E. F. Brown

Michigan State University, Ист-Лансинг, Michigan, United States

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Publications (47)153.75 Total impact

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    ABSTRACT: Aql X-1 is a prolific transient neutron star low-mass X-ray binary that exhibits an accretion outburst approximately once every year. Whether the thermal X-rays detected in intervening quiescent episodes are the result of cooling of the neutron star or due to continued low-level accretion remains unclear. In this work, we use Swift data obtained after the long and bright 2011 and 2013 outbursts, as well as the short and faint 2015 outburst, to investigate the hypothesis that cooling of the accretion-heated neutron star crust dominates the quiescent thermal emission in Aql X-1. We demonstrate that the X-ray light curves and measured neutron star surface temperatures are consistent with the expectations of the crust cooling paradigm. By using a thermal evolution code, we find that ≃1.2–3.2 MeV nucleon−1 of shallow heat release describes the observational data well, depending on the assumed mass-accretion rate and temperature of the stellar core. We find no evidence for varying strengths of this shallow heating after different outbursts, but this could be due to limitations of the data. We argue that monitoring Aql X-1 for up to ≃1 yr after future outbursts can be a powerful tool to break model degeneracies and solve open questions about the magnitude, depth, and origin of shallow heating in neutron star crusts.
    Preview · Article · Dec 2015 · Monthly Notices of the Royal Astronomical Society
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    A. W. Steiner · J. M. Lattimer · E. F. Brown
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    ABSTRACT: We investigate constraints on neutron star structure arising from the assumptions that neutron stars have crusts, that recent calculations of pure neutron matter limit the equation of state of neutron star matter near the nuclear saturation density, that the high-density equation of state is limited by causality and the largest high-accuracy neutron star mass measurement, and that general relativity is the correct theory of gravity. We explore the role of prior assumptions by considering two classes of equation of state models. In a first, the intermediate- and high-density behavior of the equation of state is parameterized by piecewise polytropes. In the second class, the high-density behavior of the equation of state is parameterized by piecewise continuous line segments. The smallest density at which high-density matter appears is varied in order to allow for strong phase transitions above the nuclear saturation density. We critically examine correlations among the pressure of matter, radii, maximum masses, the binding energy, the moment of inertia, and the tidal deformability, paying special attention to the sensitivity of these correlations to prior assumptions about the equation of state. It is possible to constrain the radii of $1.4~\mathrm{M}_{\odot}$ neutron stars to a be larger than 10 km, even without consideration of additional astrophysical observations, for example, those from photospheric radius expansion bursts or quiescent low-mass X-ray binaries. We are able to improve the accuracy of known correlations between the moment of inertia and compactness as well as the binding energy and compactness. We also demonstrate the existence of a correlation between the neutron star binding energy and the moment of inertia.
    Preview · Article · Oct 2015 · European Physical Journal A
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    ABSTRACT: When neutron stars reside in transient X-ray binaries, their crustal layers become heated during accretion outbursts and subsequently cool in quiescence. Observing and modeling this thermal response has yielded valuable insight into the physics of neutron star crusts. However, one unresolved problem is the evidence in several neutron stars for an extra energy source, located at shallow depth in the crust, that is not accounted for by standard heating models. Its origin remains puzzling, and it is currently unclear whether this additional heating occurs in all neutron stars, and if the magnitude is always the same. Here, we report on Chandra observations that cover two years after the 2012 outburst of the transient neutron star X-ray binary Swift J174805.3-244637 in the globular cluster Terzan 5. The temperature of the neutron star was elevated during the first two months following its ~8 week accretion episode, but had decayed to the pre-outburst level within ~100 days. Interpreting this as rapid cooling of the accretion-heated crust, we model the observed temperature curve with a thermal evolution code. We find that there is no need to invoke shallow heating for this neutron star, although an extra energy release up to ~1.4 MeV/nucleon is allowed by the current data (2-sigma confidence). We also present two new data points on the crust cooling curve of the 11-Hz X-ray pulsar IGR J17480-2446 in Terzan 5, which was active in 2010. The temperature of this neutron star remained significantly above its pre-outburst level, but we detect no temperature change since the previous measurements of 2013 February. This is consistent with the slower cooling expected several years post-outburst.
    Full-text · Article · May 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: 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.
