Dynamically Comptonized Spectra from Near-critical Accretion onto Neutron Stars

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

doi:10.1086/173485

Cited by 8 publications

(9 citation statements)

References 11 publications

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“…In particular, the high energy tail is populated by the strongly comptonized photons coming just from this region. A similar effect was found by Mastichiadis & Kylafis (1992, see also Zampieri, Turolla, & Treves 1993) in an accretion flow onto a neutron star. In their case the formation of an essentially flat (α ≃ 0) spectrum is due to the fact that photons experience a very large number of energetic scatterings before emerging to infinity, since no advection is present being the star surface a perfect reflector.…”

Section: Discussionsupporting

confidence: 82%

Dynamical Comptonization in spherical flows: black hole accretion and stellar winds

Turolla

Zane

Zampieri

et al. 1996

Monthly Notices of the Royal Astronomical Society

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The transport of photons in steady, spherical, scattering flows is investigated. The moment equations are solved analytically for accretion onto a Schwarzschild black hole, taking into full account relativistic effects. We show that the emergent radiation spectrum is a power law at high frequencies with a spectral index smaller (harder spectrum) than in the non-relativistic case. Radiative transfer in an expanding envelope is also analyzed. We find that adiabatic expansion produces a drift of injected monochromatic photons towards lower frequencies and the formation of a power-law, low-energy tail with spectral index −3.

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Section: Discussionsupporting

confidence: 82%

Dynamical Comptonization in spherical flows: black hole accretion and stellar winds

Turolla

Zane

Zampieri

et al. 1996

Monthly Notices of the Royal Astronomical Society

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“…at the sameṁ because the velocity is much smaller. Increasingṁ is of no avail since, as Zampieri, Turolla & Treves (1993) have shown, τ b reaches ∼ 1 forṁ ∼ 3.5 then starts to decrease. Consequently, for a spherically accreting neutron star a power-law tail may indeed form but it is always steeper (softer spectrum) than in black holes.…”

Section: Discussionmentioning

confidence: 98%

Power‐Law Tails from Dynamical Comptonization in Converging Flows

Turolla

Zane

Titarchuk

2002

ApJ

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The effects of bulk motion comptonization on the spectral formation in a converging flow onto a black hole are investigated. The problem is tackled by means of both a fully relativistic, angle-dependent transfer code and a semi-analytical, diffusion-approximation method. We find that a power-law high-energy tail is a ubiquitous feature in converging flows and that the two approaches produce consistent results at large enough accretion rates, when photon diffusion holds. Our semi-analytical approach is based on an expansion in eigenfunctions of the diffusion equation. Contrary to previous investigations based on the same method we find that, although the power-law tail at really large energies is always dominated by the flatter spectral mode, the slope of the hard X-ray portion of the spectrum is dictated by the second mode and it approaches Γ = 3 at large accretion rate, irrespective of the model parameters. The photon index in the tail is found to be largely independent on the spatial distribution of soft seed photons when the accretion rate is either quite low ( 5 in Eddington units) or sufficiently high ( 10). On the other hand, the spatial distribution of source photons controls the photon index at intermediate accretion rates, when Γ switches from the first to the second mode. Our analysis confirms that a hard tail with photon index Γ < 3 is produced by the up-scattering of primary photons onto infalling electrons if the central object is a black hole.

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“…Above l ∞ ∼ 0.1, the dynamical effects of radiation pressure and bulk motion Comptonization become also important (on this regard see e.g. Zampieri, Turolla & Treves 1993 and references therein) and are not included in our model.…”

Section: Discussionmentioning

confidence: 99%

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

Turolla¹,

Zampieri²,

Colpi³

et al. 1994

ApJ

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Stationary, spherical accretion onto an unmagnetized neutron star is here reconsidered on the wake of the seminal paper by Zel'dovich & Shakura (1969). It is found that new "hot" solutions may exist for a wide range of luminosities. These solutions are characterized by a high temperature, 10 9 ÷10 11 K, and arise from a stationary equilibrium model where the dominant radiative mechanisms are multiple Compton scattering and bremsstrahlung emission. For low luminosities, ∼ < 10 −2 L E , only the "cold" (à la Zel'dovich and Shakura) solution is present.

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Dynamically Comptonized Spectra from Near-critical Accretion onto Neutron Stars

Cited by 8 publications

References 11 publications

Dynamical Comptonization in spherical flows: black hole accretion and stellar winds

Dynamical Comptonization in spherical flows: black hole accretion and stellar winds

Power‐Law Tails from Dynamical Comptonization in Converging Flows

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

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