arXiv subjects

L. Keek

Publications and source records attributed to L. Keek.

At least 37 records · Page 2Linked to original sources

arXiv2015

Carbon production on accreting neutron stars in a new regime of stable nuclear burning

Accreting neutron stars exhibit Type I X-ray bursts from both frequent hydrogen/helium flashes as well as rare carbon flashes. The latter (superbursts) ignite in the ashes of the former. Hydrogen/helium bursts, however, are thought to produce insufficient carbon to power superbursts. Stable burning could create the required carbon, but this was predicted to only occur at much larger accretion rates than where superbursts are observed. We present models of a new steady-state regime of stable hydrogen and helium burning that produces pure carbon ashes. Hot CNO burning of hydrogen heats the neutron star envelope and causes helium to burn before the conditions of a helium flash are reached. This takes place when the mass accretion rate is around 10% of the Eddington limit: close to the rate where most superbursts occur. We find that increased heating at the base of the envelope sustains steady-state burning by steepening the temperature profile, which increases the amount of helium that burns before a runaway can ensue.

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arXiv2015

Spectral and timing analysis of the mHz QPOs in the neutron-star low-mass X-ray binary 4U 1636-53

We investigate the spectral and timing properties of the millihertz quasi-periodic oscillations (mHz QPOs) in neutron-star low-mass X-ray binary 4U 1636-53 using XMM-Newton and Rossi X-ray Timing Explorer (RXTE) observations. The mHz QPOs in the XMM-Newton/RXTE observations show significant frequency variation and disappear right before type I X-ray bursts. We find no significant correlation between the mHz QPO frequency and the temperature of the neutron-star surface, which is different from theoretical predictions. For the first time we observed the full lifetime of a mHz QPO lasting 19 ks. Besides, we also measure a frequency drift timescale around 15 ks, we speculate that this is the cooling timescale of a layer deeper than the burning depth, possibly heated by the previous burst. Moreover, the analysis of all X-ray bursts in this source shows that all type I X-ray bursts associated with the mHz QPOs are short, bright and energetic, suggesting a potential connection between mHz QPOs and He-rich X-ray bursts.

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arXiv2015

The LOFT perspective on neutron star thermonuclear bursts

This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of thermonuclear X-ray bursts on accreting neutron stars. For a summary, we refer to the paper.

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arXiv2014

X-Raying an Accretion Disk in Realtime: the Evolution of Ionized Reflection during a Superburst from 4U 1636-536

When a thermonuclear X-ray burst ignites on an accreting neutron star, the accretion disk undergoes sudden strong X-ray illumination, which can drive a range of processes in the disk. Observations of superbursts, with durations of several hours, provide the best opportunity to study these processes and to probe accretion physics. Using detailed models of ionized reflection, we perform time resolved spectroscopy of the superburst observed from 4U 1636-536 in 2001 with RXTE. The spectra are consistent with a blackbody reflecting off a photoionized accretion disk, with the ionization state dropping with time. The evolution of the reflection fraction indicates that the initial reflection occurs from a part of the disk at larger radius, subsequently transitioning to reflection from an inner region of the disk. Even though this superburst did not reach the Eddington limit, we find that a strong local absorber develops during the superburst. Including this event, only two superbursts have been observed by an instrument with sufficient collecting area to allow for this analysis. It highlights the exciting opportunity for future X-ray observatories to investigate the processes in accretion disks when illuminated by superbursts.

