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The complex and puzzling
phenomenology of thermonuclear Xray bursts
Duncan Galloway
Monash University
Andrew Cumming McGill
Jean in ‘t Zand SRON
Deepto Chakrabarty & Jake
Hartman MIT
Mike Muno Caltech
Dimitrios Psaltis Arizona
APS April 2008, St. Louis
Motivation
• Thermonuclear bursts are a key observational phenomenon
which uniquely allow us to derive information about the
neutron stars upon which they occur
• Burst oscillations -> NS spin
• Peak flux of radius-expansion bursts -> distance
• Spectrum in the burst tail -> NS radius…
• … and hence (in principle) constrain the neutron star EOS
Of course, there are also the details of the thermonuclear
burning, how it spreads over the star, the balance between
stability and instability…
Burst theory: 3 ignition regimes
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;
stable
burning
accretion
rate
Case 1
Case 2
Case 3
ignition
curves
In general, the behavior of bursts from
individual sources do
NOT
match our expectations from theory
The exceptions are the rule
• Bursts tend to decrease in frequency at higher accretion
rates, rather than increasing, as expected theoretically
• Such bursts are often much fainter than might be expected
(given the wait time and inferred accretion rate), suggesting
an “energy leak”
• A (possibly) related issue is that “normal” thermonuclear
bursts don’t produce enough carbon to power “super” bursts
• We also see bursts with inexplicably short recurrence times,
down to a few minutes, sometimes in groups of three or four
Low-mass X-ray binaries
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
•
compact objects
accreting material from a
low-mass stellar
companion
•
LMXBs are thought to be
“old” systems with weak
magnetic fields allowing
the accreting material to
fall directly onto the
surface
•
Approx. 100 are known within our Galaxy, with orbital periods between 10 min
and 16.6 d
•
Unstable nuclear burning of accumulated fuel on the neutron star surface leads to
thermonuclear (type-I) bursts
•
The Rossi X-ray Timing Explorer (RXTE) has observed more than 1200 bursts from 40 of
these systems over it’s 10-year mission lifetime, with high time resolution and signal-tonoise
Important scales
•
•
•
•
Neutron star mass ≈ 1.4 M, & radius ≈ 10 km
Distance to Galactic LMXB systems ≈ 8 kpc
Typical persistent intensity 10-9 erg cm-2 s-1
… corresponding to an accretion rate of ≈10% of the
Eddington rate (8.8104 g cm-2 s-1 or 1.310-8 M yr-1)
• Typical burst peak intensity 10-7 erg cm-2 s-1
• Characteristic burst fluence (integrated flux) of 1039 erg
• Ratio  of integrated persistent flux to burst flux is 40 (i.e.
accretion is much more efficient than thermonuclear
burning!)
Examples of X-ray bursts from RXTE
Superbursts: carbon burning?
• 1000x more energetic than
typical thermonuclear bursts
(1042 erg)
4U 1636-536
104 s
• 1000x less frequent
(recurrence times of months,
instead of hours)
• Thought to arise from unstable ignition of carbon produced
as a by-product of burning during “normal” thermonuclear
bursts
A well-behaved burster: GS 1826-24
1997-8
2000
• 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” burst conditions:
steady accretion, complete
coverage of fuel, complete burning
etc.
-> unique opportunity to test
theoretical models
… aka the textbook burster
1997-8
2000
• In RXTE 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 constant
accreted mass at ignition
• A ~10% change in  over this time
suggests solar fuel composition
(Galloway et al. 2004, ApJ 601, 466; see also
Thompson et al. 2007, ApJ accepted,
arXiv:0712.3874)
Data & model comparisons
•
•
•
•
We also compared lightcurves with predictions
by the time-dependent model of Woosley et al.
2004
Confirms that the bursts occur via ignition of
H/He in fuel with approximately solar metallicity
(i.e. CNO mass fraction)
In addition, we obtained stunning agreement
between the observed and predicted
lightcurves (this is not a fit!)
Except for a “bump” during the burst rise, which
may be an artifact of the finite time for the
burning to spread, or something arising from a
particular nuclear reaction
(Heger et al. 2007, ApJL 671, L141)
Simpler models are sometimes OK
•
•
•
•
The time-independent models are still
OK where steady H-burning is not the
dominant heating process
For example, at low accretion rates
where the recurrence time is long
enough to exhaust all the hydrogen
prior to ignition
For the bursts observed during the
2002 outburst of the millisecond
pulsar SAX J1808.4-3658, we
obtained excellent agreement
between model predictions and
observations
Can derive the distance of 3.4-3.6 kpc
(Galloway et al. 2006, ApJ 652, 559)
What about all the other
burst sources?
