Spherical accretion onto neutron stars revisited: Are hot solutions possible?

Turolla, R.; Zampieri, L.; Colpi, M.; Treves, A.

doi:10.1086/187333

Cited by 22 publications

(29 citation statements)

References 12 publications

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“…This is considerably hotter than previous calculations showed (e.g. Alme & Wilson 1973;Turolla et al 1994) but also significantly cooler than the "hot" solutions from Turolla et al (1994) and Zane et al (1998). We did not find any comparable hot solutions.…”

Section: Results Of the Model Computationscontrasting

confidence: 75%

X-ray spectra from protons illuminating a neutron star

Deufel

Dullemond

Spruit

2001

A&A

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Abstract. We consider the interaction of a slowly rotating unmagnetized neutron star with a hot (ion supported, ADAF) accretion flow. The virialized protons of the ADAF penetrate into the neutron star atmosphere, heating a surface layer. Detailed calculations are presented of the equilibrium between heating by the protons, electron thermal conduction, bremsstrahlung and multiple Compton scattering in this layer. Its temperature is of the order 40-70 keV. Its optical depth increases with the incident proton energy flux, and is of the order unity for accretion at 10 −2 -10 −1 of the Eddington rate. At these rates, the X-ray spectrum produced by the layer has a hard tail extending to 100 keV, and is similar to the observed spectra of accreting neutron stars in their hard states. The steep gradient at the base of the heated layer gives rise to an excess of photons at the soft end of the spectrum (compared to a blackbody) through an "inverse photosphere effect". The differences with respect to previous studies of similar problems are discussed, they are due mostly to a more accurate treatment of the proton penetration process and the vertical structure of the heated layer.

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Section: Results Of the Model Computationscontrasting

confidence: 75%

X-ray spectra from protons illuminating a neutron star

Deufel

Dullemond

Spruit

2001

A&A

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“…Treves & Colpi (1991) and Blaes & Madau (1993) used the Bondi accretion rate to estimate the likely luminosities of accreting neutron stars and discussed the possibility of detecting them by their EUV and X-ray emission. A number of later papers have discussed theoretical predictions for the emitted spectrum (e.g., Turolla et al 1994 ;Zane, Turolla, & Treves 1996). Despite careful searches in the ROSAT all-sky survey, the predicted large number of sources has not been found (e.g., Belloni, Zampieri, & Campana 1997).…”

Section: Astrophysical Applicationsmentioning

confidence: 99%

Three‐dimensional Magnetohydrodynamic Simulations of Spherical Accretion

Igumenshchev

Narayan

2002

ApJ

101

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We present three-dimensional numerical magnetohydrodynamic simulations of radiatively inefficient spherical accretion onto a black hole. The simulations are initialized with a Bondi Ñow and with a weak, dynamically unimportant, large-scale magnetic Ðeld. As the gas Ñows in, the magnetic Ðeld is ampliÐed. When the magnetic pressure approaches equipartition with the gas pressure, the Ðeld begins to reconnect and the gas is heated up. The heated gas is buoyant and moves outward, causing line stretching of the frozen-in magnetic Ðeld. This leads to further reconnection and more heating and buoyancy-induced motions, so that the Ñow makes a transition to a state of self-sustained convection. The radial structure of the Ñow changes dramatically from its initial Bondi proÐle, and the mass accretion rate onto the black hole decreases signiÐcantly. Motivated by the numerical results, we develop a simpliÐed analytical model of a radiatively inefficient spherical Ñow in which convective transport of energy to large radii plays an important role. In this "" convection-dominated Bondi Ñow,ÏÏ the accretion velocity is highly subsonic, and the density varies with radius as o P R~1@2 rather than the standard Bondi scaling o P R~3@2. We estimate that the mass accretion rate onto the black hole correspondingly scales as M 0 D where is a small multiple of the Schwarzschild radius of the black hole and is theaccretion radius ÏÏ at which the ambient gas in the surrounding medium is gravitationally captured by the black hole. Since the factor is typically very small, is signiÐcantly less than the Bondi R in /R a M 0 accretion rate. Convection-dominated Bondi Ñows may be relevant for understanding many astrophysical phenomena, e.g., post-supernova fallback and radiatively inefficient accretion onto supermassive black holes, stellar-mass black holes, and neutron stars.

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“…Pair production may be also expected in accretion models with shocks (see e.g. Shapiro, & Salpeter 1975) or in hydrostatic atmospheres kept at mildly relativistic temperatures by comptonization (Turolla, et al 1994;Zane, Turolla, & Treves 1996).…”

Section: Introductionmentioning

confidence: 88%

On Electrostatic Positron Acceleration in the Accretion Flow onto Neutron Stars

Turolla

Zane

Treves

et al. 1997

ApJ

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As first shown by Shvartsman (1970), a neutron star accreting close to the Eddington limit must acquire a positive charge in order for electrons and protons to move at the same speed. The resulting electrostatic field may contribute to accelerating positrons produced near the star surface in conjunction with the radiative force. We reconsider the balance between energy gains and losses, including inverse Compton (IC), bremsstrahlung and non-radiative scatterings. It is found that, even accounting for IC losses only, the maximum positron energy never exceeds ≈ 400 keV. The electrostatic field alone may produce energies ≈ 50 keV at most. We also show that Coulomb collisions and annihilation with accreting electrons severely limit the number of positrons that escape to infinity.

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Spherical accretion onto neutron stars revisited: Are hot solutions possible?

Cited by 22 publications

References 12 publications

X-ray spectra from protons illuminating a neutron star

X-ray spectra from protons illuminating a neutron star

Three‐dimensional Magnetohydrodynamic Simulations of Spherical Accretion

On Electrostatic Positron Acceleration in the Accretion Flow onto Neutron Stars

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