On the Nature of the X-ray Emission of Accreting Millisecond Pulsar SAX J1808.4-3658
By J. Poutanen, Marek Gierlinski. 2002
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We study the pulse profiles of the accreting X-ray millisecond pulsar SAX J1808.4-3658 at different energies. The two main emission components, the blackbody and the Comptonized tail, which are clearly identified in the time-averaged spectrum, show strong variability with the first component lagging the second one. The observed variability can be explained if the emission is produced by Comptonization in a hot slab (radiative shock) of Thomson optical depth ∼0.3-1 at the neutron star surface. The blackbody radiation is strongly beamed along the normal to the slab (a 'knife'-like or 'pencil'-like emission pattern), while the Comptonized emission has a broader angular distribution peaking at about 50°-60° from the slab normal (a 'fan'-like pattern). We construct a detailed model of the X-ray production accounting for the Doppler boosting, relativistic aberration and gravitational light bending in the Schwarzschild space-time. We present also accurate analytical formulae for computations of the light curves from rapidly rotating neutron stars using the formalism recently developed by Beloborodov. Our model reproduces well the pulse profiles at different energies simultaneously, corresponding phase lags, as well as the time-averaged spectrum. We constrain the compact star mass to be bounded between 1.2 and 1.6 M O .. By fitting the observed profiles, we determine the radius of the compact object to be R ∼11 km if M = 1.6 M O ., while for M = 1.2 M O . the best-fitting radius is ∼6.5 km, indicating that the compact object in SAX J1808.4-3658 can be a strange star. We obtain a lower limit on the inclination of the system of 65°.
Subfields: Astronomy and Astrophysics, Geophysics
View Paper: https://www.semanticscholar.org/paper/ce323de0e5dc4eb92d51750ee78a89f57617a945
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Citations in the corpus
- Constraining the Neutron Star Mass–Radius Relation and Dense Matter Equation of State with NICER. I. The Millisecond Pulsar X-Ray Data Set (2019, background): “…depend on R and M in different ways (Pechenick et al. 1983; Strohmayer 1992; Page 1995; Miller & Lamb 1998; Braje et al. 2000; Beloborodov 2002; Poutanen & Gierliński 2003; Cadeau et al. 2007; Morsink et al. 2007; Lo et al. 2013; Psaltis & Özel 2014; Psaltis et al. 2014; Miller & Lamb 2015).”
- The Radius of the High-mass Pulsar PSR J0740+6620 with 3.6 yr of NICER Data (2024, background): “5 See also https://xmmweb.esac.esa.int/docs/documents/ CAL-TN-0018.pdf. e.g., Miller & Lamb 1998; Nath et al. 2002; Poutanen & Gierli´nski 2003; Cadeau et al. 2007; Morsink et al. 2007; Lo et al. 2013; AlGendy & Morsink 2014; Bog-danov et al. 2019; Watts 2019).”
- Thermonuclear Burst Oscillations (2012, background): “…of papers explore the effects of (for example) different metrics, rotation, and stellar oblateness (Pechenick, Ftaclas & Cohen 1983; Chen & Shaham 1989; Strohmayer 1992; Braje, Romani & Rauch 2000; Poutanen & Gierliński 2003; Cadeau, Leahy & Morsink 2005; Cadeau et al. 2007; Morsink et al. 2007).”
- Constraining the Neutron Star Mass–Radius Relation and Dense Matter Equation of State with NICER. II. Emission from Hot Spots on a Rapidly Rotating Neutron Star (2019, methodology): “…factors are combined, the spectral flux from a surface element ¢dS seen by an observer at distance D is (see, e.g., the derivation in Section 3.1 of Poutanen & Gierliński 2003) d a a a y = - ¢ ¢ ¢ ¢ ¢ dF E R R I E d d dS D 1 , cos cos cos , 20S 1 2 3 2( ) ( ) ( ) ( ) where the surface area…”
- A NICER View of PSR J0030+0451: Millisecond Pulsar Parameter Estimation (2019, background): “…accreting material is channeled by the magnetic field onto the magnetic polar caps and the pulsed emission has two main components: thermal emission from the heated vicinity of the accretion impact zones, and nonthermal emission from the shock in the accretion funnel (Poutanen & Gierliński 2003).”
- Non‐linear X‐ray variability in X‐ray binaries and active galaxies (2005, background): “Done & Gierlinski 2003; Poutanen & Gierlinski 2003; Gilfanov, Revnivtsev &Molkov 2003 and see also Uttley 2004), makes it appear unlikely that the emission c© 0000 RAS, MNRAS 000, 000–000 Non-linear X-ray variability in XRBs and AGN 15 process itself is the origin of the non-linearity and rms-flux…”
- Thermal and Bulk Comptonization in Accretion-powered X-Ray Pulsars (2006, background): “Poutanen & Gierliński (2003) computed X-ray pulsar spectra based on the thermal Comptonization of soft radiation in a hot layer above the magnetic pole, but their model did not include a complete treatment of the bulk process, which is of central importance in X-ray pulsars.”
- Accretion to magnetized stars through the Rayleigh–Taylor instability: global 3D simulations (2008, unknown): “…of photons by high-energy electrons may lead to only a small departure of the lightcurve from the thermal ones obtained using the approximation of isotropic black-body radiation (Poutanen & Gierliński 2003) so that the QPO features may survive comptonization (see, however, Titarchuk et al. 2007).”
- Quark Deconfinement and Implications for the Radius and the Limiting Mass of Compact Stars (2004, background): “…associated with compact X-ray sources seems to indicate in some cases the existence of neutron stars with large radii in the range 12–20 km and in other cases the existence of compact stars with small radii in the range 6–9 km (Bombaci 1997, 2003; Li et al. 1999a; Poutanen & Gierlinski 2003).”