arXiv subjects

Kai Schwenzer

Publications and source records attributed to Kai Schwenzer.

At least 19 recordsLinked to original sources

arXiv2026

Energy accreted onto a compact star

We compute the energy gained by accretion onto a compact star due to matter falling from the inner edge of a thin accretion disk. We employ a controlled slow-rotation expansion based on the Hartle-Thorne metric and find that in neutron stars in general only leading order rotational corrections to the metric, describing frame dragging, are sizable. However, for accretion onto a magnetized neutron star, where the disk roughly extends to the co-rotation radius, even frame dragging effects are minor. Based on general conservation laws we then derive simple, equation-of-state-independent expressions for the spin-up and heating energy, consistently including the relevant rotational, relativistic and nuclear effects, and find that in observed accreting millisecond sources the results can strongly deviate from presently employed estimates.

gr-qc↗

arXiv2025

Connecting afterglow light curves to the GRB central engine

Gamma ray burst (GRB) afterglow light curves have the potential to inform us about presently unobserved stages in the aftermath of a neutron star merger. Using numerical simulations of short GRB afterglows we obtain an approximate quantitative connection between key aspects of the emission mechanism and the shapes of the resulting light curves. Employing simple, but efficient, parameterizations of the light curve based on a broken power law in terms of physical parameters, fitted to a large dataset of synthetic light curves, we apply basic machine learning techniques to determine the approximate connection between key input parameters of the forward shock model and the light curve parameters. Solving then the inverse problem, we find that the strength of the central engine can be reasonably accurately estimated even with very limited information. In particular, merely the position of jet-break in the on-axis, respectively the maximum in the off-axis light curve determines the kinetic energy at the tens of percent level.

astro-ph.HE↗

arXiv2024

On the potential of probing the neutron star composition in accreting X-ray binaries

Transiently accreting Low Mass X-Ray Binaries have the potential to probe the core composition of their neutron stars via deep crustal heating caused by nuclear reactions. We statistically assess this deep crustal heating scenario, taking into account the various microphysical and astrophysical uncertainties. We find that despite the sizable uncertainties there is the chance to discriminate different compositional scenarios. Several observed sources statistically challenge a minimal hadronic matter composition, where cooling proceeds exclusively via slow modified Urca reactions. Considering here two exemplary extended uniform compositions, namely ultra-dense hadronic matter with direct Urca emission and ungapped quark matter, we find that they are even within uncertainties distinguishable. We show that although exotic forms of matter are generally only expected in an inner core, which could in principle have any size, sufficiently large astrophysical data sets nonetheless have the potential to statistically discriminate compositional scenarios, in particular when further mass measurements become available.

astro-ph.HE↗

arXiv2021

Gravitational waves from isolated neutron stars

Neutron star interiors are a fantastic laboratory for high density physics in extreme environments. Probing this system with standard electromagnetic observations is, however, a challenging endeavour, as the radiation tends to be scattered by the outer layers and the interstellar medium. Gravitational waves, on the other hand, while challenging to detect, interact weakly with matter and are likely to carry a clean imprint of the high density interior of the star. In particular long lived, i.e. `continuous' signals from isolated neutron stars can carry a signature of deformations, possibly in crystalline exotic layers of the core, or allow to study modes of oscillation, thus performing gravitational wave asteroseismology of neutron star interiors. In this article we will review current theoretical models for continuous gravitational wave emission, and observational constraints, both electromagnetic and gravitational. Finally we will discuss future observational possibilities.

gr-qc↗

arXiv2020

Strengthening the bounds on the r-mode amplitude with X-ray observations of millisecond pulsars

r-mode oscillations have been shown to have a significant potential to constrain the composition of fast spinning neutron stars. Due to their high rotation rates, millisecond pulsars (MSPs) provide a unique platform to constrain the properties of such oscillations, if their surface temperatures can be inferred. We present the results of our investigations of archival X-ray data of a number of MSPs, as well as recent XMM-Newton observations of PSR J1810+1744 and PSR J2241-5236. Using the neutron star atmosphere (NSA) model and taking into account various uncertainties, we present new bounds on the surface temperature of these sources. Thereby we significantly strengthen previous bounds on the amplitude of the r-mode oscillations in millisecond pulsars and find rigorous values as low as $α\lesssim 3 \times 10^{-9}$. This is by now about three orders of magnitude below what standard saturation mechanisms in neutron stars could provide, which requires very strong dissipation in the interior, strongly pointing towards a structurally complex or exotic composition of these sources. At such low temperatures sources could even be outside of the instability region, and taking into account the various uncertainties we obtain for an observed surface temperature a simple frequency bound below which r-modes are excluded in slower spinning pulsars.

