1,004 research outputs found
Comptonization and the Spectra of Accretion-Powered X-Ray Pulsars
Accretion-powered X-ray pulsars are among the most luminous X-ray sources in
the Galaxy. However, despite decades of theoretical and observational work
since their discovery, no satisfactory model for the formation of the observed
X-ray spectra has emerged. In this paper, we report on a self-consistent
calculation of the spectrum emerging from a pulsar accretion column that
includes an explicit treatment of the bulk and thermal Comptonization occurring
in the radiation-dominated shocks that form in the accretion flows. Using a
rigorous eigenfunction expansion method, we obtain a closed-form expression for
the Green's function describing the upscattering of monochromatic radiation
injected into the column. The Green's function is convolved with
bremsstrahlung, cyclotron, and blackbody source terms to calculate the emergent
photon spectrum. We show that energization of photons in the shock naturally
produces an X-ray spectrum with a relatively flat continuum and a high-energy
exponential cutoff. Finally, we demonstrate that our model yields good
agreement with the spectra of the bright pulsar Her X-1 and the low luminosity
pulsar X Per.Comment: 6 Pages, 2 Figures, To appear in "The Multicoloured Landscape of
Compact Objects and their Explosive Progenitors" (Cefalu, Sicily, June 2006).
Eds. L. Burderi et al. (New York: AIP
Spectral Formation in X-Ray Pulsar Accretion Columns
We present the first self-consistent model for the dynamics and the radiative
transfer occurring in bright X-ray pulsar accretion columns, with a special
focus on the role of the shock in energizing the emerging X-rays. The pressure
inside the accretion column of a luminous X-ray pulsar is dominated by the
photons, and consequently the equations describing the coupled
radiative-dynamical structure must be solved simultaneously. Spectral formation
in these sources is therefore a complex, nonlinear phenomenon. We obtain the
analytical solution for the Green's function describing the upscattering of
monochromatic radiation injected into the column from the thermal mound located
near the base of the flow. The Green's function is convolved with a Planck
distribution to model the X-ray spectrum resulting from the reprocessing of
blackbody photons produced in the thermal mound. These photons diffuse through
the infalling gas and eventually escape out the walls of the column, forming
the observed X-ray spectrum. We show that the resulting column-integrated,
phase-averaged spectrum has a power-law shape at high energies and a blackbody
shape at low energies, in agreement with the observational data for many X-ray
pulsars.Comment: Accepted for publication in ApJ Letters. Several typos noticed during
the proof review were correcte
A Strong X-Ray Burst from the Low Mass X-Ray Binary EXO0748-676
We have observed an unusually strong X-ray burst as a part of our regular
eclipse timing observations of the low mass binary system EXO0748-676. The
burst peak flux was 5.2x10^-8 ergs cm^-2 s^-1, approximately five times the
normal peak X-ray burst flux observed from this source by RXTE. Spectral fits
to the data strongly suggest that photospheric radius expansion occurred during
the burst. In this Letter we examine the properties of this X-ray burst, which
is the first example of a radius expansion burst from EXO0748-676 observed by
RXTE. We find no evidence for coherent burst oscillations. Assuming that the
peak burst luminosity is the Eddington luminosity for a 1.4 solar mass neutron
star we derive a distance to EXO0748-676 of 7.7 kpc for a helium-dominated
burst photosphere and 5.9 kpc for a hydrogen-dominated burst photosphere.Comment: 15 pages including 2 figures and 1 table. Accepted for publication in
the Astrophysical Journa
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