SHOTGLAS: II. MUSE spectroscopy of blue horizontal branch stars in the core of Omega Centauri and NGC6752

Galaxie

SHOTGLAS II. MUSE spectroscopy of blue horizontal branch stars in the core of ω Centauri and NGC 6752

Aims. We want to study the population of blue horizontal branch (HB) stars in the centers of globular clusters (GC) for the first time by exploiting the unique combination of MUSE spectroscopy and HST photometry. In this work, we characterize their properties in the GCs ω Cen and NGC 6752. Methods. We use dedicated model atmospheres and synthetic spectra grids computed using a hybrid LTE/NLTE modeling approach to fit the MUSE spectra of HB stars hotter than 8 000 K in both clusters. The spectral fits provide estimates of the effective temperature (Teff), surface gravity (log g), and helium abundance of the stars. The model grids are further used to fit the HST magnitudes, meaning the spectral energy distributions (SED), of the stars. From the SED fits, we derive the average reddening, radius, luminosity, and mass of the stars in our sample. Results. The atmospheric and stellar properties that we derive for the stars in our sample are in good agreement with the theoretical expectations. In particular, the stars cooler than ∼15 000 K follow neatly the theoretical predictions for the radius, log g, and luminosity for helium-normal (Y=0.25) models. In ω Cen, we show that the majority of these cooler HB stars cannot originate from a heliumenriched population with Y >0.35. The properties of the hotter stars (radii and luminosities) are still in reasonable agreement with theoretical expectations, but the individual measurements have a large scatter. For these hot stars, we have a mismatch between the effective temperatures indicated by the MUSE spectral fits and the photometric fits, with the latter returning Teff lower by ∼3 000 K. We use three different diagnostics, namely the position of the G-jump and changes in metallicity and helium abundances to place the onset of diffusion in the stellar atmospheres at Teff between 11 000 K and 11 500 K. Our sample includes two stars known as photometric variables, we confirm one to be a bona fide extreme HB object but the other is a blue straggler star. Finally, unlike what has been reported in the literature, we do not find significant differences between the properties (e.g., log g, radius, and luminosity) of the stars in both clusters. Conclusions. We showed that our analysis method combining MUSE spectra and HST photometry of HB stars in GC is a powerful tool to characterize their stellar properties. With the availability of MUSE and HST observations of additional GCs, we have a unique opportunity to combine homogeneous spectroscopic and photometric data to study and compare the properties of blue HB stars in different GCs

X-ray detection of a nova in the fireball phase

Novae are caused by runaway thermonuclear burning in the hydrogen-rich envelopes of accreting white dwarfs, which leads to a rapid expansion of the envelope and the ejection of most of its mass. Theory has predicted the existence of a ‘fireball’ phase following directly on from the runaway fusion, which should be observable as a short, bright and soft X-ray flash before the nova becomes visible in the optical. Here we report observations of a bright and soft X-ray flash associated with the classical Galactic nova YZ Reticuli 11 h before its 9 mag optical brightening. No X-ray source was detected 4 h before and after the event, constraining the duration of the flash to shorter than 8 h. In agreement with theoretical predictions, the source’s spectral shape is consistent with a black-body of 3.27 × 10 K (28.2 eV), or a white dwarf atmosphere, radiating at the Eddington luminosity, with a photosphere that is only slightly larger than a typical white dwarf.This work was supported by the Bundesministerium für Forschung und Technologie under DLR grants 50 QR 1603, 50 QR 2103 and 50 QR 2104. G.S. acknowledges support from the Spanish MINECO grant PID2020-117252GB-I00. V.S. thanks the Deutsche Forschungsgemeinschaft (DFG) for financial support (WE1312/53-1)

EVR-CB-004: An Inflated Hot Subdwarf O Star + Unseen WD Companion in a Compact Binary Discovered with the Evryscope

We present the discovery of EVR-CB-004, a close binary with a remnant stellar core and an unseen white dwarf (WD) companion. The analysis in this work reveals that the primary is potentially an inflated hot subdwarf (sdO) and more likely is a rarer post-blue horizontal branch (post-BHB) star. Post-BHBs are the short-lived shell-burning final stage of a blue horizontal star or hot subdwarf before transitioning to a WD. This object was discovered using Evryscope photometric data in a southern all-sky hot subdwarf variability survey. The photometric light curve for EVR-CB-004 shows multicomponent variability from ellipsoidal deformation of the primary and Doppler boosting, as well as gravitational limb darkening. The binary EVR-CB-004 is one of just a handful of known systems and has a long period (6.08426 hr) and large-amplitude ellipsoidal modulation (16.0% change in brightness from maximum to minimum) for these extremely close binary systems, while the properties of the primary make it a truly unique system. It also shows a peculiar low-amplitude (less than 1%) sinusoidal light-curve variation with a period that is a 1/3 resonance of the binary period. We tentatively identify this additional variation source as a tidally induced resonant pulsation, and we suggest follow-up observations that could verify this interpretation. From the evolutionary state of the system, its components, and its mass fraction, EVR-CB-004 is a strong merger candidate to form a single high-mass (1.2 M o˙) WD. It offers a glimpse into a brief phase of remnant core evolution and secondary variation not seen before in a compact binary

TELAMON: Effelsberg Monitoring of AGN Jets with Very-High-Energy Astroparticle Emissions -- II. Polarization properties

