Coloquios

We performed full Stokes spectropolarimetric observations of loop footpoints in the active region NOAA 13363 during a C-class flare with the GREGOR Infrared Spectrograph (GRIS) on 2023 July 16. The observed spectral region included the photospheric Si I 10 827 A and Ca I 10 839 A lines and the chromospheric He I 10 830 A triplet. Simultaneously, high-cadence and high-resolution imaging observations were carried out with the improved High-resolution Fast Imager (HiFI+) in the Ca II H line and TiO bands. The observations were conducted under excellent seeing conditions, as confirmed by the Fried-parameter measurements. Speckle-restored HiFI+ Ca II H images revealed thin flare-related filaments and diffuse haze-like emissions, further confirmed by background-subtracted solar activity maps (BaSAMs), which localized chromospheric variability near the sunspot. The He I triplet showed enhanced emission during the flare events and developed intense red- and blue-shifted components, with the decisive shift of 90 km/s, suggesting the significant energy release and plasma motion triggered by the flare. Simultaneously, a delayed increase in the Si I line wing intensity was observed approximately 6 minutes after the He I emission, suggesting that the upper photosphere experienced secondary heating, possibly due to thermal conduction rather than energetic particles. This time delay and spatial correlation support a scenario where dynamic flare processes influence chromospheric and upper photospheric layers. Our results demonstrate a temporal and spatial coupling between chromospheric and upper photospheric regions, and the time delay rules out direct heating by flare-accelerated electrons, so we propose thermal conduction or ionization effects as possible mechanisms.

Measurements of the 511 keV emission reveal the presence of a steady injection of positrons that is very concentrated around the Galactic centre, and whose origin remains unknown. While astrophysical sources do not easily fit the observed morphology and intensity of this excess, MeV dark matter has been proposed as a compelling explanation with interesting consequences. In this seminar, I will introduce the "positron puzzle" and discuss its correlation to other anomalous emissions at the Galactic centre, especially focusing on the observational consequences of the dark matter explanation of the 511 keV excess. Finally, I’ll show how these observations can be used to constrain the properties of asteroid-mass primordial black holes and beyond Standard Model particles, such as sterile neutrinos or axion-like particles.

Matter ejection, in the form of either winds or jets, is ubiquitous in accreting X-ray binaries. Although it is clear that accretion and ejection are profoundly intertwined in these types of systems, the origin and the details of such an interconnection are yet to be unraveled. This is particularly true for systems where a low-magnetized neutron star (NS) accretes matter from a low-mass companion star (NS low-mass X-ray binaries, LMXBs). Indeed, unlike the case of accreting black holes, in NS LMXBs the already delicate interplay between accretion and ejection may be further complicated by the presence of, e.g., the NS magnetic field, the boundary layer and the emission from the NS surface. For instance, jets in NS LMXBs have been claimed to be more collimated than in BH LMXBs, their occurrence sometimes seems to be unrelated to the spectral state and their observed radio luminosity show a rather scattered distribution. X-ray winds on the other hand have been often detected in states where they were not expected, in particular in a class of NS LMXBs, the Accreting Millisecond X-ray Pulsars (AMXPs), where the channeling of the accretion flow along the magnetic field lines makes these systems visible as rapidly spinning X-ray pulsars. Finally, AMXPs typically drive more powerful jets than other (non-pulsating) NS LMXBs and their rapid orbital expansion can be explained by strong mass outflows. In this talk, I will review the emerging pattern of peculiar outflows in NS LMXBs, the possible implications for jet and wind-launching mechanisms in these systems and the key role that future multi-band observing campaigns will play in clarifying its physical origin.

Zoom link: https://rediris.zoom.us/j/97431924964?pwd=rpPDNaL2VrEKfs8TSZNyck8GbTnjnZ.1
Meeting ID: 974 3192 4964

Passcode: 078804

Youtube: https://youtube.com/live/ZO-hf7iNPRw?feature=share

The era of gravitational wave astronomy has dawned, allowing us not only to observe the universe but also to "listen" to it through gravitational waves. When a compact object ventures too close to a supermassive black hole, it becomes captured due to the emission of gravitational waves, eventually being swallowed whole as it crosses the event horizon. During this process, the system radiates energy, which can be viewed as a snapshot containing detailed information about the geometry of spacetime and the physical parameters of the system with extraordinary precision. Intriguingly, this information may also hold clues about the topology of spacetime, suggesting a potential link between geometry and topology in the strong-field regime of gravity. This phenomenon effectively maps the warped spacetime, serving as a unique probe of gravity in its most extreme regime. Thanks to these captures, we can now tackle fundamental questions: Do black holes truly exist? How do they accumulate their colossal mass over cosmic history? And what is the true nature of their event horizons? 

