Wednesday Colloquium


"White Dwarf Pulsars"

David Buckley (SAAO/SALT, University of Cape Town, University of the Free State)

The 2016 discovery of the unusual nature of the detached White Dwarf – Red Dwarf binary, AR Scorpii, has led to efforts to discover more examples of such systems. In this talk I will review the multi-wavelength properties of AR Sco, a system whose luminosity is dominated by non-thermal strongly polarized emission, pulsed at the ~ 2 min spin and beat periods. In many respects the system shares characteristics of a pulsar and is powered by the spin-down of the White Dwarf. I will also present preliminary results of a simultaneous multi-wavelength campaign on AR Sco, conducted in June 2020, involved optical observations (time resolved spectroscopy and photometry) from SALT, Keck and the SAAO 1-m telescope. In addition, X-ray observations were obtained with NICER and L-band radio observations with the MeerKAT radio telescope array. These observations are used to investigate the wavelength dependence of the periodic modulations seen in AR Sco, across the E-M spectrum. Finally, I will present observational results of a second AR Sco-like system, J191213.72−441045.1, discovered in 2022, which shares many similar characteristics with AR Sco, including pulsed X-ray, optical and radio emission and is also strongly polarized.


"Copernicus’s Mathematical Astronomy in "De revolutionibus": A Very Short Introduction"

Jarosław Włodarczyk (Institute for the History of Science, PAN, Warsaw)

Nicolaus Copernicus (1473–1543) propelled one of the greatest revolution in science and his heliocentric cosmology radically changed the relationship between man and the universe. However, to prove his concept of cosmos, Copernicus had to create mathematical models of the movements of planets. He devoted five of the six books of his "De revolutionibus" (On the Revolutions, 1543) to this task. As he himself argued, “mathematics is written for mathematicians”. In my lecture I shall outline Copernicus’s mathematical astronomy and its early reception in the form of the first Copernican astronomical tables which allowed to calculate the positions of the planets for any time.


"Testing cosmological models with cross-correlation measurements"

Chandra Shekhar Saraf (CAMK, Warsaw)

The well established standard ΛCDM model of cosmology suffers from several challenges and disparities like the nature of dark matter and dark energy, accelerated expansion of the Universe and tensions in cosmological parameters derived from different observations. These shortcomings have raised questions on the validity of the ΛCDM model and led to increased interests towards alternative models of cosmology (dark energy models and Modified Gravity theories). Cross-correlation measurements at different redshift bins from the next-generation CMB experiments and galaxy surveys will provide us an unprecedented opportunity to put stronger constraints on alternative models of cosmology. However, in order to accurately quantify deviations from the ΛCDM model, it is crucial to mitigate the systematic errors that may bias the cross-correlation measurements. In this presentation, we will talk about various systematics that can impact the estimation of cosmological parameters in cross-correlation studies. We will present results from the first cross-correlation measurements between Planck CMB lensing potential and the Herschel Extragalactic Legacy Project (HELP) galaxy catalogues. In addition, we will also discuss the implications of leakage of objects in tomographic cross-correlation studies, through simulations performed with expectations from the upcoming LSST survey.


"Breaking degeneracies between models using X-ray polarisation observations"

Yash Bhargava (Tata Institute for Fundamental Research, Bombay, India)

Observations of the accreting compact objects have been paramount in constraining the physical processes in extreme gravity regimes. In particular, the X-ray observations probe the spacetimes closest to the compact object and often embed the signatures of the geometry of these systems. Various persistent and transient X-ray binaries have been regularly observed using a slew of space missions and thus we now have some key insights into these enigmatic objects. Recently, there have been major headways in probing the polarisation of these objects in X-rays, due to the launch of IXPE and AstroSat. In this talk I will present some of the recent work done by us to investigate the polarisation of various X-ray binaries and how these observations have broken the degeneracies which are impossible to break using the standard spectral and timing studies.