A puzzling paucity of double peaked X-ray pulsars

Abstract

Abstract. Accretion powered pulsars exhibit a variety of lightcurves. In this paper we propose to classify the observed lightcurves as single or double pulsed. We analyze the lightcurves of 89 accretion powered pulsars and assign them to these classes. We present three datasets: first in which the classification can be easily done, second for which the classification is more difficult and not certain, and third for which we were unable to classify the pulsar because of a lack of published data. We analyze a simple model in which the angles between the magnetic and rotation axis β, and between the rotation axis and the line of sight θ are random, and show that it is inconsistent with the data. We also present a model in which the angle between the magnetic axis and the rotation axis is restricted and compare it with the data. This leads to an upper limit on the angle β < 50• . We conclude that there must be a mechanism that leads to alignment of the magnetic and spin axis in X-ray pulsars.Key words. X-rays: star -stars: neutron The lightcurves of accretion powered X-ray pulsarsAccreting neutron stars were discovered more than 30 years ago (Shklovsky 1967), with Cen X-3 beeing the first one discovered that showed pulsations (Giacconi et al. 1971). Currently we know nearly one hundred accreting neutron stars, and in more than eighty of them pulsations were identified (Liu et al. 2000(Liu et al. , 2001. Accreting neutron stars in binaries exhibit a wide range of X-ray light curves. They vary as a function of the photon energy, and moreover in the transient sources the pulse shapes change with the variation of the luminosity. The pulse period in accreting sources is identified with the rotation of a magnetized star. As the matter from the companion star falls onto the neutron star it is channeled onto the magnetic poles by the strong magnetic field of the neutron star. Thus the polar caps and/or accretion columns are the places where most of the emission takes place. Several theoretical models of radiation of magnetized accretion powered neutron stars have been proposed and the beam shape is usually described in terms of pencil beams, when most of the radiation is emitted along the magnetic field, or fan beams when most of the radiation is emitted perpendicularly to the magnetic field.The magnetized radiative transfer is solved using a difference scheme (Meszaros & Nagel 1985;Bulik et al. 1992), or using Monte Carlo scheme (Lamb et al. 1990; Send offprint requests to: T. Bulik, e-mail: bulik@camk.edu.pl Isenberg et al. 1998), for a review see . In the pencil beam model radiation from each polar cap produces one pulse in the light curve. Depending on the emission cap physics and the strength of the magnetic field each pulse may have some additional structure. When during the rotation of an accreting pulsar we see two polar caps the lightcurve should exhibit two distinct pulses (peaks), and if only one cap is seen then the lightcurve is single peaked. In the case of dominant radiation from the accretion column (fan ...

Constraining compactness and magnetic field geometry of X-ray pulsars from the statistics of their pulse profiles

“…This shows that the statistics of pulse profiles does not allow us to constrain the neutron star compactness. In contrast to the previous claims by Bulik et al (2003), the data also do not require the magnetic inclination to be confined in a narrow interval.…”

“…A significant number of light curves are produced in an non-appropriate energy range, which makes it difficult to conclude anything about the number of poles visible to the observer and details of the beam shape. Bulik et al (2003) have studied the profiles of 88 pulsars (not 89 because one of the sources, 1WGA J1958.2+3232, turned out to be an intermediate polar, Negueruela et al 2000), which have been divided into three different groups. The first group consisted of 46 pulsars, which were easy to classify.…”

Revisiting the spectral and timing properties of 4U 1909+07 with NuSTAR and Astrosat

“…The emission from one or both the poles of the neutron star produces a relatively complex pulse profile depending on the geometrical and gravitation effects (Kraus et al 1995;Bulik et al 2003;Lutovinov & Tsygankov 2009;Sasaki et al 2012). The absorption of soft X-ray photons emitted from the accretion column by the accreting material complicates the profile further.…”

The variable cyclotron line in GX 301-2

“…Detailed models for the emission pattern of XRBs, which include relativistic light propagation (Kraus et al 1996;Blum & Kraus 2000), as well as the statistical analysis of accreting neutron star pulse profiles (Bulik et al 2003), however, imply β < ∼ 20…”