The goal of the current project is to boost the study of neutron stars (NSs) by exploiting the synergies between observations of continuous gravitational waves (GWs), electromagnetic (EM) waves and theoretical studies. Even fifty years after the discovery of pulsars, several aspects of NSs, including their composition and properties, are highly uncertain. The advent of multi-messenger astronomy is a game-changer also for this field. We aim at detecting the continuous GW signals emitted by quickly rotating NSs in our Galaxy and, possibly, stable or meta-stable remnants of supernova explosions and binary mergers. The use of deep-rooted data-analysis techniques, which are mainly based on pattern recognition, and will be optimized by implementing machine-learning algorithms, and by the exploitation of modern computing architectures, like GPU clusters and/or parallel computing on HPC (high performance computing), will allow us to improve the sensitivity to detect continuous GW signals in the data taken by the advanced LIGO-Virgo detectors. EM observations of NSs in the nascent field of fast optical photometry will strongly reduce the parameter space to be searched over through the detection of fast optical pulsars that cannot be seen at other wavelengths. The discovery of continuous GWs from a spinning NS will unveil its structure and properties, making it an unparalleled laboratory for testing models of fundamental physics and astrophysics in conditions that cannot be reproduced on Earth. Bayesian inference methods will be used both to estimate the NSs parameters, in case of a detection, and to accomplish source population studies as well as model predictions.
The project is very timely and will hugely contribute to boost the newly born GW astronomy by exploiting at the best data produced by current and future GW detectors, thanks to the well-established international expertise and leadership of the proponents in all of the fields related to the project.
The current proposal can allow us to perform the first CW detection in data taken by the advanced LIGO-Virgo detectors, allowing us to achieve extremely accurate measures of NS parameters, to study effects related to the NS structure, and to investigate very basic aspects of gravitation, which are not accessible in any other way. This huge potential for discovery will be strongly enhanced by using a multi-disciplinary approach, including observations in the optical and near-infrared (IR) bands, theoretical studies to constrain the models of nucleonic matter at supranuclear density, and on GW emission mechanisms. This approach will be crucial to shrink the GW parameter space, with a consequent sensitivity improvement, and will be of paramount importance to interpret the GW results in the light of astrophysical NS models.
We expect ~ 10^9 NSs to exist in our Galaxy, but so far only ~2700 pulsars have been observed mainly in the radio band, and ~20% of them are located in binary systems. As regards the X-ray band, we know ~100 NSs that were possibly spun up by the accretion of mass transferred by a low-mass companion star, 20 of which show millisecond (ms) X-ray pulses [Nature 394:344 (1998)].
Scorpius-X1 is the most promising NS in a binary system possibly emitting CW signals, due to its brightness and closeness. Its orbital parameters are known with good accuracy [ApJ 781:14 (2014)], while its frequency evolution is completely unknown. This sets not trivial computational issues to perform CW directed searches, which unavoidably brings us to reduce the search parameter space [PRD 95:122001 (2017)], with a consequent worsening in sensitivity. A detection of an X-ray signal from a bright LMXB such as Scorpius-X1 in the near future is unlikely, since the pulse sensitivity scales with the square root of the number of photons detected and no big leap in the effective area of X-ray instruments that will operate in the next ~5 years (e.g. Chandra, XMM-Newton, Nicer) is foreseen. A ground-breaking advance in the search for CWs from bright LMXBs can be instead achieved considering a brand-new observing window to search for ms pulsars, i.e. the very fast optical photometry. The first optical ms pulsations have been unexpectedly discovered in the recycled transitional pulsar J1023+0038 [Nature Astronomy 1:854 (2017)]. Strikingly, optical pulsations were detected when the source had a disk and showed X-ray pulsations, which was a strong indication that accretion was going on. This discovery demonstrated that also weakly magnetized, quickly spinning NSs, can produce such signals, and that very fast photometry in the optical band can be more efficient than in the X-ray band to search for periodic signals.
In an analogous way to what happened for J1023+0038, we aim at observing Scorpius-X1, and similar sources, both in the optical band [320 - 900] nm and in the Near-Mid IR (NMIR) band [3.5 - 8.3] micron. We stress that this would allow us to determine the so far unknown star rotation frequency (and frequency variation), as well as to set better constraints on its orbital parameters, with an enormous positive impact on the chance to detect CWs.
We aim at extending the SiFAP capabilities to the NMIR band to explore also regions close to both the Galactic bulge and disk, where we expect a high density of interesting targets (magnetars, LMXBs and ms pulsars) to be characterized by a large content of dust that absorbs optical wavelengths. Detection of IR pulsations from NSs located in these regions not only would dramatically impact on the chance of a CW detection, but would also open new unprecedented perspectives for NSs astrophysics.
We will advance noticeably the knowledge of the astrophysical sources described in this project thanks to the joint efforts and excellent expertise of all researchers involved in the current project.