Monday, April 18, 2016

Fermi Telescope Preps to Narrow Down Gravitational Wave Sources

This visualization shows gravitational waves emitted by two black holes (black spheres) of nearly equal mass as they spiral together and merge. Credit: NASA/J. Bernard Kelly (Goddard), Chris Henze (Ames) and Tim Sandstrom (CSC Government Solutions LLC)

On Sept. 14, waves of energy traveling for more than a billion years gently rattled space-time in the vicinity of Earth. The disturbance, produced by a pair of merging black holes, was captured by the Laser Interferometer Gravitational-Wave Observatory (LIGO) facilities in Hanford, Washington, and Livingston, Louisiana. This event marked the first-ever detection of gravitational waves and opens a new scientific window on how the universe works.

Less than half a second later, the Gamma-ray Burst Monitor (GBM) on NASA's Fermi Gamma-ray Space Telescope picked up a brief, weak burst of high-energy light consistent with the same part of the sky. Analysis of this burst suggests just a 0.2-percent chance of simply being random coincidence. Gamma-rays arising from a black hole merger would be a landmark finding because black holes are expected to merge “cleanly,” without producing any sort of light.

“This is a tantalizing discovery with a low chance of being a false alarm, but before we can start rewriting the textbooks we’ll need to see more bursts associated with gravitational waves from black hole mergers,” said Valerie Connaughton, a GBM team member at the National Space, Science and Technology Center in Huntsville, Alabama, and lead author of a paper on the burst now under review by The Astrophysical Journal.

Detecting light from a gravitational wave source will enable a much deeper understanding of the event. With its wide energy range and large field of view, the GBM is the premier instrument for detecting light from short gamma-ray bursts (GRBs), which last less than two seconds. They are widely thought to occur when orbiting compact objects, like neutron stars and black holes, spiral inward and crash together. These same systems also are suspected to be prime producers of gravitational waves. 

Currently, gravitational wave observatories possess relatively blurry vision. For the September event, dubbed GW150914 after the date, LIGO scientists could only trace the source to an arc of sky spanning an area of about 600 square degrees, comparable to the angular area on Earth occupied by the United States. 

Assuming the GBM burst is connected to this event, the GBM localization and Fermi's view of Earth combine to reduce the LIGO search area by about two-thirds, to 200 square degrees. With a burst better placed for the GBM’s detectors, or one bright enough to be seen by Fermi’s Large Area Telescope, even greater improvements are possible. 

Black hole mergers were not expected to emit significant X-ray or gamma-ray signals because orbiting gas is needed to generate light. Theorists expected any gas around binary black holes would have been swept up long before their final plunge. For this reason, some astronomers view the GBM burst as most likely a coincidence and unrelated to GW150914. Others have developed alternative scenarios where merging black holes could create observable gamma-ray emission. It will take further detections to clarify what really happens when black holes collide.

NASA's Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy and with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.

Credit: NASA


  1. We are under a typical and peculiar case on the History of Science: Admitting "0.002% chance of simply being this random coincidence" is better/worse that constructing exceptions to our theoretical models predicting "gas around binary black holes would have been swept up long before their final plunge" for producing Gamma-ray Bursts in the detectors of the Fermi Gamma-ray Space Telescope . We are under our typical observational (experimental) dilemma of trying to be independent of admitting/rejecting previous theoretical approaches to data. I am inclined for the second option and I believe that GW 150914 really coincided with Plank's gamma burst indicated to arrive in that date from around [Right Ascension~75 degrees(~5 hours), Declination~-73 degrees] or [Galactic Longitude~285 degrees, Galactic Latitude~-34 degrees] 0.2 seconds after LIGO GW 150914 time of arrival.

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  3. How many times have we in the Astrophysics realm discovered we were either wrong or premature in saying this or that doesn't or shouldn't or can't happen only to find that indeed it does ?! Never say never , I say ...

  4. I wonder if the rotating black holes, or neutron stars line up their rotational axis before they merge? And would it make any difference in the energy released? They might be like big magnets which might affect their behavior as they get closer.

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