Wednesday, September 11, 2019

Very Subtle Methods In Astronomy

A new paper proposes a new way for astronomers to study gravitational wave causing events, by examining their impact on specific pulsars near events we have already detected.

The gravitational waves travel directly from the event to Earth, but they also travel directly from the event to nearby pulsars. Any effect on the pulsars (basically in the form of a glitch in the pulsar's usual pulse in the electromagnetic spectrum) must then travel from the pulsar to Earth, reaching us several years after we detect the original gravitational wave event.

The fact that we can even aspire to predict and measure something like that is amazing, but the reasoning seems sound and we will be able to determine if the predictions pan out over the next fifteen to fifty years.

Re-visiting gravitational wave events via pulsars

By now many gravitational wave (GW) signals have been detected by LIGO and Virgo, with the waves reaching earth directly from their respective sources. These waves will also travel to different pulsars and will cause (tiny) transient deformations in the pulsar shape. Some of us have recently shown that the resultant transient change in the pulsar moment of inertia may leave an observable imprint on the pulsar signals as detected on earth, especially at resonance. The pulsars may thus act as remotely stationed Weber gravitational wave detectors. This allows us to revisit the past GW events via pulsars. 
We give here a list of specific pulsars whose future signals will carry the imprints of past GW events, to be specific we constrain it within 50 years. Some interesting cases are, supernova SN1987A with earliest perturbed signals from pulsars J0709-5923 and B0559-57 expected to reach earth in 2023 and 2024 respectively, Crab supernova, with perturbed signal arrival date from pulsar J1856-3754 in 2057, and GW170814 event with its imprints on the signals on the pulsar J0437-4715 reaching earth between 2035-2043. Even the earliest recorded supernova SN185 event may become observable again via pulsar J0900-3144 with the perturbed pulsar signal reaching us sometime between 2033-2066. 
Importantly, even though the strength of the signal will depend on the interior properties of the pulsar, the expected dates of signal arrival are completely model independent, depending only on the locations of the source and the relevant pulsar.
Comments:5 pages, 1 figure, 2 tables
Subjects:High Energy Astrophysical Phenomena (astro-ph.HE); General Relativity and Quantum Cosmology (gr-qc); Nuclear Theory (nucl-th)
Cite as:arXiv:1909.04476 [astro-ph.HE]
(or arXiv:1909.04476v1 [astro-ph.HE] for this version)
In other astronomy news, some subtle methods of studying gravity, like comparing the general relativistic perihelion of planets with the Newtonian predictions (which, in the case of the planet Mercury, was on the first and most decisive empirical proofs of general relativity) is just very, very difficult to the point of being practically almost impossible for exoplanets.

Relativistic changes to particle trajectories are difficult to detect

We study the sensitivity of the computed orbits for the Kepler problem, both for continuous space, and discretizations of space. While it is known that energy can be very well preserved with symplectic methods, the semi-major-axis is in general not preserved. We study this spurious shift, as a function of the integration method used, and also as a function of an additional interpolation of forces on a 2-dimensional lattice. This is done for several choices of eccentricities, and semi-major axes. Using these results, we can predict which precisions and lattice constants allow for a detection of the relativistic perihelion advance. Such bounds are important for calculations in N-body simulations, if one wants to meaningfully add these relativistic effects.
Comments:14 pages, 8 figures
Subjects:Numerical Analysis (math.NA); Cosmology and Nongalactic Astrophysics (astro-ph.CO); Astrophysics of Galaxies (astro-ph.GA); Computational Physics (physics.comp-ph)
Cite as:arXiv:1909.04652 [math.NA]
 (or arXiv:1909.04652v1 [math.NA] for this version)

1 comment:

Alex said...

This is my first visit to your blog, your post made productive reading, thank you.
NS Capture Theory Rich Levinson