    Full-text · Article · Jan 2015
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    ABSTRACT: X-ray observations of quiescent X-ray binaries have the potential to provide insight into the structure and the composition of neutron stars. EXO 0748-676 had been actively accreting for over 24 years before its outburst ceased in late 2008. Subsequent X-ray monitoring revealed a gradual decay of the quiescent thermal emission that can be attributed to cooling of the accretion-heated neutron star crust. In this work, we report on new Chandra and Swift observations that extend the quiescent monitoring to ~5 yr post-outburst. We find that the neutron star temperature remained at ~117 eV between 2009 and 2011, but had decreased to ~110 eV in 2013. This suggests that the crust has not fully cooled yet, consistent with the fact that the temperature remains higher than observed in 1980, ~4 yr prior to the accretion phase (~95 eV). Comparing the data to thermal evolution simulations reveals that the apparent lack of cooling between 2009 and 2011 could possibly be a signature of convection driven by phase separation of light and heavy nuclei in the outer layers of the neutron star.
    Preview · Article · Mar 2014 · The Astrophysical Journal
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    ABSTRACT: The temperature in the crust of an accreting neutron star, which comprises its outermost kilometre, is set by heating from nuclear reactions at large densities, neutrino cooling and heat transport from the interior. The heated crust has been thought to affect observable phenomena at shallower depths, such as thermonuclear bursts in the accreted envelope. Here we report that cycles of electron capture and its inverse, β(-) decay, involving neutron-rich nuclei at a typical depth of about 150 metres, cool the outer neutron star crust by emitting neutrinos while also thermally decoupling the surface layers from the deeper crust. This 'Urca' mechanism has been studied in the context of white dwarfs and type Ia supernovae, but hitherto was not considered in neutron stars, because previous models computed the crust reactions using a zero-temperature approximation and assumed that only a single nuclear species was present at any given depth. The thermal decoupling means that X-ray bursts and other surface phenomena are largely independent of the strength of deep crustal heating. The unexpectedly short recurrence times, of the order of years, observed for very energetic thermonuclear superbursts are therefore not an indicator of a hot crust, but may point instead to an unknown local heating mechanism near the neutron star surface.
    Full-text · Article · Dec 2013 · Nature
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    ABSTRACT: We conducted the first long-term (60 days), multiwavelength (optical, ultraviolet, and X-ray) simultaneous monitoring of Cen X-4 with daily Swift observations, with the goal of understanding variability in the low mass X-ray binary Cen X-4 during quiescence. We found Cen X-4 to be highly variable in all energy bands on timescales from days to months, with the strongest quiescent variability a factor of 22 drop in the X-ray count rate in only 4 days. The X-ray, UV and optical (V band) emission are correlated on timescales down to less than 110 s. The shape of the correlation is a power law with index gamma about 0.2-0.6. The X-ray spectrum is well fitted by a hydrogen NS atmosphere (kT=59-80 eV) and a power law (with spectral index Gamma=1.4-2.0), with the spectral shape remaining constant as the flux varies. Both components vary in tandem, with each responsible for about 50% of the total X-ray flux, implying that they are physically linked. We conclude that the X-rays are likely generated by matter accreting down to the NS surface. Moreover, based on the short timescale of the correlation, we also unambiguously demonstrate that the UV emission can not be due to either thermal emission from the stream impact point, or a standard optically thick, geometrically thin disc. The spectral energy distribution shows a small UV emitting region, too hot to arise from the accretion disk, that we identified as a hot spot on the companion star. Therefore, the UV emission is most likely produced by reprocessing from the companion star, indeed the vertical size of the disc is small and can only reprocess a marginal fraction of the X-ray emission. We also found the accretion disc in quiescence to likely be UV faint, with a minimal contribution to the whole UV flux.
    Full-text · Article · Oct 2013 · The European Physical Journal Conferences
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    ABSTRACT: We conducted the first long-term (60 days), multiwavelength (optical, ultraviolet, and X-ray) simultaneous monitoring of Cen X-4 with daily Swift observations from June to August 2012, with the goal of understanding variability in the low mass X-ray binary Cen X-4 during quiescence. We found Cen X-4 to be highly variable in all energy bands on timescales from days to months, with the strongest quiescent variability a factor of 22 drop in the X-ray count rate in only 4 days. The X-ray, UV and optical (V band) emission are correlated on timescales down to less than 110 s. The shape of the correlation is a power law with index gamma about 0.2-0.6. The X-ray spectrum is well fitted by a hydrogen NS atmosphere (kT=59-80 eV) and a power law (with spectral index Gamma=1.4-2.0), with the spectral shape remaining constant as the flux varies. Both components vary in tandem, with each responsible for about 50% of the total X-ray flux, implying that they are physically linked. We conclude that the X-rays are likely generated by matter accreting down to the NS surface. Moreover, based on the short timescale of the correlation, we also unambiguously demonstrate that the UV emission can not be due to either thermal emission from the stream impact point, or a standard optically thick, geometrically thin disc. The spectral energy distribution shows a small UV emitting region, too hot to arise from the accretion disk, that we identified as a hot spot on the companion star. Therefore, the UV emission is most likely produced by reprocessing from the companion star, indeed the vertical size of the disc is small and can only reprocess a marginal fraction of the X-ray emission. We also found the accretion disc in quiescence to likely be UV faint, with a minimal contribution to the whole UV flux.
