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Nuclear burning on the surface of accreting
neutron stars
Duncan Galloway
University of Melbourne
Galloway,
Nuclear
burning on
the surface
of accreting
neutron stars
SINS
Summer
School,
Jan
‘07
Low-mass X-ray binaries
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•
LMXBs consist of
compact objects (in most
cases neutron stars)
accreting material from a
low-mass stellar
companion.
•
Approx. 100 are known
within our Galaxy, with
orbital periods between
10 min and 16.6 d
•
In some accreting neutron stars the magnetic field is strong enough to channel the flow,
producing X-ray pulsations at the neutron star spin
•
LMXBs are thought to be “old” systems where the neutron star magnetic field has
decayed and the accreting material can spread evenly over much of the surface
Galloway, Nuclear burning on the surface of accreting neutron stars
Discovery of thermonuclear bursts
• Netherlands/USA satellite X-ray
experiment ANS first observed bright
X-ray flashes from around the
Galactic center and elsewhere in the
early ‘70s
• Clearly originating from discrete sources
• Two characteristic types identified:
– “slow” or type-I bursts, with typical recurrence times of a
few hours, from multiple sources
– “rapid” or type-II bursts, recurring as fast as every few
seconds; but only in one source (the “Rapid Burster”)
Galloway, Nuclear burning on the surface of accreting neutron stars
Examples of X-ray bursts from RXTE
Galloway, Nuclear burning on the surface of accreting neutron stars
Bursts are interesting because
• Burst oscillations observed only during bursts trace the
neutron-star spin (e.g. Chakrabarty et al. 2003, Nature 424, 42)
• Eddington-limited bursts allow us to constrain the distance
to bursting sources (e.g. Kuulkers et al. 2003, A&A 399, 663)
• The energy spectrum in the burst tail allows us (in principle)
to constrain the blackbody radius (e.g. Özel et al. 2006, Nature 421, 1115)
• Observation of discrete emission/absorption lines during
bursts allow measurement of the surface gravitational
redshift (e.g. Cottam et al. 2004, Nature 420, 51)
• The nuclear processes are quite complex!
Galloway, Nuclear burning on the surface of accreting neutron stars
Type-I bursts: nuclear H/He burning
• Ratio of integrated burst flux (fluence) to integrated
persistent X-ray emission (arising from accretion) constrains
the burst energetics - the -value
• Compactness of the neutron star means that accretion
liberates roughly 50% of the rest-mass energy of the
accreted material
• Nuclear burning is much less efficient, at around 1%;
expected ratio is then ~50 or more, in agreement with
measurements
• Rapid (type-II) bursts, on the other hand, are thought to be
transient accretion events, confused the issue early on, and
remain rather poorly understood
Galloway, Nuclear burning on the surface of accreting neutron stars
Superbursts: carbon burning?
• Recently-discovered
phenomenon
• 1000x more energetic than
typical thermonuclear bursts
(1042 ergs instead of 1039)
• 1000x less frequent
(recurrence times of months,
instead of hours)
4U 1636-536
104 s
• Thought to arise from unstable ignition of carbon produced
as a by-product of burning during “normal” thermonuclear
bursts
Galloway, Nuclear burning on the surface of accreting neutron stars
The accreting layer
• The temperature and density structure of the accreting layer
is set primarily by the accretion rate, which we infer from the
persistent X-ray flux
• The accretion rate cannot (much) exceed the Eddington
limit, at which point the radiation pressure balances
gravitational attraction, preventing further accretion
• Many LMXBs are transients, that is they undergo outbursts
of ~weeks separated by months-years
• During these outbursts, the X-ray flux (and hence the
accretion rate) varies by many orders of magnitude, up to
LEdd
-> wide range of conditions in the fuel layer
Galloway, Nuclear burning on the surface of accreting neutron stars
Nuclear burning processes
• Accreted fuel is composed of H, He, and metals (primarily C,
N, O)
• He burns rapidly via the 3 reaction, and subsequently via
p processes
• H burns more slowly by CNO burning initially, and
subsequently via rp-process burning
• At ~1% of the Eddington accretion rate, CNO burning
becomes saturated due to the -decay wait time, and
proceeds stably between bursts
• Heating from “hot” CNO burning leads to earlier burst ignition
in this regime; cf. with thermal & compositional “inertia”
• He-burning is also thought to stabilise at around the
Eddington accretion rate (although this is not consistent with
observations)
Galloway, Nuclear burning on the surface of accreting neutron stars
Endpoint of the rp-process
• Early studies with limited reaction networks could not identify
the end-point of rp-process burning, but it is now thought to
terminate in a closed Sn-Sb-Te cycle (Schatz et al. 2001)
• Modeling suggests burning may not reach this cycle unless
the temperature and density become sufficiently high
Galloway, Nuclear burning on the surface of accreting neutron stars
Three regimes of ignition
3 cases, in order of increasing
accretion rate (e.g. Fujimoto
et al. 1981):
3) H-burning is unstable,
ignition is from H in mixed
H/He fuel;
2) H-burning stable, H is
exhausted prior to unstable
He-ignition, pure He burst;
1) H is not exhausted prior to
He-ignition, mixed burst;
Galloway, Nuclear burning on the surface of accreting neutron stars
stable
burning
accretion
rate
Case 1
Case 2
Case 3
ignition
curves
Burst ignition: case 1
• Ignition by unstable He burning
