11,038 research outputs found
Radiation spectra and polarization in magnetar bursts
We present Monte Carlo simulations of radiative transfer in magnetar
atmospheres. We include the effects of vacuum polarization, electron and proton
scattering, and free-free absorption. Simulations are performed for the
atmosphere model with the magnetic field perpendicular and also tilted with
respect to the neutron star surface, and we show that the average spectrum does
not strongly depend on the orientation of the magnetic field. We investigate
the region of the parameter space where the vacuum absorption-like feature
appears in the spectrum and we analyze the shape of the proton cyclotron line.
Our results indicate that the existence of the vacuum polarization feature
should be a general attribute of soft gamma-ray repeaters burst spectra,
provided that the energy release takes place at the sufficiently dense region,
and the atmosphere scaleheight is large enough. We discuss the existence of
such a feature in recent observational data on these sources.Comment: submitted to Ap
Quantum nature of cyclotron harmonics in thermal spectra of neutron stars
Some isolated neutron stars show harmonically spaced absorption features in
their thermal soft X-ray spectra. The interpretation of the features as a
cyclotron line and its harmonics has been suggested, but the usual explanation
of the harmonics as caused by relativistic effects fails because the
relativistic corrections are extremely small in this case. We suggest that the
features correspond to the peaks in the energy dependence of the free-free
opacity in a quantizing magnetic field, known as quantum oscillations. The
peaks arise when the transitions to new Landau levels become allowed with
increasing the photon energy; they are strongly enhanced by the square-root
singularities in the phase-space density of quantum states in the case when the
free (non-quantized) motion is effectively one-dimensional. To explore
observable properties of these quantum oscillations, we calculate models of
hydrogen neutron star atmospheres with B \sim 10^{10} - 10^{11} G (i.e.,
electron cyclotron energy E_{c,e} = 0.1 - 1 keV) and T_{eff} = 1 - 3 MK. Such
conditions are thought to be typical for the so-called central compact objects
in supernova remnants, such as 1E 1207.4-5209 in PKS 1209-51/52. We show that
observable features at the electron cyclotron harmonics form at moderately
large values of the quantization parameter, b_{eff} = E_{c,e}/kT_{eff} = 0.5 -
20. The equivalent widths of the features can reach 100 - 200 eV; they grow
with increasing b_{eff} and are lower for higher harmonics.Comment: 6 pages; shortened, references updated; published in Ap
Polarization of Thermal X-rays from Isolated Neutron Stars
Since the opacity of a magnetized plasma depends on polarization of
radiation, the radiation emergent from atmospheres of neutron stars with strong
magnetic fields is expected to be strongly polarized. The degree of linear
polarization, typically ~10-30%, depends on photon energy, effective
temperature and magnetic field. The spectrum of polarization is more sensitive
to the magnetic field than the spectrum of intensity. Both the degree of
polarization and the position angle vary with the neutron star rotation period
so that the shape of polarization pulse profiles depends on the orientation of
the rotational and magnetic axes. Moreover, as the polarization is
substantially modified by the general relativistic effects, observations of
polarization of X-ray radiation from isolated neutron stars provide a new
method for evaluating the mass-to-radius ratio of these objects, which is
particularly important for elucidating the properties of the superdense matter
in the neutron star interiors.Comment: 7 figures, to be published in Ap
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