It is widely accepted that most galaxies undergo an active phase during their evolution. The impact of the energy released by active galactic nuclei (AGN) has been proposed as a key mechanism responsible for regulating star formation (SF) by influencing the interstellar medium (ISM) of the host galaxy. Dust, gas, and molecular components are key tracers of the interplay between the supermassive black hole (SMBH) and its host. The infrared (IR) regime hosts numerous spectral features, such as fine-structure lines, dust and ice features, organic molecules, hydrogen, and water, that act as sensitive barometers of the physical conditions in the ISM. These features are essential for tracing AGN feedback from the innermost regions (tens of pc) out to kiloparsec scales. With its unprecedented sensitivity and resolution, the James Webb Space Telescope (JWST) now enables detailed measurements of the gas flow cycle in AGN. Nearby AGN provide the additional advantages of high spatial resolution and strong signal-to-noise, allowing us to disentangle the key coupling mechanisms in their different phases. In this talk, we will present recent findings and ongoing work from the Galaxy Activity, Torus, and Outflow Survey (GATOS), with a focus on results from JWST and complementary observations such as ALMA and GTC.

Fast X-ray Transients (FXTs) are minute-to-hours long flashes of X-rays, first discovered serendipitously in X-ray satellite data (mainly Chandra and XMM-Newton). They are proven to be caused by energetic extra-galactic phenomena. Currently, Einstein Probe is revolutionizing the field by discovering many FXTs and, crucially, by their low-latency announcement thereof. These extra-galactic FXTs are ubiquitous: their density rate is several hundred per year per Mpc^3. FXTs have been proposed to arise from double neutron star mergers, tidal disruption events involving an intermediate-mass black hole and a white dwarf, and from off-axis or sub-luminous gamma-ray bursts. Brief extra-galactic FXTs also arise in supernova shock breakouts. Contemporaneous multi-wavelength detections possible only in the current Einstein Probe era show that FXTs originate from more than 1 progenitor. I will discuss the most recent findings and provide some (potential) science questions to be answered using FXT observations.

MHD waves – slow, Alfvén and fast – lose their distinctiveness in certain regions of a stratified plasma, such as solar or stellar atmospheres. We discuss all three mode conversion processes, fast/slow, fast/Alfvén and slow/Alfvén and how they are affected by atmospheric structure, magnetic field orientation, and partial ionization. We also present some simulations of fast/slow coupling in shock waves.

Stellar models are a crucial ingredients for a pletora of fundamental research fields in Astrophysics: from the planet-host stars to Galactic Archaeology, from fundamental Physics to the study/understanding of far-away unresolved galaxies, from helio/astero-seismology to exotic stellar objects such as Blue Stragglers, Blue Hook stars, millisecond pulsars, supernova progenitors, etc. There are various stellar model libraries available in the literature, each one with its own pro and cons; some of them being more suitable for specific research topics. In any case, the use of any stellar model library should not ignore the knowledge of the limitations affecting each library. In this talk, we present the BaSTI_IAC stellar model library that has been developed in the context of a strong collaboration with staff members of the IAC; we discuss the main characteristics of this library, and make a comparison with some of the most commonly used model libraries available in literature. We present also some important recent applications of the BaSTI_IAC library to various scientific problems. At the end we discuss the ongoing effort to improve/extend the library as well as our wish to include additional stellar and sub-stellar mass ranges, with hope to foster new collaborations/synergies with colleagues@IAC.

The elusiveness of neutrinos is most renowned for their ability to penetrate and traverse vast amounts of matter without disturbance. While this very same property makes neutrino detection one of humanity’s most remarkable achievements, recent and forthcoming advancements in instrumentation continue to enhance our ability to harness neutrino-based information as a fundamental tool. This, in turn, provides unique insights into some of the most essential mysteries of the Universe. For instance, shortly after their discovery in the 1950s, neutrinos offered direct confirmation and deeper understanding of the fundamental fusion processes powering the Sun. Today, they are indispensable to our quest for a comprehensive understanding of Earth’s interior, the depths of the Sun, supernovae, high-energy cosmic ray emissions, and the early Universe’s structure following the Big Bang. Furthermore, neutrinos may play a crucial role in our pursuit of the origins of matter and the search for new physics beyond the Standard Model.
In this colloquium, I will highlight some of the most remarkable achievements in neutrino science to date, as well as emerging advancements that have the potential to complement all other cosmic probes — most notably, those within the IAC’s leadership in this field.

Stars and planets formed within the same molecular cloud are inextricably linked in their composition. Alpha-process elements shape planetary cores and atmospheres, with studies showing that key elemental ratios (e.g. Fe/Si, Mg/Si) in planets reflect those of their host stars. While correlations between stellar chemical abundances, planet occurrence, mass and orbital properties have been suggested, definitive confirmation remains difficult due to the subtlety of these trends. Large, homogeneous, high-precision spectral datasets are essential to uncover these relationships. Bright stars (V < 11 mag), such as PLATO's priority-one targets, provide an ideal sample for high-quality stellar and chemical abundance measurements and are expected to yield thousands of new planetary discoveries in the coming years. However, modern multi-object spectroscopic (MOS) surveys often exclude these stars due to their low on-sky density, leading to inefficiencies in conventional observing strategies. The WEAVE-TwiLight Survey (WTLS) solves this problem by introducing a groundbreaking observing mode that optimises efficiency by combining multiple fields into a single fibre configuration. It is expected to produce a homogeneous spectral dataset of ~6,000 bright stars, tailored to probe the chemical relationships between host stars and their planets. In this talk, I will give a general overview of host-star planet relations and discuss the status of the upcoming WEAVE-TwiLight Survey.