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arXiv2014

Reaction Rate and Composition Dependence of the Stability of Thermonuclear Burning on Accreting Neutron Stars

The stability of thermonuclear burning of hydrogen and helium accreted onto neutron stars is strongly dependent on the mass accretion rate. The burning behavior is observed to change from Type I X-ray bursts to stable burning, with oscillatory burning occurring at the transition. Simulations predict the transition at a ten times higher mass accretion rate than observed. Using numerical models we investigate how the transition depends on the hydrogen, helium, and CNO mass fractions of the accreted material, as well as on the nuclear reaction rates of triple alpha and the hot-CNO breakout reactions 15O(a,g)19Ne and 18Ne(a,p)21Na. For a lower hydrogen content the transition is at higher accretion rates. Furthermore, most experimentally allowed reaction rate variations change the transition accretion rate by at most 10%. A factor ten decrease of the 15O(a,g)19Ne rate, however, produces an increase of the transition accretion rate of 35%. None of our models reproduce the transition at the observed rate, and depending on the true 15O(a,g)19Ne reaction rate, the actual discrepancy may be substantially larger. We find that the width of the interval of accretion rates with marginally stable burning depends strongly on both composition and reaction rates. Furthermore, close to the stability transition, our models predict that X-ray bursts have extended tails where freshly accreted fuel prolongs nuclear burning.

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arXiv2014

Characterizing the Evolving X-Ray Spectral Features During a Superburst from 4U 1636-536

Recent studies have shown that runaway thermonuclear burning of material accreted onto neutron stars, i.e. Type I X-ray bursts, may affect the accretion disk. We investigate this by performing a detailed time-resolved spectral analysis of the superburst from 4U 1636-536 observed in 2001 with the Rossi X-ray Timing Explorer. Superbursts are attributed to the thermonuclear burning of carbon, and are approximately 1000 times more energetic than the regular short Type I bursts. This allows us to study detailed spectra for over 11 ks, compared to at most 100 s for regular bursts. A feature is present in the superburst spectra around 6.4 keV that is well fit with an emission line and an absorption edge, suggestive of reflection of the superburst off the accretion disk. The line and edge parameters evolve over time: the edge energy decreases from 9.4 keV at the peak to 8.1 keV in the tail, and both features become weaker in the tail. This is only the second superburst for which this has been detected, and shows that this behavior is present even without strong radius expansion. Furthermore, we find the persistent flux to almost double during the superburst, and return to the pre-superburst level in the tail. The combination of reflection features and increased persistent emission indicates that the superburst had a strong impact on the inner accretion disk, and it emphasizes that X-ray bursts provide a unique probe of accretion physics.

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arXiv2012

Superexpansion as a possible probe of accretion in 4U 1820-30

The ultracompact X-ray binary 4U 1820-30 is well known for its ~170-d superorbital modulation in X-ray flux and spectrum, and the exclusiveness of bursting behavior to the low hard 'island' state. In May-June 2009, there was an exceptionally long 51-d low state. This state was well covered by X-ray observations and 12 bursts were detected, 9 with the high-throughput RXTE. We investigate the character of these X-ray bursts and find an interesting change in their photospheric expansion behavior. At the lowest inferred mass accretion rates, this expansion becomes very large in 4 bursts and reaches the so-called superexpansion regime. We speculate that this is due to the geometry of the inner accretion flow being spherical and a decreasing accretion rate: when the flow geometry nearest to the neutron star is spherical and the accretion rate is low, the ram pressure of the accretion disk may become too low to counteract that of the photospheric expansion. In effect, this may provide a novel means to probe the accretion flow. Additionally, we observe a peculiar effect: the well-known cessation of X-ray bursts in the high state is too quick to be consistent with a transition to stable helium burning. We suggest an alternative explanation, that the cessation is due to the introduction of a non-nuclear heat source in the neutron star ocean.

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arXiv2012

A superburst candidate in EXO 1745--248 as a challenge to thermonuclear ignition models

We report on Chandra, RXTE, Swift/BAT and MAXI observations of a ~1 day X-ray flare and subsequent outburst of a transient X-ray source observed in October-November 2011 in the globular cluster Terzan 5. We show that the source is the same as the transient that was active in 2000, i.e., the neutron star low-mass X-ray binary EXO 1745-248. For the X-ray flare we estimate a 6-11 hr exponential decay time and a radiated energy of 2-9 x 10^42 erg. These properties, together with strong evidence of decreasing blackbody temperature during the flare decay, are fully consistent with what is expected for a thermonuclear superburst. We use the most recent superburst models and estimate an ignition column depth of ~10^12 g cm^-2 and an energy release between 0.1-2 x 10^18 erg g^-1, also consistent with expected superburst values. We conclude therefore that the flare was most probably a superburst. We discuss our results in the context of theoretical models and find that even when assuming a few days of low level accretion before the superburst onset (which is more than what is suggested by the data), the observations of this superburst are very challenging for current superburst ignition models.

