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Upper Bounds on r-Mode Amplitudes from Observations of Low-Mass X-Ray Binary Neutron StarsWe present upper limits on the amplitude of r-mode oscillations and gravitational-radiation-induced spin-down rates in low-mass X-ray binary neutron stars, under the assumption that the quiescent neutron star luminosity is powered by dissipation from a steady-state r-mode. For masses <2M solar mass we find dimensionless r-mode amplitudes in the range from about 1×10(exp−8) to 1.5×10(exp−6). For the accreting millisecond X-ray pulsar sources with known quiescent spin-down rates, these limits suggest that approx. less than 1% of the observed rate can be due to an unstable r-mode. Interestingly, the source with the highest amplitude limit, NGC 6440, could have an r-mode spin-down rate comparable to the observed, quiescent rate for SAX J1808−3658. Thus, quiescent spin-down measurements for this source would be particularly interesting. For all sources considered here, our amplitude limits suggest that gravitational wave signals are likely too weak for detection with Advanced LIGO. Our highest mass model (2.21M solar mass) can support enhanced, direct Urca neutrino emission in the core and thus can have higher r-mode amplitudes. Indeed, the inferred r-mode spin-down rates at these higher amplitudes are inconsistent with the observed spin-down rates for some of the sources, such as IGR J00291+5934 and XTE J1751−305. In the absence of other significant sources of internal heat, these results could be used to place an upper limit on the masses of these sources if they were made of hadronic matter, or alternatively it could be used to probe the existence of exotic matter in them if their masses were known.
Document ID
20140005665
Acquisition Source
Goddard Space Flight Center
Document Type
Reprint (Version printed in journal)
External Source(s)
doi:10.1088/0004-637X/773/2/140
Authors
Mahmoodifar, Simin (Maryland Univ. College Park, MD, United States)
Strohmayer, Tod (NASA Goddard Space Flight Center Greenbelt, MD United States)
Date Acquired
May 13, 2014
Publication Date
August 20, 2013
Publication Information
Publication: The Astrophysical Journal
Publisher: The Astrophysical Journal
Volume: 773
Issue: 2
Subject Category
Astrophysics
Report/Patent Number
GSFC-E-DAA-TN11747
Funding Number(s)
CONTRACT_GRANT: DEFG02-93ER-40762
Distribution Limits
Public
Copyright
Public Use Permitted.
Keywords
gravitational waves
neutron
dense matter

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