No shortage of data
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
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 (Boirin et al. 2007, A&A
465, 559)
Studies of large burst samples
H-ignition
• Long-duration X-ray satellite
missions have accumulated
large samples of bursts from
many sources
• Analysis can identify global
trends, as well as revealing
important exceptions (e.g. Cornelisse
et al. 2003, A&A
405, 1033)
He-ignition
H-rich fuel
• A comparable sample of almost
1200 bursts has been assembled from
He-ignition
H-poor
fuel Explorer (Galloway et al. 2007, astroobservations by the Rossi X-ray
Timing
ph/0608259)
• Although
Note
that theseRXTE
are has a relatively narrow field of view, the large effective
area allows us to make precision measurements of burst properties and
carefully-selected
subsamples!
the persistent emission
X-ray colors  accretion rate?
1
2
This is about where we
observe mHz oscillations;
transition to stable Heburning?
(Heger et al. 2007, ApJ 665, 1311)
3
Diverse bursts at moderate M-dot
The bursts in the accretionrate range where the burst
rate is decreasing are
highly inhomogeneous
Burst rate vs. X-ray colors
long
short
Steady He-burning at 10% Eddington?
Observations at 10-30% Eddington, including
• Infrequent, He-like bursts with ~1000
• mHz oscillations
• The occurrence of superbursts requiring efficent carbon
production
All suggest that steady He-burning occurs in this range of
accretion rates
HOWEVER
It is far from clear how steady and unsteady (i.e. bursts) Heburning can happen simultaneously!
The (a) next step: MINBAR
The MultiINstrument Burst
Archive
(MINBAR) project
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Summary and future work
• Studies of thermonuclear bursts are experiencing a
“renaissance” with much recent theoretical and
observational activity
• Wide-field observations by INTEGRAL and Swift are a
promising new source of observations, as well as existing
samples from BeppoSAX and the growing RXTE sample
• Detailed comparisons with predictions by state-of-the-art
models confirm our present understanding of the nuclear
physics
• Studies of larger burst samples can help to resolve some of
the discrepancies with the observational properties
LMXB neutron star spins
26
Burst oscillations
24
Millisecond pulsars
22
Radio pulsars
20
Number
18
– Burst oscillations
– Persistent pulsations in 7
sources
– Intermittent pulsations in up to
three more
16
14
12
10
8
6
4
2
50
10
0
15
0
20
0
25
0
30
0
35
0
40
0
45
0
50
0
55
0
60
0
65
0
70
0
0
0
• Majority of NS don’t ever exhibit
pulsations!
• Can detect pulsations (and
hence measure the spin) by
Frequency (Hz)
• LMXBs are the evolutionary
progenitors to rotation-powered
MSPs
• And so cluster at high spin frequencies (in the accreting aka “recycling”
phase)
• Despite this, the fastest-spinning NS known is a rotation powered pulsar
at 716 Hz
A neutron star spinning at 1122 Hz?
• A burst from XTE J1739-285
revealed evidence for oscillations
at 1122 Hz (Kaaret et al. 2007; astroph/0611716)
• Maximum Leahy power 42.82
• Significance from M-C
simulations is 3.97; taking into
account the number of trials, at
most 3.5
• Signal was present at high
significance only in a single time
bin of a single burst -> needs
confirmation
Discovery of thermonuclear bursts
• Instruments like the Netherlands/USA satellite
ANS first observed bright X-ray flashes from
around the Galactic center and elsewhere in
the early ‘70s
• Multiple bursts from some sources
• 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; nuclear burning is much less
efficient, at around 1%
• Expected -ratio is then ~50 or more, in agreement with measurements
… and probe the global burning physics
 (i.e. the composition of the
burst fuel) depends only upon the
burst rate
-> dominated by the steady (H)
burning
Possible role of steady Heburning?
Superbursts and the core composition
• C ignition is a plausible explanation for the superburst
properties
BUT
• Models don’t produce enough C to power them
• Furthermore, ignition occurs at
too low a column
• (Such “premature” ignition also
occurs in H/He-burning
thermonuclear bursts)
• The crust is too hot; cooling
mechanisms are inefficient
Carbon
Ignition
curve
Observed ignition
column
A possible solution:
No crust!
• The compact object is composed of “strange quark matter”,
overlaid by a layer of normal matter supplied by accretion
• The fuel layer can be electrostatically supported, and bursts
may yet occur (Cumming et al. ApJ 646, 429; See also Page & Cumming 2005, ApJL 635,
157)
• Superbursts are difficult to study because of their scarcity
(only a few have been observed; estimated recurrence times
are ~months)
• H/He bursts on the other hand, also depend on the crust
cooling, and are more frequent
-> an alternative test for strange stars
… can similarly study energetics…
He-ignition
H-poor fuel
He-ignition
H-rich fuel
H-ignition