Time Domain Studies and Fundamental Physics

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NANOgrav power-spectrum of the gravitational wave background for a spectrally free model (independent frequency bins), and a constrained running power-law spectral index.

The GBT, with the support of the VLA, has made core contributions to the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) strong evidence for the stochastic gravitational wave background from merging supermassive black holes throughout the cosmos. This discovery represents a watershed event in modern physics. The 15 year data have now been used to set a more refined limit on the power spectrum of the gravitational waves, beyond a simple amplitude and power-law spectral index, by allowing for a running logarithmic change in the spectral index with frequency. The results show no evidence that the spectral index changes with frequency. The results are used to constrain models of primordial gravitational waves from cosmic inflation during the first 10-32 s of the Universe.

Left: NANOgrav power-spectrum of the gravitational wave background for a spectrally free model (independent frequency bins), and a constrained running power-law spectral index (NANOGrav consortium 2024 arXiv:2408.10166).

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NANOgrav constraints (blue) with other cosmological measurements, on the power-law spectral index and running of the power-law index, for primordial gravitational waves from cosmic inflation (NANOGrav consortium 2024 arXiv:2408.10166)

The GBT has performed its first searches for quantum chromodynamic (QCD) axions that represent a primary candidate for dark matter. QCD axions might be converted into photons in regions of very strong magnetic field, such as around strongly magnetized neutron stars. The interaction between an axion miniclump and a neutron star might lead to a very narrow band and transient radio signal.

Right: NANOgrav constraints (blue) with other cosmological measurements, on the power-law spectral index and running of the powerlaw index, for primordial GW from cosmic inflation (NANOGrav consortium 2024 arXiv:2408.10166).

Figure-1-1-1-2.pngThe GBT is an ideal facility for such a search, given its sensitivity and spectral capabilities. The VErsatile GBT Astronomical Spectrometer (VEGAS) was used at X-band at the GBT to search for such lines from the center of the Andromeda galaxy, plus a control region well off the galaxy. Extensive methodology was developed to avoid spurious signals. The measurements were sensitive to axions with masses between 33 and 42 micro-eV. The null result from the short observations was consistent with the current model for axion dark matter, although the constraints are not strong given the limited observing time. However, the noise limited spectra are a key technical demonstration, and a guide to future experiments over wide areas in the Milky Way or longer observations of Andromeda, possibly with a dedicated radio telescope.

Left: Image showing the regions searched for Axionic dark matter in Andromeda (red dots). Right: noise limited GBT spectra for different time records, cleaned of RFI, at 8 GHz (Walters et al. arXiv:2407:13060).

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ALMA has mapped the polarized emission and Faraday rotation measures across the radio jet in M87 at 2” resolution at 90 GHz. The core has a large RM of 4.5x104 rad m-2, and time variability suggests the magnetized plasma causing the rotation measures may be due to an ionized wind from the Active Galactic Nuclei (AGN) within a few thousand gravitational radii of the black hole. The RM along the jet suggest a helical configuration for the external magnetic field on kpc-scales.

Left: Total intensity (contours) and fractional polarization (color), plus E-field vectors for M87 from ALMA at 86GHz. Right: Faraday rotation measures across the jet and core (Peng et al. 2024, arXiv:2409.12028).





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