arXiv subjects

A. Bauswein

Publications and source records attributed to A. Bauswein.

25 records · Page 2Linked to original sources

arXiv2013

Prompt merger collapse and the maximum mass of neutron stars

We perform hydrodynamical simulations of neutron-star mergers for a large sample of temperature-dependent, nuclear equations of state, and determine the threshold mass above which the merger remnant promptly collapses to form a black hole. We find that, depending on the equation of state, the threshold mass is larger than the maximum mass of a non-rotating star in isolation by between 30 and 70 per cent. Our simulations also show that the ratio between the threshold mass and maximum mass is tightly correlated with the compactness of the non-rotating maximum-mass configuration. We speculate on how this relation can be used to derive constraints on neutron-star properties from future observations.

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arXiv2013

Systematics of dynamical mass ejection, nucleosynthesis, and radioactively powered electromagnetic signals from neutron-star mergers

We investigate systematically the dynamical mass ejection, r-process nucleosynthesis, and properties of electromagnetic counterparts of neutron-star (NS) mergers in dependence on the uncertain properties of the nuclear equation of state (EoS) by employing 40 representative, microphysical high-density EoSs in relativistic, hydrodynamical simulations. The crucial parameter determining the ejecta mass is the radius R_1.35 of a 1.35 M_sun NS. NSs with smaller R_1.35 ("soft" EoS) eject systematically higher masses. These range from ~10^-3 M_sun to ~10^-2 M_sun for 1.35-1.35 M_sun binaries and from ~5*10^-3 M_sun to ~2*10^-2 M_sun for 1.2-1.5 M_sun systems (with kinetic energies between ~5*10^49 erg and 10^51 erg). Correspondingly, the bolometric peak luminosities of the optical transients of symmetric (asymmetric) mergers vary between 3*10^41 erg/s and 14*10^41 erg/s (9*10^41 erg/s and 14.5*10^41 erg/s) on timescales between ~2 h and ~12 h. If these signals with absolute bolometric magnitudes from -15.0 to -16.7 are measured, the tight correlation of their properties with those of the merging NSs might provide valuable constraints on the high-density EoS. The r-process nucleosynthesis exhibits a remarkable robustness independent of the EoS, producing a nearly solar abundance pattern above mass number 130. By the r-process content of the Galaxy and the average production per event the Galactic merger rate is limited to 4*10^-5/yr (4*10^-4/yr) for a soft (stiff) NS EoS, if NS mergers are the main source of heavy r-nuclei. The production ratio of radioactive 232Th to 238U attains a stable value of 1.64-1.67, which does not exclude NS mergers as potential sources of heavy r-material in the most metal-poor stars.

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arXiv2012

Equation-of-state dependence of the gravitational-wave signal from the ring-down phase of neutron-star mergers

Neutron-star (NS) merger simulations are conducted for 38 representative microphysical descriptions of high-density matter in order to explore the equation-of-state dependence of the postmerger ring-down phase. The formation of a deformed, oscillating, differentially rotating very massive NS is the typical outcome of the coalescence of two stars with 1.35 $M_{\odot}$ for most candidate EoSs. The oscillations of this object imprint a pronounced peak in the gravitational-wave (GW) spectra, which is used to characterize the emission for a given model. The peak frequency of this postmerger GW signal correlates very well with the radii of nonrotating NSs, and thus allows to constrain the high-density EoS by a GW detection. In the case of 1.35-1.35 $M_{\odot}$ mergers the peak frequency scales particularly well with the radius of a NS with 1.6 $M_{\odot}$, where the maximum deviation from this correlation is only 60 meters for fully microphysical EoSs which are compatible with NS observations. Combined with the uncertainty in the determination of the peak frequency it appears likely that a GW detection can measure the radius of a 1.6 $M_{\odot}$ NS with an accuracy of about 100 to 200 meters. We also uncover relations of the peak frequency with the radii of nonrotating NSs with 1.35 $M_{\odot}$ or 1.8 $M_{\odot}$, with the radius or the central energy density of the maximum-mass Tolman-Oppenheimer-Volkoff configuration, and with the pressure or sound speed at a fiducial rest-mass density of about twice nuclear saturation density. Furthermore, it is found that a determination of the dominant postmerger GW frequency can provide an upper limit for the maximum mass of nonrotating NSs. The prospects for a detection of the postmerger GW signal and a determination of the dominant GW frequency are estimated to be in the range of 0.015 to 1.2 events per year with the upcoming Advanced LIGO detector.

