Diving below the spin-down limit: Constraints on gravitational waves from the energetic young pulsar PSR J0537-6910. (arXiv:2012.12926v2 [astro-ph.HE] UPDATED)

Diving below the spin-down limit: Constraints on gravitational waves from the energetic young pulsar PSR J0537-6910. (arXiv:2012.12926v2 [astro-ph.HE] UPDATED)
The <a href="http://arxiv.org/find/astro-ph/1/au:+Collaboration_LIGO_Scientific/0/1/0/all/0/1">LIGO Scientific Collaboration</a>, the <a href="http://arxiv.org/find/astro-ph/1/au:+Collaboration_Virgo/0/1/0/all/0/1">Virgo Collaboration</a>, the <a href="http://arxiv.org/find/astro-ph/1/au:+Collaboration_KAGRA/0/1/0/all/0/1">KAGRA Collaboration</a>: <a href="http://arxiv.org/find/astro-ph/1/au:+Abbott_R/0/1/0/all/0/1">R. Abbott</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Abbott_T/0/1/0/all/0/1">T. D. Abbott</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Abraham_S/0/1/0/all/0/1">S. Abraham</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Acernese_F/0/1/0/all/0/1">F. Acernese</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ackley_K/0/1/0/all/0/1">K. Ackley</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Adams_A/0/1/0/all/0/1">A. Adams</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Adams_C/0/1/0/all/0/1">C. Adams</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Adhikari_R/0/1/0/all/0/1">R. X. Adhikari</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Adya_V/0/1/0/all/0/1">V. B. Adya</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Affeldt_C/0/1/0/all/0/1">C. Affeldt</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Agarwal_D/0/1/0/all/0/1">D. Agarwal</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Agathos_M/0/1/0/all/0/1">M. Agathos</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Agatsuma_K/0/1/0/all/0/1">K. Agatsuma</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Aggarwal_N/0/1/0/all/0/1">N. Aggarwal</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Aguiar_O/0/1/0/all/0/1">O. D. Aguiar</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Aiello_L/0/1/0/all/0/1">L. Aiello</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ain_A/0/1/0/all/0/1">A. Ain</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ajith_P/0/1/0/all/0/1">P. Ajith</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Akutsu_T/0/1/0/all/0/1">T. Akutsu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Aleman_K/0/1/0/all/0/1">K. M. Aleman</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Allen_G/0/1/0/all/0/1">G. Allen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Allocca_A/0/1/0/all/0/1">A. Allocca</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Altin_P/0/1/0/all/0/1">P. A. Altin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Amato_A/0/1/0/all/0/1">A. Amato</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Anand_S/0/1/0/all/0/1">S. Anand</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ananyeva_A/0/1/0/all/0/1">A. Ananyeva</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Anderson_S/0/1/0/all/0/1">S. B. Anderson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Anderson_W/0/1/0/all/0/1">W. G. Anderson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ando_M/0/1/0/all/0/1">M. 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We present a search for continuous gravitational-wave signals from the young,
energetic X-ray pulsar PSR J0537-6910 using data from the second and third
observing runs of LIGO and Virgo. The search is enabled by a contemporaneous
timing ephemeris obtained using NICER data. The NICER ephemeris has also been
extended through 2020 October and includes three new glitches. PSR J0537-6910
has the largest spin-down luminosity of any pulsar and is highly active with
regards to glitches. Analyses of its long-term and inter-glitch braking indices
provided intriguing evidence that its spin-down energy budget may include
gravitational-wave emission from a time-varying mass quadrupole moment. Its 62
Hz rotation frequency also puts its possible gravitational-wave emission in the
most sensitive band of LIGO/Virgo detectors. Motivated by these considerations,
we search for gravitational-wave emission at both once and twice the rotation
frequency. We find no signal, however, and report our upper limits. Assuming a
rigidly rotating triaxial star, our constraints reach below the
gravitational-wave spin-down limit for this star for the first time by more
than a factor of two and limit gravitational waves from the $l=m=2$ mode to
account for less than 14% of the spin-down energy budget. The fiducial
equatorial ellipticity is limited to less than about 3e-5, which is the third
best constraint for any young pulsar.

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