226
D. K. Galloway and L. Keek
10
-1
10
0
10
1
E (keV)
10
29
10
30
10
31
10
32
Counts (c s
– 1
cm
– 2
keV
– 1
)
NICER
PCA
1: 0F Edd
Atmosphere
NICER
PCA
Envelope
Fig. 5.6 Photon counts spectrum as a function of energy, E, for models of thermal burst emission
(burst atmosphere model from [178]). The top of the grey area marks the Planck spectrum in the
neutron star envelope during a burst. Scattering in the atmosphere modifies the spectrum (solid
line). Historically the observed spectrum is fit with a Planck model. Two such fits are shown for
NICER and RXTE/PCA in the energy bands indicated at the top. The ratio of the observationally
inferred temperature and that of the envelope spectrum is the colour correction factor
changes in the persistent spectrum induced by the burst (Sect. 5.4), in addition to
the modifications of the intrinsic spectrum discussed here.
Compton and free-free scattering of thermal emission in the neutron star atmosphere modifies the burst spectrum (Fig. 5.6). Specifically, up-scattering boosts the
energy of a fraction of the soft photons, such that the “colour temperature” measured
by fitting a blackbody is larger than the effective temperature of the photosphere
[145]. This effect is normally accounted for using a “colour-correction” factor,
f c , giving the expected ratio of the (measured) colour temperature to the effective
temperature. At fluxes close to Eddington, f c is expected to be ≈2, dropping to ≈1.4
in the burst tail (e.g. [178]).
Radiative transfer simulations are employed to calculate all scattering processes
in the atmosphere taking into account the composition, although initial models
were not fully converged [122, 124]. Current state-of-the-art spectral models are
one-dimensional, assume hydrostatic and radiative equilibrium, and have been
calculated for a range of compositions and surface gravities [140, 178, 179]. The
models have several limitations. They assume uniform emission across the entire
stellar surface, whereas the detection of burst oscillations argues that this is not
always the case (Sect. 5.5). Furthermore, the neutron star’s rotation will cause
Doppler broadening of the spectrum [12]. The impact of these effects is, however,
likely small. More importantly, the assumed equilibria imply that the models are not
representative of the PRE phase, which will require simulations that include both
radiation transfer and hydrodynamics (cf. with [145]). In a new approach, time-
D. K. Galloway and L. Keek
10
-1
10
0
10
1
E (keV)
10
29
10
30
10
31
10
32
Counts (c s
– 1
cm
– 2
keV
– 1
)
NICER
PCA
1: 0F Edd
Atmosphere
NICER
PCA
Envelope
Fig. 5.6 Photon counts spectrum as a function of energy, E, for models of thermal burst emission
(burst atmosphere model from [178]). The top of the grey area marks the Planck spectrum in the
neutron star envelope during a burst. Scattering in the atmosphere modifies the spectrum (solid
line). Historically the observed spectrum is fit with a Planck model. Two such fits are shown for
NICER and RXTE/PCA in the energy bands indicated at the top. The ratio of the observationally
inferred temperature and that of the envelope spectrum is the colour correction factor
changes in the persistent spectrum induced by the burst (Sect. 5.4), in addition to
the modifications of the intrinsic spectrum discussed here.
Compton and free-free scattering of thermal emission in the neutron star atmosphere modifies the burst spectrum (Fig. 5.6). Specifically, up-scattering boosts the
energy of a fraction of the soft photons, such that the “colour temperature” measured
by fitting a blackbody is larger than the effective temperature of the photosphere
[145]. This effect is normally accounted for using a “colour-correction” factor,
f c , giving the expected ratio of the (measured) colour temperature to the effective
temperature. At fluxes close to Eddington, f c is expected to be ≈2, dropping to ≈1.4
in the burst tail (e.g. [178]).
Radiative transfer simulations are employed to calculate all scattering processes
in the atmosphere taking into account the composition, although initial models
were not fully converged [122, 124]. Current state-of-the-art spectral models are
one-dimensional, assume hydrostatic and radiative equilibrium, and have been
calculated for a range of compositions and surface gravities [140, 178, 179]. The
models have several limitations. They assume uniform emission across the entire
stellar surface, whereas the detection of burst oscillations argues that this is not
always the case (Sect. 5.5). Furthermore, the neutron star’s rotation will cause
Doppler broadening of the spectrum [12]. The impact of these effects is, however,
likely small. More importantly, the assumed equilibria imply that the models are not
representative of the PRE phase, which will require simulations that include both
radiation transfer and hydrodynamics (cf. with [145]). In a new approach, time-