128 research outputs found
Cyclotron line signatures of thermal and magnetic mountains from accreting neutron stars
Cyclotron resonance scattering features (CRSFs) in the X-ray spectrum of an
accreting neutron star are modified differently by accretion mounds sustained
by magnetic and thermocompositional gradients. It is shown that one can
discriminate, in principle, between mounds of different physical origins by
studying how the line energy, width, and depth of a CRSF depend on the
orientation of the neutron star, accreted mass, surface temperature
distribution, and equation of state. CRSF signatures including gravitational
light bending are computed for both phase-resolved and phase-averaged spectra
on the basis of self-consistent Grad-Shafranov mound equilibria satisfying a
global flux-freezing constraint. The prospects of multimessenger X-ray and
gravitational-wave observations with future instruments are canvassed briefly.Comment: 17 pages, 12 figures, accepted for publication in MNRA
Pulsar timing noise and the minimum observation time to detect gravitational waves with pulsar timing arrays
The sensitivity of pulsar timing arrays to gravitational waves is, at some
level, limited by timing noise. Red timing noise - the stochastic wandering of
pulse arrival times with a red spectrum - is prevalent in slow-spinning pulsars
and has been identified in many millisecond pulsars. Phenomenological models of
timing noise, such as from superfluid turbulence, suggest that the timing noise
spectrum plateaus below some critical frequency, , potentially aiding the
hunt for gravitational waves. We examine this effect for individual pulsars by
calculating minimum observation times, , over which the
gravitational wave signal becomes larger than the timing noise plateau. We do
this in two ways: 1) in a model-independent manner, and 2) by using the
superfluid turbulence model for timing noise as an example to illustrate how
neutron star parameters can be constrained. We show that the superfluid
turbulence model can reproduce the data qualitatively from a number of pulsars
observed as part of the Parkes Pulsar Timing Array. We further show how a value
of , derived either through observations or theory, can be related to
. This provides a diagnostic whereby the usefulness of timing
array pulsars for gravitational-wave detection can be quantified.Comment: Accepted for publication in MNRA
Computing Fast and Reliable Gravitational Waveforms of Binary Neutron Star Merger Remnants
Gravitational waves have been detected from the inspiral of a binary
neutron-star, GW170817, which allowed constraints to be placed on the neutron
star equation of state. The equation of state can be further constrained if
gravitational waves from a post-merger remnant are detected. Post-merger
waveforms are currently generated by numerical-relativity simulations, which
are computationally expensive. Here we introduce a hierarchical model trained
on numerical-relativity simulations, which can generate reliable post-merger
spectra in a fraction of a second. Our spectra have mean fitting factors of
0.95, which compares to fitting factors of 0.76 and 0.85 between different
numerical-relativity codes that simulate the same physical system. This method
is the first step towards generating large template banks of spectra for use in
post-merger detection and parameter estimation.Comment: Submitted to PRL. 6 pages, 4 figure
Tracking continuous gravitational waves from a neutron star at once and twice the spin frequency with a hidden Markov model
Searches for continuous gravitational waves from rapidly spinning neutron
stars normally assume that the star rotates about one of its principal axes of
moment of inertia, and hence the gravitational radiation emits only at twice
the spin frequency of the star, . The superfluid interior of a star
pinned to the crust along an axis nonaligned with any of its principal axes
allows the star to emit gravitational waves at both and , even
without free precession, a phenomenon not clearly observed in known pulsars.
The dual-harmonic emission mechanism motivates searches combining the two
frequency components of a signal to improve signal-to-noise ratio. We describe
an economical, semicoherent, dual-harmonic search method, combined with a
maximum likelihood coherent matched filter, F-statistic, and improved from an
existing hidden Markov model (HMM) tracking scheme to track two frequency
components simultaneously. We validate the method and demonstrate its
performance through Monte Carlo simulations. We find that for sources emitting
gravitational waves at both and , the rate of correctly recovering
synthetic signals (i.e., detection efficiency), at a given false alarm
probability, can be improved by %-70% by tracking two frequencies
simultaneously compared to tracking a single component only. For sources
emitting at only, dual-harmonic tracking only leads to minor sensitivity
loss, producing lower detection efficiency than tracking a
single component. In directed continuous-wave searches where is unknown
and hence the full frequency band is searched, the computationally efficient
HMM tracking algorithm provides an option of conducting both the dual-harmonic
search and the conventional single frequency tracking to obtain optimal
sensitivity, with a typical run time of core-hr for one year's
observation
Axion sourcing in dense stellar matter via CP-violating couplings
Compact objects such as neutron stars and white dwarfs can source axionlike particles and QCD axions due to CP-violating axion-fermion couplings. The magnitude of the axion field depends on the stellar density and on the strength of the axion-fermion couplings. We show that even CP-violating couplings one order of magnitude smaller than existing constraints source extended axion field configurations. For axionlike particles, the axion energy is comparable to the magnetic energy in neutron stars with inferred magnetic fields of the order of 1013 G and exceeds by more than one order of magnitude the magnetic energy content of white dwarfs with inferred fields of the order of 104 G. On the other hand, the energy stored in the QCD axion field is orders of magnitude lower due to the smallness of the predicted CP-violating couplings. It is shown that the sourced axion field can polarize the photons emitted from the stellar surface, and stimulate the production of photons with energies in the radio band.F. A., P. D. L., and A. M. are supported by the Australian Research Council (ARC) Centre of Excellence for Gravitational Wave Discovery (OzGrav), through Project No. CE170100004. P. D. L. is supported through ARC Discovery Project DP220101610. J. A. P. and A. G. acknowledge support from the Generalitat Valenciana Grants No. ASFAE/2022/026 (with funding from NextGenerationEU PRTR-C17.I1) and CIPROM/2022/ 13, and from the AEI Grant No. PID2021-127495NB I00 funded by MCIN/AEI/10.13039/501100011033 and by “ESF Investing in your future.
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