Dennis Fremstad

• Fremstad, Dennis & Winther, Hans Arnold (2025). Emulating the non-linear matter power spectrum in mixed axion dark matter models. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 702. doi: 10.1051/0004-6361/202554621. Fulltekst i vitenarkiv Vis sammendrag

  In order to constrain ultra light dark matter models with current and near future weak lensing surveys, we need the predictions for the non-linear dark matter power spectrum. This is commonly extracted from numerical simulations or from semi-analytical methods. For ultra light dark matter models, such numerical simulations are often very expensive due to the need of having a very high force resolution, often limiting the simulations to very small simulation boxes that do not contain very large scales. In this work, we take a different approach by relying on fast approximate N -body simulations. In these simulations, axion physics is only included in the initial conditions, allowing us to run a large number of simulations with varying axion and cosmological parameters. From our simulation suite, we used machine learning tools to create an emulator for the ratio of the dark matter power spectrum in mixed axion models – models where dark matter is a combination of CDM and axion – to that of ΛCDM. The resulting emulator only needs to be combined with existing emulators for ΛCDM to be able to be used in parameter constraints. We compared the emulator to semi-analytical methods, but a more thorough test to full simulations to verify the true accuracy of this approach is not possible at the present time and is left for future work.

• Fremstad, Dennis; Gomez, Juan Camilo Guevara; Hudson, H. & Oliveros, J. C. Martínez (2023). An overview of HMI off-disk flare observations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 672. doi: 10.1051/0004-6361/202245788. Fulltekst i vitenarkiv Vis sammendrag

  Context. White-light continuum observations of solar flares often have coronal counterparts, including the classical ‘white-light prominence’ (WLP) phenomenon. Aims. Coronal emissions by flares, seen in the white-light continuum, have only rarely been reported. We seek to use modern data to understand the morphology of WLP events. Methods. We have identified a set of 14 examples of WLPs detected by the Heliospheric and Magnetic Imager (HMI) experiment on board the Solar Dynamics Observatory (SDO) satellite using a new online catalogue that covers 2011–2017. These WLPs invariably accompanied white-light flare (WLF) emission from the lower atmosphere from flares near the limb, as identified by hard X-ray images from the Reuven Ramaty High Energy Spectroscopic Imager (RHESSI). HMI provides full Stokes information, and we have used the linear polarisations ( Q and U ) to distinguish Thomson scattering from cool material. Results. The event morphologies fit roughly into three categories – ejection, loop, and spike – but many events show multiple phenomena. Conclusions. The coronal white-light continuum, observed by HMI analogously to the observations made by a coronagraph, reveals many examples of coronal emission and dynamics. Using the Stokes linear polarisation, we estimate the masses of hot coronal plasma in 11 of the 14 events and find them to be similar to typical coronal-mass-ejection masses, but without exceeding 10 15 g. We note that the HMI observations do not occult the bright solar disk and were not designed for coronal observations, resulting in relatively low signal-to-noise ratios. We therefore believe that future such observations with better optimisation will be even more fruitful.

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