arXiv subjects

Andrew Cumming

Publications and source records attributed to Andrew Cumming.

At least 55 records · Page 3Linked to original sources

arXiv2016

A lower limit on the heat capacity of the neutron star core

We show that observations of the core temperature of transiently-accreting neutron stars combined with observations of an accretion outburst give a lower limit to the neutron star core heat capacity. For the neutron stars in the low mass X-ray binaries KS 1731-260, MXB 1659-29, and XTE J1701-462, we show that the lower limit is a factor of a few below the core heat capacity expected if neutrons and protons in the core are paired, so that electrons provide the dominant contribution to the heat capacity. This limit rules out a core dominated by a quark color-flavor-locked (CFL) phase, which would have a much lower heat capacity. Future observations of or limits on cooling during quiescence will further constrain the core heat capacity.

astro-ph.HE↗

arXiv2016

A Survey of Chemical Separation in Accreting Neutron Stars

The heavy element ashes of rp-process hydrogen and helium burning in accreting neutron stars are compressed to high density where they freeze, forming the outer crust of the star. We calculate the chemical separation on freezing for a number of different nuclear mixtures resulting from a range of burning conditions for the rp-process. We confirm the generic result that light nuclei are preferentially retained in the liquid and heavy nuclei in the solid. This is in agreement with the previous study of a 17-component mixture of rp-process ashes by Horowitz et al. (2007), but extends that result to a much larger range of compositions. We also find an alternate phase separation regime for the lightest ash mixtures which does not demonstrate this generic behaviour. With a few exceptions, we find that chemical separation reduces the expected $Q_{\rm imp}$ in the outer crust compared to the initial rp-process ash, where $Q_{\rm imp}$ measures the mean-square dispersion in atomic number $Z$ of the nuclei in the mixture. We find that the fractional spread of $Z$ plays a role in setting the amount of chemical separation and is strongly correlated to the divergence between the two/three-component approximations and the full component model. The contrast in $Y_e$ between the initial rp-process ashes and the equilibrium liquid composition is similar to that assumed in earlier two-component models of compositionally driven convection, except for very light compositions which produce nearly negligible convective driving. We discuss the implications of these results for observations of accreting neutron stars.

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arXiv2015

The link between coherent burst oscillations, burst spectral evolution and accretion state in 4U 1728-34

Coherent oscillations and the evolution of the X-ray spectrum during thermonuclear X-ray bursts in accreting neutron-star X-ray binaries have been studied intensively but separately. We analysed all the X-ray bursts of the source 4U 1728-34 with the Rossi X-ray Timing Explorer. We found that the presence of burst oscillations can be used to predict the behaviour of the blackbody radius during the cooling phase of the bursts. If a burst shows oscillations, during the cooling phase the blackbody radius remains more or less constant for ~2 - ~8s, whereas in bursts that do not show oscillations the blackbody radius either remains constant for more than ~2 - ~8s or it shows a rapid (faster than ~2s) decrease and increase. Both the presence of burst oscillations and the time-dependent spectral behaviour of the bursts are affected by accretion rate. We also found that the rise time and convexity of the bursts' light curve are different in bursts with and without oscillations in 4U 1728--34. Bursts with oscillations have a short rise time (~0.5s) and show both positive and negative convexity, whereas bursts without oscillations have a long rise time (~1s) and mostly positive convexity. This is consistent with the idea that burst oscillations are associated with off-equator ignition.

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arXiv2015

A strong shallow heat source in the accreting neutron star MAXI J0556-332

An accretion outburst in an X-ray transient deposits material onto the neutron star primary; this accumulation of matter induces reactions in the neutron star's crust. During the accretion outburst these reactions heat the crust out of thermal equilibrium with the core. When accretion halts, the crust cools to its long-term equilibrium temperature on observable timescales. Here we examine the accreting neutron star transient MAXI J0556-332, which is the hottest transient, at the start of quiescence, observed to date. Models of the quiescent light curve require a large deposition of heat in the shallow outer crust from an unknown source. The additional heat injected is $\approx 4\textrm{-}10\,\mathrm{MeV}$ per accreted nucleon; when the observed decline in accretion rate at the end of the outburst is accounted for, the required heating increases to $\approx 6\textrm{-}16\,\mathrm{MeV}$. This shallow heating is still required to fit the lightcurve even after taking into account a second accretion episode, uncertainties in distance, and different surface gravities. The amount of shallow heating is larger than that inferred for other neutron star transients and is larger than can be supplied by nuclear reactions or compositionally driven convection; but it is consistent with stored mechanical energy in the accretion disk. The high crust temperature ($T_b \gtrsim 10^{9} \, {\rm K}$) makes its cooling behavior in quiescence largely independent of the crust composition and envelope properties, so that future observations will probe the gravity of the source. Fits to the lightcurve disfavor the presence of Urca cooling pairs in the crust.

