加载中…MeasuringtheSpinoftheGalacticCenterSupermassiveBlackHolewithTwoPulsars
(2024-12-16 08:58:18) Measuring the Spin of the Galactic Center
Supermassive
Black Hole with Two Pulsars
By 刘树岩 on Sun,
2024-12-08
Future
observations with next-generation large-area radio telescopes are
expected to discover radio pulsars closelyorbiting around Sagittarius
A*, the supermassive black hole dwelling at our Galactic Center. A
key science project
of the Square Kilometre Array
(SKA) aims to provide high-precision measurements of the spacetime
in the environ-
ment around the Galactic
Center. The discovery of highly magnetized neutron stars, called
pulsars, in this environ-
ment will allow us to initiate
novel tests of Einstein's general relativity and black hole
spacetime. The spinning of Sagittarius A* is vital to characterize
the supermassive black hole and it is crucial in testing
the
“cosmic censorship conjecture”
and “no hair theorem” of black hole spacetime in Einstein's general
relativity. Pre-
vious studies have shown that
the spin of Sagittarius A* could be measured within 1 percent
relative uncertainty
by timing radio signals from
pulsars. However, these measurements depend on the discovery of a
pulsar in a very
compact orbit of less than
half a year, a challenging prospect from observation. In order to
alleviate the problem,
a recent study, carried out by
Zexin Hu and Prof. Lijing Shao from the Kavli Institute for
Astronomy and Astrophy-
sics at Peking University,
provides a novel method of combining the observation of two pulsars
with larger orbital
periods in the Galactic Center
to precisely measure the spin of Sagittarius A*. The work has been
published in the
journal Physical Review
Letters.
Measuring the spin of
Sagittarius A* with a single pulsar suffers from the so-called
leading-order degeneracy among
spin parameters. Previous
studies thus require a pulsar with a very short orbital period—that
has strong relativistic
effects—and long observation
time span to break the degeneracy. However, this new study shows
that by properly
combining the timing of a
second pulsar, one can determine the spin of the central black hole
only with their mea-
surables that are available
with a relative short observation. With the numerical timing model
based on the post-Newtonian equations of motion, Zexin Hu and
Lijing Shao car-
ried out extensive numerical
simulations of observations. Their results suggest that combing
pulsars with orbital
periods at the range of 2 to 5
years can already reach the measurement precision expected in
previous studies.
The observation of the second
pulsar can efficiently break the parameter degeneracy and lead to
an improvement
of the measurement precision
of spin parameters by about two orders of magnitude compared to the
single-pulsar
observation of the first
pulsar. By modeling the Galactic Center pulsar population, the work
points out that in future observations, it is more
likely
to find two pulsars with large
orbital periods rather than finding a pulsar with very tight orbit.
Thus, this study pro-
vides important guidance to
the analysis of future pulsar data and creates a new possibility of
precisely studying
the supermassive black hole
and its environment in our Galaxy. This work was supported by the
National SKA Program of China, the National Natural Science
Foundation of China,
and the Max Planck Partner
Group Program funded by the Max Planck Society. Link to the journal
paper:
https://link.aps.org/doi/10.1103/PhysRevLett.133.231402
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