in a mixed H/He environment
• First burst in the simulation, and
the most intense
• Subsequent bursts illustrate the
“inertia” characteristic of this
case (enhanced @ low Z0)
• For 2nd and later bursts, ignition
takes place in the layer of ashes
produced from the previous burst
• Lower temperatures and
densities reached
(simulations from Woosley et al. ‘04)
Galloway, Nuclear burning on the surface of accreting neutron stars
Resulting composition
•
•
•
Galloway, Nuclear burning on the surface of accreting neutron stars
Lack of convection
after the start of the
burst prevents most of
the ashes being mixed
back up into the
burning layer for
further burning
Don’t reach the
endpoint of the rpprocess (cf. with
Schatz et al. 2001)
Very little 12C - not
enough to power a
superburst
Burst ignition: case 2
• Lower accretion rate, so
that enough time passes
between bursts to
exhaust H at the base
• Ignition of He in a pure
He-layer, with freshly
accreted H on top
• Burst is much shorter
and intense, and
convection plays a bigger
role
Galloway, Nuclear burning on the surface of accreting neutron stars
Lighter products, & more C
• In case 2 ignition, more carbon is
left which may accumulate and
subsequently power a superburst
• Otherwise the ashes are much
lighter than in case 1 burning
• Radial composition gradient
results in additional densitydriven (“thermohaline”)
convection
• Ashes are thus well-mixed
Galloway, Nuclear burning on the surface of accreting neutron stars
Burst ignition: case 3
•
•
•
At lowest accretion rate, unstable ignition of H in a mixed H/He environment;
perhaps the least well understood case
Short-recurrence time bursts (doublets and even triplets) characteristic of this
bursting regime
Possibly arising from ignition of unburnt or partially burnt fuel
Galloway, Nuclear burning on the surface of accreting neutron stars
The textbook burster, GS 1826-24
1997-8
2000
Galloway, Nuclear burning on the surface of accreting neutron stars
• This source, discovered in the
late 80s by the Ginga satellite, is
unique in that it consistently
exhibits highly regular bursts
• Lightcurves are extremely
consistent, and recurrence times
exhibit very little scatter within an
observation epoch
• We infer “ideal” case 1 burst
conditions: steady accretion,
complete coverage of fuel,
complete burning etc.
-> unique opportunity to test
theoretical models
Recurrence time vs. flux
• In observations spanning several
years, the persistent X-ray flux
increased by almost a factor of two
• The burst recurrence time
decreased by a similar factor,
exactly as expected for lowmetallicity ignition models
• However, the -values decreased
by 10%, indicating a slight change
in fuel composition, which requires
approximately solar metallicity
Galloway, Nuclear burning on the surface of accreting neutron stars
Lightcurve comparison
• However, using the time-dependent model
of Woosley et al. 2004 reveals that the
inconsistency is a result of the effects of
thermal- and compositional inertia
• The bursts are consistent with ignition of
H/He in fuel with approximately solar
metallicity
• In addition, we obtained stunning
agreement between the observed and
predicted lightcurves
• Except for a “bump” during the burst rise,
which may be a propagation effect, or
something arising from a particular nuclear
reaction
Galloway, Nuclear burning on the surface of accreting neutron stars
Propagation of the flame
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• Almost certainly
affects the
observational
properties, but not
really clear how
• Rise time in He
bursts is extremely
rapid (<1s) so this
limits propagation
effects
(Spitkovsky et al. 2002)
Galloway, Nuclear burning on the surface of accreting neutron stars
Unanswered questions: C ignition
• C ignition is a plausible explanation for the superburst
properties
BUT
• Models don’t produce enough C to power them
• Ignition occurs at too low a
column
• A possible explanation is that
the crust is not cooling like we
expect
• Such “premature” ignition also
occurs in normal
thermonuclear bursts
Carbon
Ignition
curve
Observed ignition
column
Galloway, Nuclear burning on the surface of accreting neutron stars
Early ignition/double bursts
• Already mentioned early ignition bursts, these appear to
occur preferentially in case 3 (unstable H) ignition
• We don’t understand how these events occur
• Future modeling might help
Galloway, Nuclear burning on the surface of accreting neutron stars
“Late” ignition
• As the accretion rate increases, we expect that the burst
rate also increases, up to the point when He burning
stabilises
• Instead we observe that bursts become less frequent, and
exhibit profiles more consistent with He-rich bursts
• One explanation is that the area over which accretion is
occuring is changing in a sufficiently diabolical way to give
exactly the opposite trend we want…
• Alternatively some have proposed yet another phase of
bursting, “delayed mixed bursts”, which burn much of their
fuel stably prior to ignition (Narayan & Heyl 2003, ApJ 599, 419)
Galloway, Nuclear burning on the surface of accreting neutron stars
Onset of stable burning
• Drop in burst rate well below the Eddington accretion rate is
not expected
• From modelling the onset of steady H-burning is expected at
around 0.92 of that level
• Around the transition
models exhibit erratic
switching between bursting
and oscillatory behaviour
“flickering burning”
• This may be observable as
quasi-periodic X-ray
oscillations (e.g. Revnitsev et al. 2001)
Galloway, Nuclear burning on the surface of accreting neutron stars