    Full-text · Article · Jul 2013 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: The transient neutron star low-mass X-ray binary and 11 Hz X-ray pulsar IGR J17480-2446 in the globular cluster Terzan 5 exhibited an 11-week accretion outburst in 2010. Chandra observations performed within five months after the end of the outburst revealed evidence that the crust of the neutron star became substantially heated during the accretion episode and was subsequently cooling in quiescence. This provides the rare opportunity to probe the structure and composition of the crust. Here, we report on new Chandra observations of Terzan 5 that extend the monitoring to ~2.2 yr into quiescence. We find that the thermal flux and neutron star temperature have continued to decrease, but remain significantly above the values that were measured before the 2010 accretion phase. This suggests that the crust has not thermally relaxed yet, and may continue to cool. Such behavior is difficult to explain within our current understanding of heating and cooling of transiently accreting neutron stars. Alternatively, the quiescent emission may have settled at a higher observed equilibrium level (for the same interior temperature), in which case the neutron star crust may have fully cooled.
    Preview · Article · Jun 2013 · The Astrophysical Journal
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    ABSTRACT: The quasi-persistent neutron star low-mass X-ray binary MXB 1659-29 went into quiescence in 2001, and we have followed its quiescent X-ray evolution since. Observations over the first 4 years showed a rapid drop in flux and temperature of the neutron star atmosphere, interpreted as cooling of the neutron star crust which had been heated during the 2.5 year outburst. However, observations taken approximately 1400 and 2400 days into quiescence were consistent with each other, suggesting the crust had reached thermal equilibrium with the core. Here we present a new Chandra observation of MXB 1659-29 taken 11 years into quiescence and 4 years since the last Chandra observation. This new observation shows an unexpected factor of ~3 drop in count rate and change in spectral shape since the last observation, which cannot be explained simply by continued cooling. Two possible scenarios are that either the neutron star temperature has remained unchanged and there has been an increase in the column density, or, alternatively the neutron star temperature has dropped precipitously and the spectrum is now dominated by a power-law component. The first scenario may be possible given that MXB 1659-29 is a near edge-on system, and an increase in column density could be due to build-up of material in, and a thickening of, a truncated accretion disk during quiescence. But, a large change in disk height may not be plausible if standard accretion disk theory holds during quiescence. Alternatively, the disk may be precessing, leading to a higher column density during this latest observation.
    Preview · Article · Jun 2013 · The Astrophysical Journal
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    ABSTRACT: Quiescent emission from the neutron star low-mass X-ray binary Cen X-4 is seen to be variable on time-scales from hundreds of seconds to years, suggesting that at least in this object, low-level accretion is important during quiescence. Here, we present results from recent XMM–Newton and Swift observations of Cen X-4, where the X-ray flux (0.5–10 keV) varies by a factor of 6.5 between the brightest and faintest states. We find a positive correlation between the X-ray flux and the simultaneous near-ultraviolet (UV) flux, where as there is no significant correlation between the X-ray and simultaneous optical (V, B) fluxes. This suggests that while the X-ray and UV emitting regions are somehow linked, the optical region originates elsewhere. Comparing the luminosities, it is plausible that the UV emission originates due to reprocessing of the X-ray flux by the accretion disc, with the hot inner region of the disc being a possible location for the UV emitting region. The optical emission, however, could be dominated by the donor star. The X-ray/UV correlation does not favour the accretion stream impact point as the source of the UV emission.
    Full-text · Article · Oct 2012 · Monthly Notices of the Royal Astronomical Society
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    N B Nguyen · F M Nunes · I J Thompson · E F Brown
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    ABSTRACT: A new three-body method is used to compute the rate of the triple-alpha capture reaction, which is the primary source of ^{12}C in stars. In this Letter, we combine the Faddeev hyperspherical harmonics and the R-matrix method to obtain a full solution to the three-body α+α+α continuum. Particular attention is paid to the long-range effects caused by the pairwise Coulomb interactions. The new rate agrees with the Nuclear Astrophysics Compilation of Reaction rates for temperatures greater than 0.07 GK, but a large enhancement at lower temperature is found (≈10^{12} at 0.02 GK). Our results are compared to previous calculations where additional approximations were made. We show that the new rate does not significantly change the evolution of stars around one solar mass. In particular, such stars still undergo a red-giant phase consistent with observations, and no significant differences are found in the final white dwarfs.