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arXiv2012

Superburst Models for Neutron Stars with Hydrogen and Helium-Rich Atmospheres

Superbursts are rare day-long Type I X-ray bursts due to carbon flashes on accreting neutron stars in low-mass X-ray binaries. They heat the neutron star envelope such that the burning of accreted hydrogen and helium becomes stable, and the common shorter X-ray bursts are quenched. Short bursts reappear only after the envelope cools down. We study multi-zone one-dimensional models of the neutron star envelope, in which we follow carbon burning during the superburst, and we include hydrogen and helium burning in the atmosphere above. We investigate both the case of a solar composition and a helium-rich atmosphere. This allows us to study for the first time a wide variety of thermonuclear burning behavior as well as the transitions between the different regimes in a self-consistent manner. For solar composition, burst quenching ends much sooner than previously expected. This is because of the complex interplay between the 3-alpha, hot CNO, and CNO breakout reactions. Stable burning of hydrogen and helium transitions via marginally stable burning (mHz QPOs) to less energetic bursts with short recurrence times. We find a short-lived bursting mode where weaker and stronger bursts alternate. Eventually the bursting behavior changes back to that of the pre-superburst bursts. Because of the scarcity of observations, this transition has not been directly detected after a superburst. Using the MINBAR burst catalog we identify the shortest upper limit on the quenching time for 4U 1636-536, and derive further constraints on the time scale on which bursts return.

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arXiv2012

Photospheric radius expansion in superburst precursors from neutron stars

Thermonuclear runaway burning of carbon is in rare cases observed from accreting neutron stars as day-long X-ray flares called superbursts. In the few cases where the onset is observed, superbursts exhibit a short precursor burst at the start. In each instance, however, the data was of insufficient quality for spectral analysis of the precursor. Using data from the propane anti-coincidence detector of the PCA instrument on RXTE, we perform the first detailed time resolved spectroscopy of precursors. For a superburst from 4U 1820-30 we demonstrate the presence of photospheric radius expansion. We find the precursor to be 1.4-2 times more energetic than other short bursts from this source, indicating that the burning of accreted helium is insufficient to explain the full precursor. Shock heating would be able to account for the lacking energy. We argue that this precursor is a strong indication that the superburst starts as a detonation, and that a shock induces the precursor. Furthermore, we employ our technique to study the superexpansion phase of the same superburst in greater detail.

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arXiv2012

Millihertz quasi-periodic oscillations and thermonuclear bursts from Terzan 5: A showcase of burning regimes

We present a comprehensive study of the thermonuclear bursts and millihertz quasi-periodic oscillations (mHz QPOs) from the neutron star (NS) transient and 11 Hz X-ray pulsar IGR J17480-2446, located in the globular cluster Terzan 5. The increase in burst rate that we found during its 2010 outburst, when persistent luminosity rose from 0.1 to 0.5 times the Eddington limit, is in qualitative agreement with thermonuclear burning theory yet opposite to all previous observations of thermonuclear bursts. Thermonuclear bursts gradually evolved into a mHz QPO when the accretion rate increased, and vice versa. The mHz QPOs from IGR J17480-2446 resemble those previously observed in other accreting NSs, yet they feature lower frequencies (by a factor ~3) and occur when the persistent luminosity is higher (by a factor 4-25). We find four distinct bursting regimes and a steep (close to inverse cubic) decrease of the burst recurrence time with increasing persistent luminosity. We compare these findings to nuclear burning models and find evidence for a transition between the pure helium and mixed hydrogen/helium ignition regimes when the persistent luminosity was about 0.3 times the Eddington limit. We also point out important discrepancies between the observed bursts and theory, which predicts brighter and less frequent bursts, and suggest that an additional source of heat in the NS envelope is required to reconcile the observed and expected burst properties. We discuss the impact of NS magnetic field and spin on the expected nuclear burning regimes, in the context of this particular pulsar.

