Analyzing X-ray pulsar profiles: geometry and beam pattern of A 0535+26

📝 Abstract

We applied a decomposition method to the energy dependent pulse profiles of the accreting binary pulsar A 0535+26, in order to identify the contribution of the two magnetic poles of the neutron star and to obtain constraints on the geometry of the system and on the beam pattern. We analyzed pulse profiles obtained from RXTE observations in the X-ray regime. Basic assumptions of the method are that the asymmetry observed in the pulse profiles is caused by non-antipodal magnetic poles and that the emission regions have axisymmetric beam patterns. Constraints on the geometry of the pulsar and a possible solution of the beam pattern are given. We interpreted the reconstructed beam pattern in terms of a geometrical model of a hollow column plus a halo of scattered radiation on the neutron star surface, which includes relativistic light deflection.

💡 Analysis

We applied a decomposition method to the energy dependent pulse profiles of the accreting binary pulsar A 0535+26, in order to identify the contribution of the two magnetic poles of the neutron star and to obtain constraints on the geometry of the system and on the beam pattern. We analyzed pulse profiles obtained from RXTE observations in the X-ray regime. Basic assumptions of the method are that the asymmetry observed in the pulse profiles is caused by non-antipodal magnetic poles and that the emission regions have axisymmetric beam patterns. Constraints on the geometry of the pulsar and a possible solution of the beam pattern are given. We interpreted the reconstructed beam pattern in terms of a geometrical model of a hollow column plus a halo of scattered radiation on the neutron star surface, which includes relativistic light deflection.

📄 Content

arXiv:1012.3077v1 [astro-ph.HE] 14 Dec 2010 Astronomy & Astrophysics manuscript no. 14728 c⃝ESO 2018 March 13, 2018 Analyzing X-ray pulsar profiles: geometry and beam pattern of A 0535+26 I. Caballero1, U. Kraus2, A. Santangelo3, M. Sasaki3, and P. Kretschmar4 1 CEA Saclay, DSM/IRFU/SAp –UMR AIM (7158) CNRS/CEA/Universit´e P. Diderot, Orme des Merisiers Bat. 709, 91191 Gif-sur- Yvette, France e-mail: isabel.caballero@cea.fr 2 Institut f¨ur Physik und Technik, Universit¨at Hildesheim, Marienburger Platz 22, 31141, Hildesheim, Germany 3 Institute f¨ur Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, Eberhard Karls Universit¨at T¨ubingen, Sand 1, 72076 T¨ubingen, Germany 4 ISOC, European Space Astronomy Centre (ESAC), PO Box 78, 28691 Villanueva de la Ca˜nada, Spain ABSTRACT Aims. We applied a decomposition method to the energy dependent pulse profiles of the accreting binary pulsar A 0535+26, in order to identify the contribution of the two magnetic poles of the neutron star and to obtain constraints on the geometry of the system and on the beam pattern. Methods. We analyzed pulse profiles obtained from RXTE observations in the X-ray regime. Basic assumptions of the method are that the asymmetry observed in the pulse profiles is caused by non-antipodal magnetic poles and that the emission regions have ax- isymmetric beam patterns. Results. Constraints on the geometry of the pulsar and a possible solution of the beam pattern are given. We interpreted the recon- structed beam pattern in terms of a geometrical model of a hollow column plus a halo of scattered radiation on the neutron star surface, which includes relativistic light deflection. Key words. X-rays: binaries – Pulsars: individual: A 0535+26

1. Introduction The Be/X-ray binary A 0535+26 was discovered by Ariel V in 1975 (Rosenberg et al. 1975) during a giant outburst. The system consists of a neutron star orbiting the optical com- panion HDE 245770 on an eccentric orbit (e = 0.47 ± 0.02) of orbital period Porb = 111.1 ± 0.3 d (Finger et al. 2006). The source presents quiescent X-ray emission with a luminos- ity of LX ≲1035−36 erg s−1, sometimes interrupted by “nor- mal” outbursts (LX ≈1037 erg s−1) linked to the periastron pas- sages of the neutron star, and less frequent “giant” outbursts (LX

1037 erg s−1) of longer duration and less clearly re- lated to the orbital phase (see, e.g., Giovannelli & Graziati 1992; Kendziorra et al. 1994; Finger et al. 1996). The system presents two cyclotron resonant scattering features at E ∼45 keV and E ∼100 keV, from which a magnetic field strength of B ∼ 4 × 1012 G is inferred (Kendziorra et al. 1994; Grove et al. 1995; Kretschmar et al. 2005; Caballero et al. 2007). Pulsations are observed to have a period of Pspin ∼103 s, typi- cally with spin-up during stronger outbursts and spin-down dur- ing quiescent periods1. The pulse profile evolves from a com- plex profile at lower energies to a simpler, two-peaked structure at higher energies. This behavior is observed in several accret- ing X-ray pulsars. Similar to other sources (e.g., Staubert et al. 1980), individual pulses show strong pulse-to-pulse variations, while the average pulse profile is rather stable, with slower vari- ations over the course of an outburst (Caballero et al. 2008b). The basic concept of pulsed emission is well understood. Pulsed emission originates in regions close to the magnetic poles 1 see, e.g., the results of Fermi-GBM monitoring at http://gammaray.nsstc.nasa.gov/gbm/science/pulsars/ of a rotating neutron star with the magnetic axis misaligned with respect to the rotation axis. In contrast, physical model- ing of the pulsed emission turns out to be a complex task. Many processes are in fact involved in modeling pulse profiles, from the modeling of the emission regions and their local emission pattern to the formation of the pulse profiles seen by a dis- tant observer. Comparison of model calculations with observa- tions has been performed for instance by Wang & Welter (1981), Meszaros & Nagel (1985), and Leahy (1991). A proper model calculation should include relativistic light deflection, which has a significant effect on the pulse shape2 (Riffert & Meszaros 1988). For slowly rotating neutron stars, the metric around a neutron star can be approximated by the Schwarzschild metric (see, e.g., Pechenick et al. 1983). Due to the strong gravitational field around the neutron star, the X-rays will be observed at red-shifted energies. Geometrical models of filled and hollow accretion columns of accreting neutron stars, including relativistic light deflection, were computed in Kraus (2001) and Kraus et al. (2003). These models give the beam pat- tern or energy-dependent flux of one emission region as a func- tion of the angle, as seen by a distant observer. Introducing the rotation of the pulsar and its geometry, i.e., the orientation of the rotation axis with respect to the direction of observation and the location of the two poles,

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