astro-ph.HE↗

arXiv2019

Signatures for quark matter from multi-messenger observations

We review the prospects for detecting quark matter in neutron star cores. We survey the proposed signatures and emphasize the importance of data from neutron star mergers, which provide access to dynamical properties that operate on short timescales that are not probed by other neutron star observables.

nucl-th↗

arXiv2017

On the importance of viscous dissipation and heat conduction in binary neutron-star mergers

Inferring the properties of dense matter is one of the most exciting prospects from the measurement of gravitational waves from neutron star mergers. However, it will require reliable numerical simulations that incorporate viscous dissipation and energy transport if these can play a significant role within the survival time of the post-merger object. We calculate timescales for typical forms of dissipation and find that thermal transport and shear viscosity will not be important unless neutrino trapping occurs, which requires temperatures above about 10 MeV and gradients over lengthscales of 0.1 km or less. On the other hand, if direct-Urca processes remain suppressed, leaving modified-Urca processes to establish flavor equilibrium, then bulk viscous dissipation could provide significant damping to density oscillations observed right after the merger. When comparing with data from a state-of-the-art merger simulation we find that the bulk viscosity takes values close to its resonant maximum in a typical neutron-star merger, motivating a more careful assessment of the role of bulk viscous dissipation in the gravitational-wave signal from merging neutron stars.

gr-qc↗

arXiv2016

X-ray bounds on the r-mode amplitude in millisecond pulsars

r-mode astroseismology provides a unique way to study the internal composition of compact stars. Due to their precise timing, recycled millisecond radio pulsars present a particularly promising class of sources. Although their thermal properties are still poorly constrained, X-ray data is very useful for astroseismology since r-modes could strongly heat a star. Using known and new upper bounds on the temperatures and luminosities of several non-accreting millisecond radio pulsars we derive bounds on the r-mode amplitude as low as $α\lesssim10^{-8}$ and discuss the impact on scenarios for their internal composition.

astro-ph.HE↗

arXiv2015

Phase conversion dissipation in multicomponent compact stars

We propose a mechanism for the damping of density oscillations in multicomponent compact stars. The mechanism is the periodic conversion between different phases, i.e., the movement of the interface between them, induced by pressure oscillations in the star. The damping grows nonlinearly with the amplitude of the oscillation. We study in detail the case of r-modes in a hybrid star with a sharp interface, and we find that this mechanism is powerful enough to saturate the r-mode at very low saturation amplitude, of order $10^{-10}$, and is therefore likely to be the dominant r-mode saturation mechanism in hybrid stars with a sharp interface.

astro-ph.SR↗

arXiv2015

R-mode astronomy

Next generation gravitational wave detectors will start taking data in the near future. Here we discuss the chances to detect the continuous emission from r-mode oscillations in compact stars and study which properties of compact stars we can infer from such novel data. In particular we show that the combination of the gravitational wave data with electromagnetic multi-messenger observations could give us detailed insight into compact star properties, ranging from precise mass-radius measurements to the determination of the equation of state and the phase structure of dense matter.

gr-qc↗

arXiv2014

What the Timing of Millisecond Pulsars Can Teach us about Their Interior

The cores of compact stars reach the highest densities in nature and therefore could consist of novel phases of matter. We demonstrate via a detailed analysis of pulsar evolution that precise pulsar timing data can constrain the star's composition, through unstable global oscillations (r-modes) whose damping is determined by microscopic properties of the interior. If not efficiently damped, these modes emit gravitational waves that quickly spin down a millisecond pulsar. As a first application of this general method, we find that ungapped interacting quark matter is consistent with both the observed radio and x-ray data, whereas for ordinary nuclear matter some additional enhanced damping mechanism is required.

astro-ph.HE↗

arXiv2014

Probing dense matter in compact star cores with radio pulsar data

Astrophysical observations of compact stars provide, in addition to collider experiments, the other big source of information on matter under extreme conditions. The largest and most precise data set about neutron stars is the timing data of radio pulsars. We show how this unique data can be used to learn about the ultra-dense matter in the compact star interior. The method relies on astro-seismology based on special global oscillation modes (r-modes) that emit gravitational waves. They would prevent pulsars from spinning with their observed high frequencies, unless the damping of these modes, determined by the microscopic properties of matter, can prevent this. We show that for each form of matter there is a distinct region in a frequency/spindown-rate diagram where r-modes can be present. We find that stars containing ungapped quark matter are consistent with both the observed radio and x-ray data, whereas, even when taking into account the considerable uncertainties, neutron star models with standard viscous damping are inconsistent with both data sets and additional damping mechanisms would be required.

