NASA Missions Help Pinpoint the Source of a Unique X-ray, Radio Burst
This is the first time one of these mysterious, repeating radio burst has been identified in our own galaxy, and the first identification of an object that caused one.
On
April 28, a supermagnetized stellar remnant known as a magnetar blasted out a simultaneous
mix of X-ray and radio signals never observed before. The flare-up included the
first fast radio burst (FRB) ever seen from within our Milky Way galaxy and shows
that magnetars can produce these mysterious and powerful radio blasts
previously only seen in other galaxies.
"Before
this event, a wide variety of scenarios could explain the origin of FRBs,"
said Chris Bochenek, a doctoral student in astrophysics at Caltech who led one
study of the radio event. "While there may still be exciting twists in the
story of FRBs in the future, for me, right now, I think it's fair to say that
most FRBs come from magnetars until proven otherwise."
A
magnetar is a type of isolated neutron star, the crushed, city-size remains of
a star many times more massive than our Sun. What makes a magnetar so special
is its intense magnetic field. The field can be 10 trillion times stronger than
a refrigerator magnet's and up to a thousand times stronger than a typical
neutron star's. This represents an enormous storehouse of energy that
astronomers suspect powers magnetar outbursts.
The
X-ray portion of the synchronous bursts was detected by several satellites,
including NASA's Wind mission.
The
radio component was discovered by the Canadian Hydrogen Intensity Mapping Experiment (CHIME), a radio telescope located
at Dominion Radio Astrophysical Observatory in British Columbia and led by
McGill University in Montreal, the University of British Columbia, and the University
of Toronto.
A
NASA-funded project called Survey for Transient
Astronomical Radio Emission 2 (STARE2) also detected the radio burst seen
by CHIME. Consisting of a trio of detectors in California and Utah and operated
by Caltech and NASA's Jet Propulsion
Laboratory
in Southern California, STARE 2 is led by Bochenek, Shri Kulkarni at Caltech, and
Konstantin Belov at JPL. They determined the burst's energy was comparable to
FRBs.
By
the time these bursts occurred, astronomers had already been monitoring their
source for more than half a day.
Late
on April 27, NASA's Neil Gehrels Swift Observatory spotted a new round of
activity from a magnetar called SGR 1935+2154 (SGR 1935 for short) located in
the constellation Vulpecula. It was the object's most prolific flare-up yet - a storm of rapid-fire X-ray bursts, each lasting less
than a second. The storm, which raged for hours, was picked up at various times
by Swift, NASA's Fermi Gamma-ray Space Telescope, and NASA's Neutron star Interior Composition Explorer (NICER), an X-ray telescope
mounted on the International Space Station.
About
13 hours after the storm subsided, when the magnetar was out of view for Swift,
Fermi and NICER, one special X-ray burst erupted. The blast was seen by the
European Space Agency's INTEGRAL mission, the China National
Space Administration's Huiyan X-ray satellite, and the Russian Konus instrument on Wind. As the half-second-long
X-ray burst flared, CHIME and STARE2 detected the radio burst, which lasted only
a thousandth of a second.
"The
radio burst was far brighter than anything we had seen before, so we
immediately knew it was an exciting event," said Paul Scholz, a researcher
at the University of Toronto's Dunlap Institute for
Astronomy & Astrophysics and a member of the CHIME/FRB Collaboration. "We've
studied magnetars in our galaxy for decades, while FRBs are an extragalactic
phenomenon whose origins have been a mystery. This event shows that the two
phenomena are likely connected."
Papers
from both the CHIME/FRB Collaboration and the STARE2 team were published on Nov.
4 in the journal Nature.
SGR
1935's distance remains poorly established, with estimates ranging from 14,000
to 41,000 light-years. Assuming it lies at the nearer end of this range, the
X-ray portion of the simultaneous bursts carried as much energy as our Sun
produces over a month. Intriguingly, however, it was not as powerful as some of
the flares in the magnetar's storm eruption.
"The
bursts seen by NICER and Fermi during the storm are clearly different in their spectral
characteristics from the one associated with the radio blast," said George
Younes, a researcher at George Washington University in Washington and the lead
author of two papers analyzing the burst storm that are now undergoing peer
review. "We attribute this difference to the location of the X-ray flare on
the star's surface, with the FRB-associated burst likely occurring at or close
to the magnetic pole. This may be key to understanding the origin of the
exceptional radio signal."
SGR
1935's radio burst was thousands of times brighter than any radio emissions
from magnetars in our galaxy. If this event had occurred in another galaxy, it would
have been indistinguishable from some of the weaker FRBs observed.
In
addition, the radio pulse arrived during an X-ray burst, something that has
never before been seen in association with FRBs. Taken together, the
observations strongly suggest that SGR 1935 produced the Milky Way's equivalent
of an FRB, which means magnetars in other galaxies likely produce at least some
of these signals.
For ironclad proof of the magnetar connection,
researchers ideally would like to find an FRB outside of our galaxy that
coincides with an X-ray burst from the same source. This combination may only
be possible for nearby galaxies, which is why CHIME, STARE2 and NASA's
high-energy satellites will keep watching the skies.
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