...
Abstract: Blazars are active galactic nuclei with relativistic jets directed toward Earth. Relativistic beaming amplifies their brightness, making them appear extremely luminous across multiple wavelengths, from radio to gamma-rays. Consequently, blazars are the most numerous source class among very high-energy gamma-ray emitters. However, determining their redshifts is often challenging because jet emission can obscure spectral lines from the host galaxy or intervening matter. In this talk, I will introduce two methods for estimating blazar distances by associating them with their "cosmic neighborhood". These techniques involve analyzing the optical fields around a blazar using either multi-object spectroscopy or multi-band photometry, combined with the assumption that blazars are typically located in galaxy-rich environments. Accurate redshift estimation for high-redshift blazars is crucial for advancing our understanding of extragalactic very high-energy gamma-ray sources and their interactions with the surrounding universe. Slides available here: https://www.ntnu.no/wiki/download/attachments/195538250/NTNU_seminar_Koljonen.pdf?api=v2
The (planned) seminars in 2025 are
- 14.01. Matteo Imbrogno (Università di Tor Vergata of Rome): A tale of serendipity: quasi-periodic oscillations in pulsating ULXs and a search for new X-ray pulsators
Abstract: The discovery of pulsating ultraluminous X-ray sources (PULXs) has revealed that accreting neutron stars can shine at extreme luminosities, well above their Eddington limit. This finding has caused a shift in the ULX paradigm and poses significant challenges to our understanding of the physics of accretion onto compact objects. Given their rarity, every new insight into their complex phenomenology can bring us closer to a deeper understanding of these sources. A possible way to find more PULXs is by searching the archives of X-ray missions with good imaging and timing capabilities using a data mining approach. In the process, serendipitous sources are always behind the corner. Both the study of PULXs and the search for new X-ray pulsators through data mining have defined the course of my PhD. I will report my discovery of mHz QPOs in the X-ray flux of the PULXs. These mHz QPOs could represent a signature of super-Eddington accretion. I will also talk about a new pulsar (likely a new candidate magnetar) I discovered in the Large Magellanic Cloud thanks to a data mining project aimed at searching for new pulsators in the XMM-Newton archive.