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arXiv2014

The Thermal Stability of Helium Burning on Accreting Neutron Stars

Thermonuclear burning on the surface of accreting neutron stars is observed to stabilize at accretion rates almost an order of magnitude lower than theoretical models predict. One way to resolve this discrepancy is by including a base heating flux that can stabilize the layer. We focus our attention on pure helium accretion, for which we calculate the effect of a base heating flux on the critical accretion rate at which thermonuclear burning stabilizes. We use the MESA stellar evolution code to calculate $\dot m_{\rm crit}$ as a function of the base flux, and derive analytic fitting formulae for $\dot m_{\rm crit}$ and the burning temperature at that critical accretion rate, based on a one-zone model. We also investigate whether the critical accretion rate can be determined by examining steady-state models only, without time-dependent simulations. We examine the argument that the stability boundary coincides with the turning point $dy_{\rm burn}/d\dot m=0$ in the steady-state models, and find that it does not hold outside of the one-zone, zero base flux case. A linear stability analysis of a large suite of steady-state models is also carried out, which yields critical accretion rates a factor of $\sim3$ larger than the MESA result, but with a similar dependence on base flux. Lastly, we discuss the implications of our results for the ultracompact X-ray binary 4U~1820-30.

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arXiv2014

Time-dependent, compositionally driven convection in the oceans of accreting neutron stars

We discuss the effect of chemical separation as matter freezes at the base of the ocean of an accreting neutron star, and the subsequent enrichment of the ocean in light elements and inward transport of heat through convective mixing. We extend the steady-state results of Medin & Cumming 2011 to transiently accreting neutron stars, by considering the time-dependent cases of heating during accretion outbursts and cooling during quiescence. Convective mixing is extremely efficient, flattening the composition profile in about one convective turnover time (weeks to months at the base of the ocean). During accretion outbursts, inward heat transport has only a small effect on the temperature profile in the outer layers until the ocean is strongly enriched in light elements, a process that takes hundreds of years to complete. During quiescence, however, inward heat transport rapidly cools the outer layers of the ocean while keeping the inner layers hot. We find that this leads to a sharp drop in surface emission at around a week followed by a gradual recovery as cooling becomes dominated by the crust. Such a dip should be observable in the light curves of these neutron star transients, if enough data is taken at a few days to a month after the end of accretion. If such a dip is definitively observed, it will provide strong constraints on the chemical composition of the ocean and outer crust.

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arXiv2014

A signature of chemical separation in the cooling curves of transiently accreting neutron stars

We show that convection driven by chemical separation can significantly affect the cooling curves of accreting neutron stars after they go into quiescence. We calculate the thermal relaxation of the neutron star ocean and crust including the thermal and compositional fluxes due to convection. After the inward propagating cooling wave reaches the base of the neutron star ocean, the ocean begins to freeze, driving chemical separation. The resulting convection transports heat inward, giving much faster cooling of the surface layers than found assuming the ocean cools passively. The light curves including convection show a rapid drop in temperature weeks after outburst. Identifying this signature in observed cooling curves would constrain the temperature and composition of the ocean as well as offer a real time probe of the freezing of a classical multicomponent plasma.

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arXiv2014

Carbon Synthesis in Steady-State Hydrogen and Helium Burning On Accreting Neutron Stars

Superbursts from accreting neutron stars probe nuclear reactions at extreme densities ($ρ\approx 10^{9}~g\,cm^{-3}$) and temperatures ($T>10^9~K$). These bursts ($\sim$1000 times more energetic than type I X-ray bursts) are most likely triggered by unstable ignition of carbon in a sea of heavy nuclei made during the rp-process of regular type I X-ray bursts (where the accumulated hydrogen and helium are burned). An open question is the origin of sufficient amounts of carbon, which is largely destroyed during the rp-process in X-ray bursts. We explore carbon production in steady-state burning via the rp-process, which might occur together with unstable burning in systems showing superbursts. We find that for a wide range of accretion rates and accreted helium mass fractions large amounts of carbon are produced, even for systems that accrete solar composition. This makes stable hydrogen and helium burning a viable source of carbon to trigger superbursts. We also investigate the sensitivity of the results to nuclear reactions. We find that the $^{14}$O($α$,p)$^{17}$F reaction rate introduces by far the largest uncertainties in the $^{12}$C yield.

