Low-luminosity accretion onto magnetized neutron stars

Langer, Steven H.; Rappaport, S.

doi:10.1086/160028

Cited by 74 publications

(99 citation statements)

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“…The hard emission from X-ray pulsars is believed to originate in the accretion column, as a result of Comptonization of soft photons coming from the neutron star thermal mound by high-energy electrons in the accretion flow (Becker & Wolff 2007). We speculate that as the accretion rate increases, the photons spend more time on average being upscattered in the flow before escaping from the accretion column, which results in harder X-ray spectra (Langer & Rappaport 1982;Becker & Wolff 2007).…”

Section: Spectral Analysismentioning

confidence: 91%

Discovery of a quasi-periodic oscillation in the X-ray pulsar 1A 1118-615: correlated spectral and aperiodic variability

Reig

2010

A&A

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Aims. Our goal is to investigate the X-ray timing and spectral variability of the high-mass X-ray binary 1A 1118-615 during a type-II outburst.Methods. We performed a detailed color, spectral, and timing analysis of a giant outburst from 1A 1118-615 using RXTE data. Results. We report the discovery of a variable quasi-periodic oscillation (QPO) in the power spectral density of 1A 1118-615, with a centroid frequency of ∼0.08 Hz. The centroid frequency of the QPO correlates with the X-ray flux, as expected according to the most accredited models for QPO production. For energies above ∼4 keV, the QPO rms variability decreases as the energy increases. Pulse profiles display an energy dependence with a two-peak profile at lower energies and a single peak at higher energies. From the spectral analysis we confirm the presence of a cyclotron absorption feature at ∼60 keV, the highest value measured for an X-ray pulsar. We find that the spectral parameters (photon index, cutoff energy, iron fluorescence line strength) display a marked dependence with flux. We detect two different levels of neutral hydrogen column density, possibly owing to to the Be companion activity. We report for the first time a correlation between the timing and spectral parameters in an X-ray pulsar. All correlations found between spectral/timing parameters and X-ray flux are present up to a flux of ∼6 × 10 −9 erg cm −2 s −1 , when a saturation level is reached. We propose that the observed saturation corresponds to the minimum extent of the neutron star magnetosphere. We estimate the magnetic field of the neutron star from two independent ways, using results from spectral (cyclotron line energy) and timing (QPO frequency) analysis, obtaining consistent values of ∼7−8 × 10 12 G. Results from the comprehensive spectral and timing analysis are discussed in comparison with other X-ray pulsars.

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Section: Spectral Analysismentioning

confidence: 91%

Discovery of a quasi-periodic oscillation in the X-ray pulsar 1A 1118-615: correlated spectral and aperiodic variability

Reig

2010

A&A

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“…2. For sources to the left of this line, we expect that the effect of Coulomb interactions is reduced, and the final stopping occurs via passage through a discontinuous, gas-mediated shock (Langer & Rappaport 1982). Hence we anticipate that the emission region approaches the stellar surface as L X is reduced below L coul .…”

Section: Comparison Of L X With L Coulmentioning

confidence: 96%

“…The associated drop in the density causes the Thomson depth τ to fall below the value τ * ∼ 20 required for the gas to be effectively stopped via Coulomb interactions. It is not completely clear what happens in this case, but we expect that the final phase of deceleration will occur via passage through a gas-mediated shock near the stellar surface (Langer & Rappaport 1982). By combining Eqs.…”

Section: Transition From Coulomb Stopping To Gas Shockmentioning

confidence: 99%

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Spectral formation in accreting X-ray pulsars: bimodal variation of the cyclotron energy with luminosity