We present recent results of the TELAMON program, which is using the Effelsberg 100-m telescope to monitor the radio spectra of active galactic nuclei (AGN) under scrutiny in astroparticle physics, namely TeV blazars and neutrino-associated AGN. Our sample includes all known Northern TeV-emitting blazars as well as blazars positionally coincident with IceCube neutrino alerts. Polarization can give additional insight into the source properties, as the polarized emission is often found to vary on different timescales and amplitudes than the total intensity emission. Here, we present an overview of the polarization properties of the TeV-emitting TELAMON sources at four frequencies in the 20 mm and 7 mm bands. While at 7 mm roughly $82\,\%$ of all observed sources are found to be significantly polarized, for 20 mm the percentage is $\sim58\,\%$. We find that most of the sources exhibit mean fractional polarizations of $<5\%$, matching the expectations of rather low polarization levels in these sources from previous studies at lower radio frequencies. Nevertheless, we demonstrate examples of how the polarized emission can provide additional information over the total intensity.Comment: 9 pages, 2 figures, Proceedings of the 38th International Cosmic Ray Conference (ICRC2023

SHOTGLAS

Aims. We want to study the population of blue horizontal branch (HB) stars in the centres of globular clusters (GC) for the first time by exploiting the unique combination of MUSE spectroscopy and HST photometry. In this work, we characterize their properties in the GCs ω Cen and NGC 6752. Methods. We use dedicated model atmospheres and grids of synthetic spectra computed using a hybrid LTE/NLTE modeling approach to fit the MUSE spectra of HB stars hotter than 8000 K in both clusters. The spectral fits provide estimates of the effective temperature (Teff), surface gravity (log ɡ), and helium abundance of the stars. The model grids are further used to fit the HST magnitudes of the stars, that is, their spectral energy distributions (SEDs). From the SED fits, we derive the average reddening, radius, luminosity, and mass of the stars in our sample. Results. The atmospheric and stellar properties that we derive for the stars in our sample are in good agreement with theoretical expectations. In particular, the stars cooler than ~15 000 K closely follow the theoretical predictions on radius, log ɡ, and luminosity for helium-normal (Y = 0.25) models. In ω Cen, we show that the majority of these cooler HB stars cannot originate from a helium-enriched population with Y > 0.35. The properties of the hotter stars (radii and luminosities) are still in reasonable agreement with theoretical expectations, but the individual measurements show a large scatter. For these hot stars, we find a mismatch between the effective temperatures indicated from the MUSE spectral fits and the photometric fits, with the latter returning Teff lower by ~3000 K. We use three different diagnostics, namely the position of the G-jump and changes in metallicity and helium abundances, to place the onset of diffusion in the stellar atmospheres at Teff between 11 000 and 11 500 K. Our sample includes two stars known as photometric variables; we confirm one to be a bona fide extreme HB object but the other is a blue straggler star. Finally, unlike what has been reported in the literature, we do not find significant differences between the properties (e.g., log ɡ, radius, and luminosity) of the stars in the two clusters. Conclusions. We show that our analysis method – combining MUSE spectra and HST photometry of HB stars in GCs – is a powerful tool for characterising their stellar properties. With the availability of MUSE and HST observations of additional GCs, we have a unique opportunity to combine homogeneous spectroscopic and photometric data to study and compare the properties of blue HB stars in different GCs

The SRG/eROSITA all-sky survey. First X-ray catalogues and data release of the western Galactic hemisphere

International audienceThe eROSITA telescope array aboard the Spektrum Roentgen Gamma (SRG) satellite began surveying the sky in December 2019, with the aim of producing all-sky X-ray source lists and sky maps of an unprecedented depth. Here we present catalogues of both point-like and extended sources using the data acquired in the first six months of survey operations (eRASS1; completed June 2020) over the half sky whose proprietary data rights lie with the German eROSITA Consortium. We describe the observation process, the data analysis pipelines, and the characteristics of the X-ray sources. With nearly 930 000 entries detected in the most sensitive 0.2–2.3 keV energy range, the eRASS1 main catalogue presented here increases the number of known X-ray sources in the published literature by more than 60%, and provides a comprehensive inventory of all classes of X-ray celestial objects, covering a wide range of physical processes. A smaller catalogue of 5466 sources detected in the less sensitive but harder 2.3–5 keV band is the result of the first true imaging survey of the entire sky above 2 keV. We present methods to identify and flag potential spurious sources in the catalogues, which we applied for this work, and we tested and validated the astrometric accuracy via cross-comparison with other X-ray and multi-wavelength catalogues. We show that the number counts of X-ray sources in eRASSl are consistent with those derived over narrower fields by past X-ray surveys of a similar depth, and we explore the number counts variation as a function of the location in the sky. Adopting a uniform all-sky flux limit (at 50% completeness) of F 05–2 keV > 5 × 10 −14 erg s −1 cm −2 , we estimate that the eROSITA all-sky survey resolves into individual sources about 20% of the cosmic X-ray background in the 1–2 keV range. The catalogues presented here form part of the first data release (DR1) of the SRG/eROSITA all-sky survey. Beyond the X-ray catalogues, DR1 contains all detected and calibrated event files, source products (light curves and spectra), and all-sky maps. Illustrative examples of these are provided