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arXiv2014

Hall Attractor in Axially Symmetric Magnetic Fields in Neutron Star Crusts

We have found an attractor for an axially symmetric magnetic field evolving under the Hall effect and subdominant ohmic dissipation, resolving the question of the long term fate of the magnetic field in neutron star crusts. The electron fluid is in isorotation, analogous to Ferraro's law, with its angular velocity being approximately proportional to the poloidal magnetic flux, $Ω\propto Ψ$. This equilibrium is the long term configuration of a magnetic field evolving because of the Hall effect and ohmic dissipation. For an initial dipole dominated field the attractor consists mainly of a dipole and an octupole component accompanied by an energetically negligible quadrupole toroidal field. The field dissipates in a self-similar way: although higher multipoles should have been decaying faster, the toroidal field mediates transfer of energy into them from the lower ones, leading to an advection diffusion equilibrium and keeping the ratio of the poloidal multipoles almost constant. This has implications for the structure of the intermediate age neutron stars, suggesting that their poloidal field should consist of a dipole and a octupole component accompanied by a very weak toroidal quadrupole. For initial conditions that have a higher multipole $\ell$ structure the attractor consists mainly of $\ell$ and $\ell+2$ poloidal components.

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arXiv2014

Constraining the initial entropy of directly-detected exoplanets

The post-formation, initial entropy S_i of a gas giant planet is a key witness to its mass-assembly history and a crucial quantity for its early evolution. However, formation models are not yet able to predict reliably S_i, making unjustified the use solely of traditional, 'hot-start' cooling tracks to interpret direct imaging results and calling for an observational determination of initial entropies to guide formation scenarios. Using a grid of models in mass and entropy, we show how to place joint constraints on the mass and initial entropy of an object from its observed luminosity and age. This generalises the usual estimate of only a lower bound on the real mass through hot-start tracks. Moreover, we demonstrate that with mass information, e.g. from dynamical stability analyses or radial velocity, tighter bounds can be set on the initial entropy. We apply this procedure to 2M1207 b and find that its initial entropy is at least 9.2 kB/baryon, assuming that it does not burn deuterium. For the planets of the HR 8799 system, we infer that they must have formed with S_i > 9.2 kB/baryon, independent of uncertainties about the age of the star. Finally, a similar analysis for beta Pic b reveals that it must have formed with S_i > 10.5 kB/baryon, using the radial-velocity mass upper limit. These initial entropy values are respectively ca. 0.7, 0.5, and 1.5 kB/baryon higher than the ones obtained from core accretion models by Marley et al., thereby quantitatively ruling out the coldest starts for these objects and constraining warm starts, especially for beta Pic b.

astro-ph.EP↗

arXiv2014

Shedding Light on the Eccentricity Valley: Gap Heating and Eccentricity Excitation of Giant Planets in Protoplanetary Disks

We show that the first order (non co-orbital) corotation torques are significantly modified by entropy gradients in a non-barotropic protoplanetary disk. Such non-barotropic torques can dramatically alter the balance that, for barotropic cases, results in the net eccentricity damping for giant gap-clearing planets embedded in the disk. We demonstrate that stellar illumination can heat the gap enough for the planet's orbital eccentricity to instead be excited. We also discuss the "Eccentricity Valley" noted in the known exoplanet population, where low-metallicity stars have a deficit of eccentric planets between $\sim 0.1$ and $\sim 1$ AU compared to metal-rich systems (Dawson & Murray-Clay 2013). We show that this feature in the planet distribution may be due to the self-shadowing of the disk by a rim located at the dust sublimation radius $\sim 0.1$ AU, which is known to exist for several T Tauri systems. In the shadowed region between $\sim 0.1$ and $\sim 1$ AU lack of gap insolation allows disk interactions to damp eccentricity. Outside such shadowed regions stellar illumination can heat the planetary gaps and drive eccentricity growth for giant planets. We suggest that the self-shadowing does not arise at higher metallicity due to the increased optical depth of the gas interior to the dust sublimation radius.

