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We present one of the largest uniform optical spectroscopic surveys of X-ray selected sources to date that were observed as a pilot study for the Black Hole Mapper (BHM) survey. The BHM program of the Sloan Digital Sky Survey (SDSS)-V is designed to provide optical spectra for hundreds of thousands of X-ray selected sources from the SRG/eROSITA all-sky survey. This significantly improves our ability to classify and characterise the physical properties of large statistical populations of X-ray emitting objects. Our sample consists of 13 079 sources in the eROSITA eFEDS performance verification field, 12 011 of which provide reliable redshifts from 0 ≲ z ≤ 5.8. The vast majority of these objects were detected as point-like sources (X-ray flux limitF_{0.5 − 2 keV} ≳ 6.5 × 10⁻¹⁵erg/s/cm²) and were observed for about 20 years with fibre-fed SDSS spectrographs. After including all available redshift information for the eFEDS sources from the dedicated SDSS-V plate programme and archival data, we visually inspected the SDSS optical spectra to verify the reliability of these redshift measurements and the performance of the SDSS pipeline. The visual inspection allowed us to recover reliable redshifts (for 99% of the spectra with a signal-to-noise ratio of > 2) and to assign classes to the sources, and we confirm that the vast majority of our sample consists of active galactic nuclei (AGNs). Only ∼3% of the eFEDS/SDSS sources are Galactic objects. We analysed the completeness and purity of the spectroscopic redshift catalogue, in which the spectroscopic completeness increases from 48% (full sample) to 81% for a cleaner, brighter (r_AB < 21.38) sample that we defined by considering a high X-ray detection likelihood, a reliable counterpart association, and an optimal sky coverage. We also show the diversity of the optical spectra of the X-ray selected AGNs and provide spectral stacks with a high signal-to-noise ratio in various sub-samples with different redshift and optical broad-band colours. Our AGN sample contains optical spectra of (broad-line) quasars, narrow-line galaxies, and optically passive galaxies. It is considerably diverse in its colours and in its levels of nuclear obscuration. 

Abstract Quantum Chromodynamics predicts a phase transition from hadronic matter to quark–gluon plasma (QGP) at high temperatures and energy densities, where quarks and gluons (partons) are no longer confined within hadrons. The QGP forms in ultrarelativistic heavy-ion collisions. Anisotropic flow coefficients, quantifying the azimuthal expansion of produced matter, probe QGP properties. Flow measurements in high-energy heavy-ion collisions show a distinctive grouping of anisotropic flow for baryons and mesons at intermediate transverse momentum – a feature associated with flow imparted at the quark level, confirming QGP existence. The observation of QGP-like features in proton–proton and proton–ion collisions has sparked debate about QGP formation in smaller systems. For the first time, we demonstrate the distinctive grouping of anisotropic flow for baryons and mesons in high-multiplicity proton–lead and proton–proton collisions at the Large Hadron Collider (LHC). These results are described by a model including hydrodynamic flow followed by hadron formation via quark coalescence, consistent with the formation of partonic flowing systems in these collisions. 

A<sc>bstract</sc> The yields of prompt and non-prompt J/ψand the fraction of non-prompt J/ψare measured at midrapidity (|y| < 0.9) via the dielectron decay channel as a function of the midrapidity charged-particle multiplicity (|η| < 0.9) in pp collisions at$$\sqrt{s}=13$$TeV. The J/ψyields and the multiplicity are normalized by their average value in inelastic collisions. The multiplicity-dependent yield ratio between prompt J/ψand D⁰is reported. The multiplicity is further divided into three azimuthal regions with respect to the J/ψmomentum: toward the J/ψemission direction, transverse, or opposite to it. A stronger-than-linear increase of the self-normalized yields is observed for both prompt and non-prompt J/ψproduction, with similar trends. This behaviour is also observed in the toward region, while a weaker increase is observed in the transverse and away regions. 

Abstract Two-particle angular correlations probe particle production mechanisms and the underlying event-wide phenomena present in hadronic collisions. The correlations are examined as a function of rapidity and azimuthal-angle differences ($$\Delta y, \Delta \varphi $$ $\Delta y,\Delta \phi$ ) for pairs of like- and unlike-sign pions, kaons, and (anti-)protons produced in pp collisions at$$\sqrt{s}$$ $\sqrt{s}$ = 13 TeV, measured by the ALICE experiment. Two-particle correlation functions are provided together with$$\Delta y$$ $\Delta y$ and$$\Delta \varphi $$ $\Delta \phi$ projections and compared to Monte Carlo (MC) model predictions. For the first time, the measurement is performed as a function of the event’s charged-particle density. Previous studies conducted for pp collisions at$$\sqrt{s}$$ $\sqrt{s}$ = 7 TeV at ALICE revealed a near-side anticorrelation for baryon–baryon and antibaryon–antibaryon pairs, whose origin remains unresolved. Here, an additional approach is introduced to study the multiplicity dependence and the expected inverse multiplicity scaling of the correlation function. This method highlights qualitative differences in the underlying sources of correlations, such as quantum-statistics effects, final-state interactions, and resonance decays. The puzzling near-side anticorrelation in baryon baryon measurements is observed across all multiplicity classes and continues to challenge current particle-production models. Furthermore, the multiplicity dependence of the correlations between mesons provides an independent probe of the sensitivity of current MC models to soft-QCD effects and hadronization dynamics. The presented measurements, together with the baryon results, enrich the experimental picture of two-particle correlations in pp collisions and serve as valuable input for ongoing theoretical developments. 

Abstract Rotating nonaxisymmetric neutron stars (NSs) are promising sources for continuous gravitational waves (CWs). CWs may, if detected, inform us about the internal structure and equation of state of NSs. Here, we present a narrowband search for CWs from known pulsars, for which a matched-filter search can be applied. Narrowband searches are robust to mismatches between electromagnetic (EM) and gravitational emissions, in contrast to fully targeted searches where they are assumed to be phase-locked. In this work, we search for the CW counterparts emitted by 34 pulsars using data from the first and second parts of the fourth LIGO–Virgo–KAGRA observing run. We use the 5n-vector narrowband pipeline, which applies frequency-domain matched filtering. In previous searches, it covered a narrow range in the frequency—frequency time derivative (f— $\dot{f}$ ) space. Here, we also explore a range in the second time derivative of the frequency $\ddot{f}$ around the EM observations. Additionally, for the first time, we target sources in a binary system with this kind of search. We find no evidence for CWs and therefore set upper limits on the strain amplitude emitted by each pulsar. For 20 analyses, we report an upper limit below the theoretical spin-down limit. The tightest constraint is for pulsar PSR J0534+2200 (the Crab pulsar), for which our strain upper limit on the CW amplitude corresponds to ≤0.04% of the spin-down power being radiated in the CW channel.