234
D. K. Galloway and L. Keek
line depends on the ionization state of iron in the disk, quantified by the ionization
parameter ξ = 4πF n −1 , with F the illuminating flux measured at the disk and n its
number density. Reflection further contributes a number of emission features below
3 keV as well as a free-free continuum. The strength of these features depends on
the density and composition of the reflecting material (Fig. 5.8).
The reflection spectrum undergoes relativistic Doppler broadening from the
motion of gas in the disk, which is stronger if the inner disk is closer to the neutron
star (cf. with Sect. 5.1.2). A reflection spectrum, therefore, contains information
on the location of the inner disk, its ionization state, composition, and density, and
the geometry of the disk. Current reflection models cover only part of this large
parameter space, making accurate interpretation of the spectra challenging. Thus
far, reflection has been detected in two superbursts [9, 101, 103, 104, 172] and one
intermediate duration burst [105]. The reflection signals hint at a strong impact of
these bursts on their environment, showing the evolution of the disk inner radius and
its ionization parameter.
5.4.2 Anisotropic Emission
An interesting consequence of reflection combined with obscuration of the neutron
star by the disk is that the observed burst flux depends on the inclination angle of
the disk with respect to the line of sight. Both the burst flux from the neutron star
and the flux from the disk are anisotropic. If the inclination angle is known, the
anisotropy factors ξ can be calculated, which are defined as L = 4πd 2 ξF , with L
and F the intrinsic luminosity and the observed flux, respectively, of either the star
or the disk, and with source distance d [47, 70, 113].
Unfortunately, the inclination angle of most bursting systems is poorly constrained (e.g. [55]). Moreover, most predictions for ξ assume that the accretion disk
is thin and extends down to the surface of the neutron star [47, 113]. At lower mass
accretion rates, however, the disk is truncated at some distance from the star. Also,
different disk shapes will alter the anisotropy ξ [70]. Lack of information on the disk
geometry hampers accurate predictions of ξ . The observation of reflection fractions
in excess of 0.5 (the maximum for thin disks) indicates a concave geometry. For
the two superbursts observed with RXTE/PCA large reflection fractions in excess of
unity were inferred [9, 103], suggesting such a disk shape. However, because of the
limited data quality alternate interpretations are possible that are consistent with a
thin disk [104].
The anisotropy has important consequences for a range of measured quantities,
from the burst peak flux and fluence, to the mass accretion rate derived from
the persistent flux. It impacts distance measurements that use the peak flux
of Eddington-limited bursts. Different values of the Eddington flux have been
derived from bursts of the same source [52], but the effect of variations in ξ in
different accretion states has not been investigated yet. Radius measurements are
impacted as well, as they are often derived from the normalization of the spectrum.
D. K. Galloway and L. Keek
line depends on the ionization state of iron in the disk, quantified by the ionization
parameter ξ = 4πF n −1 , with F the illuminating flux measured at the disk and n its
number density. Reflection further contributes a number of emission features below
3 keV as well as a free-free continuum. The strength of these features depends on
the density and composition of the reflecting material (Fig. 5.8).
The reflection spectrum undergoes relativistic Doppler broadening from the
motion of gas in the disk, which is stronger if the inner disk is closer to the neutron
star (cf. with Sect. 5.1.2). A reflection spectrum, therefore, contains information
on the location of the inner disk, its ionization state, composition, and density, and
the geometry of the disk. Current reflection models cover only part of this large
parameter space, making accurate interpretation of the spectra challenging. Thus
far, reflection has been detected in two superbursts [9, 101, 103, 104, 172] and one
intermediate duration burst [105]. The reflection signals hint at a strong impact of
these bursts on their environment, showing the evolution of the disk inner radius and
its ionization parameter.
5.4.2 Anisotropic Emission
An interesting consequence of reflection combined with obscuration of the neutron
star by the disk is that the observed burst flux depends on the inclination angle of
the disk with respect to the line of sight. Both the burst flux from the neutron star
and the flux from the disk are anisotropic. If the inclination angle is known, the
anisotropy factors ξ can be calculated, which are defined as L = 4πd 2 ξF , with L
and F the intrinsic luminosity and the observed flux, respectively, of either the star
or the disk, and with source distance d [47, 70, 113].
Unfortunately, the inclination angle of most bursting systems is poorly constrained (e.g. [55]). Moreover, most predictions for ξ assume that the accretion disk
is thin and extends down to the surface of the neutron star [47, 113]. At lower mass
accretion rates, however, the disk is truncated at some distance from the star. Also,
different disk shapes will alter the anisotropy ξ [70]. Lack of information on the disk
geometry hampers accurate predictions of ξ . The observation of reflection fractions
in excess of 0.5 (the maximum for thin disks) indicates a concave geometry. For
the two superbursts observed with RXTE/PCA large reflection fractions in excess of
unity were inferred [9, 103], suggesting such a disk shape. However, because of the
limited data quality alternate interpretations are possible that are consistent with a
thin disk [104].
The anisotropy has important consequences for a range of measured quantities,
from the burst peak flux and fluence, to the mass accretion rate derived from
the persistent flux. It impacts distance measurements that use the peak flux
of Eddington-limited bursts. Different values of the Eddington flux have been
derived from bursts of the same source [52], but the effect of variations in ξ in
different accretion states has not been investigated yet. Radius measurements are
impacted as well, as they are often derived from the normalization of the spectrum.