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arXiv2012

Direct MD simulation of liquid-solid phase equilibria for three-component plasma

The neutron rich isotope 22Ne may be a significant impurity in carbon and oxygen white dwarfs and could impact how the stars freeze. We perform molecular dynamics simulations to determine the influence of 22Ne in carbon-oxygen-neon systems on liquid-solid phase equilibria. Both liquid and solid phases are present simultaneously in our simulation volumes. We identify liquid, solid, and interface regions in our simulations using a bond angle metric. In general we find good agreement for the composition of liquid and solid phases between our MD simulations and the semi analytic model of Medin and Cumming. The trace presence of a third component, neon, does not appear to strongly impact the chemical separation found previously for two component carbon and oxygen systems. This suggests that small amounts of 22Ne may not qualitatively change how the material in white dwarf stars freezes. However, we do find systematically lower melting temperatures (higher Gamma) in our MD simulations compared to the semi analytic model. This difference seems to grow with impurity parameter Q_imp and suggests a problem with simple corrections to the linear mixing rule for the free energy of multicomponent solid mixtures that is used in the semi analytic model.

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arXiv2012

Spectral and timing properties of the magnetar CXOU~J164710.2-455216

We report on spectral and timing properties of the magnetar CXOU J164710.2-455216 in the massive star cluster Westerlund 1. Using 11 archival observations obtained with Chandra and XMM-Newton over approximately 1000 days after the source's 2006 outburst, we study the flux and spectral evolution of the source. We show that the hardness of the source, as quantified by hardness ratio, blackbody temperature or power-law photon index, shows a clear correlation with the 2--10 keV absorption-corrected flux and that the power-law component flux decayed faster than the blackbody component for the first ~100 days. We also measure the timing properties of the source by analyzing data spanning approximately 2500 days. The measured period and period derivative are 10.610644(17) s (MJD 53999.06) and <4 X 10^{-13} s s^-1 (90% confidence) which imply that the spin-inferred dipolar magnetic field of the source is less than 7 X 10^{13} G. This is significantly smaller than was suggested previously. We find evidence for a second flux increase, suggesting a second outburst between MJDs 55068 and 55832. Finally, based on a crustal cooling model, we find that the source's cooling curve can be reproduced if we assume that the energy was deposited in the outer crust and that the temperature profile of the star right after the 2006 outburst was relatively independent of density.

astro-ph.HE↗

arXiv2012

Chandra observations of SGR 1627-41 near quiescence

We report on an observation of SGR 1627-41 made with the Chandra X-ray Observatory on 2011 June 16. Approximately three years after its outburst activity in 2008, the source's flux has been declining, as it approaches its quiescent state. For an assumed power-law spectrum, we find that the absorbed 2--10 keV flux for the source is $1.0^{+0.3}_{-0.2} \times 10^{-13} erg cm^{-2} s^{-1}$ with a photon index of $2.9 \pm 0.8$ ($N_H=1.0\times10^{23}$ cm^{-2}). This flux is approximately consistent with that measured at the same time after the source's outburst in 1998. With measurements spanning 3 years after the 2008 outburst, we analyze the long-term flux and spectral evolution of the source. The flux evolution is well described by a double exponential with decay times of 0.5 $\pm$ 0.1 and 59 $\pm$ 6 days, and a thermal cooling model fit suggests that SGR 1627-41 may have a hot core ($T_c ~ 2\times 10^8$ K). We find no clear correlation between flux and spectral hardness as found in other magnetars. We consider the quiescent X-ray luminosities of magnetars and the subset of rotation-powered pulsars with high magnetic fields ($B >~ 10^{13}$ G) in relation to their spin-inferred surface magnetic-field strength, and find a possible trend between the two quantities.