Becker

Klochkov

Schönherr

et al. 2012

A&A

285

495

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Context. Accretion-powered X-ray pulsars exhibit significant variability of the cyclotron resonance scattering feature (CRSF) centroid energy on pulse-to-pulse timescales, and also on much longer timescales. Two types of spectral variability are observed. For sources in group 1, the CRSF energy is negatively correlated with the variable source luminosity, and for sources in group 2, the opposite behavior is observed. The physical basis for this bimodal behavior is currently not well understood. Aims. We explore the hypothesis that the accretion dynamics in the group 1 sources is dominated by radiation pressure near the stellar surface, and that Coulomb interactions decelerate the gas to rest in the group 2 sources. Methods. We derive a new expression for the critical luminosity, L crit , such that radiation pressure decelerates the matter to rest in sources with X-ray luminosity L X > L crit . The formula for L crit is based on a simple physical model for the structure of the accretion column in luminous X-ray pulsars that takes into account radiative deceleration, the energy dependence of the cyclotron cross section, the thermodynamics of the accreting gas, the dipole structure of the pulsar magnetosphere, and the diffusive escape of radiation through the column walls. We show that for typical neutron star parameters, L crit = 1.5 × 10 37 B 16/15 12 erg s −1 , where B 12 is the surface magnetic field strength in units of 10 12 G. Results. The formula for the critical luminosity is evaluated for five sources, using the maximum value of the CRSF centroid energy to estimate the surface magnetic field strength B 12 . The results confirm that the group 1 sources are supercritical (L X > L crit ) and the group 2 sources are subcritical (L X < L crit ), although the situation is less clear for those highly variable sources that cross over the line L X = L crit . We also explain the variation of the CRSF energy with luminosity as a consequence of the variation of the characteristic emission height. The sign of this dependence is opposite in the supercritical and subcritical cases, hence creating the observed bimodal behavior. Conclusions. We have developed a new model for the critical luminosity in accretion-powered X-ray pulsars that explains the bimodal dependence of the CRSF centroid energy on the X-ray luminosity L X . Our model provides a physical basis for the observed variation of the CRSF energy as a function of L X for both the group 1 (supercritical) and the group 2 (subcritical) sources as a result of the variation of the emission height in the column.

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“…Our knowledge of plasma physics in a strong magnetic field is insufficient to decide if a collisionless shock forms or not (Langer & Rappaport 1982;Arons et al 1987). However, if a collisionless shock forms (which is an assumption) then its height above the NS surface depends on the mass accretion rate: the lower the mass accretion rate, the higher the location of the collisionless shock.…”

Section: Sub-critical X-ray Pulsarsmentioning

confidence: 99%

On the radiation beaming of bright X-ray pulsars and constraints on neutron star mass–radius relation

Mushtukov

Verhagen

Tsygankov

et al. 2017

Monthly Notices of the Royal Astronomical Society

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The luminosity of accreting magnetised neutron stars can largely exceed the Eddington value due to appearance of accretion columns. The height of the columns can be comparable to the neutron star radius. The columns produce the X-rays detected by the observer directly and illuminate the stellar surface, which reprocesses the X-rays and causes additional component of the observed flux. The geometry of the column and the illuminated part of the surface determines the radiation beaming. Curved spacetime affects the angular flux distribution. We construct a simple model of the beam patterns formed by direct and reflected flux from the column. We take into account the possibility of appearance of accretion columns, whose height is comparable to the neutron star radius. We argue that depending on the compactness of the star the flux from the column can be either strongly amplified due to gravitational lensing, or significantly reduced due to column eclipse by the star. The eclipses of high accretion columns result in specific features in pulse profiles. Their detection can put constraints on the neutron star radius. We speculate that column eclipses are observed in X-ray pulsar V 0332+53, leading us to the conclusion of large neutron star radius in this system (∼ 15 km if M ∼ 1.4M ). We point out that the beam pattern can be strongly affected by scattering in the accretion channel at high luminosity, which has to be taken into account in the models reproducing the pulse profiles.

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Low-luminosity accretion onto magnetized neutron stars

Cited by 74 publications

References 0 publications

Discovery of a quasi-periodic oscillation in the X-ray pulsar 1A 1118-615: correlated spectral and aperiodic variability

Discovery of a quasi-periodic oscillation in the X-ray pulsar 1A 1118-615: correlated spectral and aperiodic variability

Spectral formation in accreting X-ray pulsars: bimodal variation of the cyclotron energy with luminosity

On the radiation beaming of bright X-ray pulsars and constraints on neutron star mass–radius relation

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