GRB 140903A
Credit
X-ray: NASA/CXC/Univ. of Maryland/E.
Troja et al,
Optical: Lowell Observatory's Discovery Channel
Telescope/E.Troja et al.
Illustration: NASA/CXC/M.Weiss
*********************************************************
Gamma-ray bursts,
or GRBs, are some of the most violent and energetic events in the
Universe. Although these events are the most luminous explosions in the
universe, a new study using NASA's Chandra X-ray
Observatory, NASA's Swift satellite and other telescopes suggests that
scientists may be missing a majority of these powerful cosmic
detonations.
Astronomers think that some GRBs are the product of the collision and merger of two neutron stars or a neutron star and a black hole.
The new research gives the best evidence to date that such collisions
will generate a very narrow beam, or jet, of gamma rays. If such a
narrow jet is not pointed toward Earth, the GRB produced by the
collision will not be detected.
Collisions between two neutron stars or a neutron star and black hole are expected to be strong sources of gravitational waves
that could be detected whether or not the jet is pointed towards the
Earth. Therefore, this result has important implications for the number
of events that will be detectable by the Laser Interferometry
Gravitational-Wave Observatory (LIGO) and other gravitational wave
observatories.
On September 3, 2014, NASA's Swift observatory picked up a GRB -
dubbed GRB 140903A due to the date it was detected. Scientists used
optical observations with the Gemini Observatory telescope in Hawaii to
determine that GRB 140903A was located in a galaxy about 3.9 billion light years away, relatively nearby for a GRB.
The large panel in the graphic is an illustration
showing the aftermath of a neutron star merger, including the
generation of a GRB. In the center is a compact object - either a black
hole or a massive neutron star - and in red is a disk of material left
over from the merger, containing material falling towards the compact
object. Energy from this infalling material drives the GRB jet shown in
yellow. In orange is a wind of particles blowing away from the disk and
in blue is material ejected from the compact object and expanding at
very high speeds of about one tenth the speed of light.
The image on the left of the two smaller panels shows an optical view
from the Discovery Channel Telescope (DCT) with GRB 140903A in the
middle of the square and a close-up X-ray view from Chandra on the
right. The bright star in the optical image is unrelated to the GRB.
The gamma-ray blast lasted less than two seconds. This placed it into
the "short GRB" category, which astronomers think are the output from
neutron star-neutron star or black hole-neutron star collisions
eventually forming either a black hole or a neutron star with a strong
magnetic field. (The scientific consensus is that GRBs that last longer
than two seconds result from the collapse of a massive star.)
About three weeks after the Swift discovery of GRB 140903A, a team of
researchers led by Eleonora Troja of the University of Maryland,
College Park (UMD), observed the aftermath of the GRB in X-rays with
Chandra. Chandra observations of how the X-ray emission from this GRB
decreases over time provide important information about the properties
of the jet.
Specifically, the researchers found that the jet is beamed into an
angle of only about five degrees based on the X-ray observations, plus
optical observations with the Gemini Observatory and the DCT and radio
observations with the National Science Foundation's Karl G. Jansky Very
Large Array. This is roughly equivalent to a circle with the diameter of
your three middle fingers held at arms length. This means that
astronomers are detecting only about 0.4% of this type of GRB when it
goes off, since in most cases the jet will not be pointed directly at
us.
Previous studies by other astronomers had suggested that these
mergers could produce narrow jets. However, the evidence in those cases
was not as strong because the rapid decline in light was not observed at
multiple wavelengths, allowing for explanations not involving jets.
Several pieces of evidence link this event to the merger of two
neutron stars, or between a neutron star and black hole. These include
the properties of the gamma ray emission, the old age and the low rate
of stars forming in the GRB's host galaxy and the lack of a bright supernova. In some previous cases strong evidence for this connection was not found.
New studies have suggested that such mergers could be the production site of elements
heavier than iron, such as gold. Therefore, the rate of these events is
also important to estimate the total amount of heavy elements produced
by these mergers and compare it with the amounts observed in the Milky Way galaxy.
A paper describing these results was recently accepted for publication in The Astrophysical Journal and is available online.
The first author of this paper is Eleonora Troja and the co-authors are
T. Sakamoto (Aoyama Gakuin University, Japan), S.Cenko (GSFC), A. Lien
(University of Maryland, Baltimore), N. Gehrels (GSFC), A. Castro-Tirado
(IAA-CSIC, Spain), R. Ricci (INAF-Istituto di Radioastronomia, Italy),
J. Capone, V. Toy, & A. Kutyrev (UMD), N. Kawai (Tokyo Institute of
Technology, Japan), A. Cucchiara (GSFC), A. Fruchter (STScI),
J.Gorosabel (UMD), S. Jeong (IAA-CSIC), A. Levan (University of Warwick,
UK), D. Perley (University of Copenhagen, Denmark), R.Sanchez-Ramirez
(Instituto de Astrof 퀱sica de Andaluc 퀱a, Spain), N.Tanvir
(University of Leicester, UK), S. Veilleux (UMD).
NASA's Marshall Space Flight Center in Huntsville, Alabama, manages
the Chandra program for NASA's Science Mission Directorate in
Washington. The Smithsonian Astrophysical Observatory in Cambridge,
Massachusetts, controls Chandra's science and flight operations.
Fast Facts for GRB 140903A:
Scale: X-ray image is 15 arcsec across (about 244,00 light years)
Category: Miscellaneous Objects, Black Holes
Coordinates (J2000): RA 15h 52m 03.27s | Dec +27° 36' 09.30"
Constellation: Corona Borealis
Observation Date: 06 Sep and 18 Sep 2014
Observation Time: 22 hours 13 min.
Obs. ID: 15873, 15986
Instrument: ACIS
References: Troja, E. et al, 2016, ApJ (accepted); arXiv:1605.03573
Color Code: X-ray (Blue), Optical (Yellow)
Distance Estimate: About 3.9 billion light years (z=0.351)
Source: NASA’s Chandra X-ray Observatory
