5 Thermonuclear X-ray Bursts
229
mechanisms which may act to remove elements that might give rise to features,
from the upper layers of the photosphere.
Gravitational settling is thought to rapidly separate accreted metals from the
photosphere on a timescale of 10 −3 s [19, 149]. The metals that can produce lines
and edges may, therefore, usually be absent from the neutron star surface. Just a few
meters below the surface, a plethora of metals is created by nuclear burning during
bursts. These elements may be revealed at the neutron star surface—only during
the most powerful bursts—because of two processes. First, convection at the onset
of the burst transports the burning ashes towards the surface. Convection typically
does not reach the surface, but the more powerful the initial burning, the closer
to the surface the metal-rich ashes are convected. Secondly, if the burst has PRE,
a wind during the Eddington-limited phase may blow off the upper atmosphere,
facilitating the exposure of heavy metals [196]. The appearance of metals in the
photosphere can also give rise to a changing colour correction factor [92]. In bursts
where the fuel is helium rich, most of the burning happens at the burst onset, and
the Eddington limit is reached. Powerful helium bursts are, therefore, the prime
candidates for observing absorption features during the PRE phase. For example, a
weak absorption line was detected during the PRE phase of an intermediate duration
burst from GRS 1741.9−2853 [10].
The energy at which a line or edge is observed is reduced by the gravitational
redshift of the neutron star, that also provides a measure of the star’s compactness.
Discrete features are also expected to be affected by relativistic Doppler broadening,
since most bursting neutron stars rotate rapidly (e.g. [11]). These effects reduces
the detectability of such features against the continuum, increasing the difficulty of
constraining the equation of state.
The studies of EXO 0748−676 provide an illustrative example. The source was a
calibration target for XMM-Newton, and consequently a large number of bursts were
observed with the Reflection Grating Spectrometer. Stacking the spectra of a series
of bursts revealed a tentative detection of an iron absorption line [29, 154]. The
inferred gravitational redshift of the line was employed to constrain the mass and
radius of this neutron star [143], favoring a soft equation of state. These analyses
assumed a low neutron star spin frequency, supported by the detection of a weak
burst oscillation at 45 Hz [190]. Later, a much stronger burst oscillation was detected
in two bursts, at a significantly higher frequency: 552 Hz [54]. If the later detection
instead corresponded to the neutron star spin in this system, the expected Doppler
broadening was, therefore, substantially larger than assumed, casting doubt on the
line origin. Furthermore, a larger sample of bursts observed with XMM-Newton [30]
and Chandra [99] do not exhibit the line feature. The status of the line detection from
EXO 0748−676 remains, therefore, uncertain. However, the potential for absorption
features from the neutron star surface to constrain the neutron star equation of state
remains high.
Because a wider range of energies is absorbed to produce an edge, edges typically
have a larger equivalent width than lines [196], and consequently should be easier
to detect. The strongest absorption features associated with the surface of neutron
stars are nickel edges observed in the PRE phase of three superexpansion bursts
229
mechanisms which may act to remove elements that might give rise to features,
from the upper layers of the photosphere.
Gravitational settling is thought to rapidly separate accreted metals from the
photosphere on a timescale of 10 −3 s [19, 149]. The metals that can produce lines
and edges may, therefore, usually be absent from the neutron star surface. Just a few
meters below the surface, a plethora of metals is created by nuclear burning during
bursts. These elements may be revealed at the neutron star surface—only during
the most powerful bursts—because of two processes. First, convection at the onset
of the burst transports the burning ashes towards the surface. Convection typically
does not reach the surface, but the more powerful the initial burning, the closer
to the surface the metal-rich ashes are convected. Secondly, if the burst has PRE,
a wind during the Eddington-limited phase may blow off the upper atmosphere,
facilitating the exposure of heavy metals [196]. The appearance of metals in the
photosphere can also give rise to a changing colour correction factor [92]. In bursts
where the fuel is helium rich, most of the burning happens at the burst onset, and
the Eddington limit is reached. Powerful helium bursts are, therefore, the prime
candidates for observing absorption features during the PRE phase. For example, a
weak absorption line was detected during the PRE phase of an intermediate duration
burst from GRS 1741.9−2853 [10].
The energy at which a line or edge is observed is reduced by the gravitational
redshift of the neutron star, that also provides a measure of the star’s compactness.
Discrete features are also expected to be affected by relativistic Doppler broadening,
since most bursting neutron stars rotate rapidly (e.g. [11]). These effects reduces
the detectability of such features against the continuum, increasing the difficulty of
constraining the equation of state.
The studies of EXO 0748−676 provide an illustrative example. The source was a
calibration target for XMM-Newton, and consequently a large number of bursts were
observed with the Reflection Grating Spectrometer. Stacking the spectra of a series
of bursts revealed a tentative detection of an iron absorption line [29, 154]. The
inferred gravitational redshift of the line was employed to constrain the mass and
radius of this neutron star [143], favoring a soft equation of state. These analyses
assumed a low neutron star spin frequency, supported by the detection of a weak
burst oscillation at 45 Hz [190]. Later, a much stronger burst oscillation was detected
in two bursts, at a significantly higher frequency: 552 Hz [54]. If the later detection
instead corresponded to the neutron star spin in this system, the expected Doppler
broadening was, therefore, substantially larger than assumed, casting doubt on the
line origin. Furthermore, a larger sample of bursts observed with XMM-Newton [30]
and Chandra [99] do not exhibit the line feature. The status of the line detection from
EXO 0748−676 remains, therefore, uncertain. However, the potential for absorption
features from the neutron star surface to constrain the neutron star equation of state
remains high.
Because a wider range of energies is absorbed to produce an edge, edges typically
have a larger equivalent width than lines [196], and consequently should be easier
to detect. The strongest absorption features associated with the surface of neutron
stars are nickel edges observed in the PRE phase of three superexpansion bursts