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arXiv2011

Measuring neutron-star properties via gravitational waves from binary mergers

We demonstrate by a large set of merger simulations for symmetric binary neutron stars (NSs) that there is a tight correlation between the frequency peak of the postmerger gravitational-wave (GW) emission and the physical properties of the nuclear equation of state (EoS), e.g. expressed by the radius of the maximum-mass Tolman-Oppenheimer-Volkhoff configuration. Therefore, a single measurement of the peak frequency of the postmerger GW signal will constrain the NS EoS significantly. For plausible optimistic merger-rate estimates a corresponding detection with Advanced LIGO is likely to happen within an operation time of roughly a year.

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arXiv2010

Testing Approximations of Thermal Effects in Neutron Star Merger Simulations

We perform three-dimensional relativistic hydrodynamical calculations of neutron star mergers to assess the reliability of an approximate treatment of thermal effects in such simulations by combining an ideal-gas component with zero-temperature, micro-physical equations of state. To this end we compare the results of simulations that make this approximation to the outcome of models with a consistent treatment of thermal effects in the equation of state. In particular we focus on the implications for observable consequences of merger events like the gravitational-wave signal. It is found that the characteristic gravitational-wave oscillation frequencies of the post-merger remnant differ by about 50 to 250 Hz (corresponding to frequency shifts of 2 to 8 per cent) depending on the equation of state and the choice of the characteristic index of the ideal-gas component. In addition, the delay time to black hole collapse of the merger remnant as well as the amount of matter remaining outside the black hole after its formation are sensitive to the description of thermal effects.

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arXiv2010

Discriminating Strange Star Mergers from Neutron Star Mergers by Gravitational-Wave Measurements

We perform three-dimensional relativistic hydrodynamical simulations of the coalescence of strange stars (SSs) and explore the possibility to decide on the strange matter hypothesis by means of gravitational-wave (GW) measurements. Selfbinding of strange quark matter (SQM) and the generally more compact stars yield features that clearly distinguish SS from neutron star (NS) mergers, e.g. hampering tidal disruption during the plunge of quark stars. Furthermore, instead of forming dilute halos around the remnant as in the case of NS mergers, the coalescence of SSs results in a differentially rotating hypermassive object with a sharp surface layer surrounded by a geometrically thin, clumpy high-density SQM disk. We also investigate the importance of including non-zero temperature equations of state (EoSs) in NS and SS merger simulations. In both cases we find a crucial sensitivity of the dynamics and outcome of the coalescence to thermal effects, which, e.g., determine the outer remnant structure and the delay time of the dense remnant core to black hole collapse. For comparing and classifying the GW signals, we use a number of characteristic quantities like the maximum frequency during inspiral or the dominant frequency of oscillations of the postmerger remnant. In general, these frequencies are higher for SS mergers. If not, additional features of the GW luminosity spectrum may help to discriminate coalescence events of the different types. Future GW measurements may thus help to decide on the existence of SQM stars. (abridged)

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arXiv2009

Mass Ejection by Strange Star Mergers and Observational Implications

We determine the Galactic production rate of strangelets as a canonical input to calculations of the measurable cosmic ray flux of strangelets by performing simulations of strange star mergers and combining the results with recent estimates of stellar binary populations. We find that the flux depends sensitively on the bag constant of the MIT bag model of QCD and disappears for high values of the bag constant and thus more compact strange stars. In the latter case strange stars could coexist with ordinary neutron stars as they are not converted by the capture of cosmic ray strangelets. An unambiguous detection of an ordinary neutron star would then not rule out the strange matter hypothesis.

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