and, for the first time, at hard X-ray energies with NuSTAR. Gamma ray bursts like the one that occurred on April 27 mark the explosive transformation of a mssive star into a black hole, and these new data provide an unprecedented look at this process. Astronomers are using these data, along with optical ground-based data from the RAPTOR telescopes, to create a detailed model of the structure of the shock waves which generate the observed emission.

NASA Sees 'Watershed' Cosmic Blast in Unique Detail

NuSTAR Observations of GRB 130427A Establish a Single Component Synchrotron Afterglow Origin for the Late Optical to Multi-GeV Emission

• Astrophysical Journal Letters 779(1) L1 (21 Nov 2013) DOI: 10.1088/2041-8205/779/1/L1
  arXiv.org > astro-ph > arXiv:1311.5245 > 20 Nov 2013

GRB 130427A: A Nearby Ordinary Monster - A. Maselli et al

• Science Express (Online 21 Nov 2013) DOI: 10.1126/science.1242279
  arXiv.org > astro-ph > arXiv:1311.5254 > 20 Nov 2013

The Bright Optical Flash and Afterglow from the Gamma-Ray Burst GRB 130427A - W. T. Vestrand et al

• Science Express (Online 21 Nov 2013) DOI: 10.1126/science.1242316
  arXiv.org > astro-ph > arXiv:1311.5489 > 21 Nov 2013

The First Pulse of the Extremely Bright GRB 130427A: A Test Lab for Synchrotron Shocks - R. Preece et al

• Science Express (Online 21 Nov 2013) DOI: 10.1126/science.1242302
  arXiv.org > astro-ph > arXiv:1311.5581 > 21 Nov 2013

Fermi-LAT Observations of the Gamma-Ray Burst GRB 130427A - Fermi-LAT collaboration, Fermi-GBM collaboration

• Science Express (Online 21 Nov 2013) DOI: 10.1126/science.1242353
  arXiv.org > astro-ph > arXiv:1311.5623 > 22 Nov 2013 (v1)

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Author: Dr. Michael F. Corcoran

[MargaritaMc]

Smithsonian Astrophysical Observatory:
First Confirmed Reverse Shock in a Gamma Ray Burst

Friday, November 29, 2013

Science Update - A look at CfA discoveries from recent journals

Gamma ray bursts (GRBs) are the brightest events in the known universe. These flashes of high-energy light occur about once a day, randomly, from around the sky. 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 brightnesses enables them to be seen across cosmological distances and times, providing windows into the young universe.

The somewhat longer lasting variety of GRB is associated with the death of massive stars. The details of these bursts reflect the nature of the progenitor stars, the structure of the explosion environment, and the composition of the ejecta. Even after the explosion ends, the powerful ejecta generate an afterglow that can be analyzed as the particles plow into the circumstellar material around the progenitor star. Studies of the afterglow find two light signatures: one produced when a forward moving shock slams into the material, and a second kind resulting when a backward moving shock (the "reverse shock") is produced (roughly similar to the way a water wave, encountering an obstacle, spawns a backward moving wave). Both the forward and reverse shocks reveal different details of the cataclysm; the reverse shock is a particularly valuable probe of particle velocities in the burst.

Although in the past there have been some diagnostic hints found of a reverse shock in the radiation, the conclusions were indecisive because they lacked a full decomposition of the radiation into both its forward and reverse components. Each of these components radiates over a very broad band of wavelengths, with each kind if shock characterized by a distinct intensity peak at a different wavelength band. One problem has been that observations over the full range from optical to radio are needed to sort things out, but there is very little time during a burst to collect all the data.

CfA astronomers ... present extensive observations of a burst that occurred on April 27. They obtained data from radio and submillimeter wavelengths through the infrared, optical, ultraviolet, and X-rays, and all in the short period from 0.67 days to 12 days after the burst. Their careful analysis convincingly finds the signature of the reverse shock, and determines the characteristic velocity of the ejecta as being 99.997% of the speed of light - very fast indeed. This benchmark dataset offers a so-far unique view of the reverse shock, helps to confirm theoretical models, and illustrates to power of detailed, multi-wavelength modeling of GRBs.

Reference: 

"A Reverse Shock in GRB 130427A," T. Laskar, E. Berger, B. A. Zauderer, R. Margutti, A. M. Soderberg, S. Chakraborti, R. Lunnan, R. Chornock, P. Chandra, and A. Ray, ApJ 776, 119, 2013.
http://www.cfa.harvard.edu/news/su201347