Where Are the r-modes? Chandra Observations of Millisecond Pulsars

• Published in: The Astrophysical journal Vol. 840; no. 2; pp. 94 - 101
• Main Authors: Mahmoodifar, Simin, Strohmayer, Tod
• Format: Journal Article
• Language: English
• Published: Goddard Space Flight Center The American Astronomical Society 10.05.2017; IOP Publishing
• Subjects: AMPLITUDES; Astrophysics; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; Binary stars; COMPARATIVE EVALUATIONS; Deposition; EMISSION; Emission analysis; Fluxes; HYDROGEN; KEV RANGE; MAGNETIC FIELDS; MASS; Millisecond pulsars; NEUTRON STARS; Neutrons; OSCILLATIONS; PULSARS; pulsars: individual (PSR J1640+2224, PSR J1709+2313); SMALL ANGLE SCATTERING; SOFT X RADIATION; Soft x rays; Spectral analysis; Spectrum analysis; SPIN; stars: neutron; stars: oscillations; Surface temperature; SURFACES; X ray binaries; X ray stars; X-RAY DIFFRACTION; X-ray fluxes; X-rays: stars; Oscillations; Stars - Pulsars; X Rays; Stars: Neutron
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aa6d62

We present the results of Chandra observations of two non-accreting millisecond pulsars, PSRs J1640+2224 (J1640) and J1709+2313 (J1709), with low inferred magnetic fields and spin-down rates in order to constrain their surface temperatures, obtain limits on the amplitude of unstable r-modes in them, and make comparisons with similar limits obtained for a sample of accreting low-mass X-ray binary (LMXB) neutron stars. We detect both pulsars in the X-ray band for the first time. They are faint, with inferred soft X-ray fluxes (0.3-3 keV) of 6 × 10−15 and 3 × 10−15 erg cm−2 s−1 for J1640 and J1709, respectively. Spectral analysis assuming hydrogen atmosphere emission gives global effective temperature upper limits (90% confidence) of 3.3-4.3 × 105 K for J1640 and 3.6-4.7 × 105 K for J1709, where the low end of the range corresponds to canonical neutron stars (M = 1.4 M ), and the upper end corresponds to higher-mass stars (M = 2.21 M ). Under the assumption that r-mode heating provides the thermal support, we obtain dimensionless r-mode amplitude upper limits of 3.2-4.8 × 10−8 and 1.8-2.8 × 10−7 for J1640 and J1709, respectively, where again the low end of the range corresponds to lower-mass, canonical neutron stars (M = 1.4 M ). These limits are about an order of magnitude lower than those we derived previously for a sample of LMXBs, except for the accreting millisecond X-ray pulsar SAX J1808.4-3658, which has a comparable amplitude limit to J1640 and J1709.