Artist's conception of a gamma ray burst. Jet
of fast-moving material is propelled outward through spherical shell of
ejected material from initial explosion of a massive star and its
collapse into a black hole. Credit: Bill Saxton, NRAO/AUI/NSF. Hi-res image
Series of radio images of FIRST J1419+3940 from 1993 to 2017 show its slow fade.
Credit: Law et al., Bill Saxton, NRAO/AUI/NSF
Credit: Law et al., Bill Saxton, NRAO/AUI/NSF
Astronomers comparing data from an ongoing major survey of the sky
using the National Science Foundation’s Karl G. Jansky Very Large Array
(VLA) to data from earlier surveys likely have made the first discovery
of the afterglow of a powerful gamma ray burst that produced no gamma
rays detectable at Earth. The unprecedented discovery of this “orphan”
gamma ray burst (GRB) offers key clues to understanding the aftermath of
these highly energetic events.
“GRBs emit their gamma rays in narrowly focused beams. In this case,
we believe the beams were pointed away from Earth, so gamma ray
telescopes did not see this event. What we found is the radio emission
from the explosion’s aftermath, acting over time much as we expect for a
GRB,” said Casey Law, of the University of California, Berkeley.
While searching through data from the first epoch of observing for
the VLA Sky Survey (VLASS) in late 2017, the astronomers noted that an
object that appeared in images from an earlier VLA survey in 1994 did
not appear in the VLASS images. They then searched for additional data
from the VLA and other radio telescopes. They found that observations of
the object’s location in the sky dating back as far as 1975 had not
detected it until it first appeared in a VLA image from 1993.
The object then appeared in several images made with the VLA and the
Westerbork telescope in the Netherlands from 1993 through 2015. The
object, dubbed FIRST J1419+3940, is in the outskirts of a galaxy more
than 280 million light-years from Earth.
“This is a small galaxy with active star formation, similar to others
in which we have seen the type of GRBs that result when a very massive
star explodes,” Law said.
The strength of the radio emission from J1419+3940 and the fact that
it slowly evolved over time support the idea that it is the afterglow of
such a GRB, the scientists said. They suggested that the explosion and
burst of gamma rays should have been seen sometime in 1992 or 1993.
However, after searching databases from gamma ray observatories, “We
could find no convincing candidate for a detected GRB from this galaxy,”
Law said.
While there are other possible explanations for the object’s behavior, the scientists said that a GRB is the most likely.
“This is exciting, and not just because it probably is the first
‘orphan’ GRB to be discovered. It also is the oldest well-localized GRB,
and the long time period during which it has been observed means it can
give us valuable new information about GRB afterglows,” Law said.
“Until now, we’ve never seen how the afterglows of GRBs behave at
such late times,” noted Brian Metzger of Columbia University, co-author
of the study. “If a neutron star is responsible for powering the GRB and
is still active, this might give us an unprecedented opportunity to
view this activity as the expanding ejecta from the supernova explosion
finally becomes transparent.”
“I’m delighted to see this discovery, which I expect will be the
first of many to come from the unique investment the National Radio
Astronomy Observatory (NRAO) and the National Science Foundation are
making in VLASS,” said NRAO Director Tony Beasley.
VLASS is the largest observing project in the history of the VLA.
Begun in 2017, the survey will use 5,500 hours of observing time over
seven years. The survey will make three complete scans of the sky
visible from the VLA, roughly 80 percent of the sky. Initial images from
the first round of observations now are available to astronomers.
VLASS follows two earlier sky surveys done with the VLA. The NRAO VLA
Sky Survey (NVSS), like VLASS, was an all-sky survey done from 1993 to
1996, and the FIRST (Faint Images of the Radio Sky at Twenty
centimeters) survey studied a smaller portion of the sky in more detail
from 1993 to 2002. The astronomers discovered FIRST J1419+3940 by
comparing a 1994 image from the FIRST survey to the VLASS 2017 data.
From 2001 to 2012, the VLA underwent a major upgrade, greatly
increasing its sensitivity, or ability to image faint objects. The
upgrade made possible a new, improved survey offering a rich scientific
payoff. The earlier surveys have been cited more than 4,500 times in
scientific papers, and scientists expect VLASS to be a valuable resource
for research in the coming years.
Law and his colleagues are publishing their findings in the Astrophysical Journal Letters.
The National Radio Astronomy Observatory is a facility of the
National Science Foundation, operated under cooperative agreement by
Associated Universities, Inc.
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