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EI2GYB > ASTRO 06.09.21 10:29l 89 Lines 4858 Bytes #999 (0) @ WW
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Subj: New Observations Challenge Popular Radio Burst Model
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Sent: 210906/0925Z @:EI2GYB.DGL.IRL.EURO #:14150 BPQ6.0.22
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New Observations Challenge Popular Radio Burst Model
Strange behavior caught by two radio observatories may send theorists back to
the drawing board.
Fourteen years ago, the first fast radio burst (FRB) was discovered. By now,
many hundreds of these energetic, millisecond-duration bursts from deep space
have been detected (most of them by the CHIME radio observatory in British
Columbia, Canada), but astronomers still struggle to explain their enigmatic
properties. A new publication in this week's Nature "adds a new piece to the
puzzle," says Victoria Kaspi (McGill University, Canada). "In this field of
research, surprising twists are almost as common as new results."
Most astronomers agree that FRBs are probably explosions on the surfaces of
highly magnetized neutron stars (so-called magnetars). But it's unclear why
most FRBs appear to be one-off events, while others flare repeatedly. In some
cases, these repeating bursts show signs of periodicity, and scientists had
come up with an attractive model to explain this behavior, involving stellar
winds in binary systems.
However, new observations by European radio telescopes may rule out this model.
Astronomers knew that FRB 20180916B, located in a galaxy some 460 million
light-years away, produces multiple bursts about every 16 days, during a
'window' that lasts for a few days. "The idea was that the magnetar is part of
a binary system with a 16.29-day period," says In‚s Pastor-Marazuela
(University of Amsterdam and ASTRON, The Netherlands), the first author of the
new paper. If the companion star had a thick stellar wind that absorbs radio
waves, the bursts would only be visible when the magnetar was on 'our' side of
the orbit, she explains.
However, simultaneous observations of FRB 20180916B by the Low-Frequency Array
(LOFAR) and the 14-dish Westerbork Synthesis Radio Telescope (WSRT) in the
Netherlands challenge the predictions of this model. Since stellar winds should
better absorb lower-frequency radio waves than higher-frequency ones,
astronomers expected that the bursts observed by LOFAR (down to 110 megahertz)
would only be visible in a narrower time window than the bursts observed by
WSRT (at around 1.4 gigahertz). "We found the exact opposite," says coauthor
Joeri van Leeuwen (University of Amsterdam and ASTRON, The Netherlands).
Moreover, the peak in the number of high-frequency bursts preceded the
low-frequency peak by a few days, which also isn't expected in the binary wind
model.
"I agree that the observations are challenging for the model," says Kaspi,
who's part of a team that has independently studied the LOFAR data (which are
publicly available) but didn't have access to the simultaneous Westerbork
observations. However, she's not yet convinced that the binary idea is
completely ruled out. "We need more sources and better statistics."
What could be an alternative explanation? Perhaps, says Pastor-Marazuela, the
16.29-day period is actually the rotation period of the burst source, instead
of its orbital period. If the explosions originate in a small, localized region
of the magnetar's surface, this region will be carried in and out of sight by
the compact object's rotation.
Kaspi counters that a rotation period of 16.29 days would be incredibly long:
Magnetars (and neutron stars in general) usually complete tens, hundreds, or
even a few thousand revolutions per minute. "But nature can be very creative,"
she adds. "Never say never."
FRB 20180916B could be a very unusual case, says van Leeuwen. In particular, he
is surprised by the fact that no single burst was detected by both LOFAR and
WSRT, even though the two facilities were observing simultaneously. "It's
something I had never expected," he says. But even if this particular source is
special, it could shed more light on the properties of FRBs in general. "Think
of Oliver Sacks, the famous neurologist," van Leeuwen says. "He learned a lot
about the human brain by studying his most interesting patients."
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