On timing and spectral characteristics of the X-ray pulsar 4U 0115+63: Evolution of the pulsation period and the cyclotron line energy

Abstract

An overview of the results of observations for the transient X-ray pulsar 4U 0115+63, a member of a binary system with a Be star, since its discovery to the present day (∼40 years) based on data from more than dozen observatories and instruments is presented. A overall light curve and the history of change in the spin frequency of the neutron star over the entire history of its observations, which also includes the results of recent measurements made by the INTEGRAL observatory during the 2004, 2008, and 2011 outbursts, are provided. The source's energy spectra have also been constructed from the INTEGRAL data obtained during the 2011 outburst for a dynamic range of its luminosities (10 37 − 7 × 10 37 erg s −1 ). We show that apart from the fundamental harmonic of the cyclotron absorption line at energy ∼11 keV, its four higher harmonics at energies ≃ 24, 35.6, 48.8, and 60.7 keV are detected in the spectrum. We have performed a detailed analysis of the source's spectra in the 4-28 keV energy band based on all of the available RXTE archival data obtained during bright outbursts in 1995 − 2011. We have confirmed that modifying the source's continuum model can lead to the disappearance of the observed anticorrelation between the energy of the fundamental harmonic of the cyclotron absorption line and the source's luminosity. Thus, the question about the evolution of the cyclotron absorption line energy with the luminosity of the X-ray pulsar 4U 0115+63 remains open and a physically justified radiation model for X-ray pulsars is needed to answer it. INTRODUCTIONThe X-ray pulsar 4U 0115+63 has a long history of observations and is well suited for testing models for the evolution of binary systems, models for the structure and evolution of decretion disks around Be stars, and models for the structure of emitting regions (accretion columns) in neutron stars. The source was discovered in the UHURU satellite survey (Giacconi et al., 1972;Forman et al., 1978). Later on, this object was found in the Vela-5B satellite archival data since 1969 (Whitlock et al., 1989). Accurate measurements of its position in the sky were subsequently made by the SAS-3, Ariel-V, and HEAO-1 observatories Johnston et al., 1978). These measurements allowed the optical component in this binary system

High-mass X-ray binaries in the Milky Way

“…Particularly Mihara et al (1998Mihara et al ( , 2004 found that the position of the fundamental cyclotron line in the energy spectrum depends on the pulsar luminosity. Later this effect was confirmed using the RXTE and INTEGRAL data (Nakajima et al, 2006;Tsygankov et al, 2007;Klochkov et al, 2011;Boldin et al, 2013). Such a behaviour can be explained either by the modification of the emitting regions in the vicinity of the neutron star or by artificial effects due to poor knowledge of the spectral continuum Müller et al, 2013;Boldin et al, 2013).…”

“…An important result of INTEGRAL is the discovery of an anti-correlation between the cyclotron energy and the X-ray luminosity in the transient Xray pulsars V 0332+53 (Tsygankov et al, 2006;Mowlavi et al, 2006) and 4U 0115+63 (Nakajima et al (2006); Tsygankov et al (2007); but see Müller et al (2013) and Boldin et al (2013) for the influence of the continuum spectral shape). This result initiated a systematic study of the cyclotron energy properties as a function of the source luminosity.…”

Luminosity-dependent changes of the cyclotron line energy and spectral hardness in Cepheus X-4

“…To ensure that this component does not influence the CRSF centroid energy (see e.g. Boldin et al 2013), we constructed the contour plots for the CRSF energy versus the parameters of the 10 keV component (Fig. 5).…”

Atmospheres and radiating surfaces of neutron stars

“…Equations (16) and (17) are a particular case of this expansion. Petri [76] developed a technique of expansion of electromagnetic fields around a rotating magnetized star over vector spherical harmonics, which allows one to find a solution of the Maxwell equations in the GR for an arbitrary multipole component of the magnetic field.…”

“…The discovery of the cyclotron line in the spectrum of the X-ray pulsar in the binary system Hercules X-1 [12] gave a striking confirmation of this idea. About 20 accreting X-ray pulsars are currently known to reveal the electron cyclotron line and sometimes several its harmonics at energies of tens keV, corresponding to B ≈ (1 -4) × 10 12 G (e.g., [13][14][15][16]). …”