242
D. K. Galloway and L. Keek
makes intermediate duration bursts the prime candidates for detecting redshifted
spectral features from the neutron star surface to constrain the dense matter EOS
(Sect. 5.3.2).
5.7.2 Superbursts
X-ray bursts that last between several hours and a day are termed “superbursts”.
Their duration corresponds to the cooling timescale at a column depth of 10 11 –
10 12 g cm −2s . At a mass accretion rate of 0.1 ˙
M Edd , it takes roughly a year to
accumulate such a layer. During that time all superbursting sources exhibit short
bursts. Therefore, the freshly accreted hydrogen and helium burns at a smaller
depth, and superbursts ignite in the carbon-rich ashes (Fig. 5.2). Most superbursts
are observed from sources with accretion rates of the order of 0.1 ˙
M Edd , but exceptional cases are known: 4U 0614+09 with ∼0.01 ˙
M Edd exhibited two superbursts
[110, 164], and GX 17+2 at ∼ 1 ˙
M Edd showed four [78]. Because their duration is
often much longer than the typical satellite (low-Earth) orbit of ≈90 min, the start is
in many cases obscured by Earth occultations. Combined with the sparse sampling
and limited data quality of all-sky instruments, this limits in many cases our ability
to measure the duration and fluence of the event, and we refer instead to candidate
superbursts. The long recurrence time makes superbursts the rarest burst category:
since their discovery [27, 172] only 25 (candidates) have been observed (see [75]
for a recent review and list of detections).
In the rare cases when the start of a superburst was observed, a short precursor
is visible directly prior to the superburst. After the carbon ignition ∼100 m below
the neutron star surface, a sound wave may travel toward the surface ahead of the
carbon flame [94, 100, 194, 195]. As it propagates to lower density regions, it may
speed up and exceed the speed of sound, turning into a shock. When it reaches the
surface, the kinetic energy of the shock heats the outer layers, producing a short Xray burst. Furthermore, any hydrogen or helium present there will be burned, adding
to the energy of the precursor burst. Only in one observation could the precursor’s
fluence be accurately determined: it was 40–100% larger than the fluence of the
regular short helium bursts observed from this source, indicating that another source
of energy contributed, such as shock heating [93].
Following the precursor, the rise of a superburst light curve typically is rather
slow: in 2001, the superburst flux from 4U 1636−536 reached its peak only
after ∼900 s (Fig. 5.3 bottom). With the exception of the 1999 superburst from
4U 1820−20, which reached the Eddington limit [172], most superbursts likely
have such a slow rise. The shape of the light curve is attributed to the cooling of
an envelope with a particular temperature profile [104]. This profile is left behind
by the radially outward moving carbon flame. Fits with cooling models measure a
profile close to T ∝ y 1/4 . This suggests that as the carbon flame moved into lower
density regions near the surface, the fraction of carbon that burned decreased, and
the flame likely stalled before reaching the photosphere.
D. K. Galloway and L. Keek
makes intermediate duration bursts the prime candidates for detecting redshifted
spectral features from the neutron star surface to constrain the dense matter EOS
(Sect. 5.3.2).
5.7.2 Superbursts
X-ray bursts that last between several hours and a day are termed “superbursts”.
Their duration corresponds to the cooling timescale at a column depth of 10 11 –
10 12 g cm −2s . At a mass accretion rate of 0.1 ˙
M Edd , it takes roughly a year to
accumulate such a layer. During that time all superbursting sources exhibit short
bursts. Therefore, the freshly accreted hydrogen and helium burns at a smaller
depth, and superbursts ignite in the carbon-rich ashes (Fig. 5.2). Most superbursts
are observed from sources with accretion rates of the order of 0.1 ˙
M Edd , but exceptional cases are known: 4U 0614+09 with ∼0.01 ˙
M Edd exhibited two superbursts
[110, 164], and GX 17+2 at ∼ 1 ˙
M Edd showed four [78]. Because their duration is
often much longer than the typical satellite (low-Earth) orbit of ≈90 min, the start is
in many cases obscured by Earth occultations. Combined with the sparse sampling
and limited data quality of all-sky instruments, this limits in many cases our ability
to measure the duration and fluence of the event, and we refer instead to candidate
superbursts. The long recurrence time makes superbursts the rarest burst category:
since their discovery [27, 172] only 25 (candidates) have been observed (see [75]
for a recent review and list of detections).
In the rare cases when the start of a superburst was observed, a short precursor
is visible directly prior to the superburst. After the carbon ignition ∼100 m below
the neutron star surface, a sound wave may travel toward the surface ahead of the
carbon flame [94, 100, 194, 195]. As it propagates to lower density regions, it may
speed up and exceed the speed of sound, turning into a shock. When it reaches the
surface, the kinetic energy of the shock heats the outer layers, producing a short Xray burst. Furthermore, any hydrogen or helium present there will be burned, adding
to the energy of the precursor burst. Only in one observation could the precursor’s
fluence be accurately determined: it was 40–100% larger than the fluence of the
regular short helium bursts observed from this source, indicating that another source
of energy contributed, such as shock heating [93].
Following the precursor, the rise of a superburst light curve typically is rather
slow: in 2001, the superburst flux from 4U 1636−536 reached its peak only
after ∼900 s (Fig. 5.3 bottom). With the exception of the 1999 superburst from
4U 1820−20, which reached the Eddington limit [172], most superbursts likely
have such a slow rise. The shape of the light curve is attributed to the cooling of
an envelope with a particular temperature profile [104]. This profile is left behind
by the radially outward moving carbon flame. Fits with cooling models measure a
profile close to T ∝ y 1/4 . This suggests that as the carbon flame moved into lower
density regions near the surface, the fraction of carbon that burned decreased, and
the flame likely stalled before reaching the photosphere.