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arXiv2011

Multi-Zone Models of Superbursts from Accreting Neutron Stars

Superbursts are rare and energetic thermonuclear carbon flashes observed to occur on accreting neutron stars. We create the first multi-zone models of series of superbursts using a stellar evolution code. We self-consistently build up the fuel layer at different rates, spanning the entire range of observed mass accretion rates for superbursters. For all models light curves are presented. They generally exhibit a shock breakout, a precursor burst due to shock heating, and a two-component power-law decay. Shock heating alone is sufficient for a bright precursor, that follows the shock breakout on a short dynamical time scale due to the fall-back of expanded layers. Models at the highest accretion rates, however, lack a shock breakout, precursor, and the first power law decay component. The ashes of the superburst that form the outer crust are predominantly composed of iron, but a superburst leaves a silicon-rich layer behind in which the next one ignites. Comparing the model light curves to an observed superburst from 4U 1636-53, we find for our accretion composition the best agreement with a model at three times the observed accretion rate. We study the dependence on crustal heating of observables such as the recurrence time and the decay time scale. It remains difficult, however, to constrain crustal heating, if there is no good match with the observed accretion rate, as we see for 4U 1636-53.

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arXiv2010

Multi-Instrument X-ray Observations of Thermonuclear Bursts with Short Recurrence Times

Type I X-ray bursts from low-mass X-ray binaries result from a thermonuclear runaway in the material accreted onto the neutron star. Although typical recurrence times are a few hours, consistent with theoretical ignition model predictions, there are also observations of bursts occurring as promptly as ten minutes or less after the previous event. We present a comprehensive assessment of this phenomenon using a catalog of 3387 bursts observed with the BeppoSAX/WFCs and RXTE/PCA X-ray instruments. This catalog contains 136 bursts with recurrence times of less than one hour, that come in multiples of up to four events, from 15 sources. Short recurrence times are not observed from so-called ultra-compact binaries, indicating that hydrogen burning processes play a crucial role. As far as the neutron star spin frequency is known, these sources all spin fast at over 500 Hz; the rotationally induced mixing may explain burst recurrence times of the order of 10 min. Short recurrence time bursts generally occur at all mass accretion rates where normal bursts are observed, but for individual sources the short recurrence times may be restricted to a smaller interval of accretion rate. The fraction of such bursts is roughly 30%. We also report the shortest known recurrence time of 3.8 minutes.

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arXiv2009

The effect of rotation on the stability of nuclear burning in accreting neutron stars

Hydrogen and/or helium accreted by a neutron star from a binary companion may undergo thermonuclear fusion. At different mass accretion rates different burning regimes are discerned. Theoretical models predict helium fusion to proceed as a thermonuclear runaway for accretion rates below the Eddington limit and as stable burning above this limit. Observations, however, place the boundary close to 10% of the Eddington limit. We study the effect of rotationally induced transport processes on the stability of helium burning. For the first time detailed calculations of thin helium shell burning on neutron stars are performed using a hydrodynamic stellar evolution code including rotation and rotationally induced magnetic fields. We find that in most cases the instabilities from the magnetic field provide the dominant contribution to the chemical mixing, while Eddington-Sweet circulations become important at high rotation rates. As helium is diffused to greater depths, the stability of the burning is increased, such that the critical accretion rate for stable helium burning is found to be lower. Combined with a higher heat flux from the crust, as suggested by recent studies, turbulent mixing could explain the observed critical accretion rate. Furthermore, close to this boundary we find oscillatory burning, which previous studies have linked to mHz QPOs. In models where we continuously lower the heat flux from the crust, the period of the oscillations increases by up to several tens of percents, similar to the observed frequency drift, suggesting that this drift could be caused by the cooling of deeper layers.