astro-ph.HE↗

arXiv2012

Ohmic Dissipation in the Interiors of Hot Jupiters

We present models of ohmic heating in the interiors of hot jupiters in which we decouple the interior and the wind zone by replacing the wind zone with a boundary temperature Tiso and magnetic field Bphi0. Ohmic heating influences the contraction of gas giants in two ways: by direct heating within the convection zone, and by heating outside the convection zone which increases the effective insulation of the interior. We calculate these effects, and show that internal ohmic heating is only able to slow the contraction rate of a cooling gas giant once the planet reaches a critical value of internal entropy. We determine the age of the gas giant when ohmic heating becomes important as a function of mass, Tiso and induced Bphi0. With this survey of parameter space complete, we then adopt the wind zone scalings of Menou (2012) and calculate the expected evolution of gas giants with different levels of irradiation. We find that,with this prescription of magnetic drag, it is difficult to inflate massive planets or those with strong irradiation using ohmic heating, meaning that we are unable to account for many of the observed hot jupiter radii. This is in contrast to previous evolutionary models that assumed that a constant fraction of the irradiation is transformed into ohmic power.

astro-ph.EP↗

arXiv2012

Constraints on neutron star mass and radius in GS 1826-24 from sub-Eddington X-ray bursts

We investigate the constraints on neutron star mass and radius in GS 1826-24 from models of lightcurves and spectral evolution of type I X-ray bursts. This source shows remarkable agreement with theoretical calculations of burst energies, recurrence times, and lightcurves. We first exploit this agreement to set the overall luminosity scale of the observed bursts. When combined with a measured blackbody normalization, this leads to a distance and anisotropy independent measurement of the ratio between the redshift 1+z and color correction factor f_c. We find 1+z=1.19-1.28 for f_c=1.4-1.5. We then compare the evolution of the blackbody normalization with flux in the cooling tail of bursts with predictions from spectral models of Suleimanov et al. (2011b). The observations are well described by the models at luminosities greater than about one third of the peak luminosity, with deviations emerging at luminosities below that. We show that this comparison leads to distance independent upper limits on R_\infty and neutron star mass of R_\infty\lesssim 9.0-13.2 km and M<1.2-1.7 M_\odot, respectively, for solar abundance of hydrogen at the photosphere and a range of metallicity and surface gravity. The radius limits are low in comparison to previous measurements. This may be indicative of a subsolar hydrogen fraction in the GS 1826-24 photosphere, or of larger color corrections than that predicted by spetral models. Our analysis also gives an upper limit on the distance to GS 1826-24 of d<4.0-5.5 kpc ξ_b^{-1/2}, where ξ_b is the degree of anisotropy of the burst emission.

astro-ph.SR↗

arXiv2010

Compositionally-driven convection in the oceans of accreting neutron stars

We discuss the effect of chemical separation as matter freezes at the base of the ocean of an accreting neutron star, and argue that the retention of light elements in the liquid acts as a source of buoyancy that drives a slow but continual mixing of the ocean, enriching it substantially in light elements, and leading to a relatively uniform composition with depth. We first consider the timescales associated with different processes that can redistribute elements in the ocean, including convection, sedimentation, crystallization, and diffusion. We then calculate the steady state structure of the ocean of a neutron star for an illustrative model in which the accreted hydrogen and helium burns to produce a mixture of O and Se. Even though the H/He burning produces only 2% oxygen by mass, the steady state ocean has an oxygen abundance more than ten times larger, almost 40% by mass. Furthermore, we show that the convective motions transport heat inwards, with a flux of ~ 0.2 MeV per nucleon for an O-Se ocean, heating the ocean and steepening the outwards temperature gradient. The enrichment of light elements and heating of the ocean due to compositionally-driven convection likely have important implications for carbon ignition models of superbursts.

astro-ph.SR↗

arXiv2010

Continued cooling of the crust in the neutron star low-mass X-ray binary KS 1731-260

Some neutron star low-mass X-ray binaries have very long outbursts (lasting several years) which can generate a significant amount of heat in the neutron star crust. After the system has returned to quiescence, the crust then thermally relaxes. This provides a rare opportunity to study the thermal properties of neutron star crusts, putting constraints on the thermal conductivity and hence the structure and composition of the crust. KS 1731-260 is one of only four systems where this crustal cooling has been observed. Here, we present a new Chandra observation of this source approximately 8 years after the end of the last outburst, and 4 years since the last observation. We find that the source has continued to cool, with the cooling curve displaying a simple power-law decay. This suggests that the crust has not fully thermally relaxed yet, and may continue to cool further. A simple power law decay is in contrast to theoretical cooling models of the crust, which predict that the crust should now have cooled to the same temperature as the neutron star core.

astro-ph.HE↗