    Full-text · Article · Oct 2012 · Physical Review Letters
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    ABSTRACT: We present details of a statistical study investigating the role of the central density of the progenitor white dwarf on the brightness of a Type Ia supernova. We present results from a suite of two-dimensional simulations varying the central density at flame ignition. We find that the production of Fe-group material does not significantly change with increased progenitor central density, but that the mass of stable Fe-group isotopes is tightly correlated with central density. The result is a decrease in the production of 56Ni, which we attribute to a higher rate of neutronization occurring at higher density. We present details of our models including the distribution of 56Ni and quantify trends of 56Ni production. We also relate the variations in central density to the age of the host galaxy stellar population through the main-sequence lifetime and the white dwarf cooling time, which is the elapsed time between the formation of the white dwarf and the onset of accretion. This density-age relationship, along with our results, allows us to obtain the observed relationship between the age of the host galaxy and the average brightness of an event. This work was supported by NASA under grant No. NNX09AD19G and utilized resources at the New York Center for Computational Sciences at Stony Brook University/Brookhaven National Laboratory, which is supported by the U.S. Department of Energy under Contract No. DE-AC02-98CH10886 and by the State of New York.
    No preview · Article · Jan 2012
  • N.B. Nguyen · F.M. Nunes · E.F. Brown · I.J. Thompson
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    ABSTRACT: A new three-body method (R-matrix hyperspherical harmonics) has been developed to calculate the triple alpha reaction rate at low temperature. This new method provides rates that are larger by several orders of magnitude than rates included in the standard databases, at temperatures below 0.06 GK. Rates above T=0.07 GK are consistent with previously used values. The modification of the triple-alpha rate at these low energies does not affect low-mass stellar evolution. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.
    No preview · Article · Jan 2012
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    ABSTRACT: We present results from time-of-flight nuclear mass measurements at the National Superconducting Cyclotron Laboratory that are relevant for neutron star crust models. The masses of 16 neutron-rich nuclei in the scandium-nickel range were determined simultaneously, with the masses of (61)V, (63)Cr, (66)Mn, and (74)Ni measured for the first time with mass excesses of -30.510(890) MeV, -35.280(650) MeV, -36.900(790) MeV, and -49.210(990) MeV, respectively. With these results the locations of the dominant electron capture heat sources in the outer crust of accreting neutron stars that exhibit super bursts are now experimentally constrained. We find the experimental Q value for the (66)Fe→(66)Mn electron capture to be 2.1 MeV (2.6σ) smaller than predicted, resulting in the transition occurring significantly closer to the neutron star surface.
    Full-text · Article · Oct 2011 · Physical Review Letters
  • Nathalie Degenaar · R. Wijnands · E. F. Brown
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    ABSTRACT: A profound reason to study neutron stars is that these stellar remnants provide a gateway to explore the behavior of matter under extreme physical conditions that are unattainable on Earth. We present new observational evidence for the temporary heating and subsequent cooling of the outer layers of a recently discovered neutron star that resides in a transient X-ray binary. Our findings provide strong support for the hypothesis that the accretion process causes a release of heat energy in a chain of nuclear reactions occurring deep inside the neutron star crust. Studies of the cooling of an accretion-heated neutron star crust can probe important stellar parameters, such as the thermal conductivity of the crust and the dominant neutrino emission mechanism operating in the core. This allows the investigation of a variety of processes occurring in the extreme density and pressure environment of the neutron star interior.
    No preview · Article · Sep 2011
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    N. Degenaar · E. F. Brown · R. Wijnands
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    ABSTRACT: The temporal heating and subsequent cooling of the crusts of transiently accreting neutron stars carries unique information about their structure and a variety of nuclear reaction processes. We report on a new Chandra Director's Discretionary Time observation of the globular cluster Terzan 5, aimed to monitor the transiently accreting 11-Hz X-ray pulsar IGR J17480-2446 after the cessation of its recent 10-week long accretion outburst. During the observation, which was performed ~125 days into quiescence, the source displays a thermal spectrum that fits to a neutron star atmosphere model with a temperature for an observer at infinity of kT~92 eV. This is ~10% lower than found ~75 days earlier, yet ~20% higher than the quiescent base level measured prior to the recent outburst. This can be interpreted as cooling of the accretion-heated neutron star crust, and implies that crust cooling is observable after short accretion episodes. Comparison with neutron star thermal evolution simulations indicates that substantial heat must be released at shallow depth inside the neutron star, which is not accounted for in current nuclear heating models.