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arXiv2009

Long tails on thermonuclear X-ray bursts from neutron stars: a signature of inward heating?

We report the discovery of one-hour long tails on the few-minutes long X-ray bursts from the `clocked burster' GS 1826-24. We propose that the tails are due to enduring thermal radiation from the neutron star envelope. The enduring emission can be explained by cooling of deeper NS layers which were heated up through inward conduction of heat produced in the thermonuclear shell flash responsible for the burst. Similar, though somewhat shorter, tails are seen in bursts from EXO 0748-676 and 4U 1728-34. Only a small amount of cooling is detected in all these tails. This is either due to compton up scattering of the tail photons or, more likely, to a NS that is already fairly hot due to other, stable, nuclear processes.

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arXiv2008

On burning regimes and long duration X-ray bursts

Hydrogen and helium accreted onto a neutron star undergo thermonuclear burning. Explosive burning is observed as a type I X-ray burst. We describe the different burning regimes and focus on some of the current inconsistencies between theory and observations. Of special interest are the rare kinds of X-ray bursts such as carbon-fueled superbursts and helium-fueled intermediately long X-ray bursts. These bursts are thought to originate deeper in the neutron star envelope, such that they are probes of the thermal properties of the crust. We investigate the possibility of observing superbursts with the wide-field instruments INTEGRAL-ISGRI and Swift-BAT. We find that only the brightest bursts are detectable.

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arXiv2008

An X-ray and optical study of the ultracompact X-ray binary A1246-58

Results are discussed of an X-ray and optical observation campaign of the low-mass X-ray binary A1246-58 performed with instruments on Satellite per Astronomia X ('BeppoSAX'), the Rossi X-ray Timing Explorer (RXTE), the X-ray Multi-mirror Mission ('XMM-Newton'), the Swift mission, and the Very Large Telescope. Spectra and flux time histories are studied. The most important results are the lack of hydrogen spectral features in the optical spectrum, supporting the proposition that this is an ultracompact X-ray binary (UCXB), the determination of a 4.3 kpc distance from time-resolved spectroscopy of thermonuclear X-ray bursts, and the detection of intermediately long thermonuclear bursts as seen in a number of other UCXBs. There is evidence for a Ne/O abundance ratio in the line of sight that is higher than solar and variable. This may be due to different changes in the ionization degrees of Ne and O, which may be related to the variable irradiating flux. We discuss the spectral variability and the peculiarities of the long-term light curve.

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arXiv2007

First superburst from a classical low-mass X-ray binary transient

We report the analysis of the first superburst from a transiently accreting neutron star system with the All-Sky Monitor (ASM) on the Rossi X-ray Timing Explorer. The superburst occurred 55 days after the onset of an accretion outburst in 4U 1608-522. During that time interval, the accretion rate was at least 7% of the Eddington limit. The peak flux of the superburst is 22 to 45% of the Eddington limit, and its radiation energy output is between 4e41 and 9e41 erg for a distance of 3.2 kpc. Fits of cooling models to the superburst light curve indicate an ignition column depth between 1.5e12 and 4.1e12 g/cm2. Extrapolating the accretion history observed by the ASM, we derive that this column was accreted over a period of 26 to 72 years. The superburst characteristics are consistent with those seen in other superbursting low-mass X-ray binaries. However, the transient nature of the hosting binary presents significant challenges for superburst theory, requiring additional ingredients for the models. The carbon that fuels the superburst is thought to be produced mostly during the accretion outbursts and destroyed in the frequent type-I X-ray bursts. Mixing and sedimentation of the elements in the neutron star envelope may significantly influence the balance between the creation and destruction of carbon. Furthermore, predictions for the temperature of the neutron star crust fail to reach the values required for the ignition of carbon at the inferred column depth.

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