General Relativistic Constraints on Emission Models of Anomalous X-Ray Pulsars

Most models of anomalous X-ray pulsars (AXPs) account for the observed X-ray spectra and pulsations by means of radiation processes that occur on the surfaces of neutron stars. For any such model, general relativistic deflection of light severely suppresses the amplitude of the observed pulsations. We calculate the expected pulsation amplitudes of AXPs according to various models and compare the results with observations. We show that the high (≲70%) pulse amplitudes observed in some AXPs can be accounted for only if the surface emission is localized (spot radius < 40°) and strongly beamed (e.g., radially peaked with cosn θ' and n ≳ 2, where θ' is the angle to the normal). These constraints are incompatible with those cooling and magnetar models in which the observed X-rays originate as thermal emission from the neutron star surface. Accretion models, on the other hand, are compatible with observations for a wide range of parameters. Finally, definitive conclusions cannot be reached on magnetospheric models, since their localization and beaming properties are not well understood.

[1]  R. Mignani,et al.  A search for the optical/infrared counterpart of the anomalous X-ray pulsar 1E 1841-045 , 2001 .

[2]  M. Kerkwijk,et al.  An optical counterpart to the anomalous X-ray pulsar 4U0142+61 , 2000, Nature.

[3]  R. Perna,et al.  Emission Spectra of Fallback Disks around Young Neutron Stars , 1999, astro-ph/9912297.

[4]  L. Hernquist,et al.  An Accretion Model for Anomalous X-Ray Pulsars , 1999, astro-ph/9912137.

[5]  Pavlov,et al.  The Compact Central Object in Cassiopeia A: A Neutron Star with Hot Polar Caps or a Black Hole? , 1999, The Astrophysical journal.

[6]  G. Vasisht,et al.  On the Spin History of the X-Ray Pulsar in Kes 73: Further Evidence for an Ultramagnetized Neutron Star , 1999, astro-ph/9906122.

[7]  A. Melatos.  Bumpy Spin-Down of Anomalous X-Ray Pulsars: The Link with Magnetars , 1999, astro-ph/9904403.

[8]  J. Heyl,et al.  Do magnetars glitch? Timing irregularities in anomalous X-ray pulsars , 1998, astro-ph/9808125.

[9]  K. Torii,et al.  Discovery of a 7 Second X-Ray Pulsar, AX J1845.0–0300 , 1998 .

[10]  J. Heyl,et al.  Almost analytic models of ultramagnetized neutron star envelopes , 1998, astro-ph/9805175.

[11]  A. Harding,et al.  Pulsar X-Ray and Gamma-Ray Pulse Profiles: Constraint on Obliquity and Observer Angles , 1998, astro-ph/9802038.

[12]  L. Stella,et al.  New limits on the orbital parameters of 1E 1048.1–5937 and 1E 2259+586 from RXTE observations , 1997, astro-ph/9710337.

[13]  J. Heyl,et al.  The Thermal Evolution of Ultramagnetized Neutron Stars , 1997, astro-ph/9710218.

[14]  K. Torii,et al.  Discovery of an 11-s X-Ray Pulsar in the Galactic-Plane Section of the Scorpius Constellation , 1997 .

[15]  J. Heyl,et al.  Powering Anomalous X-Ray Pulsars by Neutron Star Cooling , 1997, astro-ph/9708179.

[16]  G. Chabrier,et al.  Cooling Neutron Stars with Accreted Envelopes , 1997 .

[17]  Christopher Thompson,et al.  The Soft Gamma Repeaters as Very Strongly Magnetized Neutron Stars. II. Quiescent Neutrino, X-Ray, and Alfvén Wave Emission , 1996 .

[18]  J. Swank,et al.  Pulse frequency changes of 1E 2259+586 and the binary interpretation , 1996 .

[19]  C. Thompson,et al.  The soft gamma repeaters as very strongly magnetized neutron stars - I. Radiative mechanism for outbursts , 1995 .

[20]  S. Mereghetti,et al.  The Very low mass X-ray binary pulsars: A New class of sources? , 1995 .

[21]  C. Hellier.  A ROSAT observation of the X-ray pulsars X0142 + 614 and X0146 + 612 , 1994 .

[22]  Dany Page.  Surface temperature of a magnetized neutron star and interpretation of the ROSAT data. 1: Dipole fields , 1994 .

[23]  H. Ruder,et al.  Fitting pulse profiles of X-ray pulsars : the effects of relativistic light deflection , 1993 .

[24]  C. Pethick,et al.  Neutron star crusts. , 1993, Physical review letters.

[25]  Christopher Thompson,et al.  Formation of very strongly magnetized neutron stars - Implications for gamma-ray bursts , 1992 .

[26]  J. Crivello,et al.  The electron beam‐induced cationic polymerization of epoxy resins , 1992 .

[27]  R. Wiringa,et al.  Equation of state for dense nucleon matter. , 1988, Physical review. C, Nuclear physics.

[28]  P. Mészáros,et al.  X-ray pulsar models. II. Comptonized spectra and pulse shapes , 1985 .

[29]  C. Ftaclas,et al.  Hot spots on neutron stars - The near-field gravitational lens , 1982 .

[30]  W. Nagel.  Radiative transfer in a strongly magnetized plasma. I - Effects of anisotropy. II - Effects of Comptonization , 1981 .

[31]  P. Ghosh,et al.  Accretion by rotating magnetic neutron stars. II. Radial and vertical structure of the transition zone in disk accretion. , 1979 .

[32]  V. Pandharipande,et al.  Nuclear matter calculations with mean scalar fields , 1975 .

[33]  V. Pandharipande,et al.  A model neutron solid with π0 condensate , 1975 .

[34]  V. Pandharipande.  Dense neutron matter with realistic interactions , 1971 .