Fundamental Physics Using Black Widow, Redback and Transitional Pulsar Binaries

The Spiders ERC project aims to use an innovative approach to combining multi-wavelength observations and theory in order to facilitate a paradigm shift in the use of neutron stars as a probe of high energy physics. The compactness, rapid rotation and large magnetic fields of neutron stars make them, along with black holes, the most extreme objects found in our Universe. Neutron stars are therefore formidable laboratories to study fundamental physics, as they allow us to probe regimes simply unavailable to Earth-based experiments. However, in spite of their potential utility, there are still a number of important outstanding questions in this field. Key amongst these are: What is the neutron star equation of state? How do neutron stars become the fastest rotating stellar objects in the Universe? How do millisecond pulsars affect their environment? Answering these questions has a broad multi-disciplinary impact, far beyond the field of neutron star research alone; however, at present our ability to answer these questions is limited by cherry picking individual objects for study. We want to move away from the biases inherent in the idiosyncratic nature of particular sources to a comprehensive population analysis, and, specifically, use a particular class of neutron star systems, known as “Spider binaries”, to discover the most massive and the fastest spinning neutron stars that exist. Such extreme cases are of crucial importance for constraining the neutron star equation of state and thus determining the boundary for stellar mass black hole formation. This knowledge in turn can tell us about possible phase transitions of matter at high densities and provide observable quantities that can then be tested against Earth-based experiments, feeding back into high energy physics theories. The Spiders ERC project will achieve these goals through a transformational observational approach to finding new Spider systems, using the newest optical telescopes and drawing on innovative techniques from other fields to improve parameter estimation, coupled with cutting-edge Spider binary modelling - the utility of which has previously been limited by a lack of matching observational technology. Together this approach offers a combined outcome much greater than either one could achieve independently.

The work funded through this grant has concentrated in three main areas: 1) the characterisation the population of Spider systems using multi-wavelength observations, 2) the search for new Spider systems, and 3) the development of our theoretical understanding of the physical processes taking place in these systems.

Here are highlights related to each area:
1) Characterisation
- We have conducted over 100 hours of observations with world-class optical telescopes (>3 meters) in order to obtain photometric light curves for the largest sample of Spider binaries ever collected thus far. Nearly all of these data has been reduced, and there are currently 6 papers in preparation relating to these data.
- Three publications detailing observations and modelling of Spider binary systems have been published or are under journal review.
- Our group obtained and modelled the optical light curve of the second-fastest spinning MSP yet to be discovered, making it the most extreme black widow system to have been studied in this way.
- We have been involved with the first discovery of the long-term optical variability in Spider binary light curves, which indicates that the companion stars in these systems are likely undergoing physical change of time scales of a few years. The underlying reasons for these changes might be connected to the observed transitions seen in some other systems upon which states of active accretion onto the neutron stars are triggered.

2) Search
- We have developed a new method which utilises information from optical observations in order to constrain the range of parameters enabling searches for gamma ray and radio pulsations from Spider MSPs. This method has already led to two new discoveries.
- We have applied machine learning techniques to classify the optical light curves of sources observed with survey telescopes, with a specific focus on identifying light curves which resemble those of Spider systems. Initial results are promising, but progress has been hampered by a delay in the acquisition of new survey data.

3) Theoretical understanding
- We have been working on improving the modelling of Spider timing data by incorporating the various physical components contributing to tides as well as effects of general relativity. This has led to the prediction that these binary systems should not have a perfectly circular orbit, and the measurement of this effect can provide a proxy to understand the internal structure of the companion star in these systems. This result is not just limited to Spider binary systems, but has wide ranging implications for all binary star systems.

The detection of the gravitational quadrupole moment of the companion star in Spider systems represents an important step forward as it allows for the first time to peer into the internal distribution of matter of the companion star. Eventually, we plan to use this result to put constraints on stellar evolution in Spider systems, but also to place physically sound priors on optical light-curve fitting.

We expect to publish a full database of Spider binary physical parameters at this end of the project, along with a number of publications summarising the individual characteristic of these systems.

Periodic Reporting for period 3 - Spiders (Fundamental Physics Using Black Widow, Redback and Transitional Pulsar Binaries)

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