    Preview · Article · Jul 2011 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: Type Ia supernovae (SNeIa) are bright astrophysical explosions that form a remarkably homogeneous class of objects used to study the expansion history of the Universe and the nature of Dark Energy. However, details of the explosion mechanism and the influence of properties of the host stellar population remain incompletely understood. The most widely accepted scenario for a SNIa to occur is the explosion of a near-Chandrasekhar mass white dwarf. Under this scenario, the thermonuclear explosion begins as a deflagration (subsonic burning) near the center that transitions to a detonation (supersonic burning) some time later. Turbulence, particularly its interaction with the flame, plays a key role throughout the evolution of the explosion process. Pre-existing turbulence from a vigorous convection field encompassing roughly 70% of the star will influence the evolution of the early flame, while turbulence generated by fluid instabilities is thought to interact with the flame such that a deflagration-to-detonation transition (DDT) occurs. As I will show, the DDT density strongly influences the yield of radioactive 56Ni that powers the light curve. While the conditions under which a DDT occurs remains an area of active research, it is thought to be influenced by the metallicity of the progenitor. I will offer an explanation of observed trends in the peak brightness of SNeIa with host galaxy metallicity from results of a suite of two-dimensional simulations in which the DDT density was varied. I will also present improvements to our flame model for the enhancement of burning by turbulence and discuss its application in three-dimensional simulations of the impact of pre-existing turbulence on the early flame evolution. This work was supported by NASA under grant No. NNX09AD19G.
    No preview · Article · Jan 2011
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    ABSTRACT: Type Ia supernovae (SNe Ia) are the premier standard candle for measuring the expansion history of the universe. SNe Ia make good standard candles only because their light curves can be calibrated. However, observations indicate even after calibration SNe Ia light curves have some dependence on properties of the host galaxy. Numerical models are steadily becoming more refined and can begin to probe the connection between the properties of the progenitor white dwarf and the outcome of the explosion. We perform numerical calculations to examine the effect of metallicity on the nucleosynthesis taking place in SNe Ia. Detailed yields resulting from explosive burning of the carbon/oxygen plasma in our models are examined using post-processing through a 532-nuclide reaction network. We explore how the production of elements from silicon to titanium varies with metallicity of the progenitor star. Our calculations suggest systematic trends in the silicon-group elements that may be observable. There is a clear trend with increasing metallicity of increasing silicon production while all other intermediate mass elements are produced in smaller abundances. We find, for example, that calcium follows a nearly linear trend of decreasing production with increasing metallicity. This work was supported by the US Department of Energy, Office of Nuclear Physics, under contract DE-AC02-06CH11357.
    No preview · Article · Jan 2011
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    ABSTRACT: We present a study of type Ia supernovae in the single-degenerate scenario, in which a white dwarf accretes mass from a companion star until it approaches the Chandrasekhar limiting mass and an explosion ensues. We investigate progenitor models with a range of central densities to study the influence of this parameter on explosion outcome. We present a suite of simulations from a well-controlled statistical study that allows us to quantify the effects of a variety of initial conditions. We present details of the models, including the mass and distribution of 56Ni, the radioactive decay of which powers the light curve. Our results indicate that progenitors with a higher central density produce less 56Ni and hence a dimmer event. We combine our results with those from previous studies by our collaboration to explore trends in explosion brightness that follow from properties related to the morphology and color of the host galaxy. This work was supported by NASA under grant No. NNX09AD19G and utilized resources at the New York Center for Computational Sciences at Stony Brook University/Brookhaven National Laboratory, which is supported by the U.S. Department of Energy under Contract No. DE-AC02-98CH10886 and by the State of New York.
    No preview · Article · Jan 2011

Publication Stats

415 Citations
153.75 Total Impact Points

Institutions

  • 2005-2015
    • Michigan State University
      • Department of Physics and Astronomy
      Ист-Лансинг, Michigan, United States
  • 2013
    • University of Amsterdam
      • Astronomical Institute Anton Pannekoek
      Amsterdamo, North Holland, Netherlands
  • 2003-2007
    • University of Chicago
      • Department of Astronomy and Astrophysics
      Chicago, Illinois, United States