COHERENT TIMING OF THE ACCRETING MILLISECOND PULSAR NGC 6440 X-2

We study the 205.9 Hz pulsations of the accreting millisecond X-ray pulsar NGC 6440 X-2 across all outbursts observed with the Rossi X-ray Timing Explorer over a period of 800 days. We find the pulsations are highly sinusoidal with a fundamental amplitude of 5%–15% rms and a second harmonic that is only occasionally detected with amplitudes of ≲2% rms. By connecting the orbital phase across multiple outbursts, we obtain an accurate orbital ephemeris for this source and constrain its 57 minute orbital period to sub-millisecond precision. We do not detect an orbital period derivative to an upper limit of ∣ P ˙ ∣ ≤ 8 × 10 − 11 s s − 1 . ?> We investigate the possibility of coherently connecting the pulse phase across all observed outbursts, but find that due to the poorly constrained systematic uncertainties introduced by a flux-dependent bias in the pulse phase, multiple statistically acceptable phase-connected timing solutions exist.

[1]  Deepto Chakrabarty,et al.  PROSPECTS FOR MEASURING NEUTRON-STAR MASSES AND RADII WITH X-RAY PULSE PROFILE MODELING , 2013, 1311.1571.

[2]  A. Patruno,et al.  1 Hz FLARING IN THE ACCRETING MILLISECOND PULSAR NGC 6440 X-2: DISK TRAPPING AND ACCRETION CYCLES , 2013, 1304.6430.

[3]  T. Strohmayer,et al.  UPPER BOUNDS ON r-MODE AMPLITUDES FROM OBSERVATIONS OF LOW-MASS X-RAY BINARY NEUTRON STARS , 2013, 1302.1204.

[4]  P. Bult,et al.  ACCELERATED ORBITAL EXPANSION AND SECULAR SPIN-DOWN OF THE ACCRETING MILLISECOND PULSAR SAX J1808.4−3658 , 2011, 1111.6967.

[5]  Caroline R. D'Angelo,et al.  Accretion discs trapped near corotation , 2011, 1108.3833.

[6]  A. Patruno,et al.  SPIN EQUILIBRIUM WITH OR WITHOUT GRAVITATIONAL WAVE EMISSION: THE CASE OF XTE J1814-338 AND SAX J1808.4-3658 , 2011, 1106.6264.

[7]  M. Servillat,et al.  DISCOVERY OF A SECOND TRANSIENT LOW-MASS X-RAY BINARY IN THE GLOBULAR CLUSTER NGC 6440 , 2009, 0911.0444.

[8]  R. Wijnands,et al.  DISCOVERY OF A 205.89 Hz ACCRETING MILLISECOND X-RAY PULSAR IN THE GLOBULAR CLUSTER NGC 6440 , 2009, 0911.0435.

[9]  Diego Altamirano,et al.  The long-term evolution of the accreting millisecond X-ray pulsar Swift J1756.9−2508 , 2009, 0910.2920.

[10]  R. Wijnands,et al.  AN ALTERNATIVE INTERPRETATION OF THE TIMING NOISE IN ACCRETING MILLISECOND PULSARS , 2009, 0902.4118.

[11]  R. Taam,et al.  FURTHER CONSTRAINTS ON THERMAL QUIESCENT X-RAY EMISSION FROM SAX J1808.4-3658 , 2008, 0810.0497.

[12]  D. Leahy,et al.  Limits on Mass and Radius for the Millisecond-Period X-Ray Pulsar SAX J1808.4–3658 , 2007, astro-ph/0703287.

[13]  Arnold H. Rots,et al.  Absolute Timing of the Crab Pulsar with the Rossi X-Ray Timing Explorer , 2004 .

[14]  M. Klis,et al.  The Atoll Source States of 4U 1608-52 , 2003, astro-ph/0307041.

[15]  S. Rappaport,et al.  Theoretical Considerations on the Properties of Accreting Millisecond Pulsars , 2003, astro-ph/0304374.

[16]  L. Bildsten.  Hot White Dwarf Donors in Ultracompact X-Ray Binaries , 2002, astro-ph/0208164.

[17]  Juri Poutanen,et al.  On the Nature of the X-ray Emission of Accreting Millisecond Pulsar SAX J1808.4-3658 , 2002, astro-ph/0303084.

[18]  D. Psaltis,et al.  Correlations in Quasi-periodic Oscillation and Noise Frequencies among Neutron Star and Black Hole X-Ray Binaries , 1999, astro-ph/9902130.

[19]  Lars Bildsten,et al.  Gravitational Radiation and Rotation of Accreting Neutron Stars , 1998, astro-ph/9804325.

[20]  J. Greenstein,et al.  Binary Stars among Cataclysmic Variables. II. Nova WZ Sagittae: a Possible Radiator of Gravitational Waves. , 1962 .