nucl-th↗

arXiv2014

Gravitational wave emission from oscillating millisecond pulsars

Neutron stars undergoing r-mode oscillation emit gravitational radiation that might be detected on earth. For known millisecond pulsars the observed spindown rate imposes an upper limit on the possible gravitational wave signal of these sources. Taking into account the physics of r-mode evolution, we show that only sources spinning at frequencies above a few hundred Hertz can be unstable to r-modes, and we derive a more stringent universal r-mode spindown limit on their gravitational wave signal, exploiting the fact that the r-mode saturation amplitude is insensitive to the structural properties of individual sources. We find that this refined bound limits the gravitational wave strain from millisecond pulsars to values below the detection sensitivity of next-generation detectors. Young sources are therefore a more promising option for the detection of gravitational waves emitted by r-modes and to probe the interior composition of compact stars in the near future.

gr-qc↗

arXiv2013

Gravitational wave emission and spindown of young pulsars

The rotation frequencies of young pulsars are systematically below their theoretical Kepler limit. R-modes have been suggested as a possible explanation for this observation. With the help of semi-analytic expressions that make it possible to assess the uncertainties of the r-mode scenario due to the impact of uncertainties in underlying microphysics, we perform a quantitative analysis of the spin-down and the emitted gravitational waves of young pulsars. We find that the frequency to which r-modes spin down a young neutron star is surprisingly insensitive both to the microscopic details and the saturation amplitude. Comparing our result to astrophysical data, we show that for a range of sufficiently large saturation amplitudes r-modes provide a viable spindown scenario and that all observed young pulsars are very likely already outside the r-mode instability region. Therefore the most promising sources for gravitational wave detection are unobserved neutron stars associated with recent supernovae, and we find that advanced LIGO should be able to see several of them. We find the remarkable result that the gravitational wave strain amplitude is completely independent of both the r-mode saturation amplitude and the microphysics, and depends on the saturation mechanism only within some tens of per cent. However, the gravitational wave frequency depends on the amplitude and we provide the required expected timing parameter ranges to look for promising sources in future searches.

gr-qc↗

arXiv2013

On the consistency of the spindown behavior of young and old pulsars

We study the spindown of pulsars due to gravitational wave emission and show that r-modes in neutron stars provide a quantitative explanation for the observed low rotation frequencies of young pulsars if the r-mode saturation amplitude is sufficiently large. In contrast for such large saturation amplitudes old hadronic millisecond pulsars would spin down much faster than observed. We discuss resolutions of this apparent discrepancy that could make this mechanism consistent with the observational data.

astro-ph.SR↗

arXiv2012

How long-range interactions tune the damping in compact stars

Long-range interactions lead to non-Fermi liquid effects in dense matter. We show that, in contrast to other material properties, their effect on the bulk viscosity of quark matter is significant since they shift its resonant maximum and can thereby change the viscosity by many orders of magnitude. This is of importance for the damping of oscillations of compact stars, like in particular unstable r-modes, and the quest to detect signatures of deconfined matter in astrophysical observations. We find that, in contrast to neutron stars with standard damping mechanisms, compact stars that contain ungapped quark matter are consistent with the observed data on low mass x-ray binaries.

nucl-th↗

arXiv2012

Impact of r-modes on the cooling of neutron stars

Studying the frequency and temperature evolution of a compact star can give us valuable information about the microscopic properties of the matter inside the star. In this paper we study the effect of dissipative reheating of a neutron star due to r-mode oscillations on its temperature evolution. We find that there is still an impact of an r-mode phase on the temperature long after the star has left the instability region and the r-mode is damped completely. With accurate temperature measurements it may be possible to detect this trace of a previous r-mode phase in observed pulsars.

astro-ph.HE↗

arXiv2012

Viscous damping of r-modes: Large amplitude saturation

We analyze the viscous damping of r-mode oscillations of compact stars, taking into account non-linear viscous effects in the large-amplitude regime. The qualitatively different cases of hadronic stars, strange quark stars, and hybrid stars are studied. We calculate the viscous damping times of r-modes, obtaining numerical results and also general approximate analytic expressions that explicitly exhibit the dependence on the parameters that are relevant for a future spindown evolution calculation. The strongly enhanced damping of large amplitude oscillations leads to damping times that are considerably lower than those obtained when the amplitude dependence of the viscosity is neglected. Consequently, large-amplitude viscous damping competes with the gravitational instability at all physical frequencies and could stop the r-mode growth in case this is not done before by non-linear hydrodynamic mechanisms.

astro-ph.HE↗