Astronomy and Space

Abstract We report the discovery and timing of two pulsars from a sample of four circularly polarized sources identified in radio continuum images taken as part of the Australian SKA Pathfinder Variables and Slow Transients survey. Observations with the Parkes (Murriyang) radio telescope confirmed both sources as normal pulsars with high dispersion measures (DMs). PSR J1646−4451 has a spin period of 217 ms and a DM of 928 cm −3 pc, while PSR J1837−0616 exhibits a spin period of 118 ms and a DM of 793 cm −3 pc. These pulsars show extreme pulse broadening due to scattering, with measured scattering timescales of 117 ms and 75 ms at observing frequencies of ∼1.8 GHz, respectively. These measurements imply extrapolated scattering timescales at 1 GHz of ∼1346 ms and 740 ms, placing them among the most heavily scattered known pulsars. Our findings underscore the potential of using circular polarization in radio continuum images as a tool for identifying highly scattered pulsars. Future wide-field radio continuum surveys are poised to uncover a broader population of extreme pulsars, particularly those that are heavily scattered at 1.4 GHz, intrinsically faint, or residing in binaries—offering valuable insights into both pulsar demographics and the complex structure of the ionized interstellar medium.

ABSTRACT We perform a non-local thermodynamic equilibrium spectroscopic analysis of eight OB-type stars in the solar neighbourhood, using high-resolution spectra obtained with the 0.8-m telescope at Ankara University Kreiken Observatory together with archival European Southern Observatory/Ultraviolet and Visual Echelle Spectrograph data. Stellar parameters were determined from hydrogen Balmer lines, silicon ionization equilibria, and helium line profiles, while chemical abundances were derived for He, C, N, O, Ne, Mg, Al, Si, S, and Fe. HD 37032 is newly identified as a He-rich peculiar star ([He/H] $\approx +0.33$ dex), with asymmetric H $\alpha$ and [O i] $\lambda$5577 features revealing the presence of circumstellar material. HD 46660, HD 46883, and HD 7694 likewise exhibit mild helium enrichment relative to the solar abundance. HD 29309 is found to be a double-lined spectroscopic binary hosting a Barium-star secondary. A Hertzsprung–Russell diagram analysis indicates that most of the stars are located close to the main sequence, whereas HD 46883 is classified as a blue supergiant with no strong signatures of CNO mixing. These findings provide new constraints of the chemical diversity and evolutionary pathways of early-type stars in the local Galactic environment.

Since April 2023, solar flares have been simultaneously observed by the Spectrometer/Telescope for Imaging X-ray (STIX) onboard ESA’s Solar Orbiter and by the Hard X-ray Imager (HXI) onboard the Chinese ASO-S mission. The two telescopes independently measure 2D Fourier components (visibilities) of the flaring X-ray radiation from different vantage points. However, by combining their datasets, it is possible to obtain a sampling of the 3D Fourier transform of thermal hard X-ray sources in solar flares. This combined dataset allows reconstructing the 3D morphology of the flaring sources by solving an inverse imaging problem.In this presentation, we describe the methodology we developed for 3D reconstruction of thermal hard X-ray sources in solar flares from combined STIX/HXI data. We present the results obtained in the case of the X9.1 GOES class event which occurred on October 3, 2024. During this event, the two instruments were in an ideal configuration, where the separation angle between them and the flaring site was approximately 90 degrees. We validate the 3D reconstruction by comparing them with the 2D images independently reconstructed from STIX and HXI data. Finally, we determine the altitude of the reconstructed X-ray source above the solar surface as a function of time, and we derive estimates of its radial velocity.

Although strong electric fields have been observed in lightning clouds, these fields are well below the limit to spontaneously initiate a spark that could be the beginning of a lightning flash. Understanding the lightning initiation process is thus one of (if not The) main topics in lightning research.In this work we present very high frequency (VHF) radio observations using the LOFAR radio telescope [1]. Because of the high resolution and high sensitivity of LOFAR we could observe the faint initiating event for multiple lightning flashes. The new imaging procedure (called ATRI-D) was shown to be able to distinguish different emission sites of VHF pulses on an airplane flying at an altitude of 8 km [2].The propagating tip of this apparent initiating event carries positive charge, as is generally expected. Our observations show that the propagation speeds of this positive initiating event (PIE) are very similar at about 5 x 10^6 m/s. Very surprisingly, both the e-folding rates in VHF-intensity and peak intensities differ significantly for the investigated flashes and show no correlation with altitude. Additionally, these structures are extremely narrow, with diameters under 0.8 meters, and maintain this confinement over propagation distances exceeding 100 meters. Even more surprising is that subsequent dart leaders do not follow the path of the PIE, implying that the PIE has not formed a well-conducting structure and does not transform into a positive leader.Lightning initiation is shown to be a very subtle process, in spite of the vigor of a lightning flash, and the high resolution and sensitivity of LOFAR shows, for multiple lightning flashes, that the initiating event is a very weakly radiating, positively charged propagating structure.1) Olaf Scholten, Steven A. Cummer, Joseph R Dwyer, et al.; A Comprehensive analysis of High Resolution VHF Observations with LOFAR of the Positive Initiating Event for Several Lightning Flashes. ESS Open Archive . December 12, 2025. https://doi.org/10.22541/essoar.176556304.42772793/v12) O. Scholten, M. Lourens, et al. (2025) ; Measuring location and properties of very high frequency sources emitted from an aircraft flying through high clouds. Nature Communications, 16 (1), 10572. https://doi.org/10.1038/s41467-025-65667-2

With gradual stepwise reduction in rigidity of the Earth model, the three fluid Love numbers of the Earth were determined as; (hf, kf , lf ) = (1.935, 0.935, 1.07) for PREM and (hf, kf , lf ) = (1.937, 0.937, 1.07) for ak135-F Earth models. Also analytical evaluation of the Earth’s secular Love number as well as fluid Love number, originally defined by Munk and MacDonald, were made by using updated Earth’s physical property and minor enhancement in the formulation. The permanent tide of the Earth was calculated as follows: the vertical displacement is 0.1924m at the equator and -0.3801m at the pole, while the horizontal displacement is 0.317m at the mid latitude.

Abstract Galaxy mergers have long been proposed as a mechanism for funneling gas toward galactic centres, potentially triggering accretion onto supermassive black holes (SMBHs) and igniting active galactic nuclei (AGN). While simulations often support this scenario, observational studies have yielded conflicting results regarding the AGN-merger connection. In this study, we analyze 31 galaxies from cosmological zoom-in simulations spanning redshifts 0.5 < z < 3. We identify mergers using detailed merger trees based on six-dimensional dark matter particle information and identify AGN activity through SMBH accretion histories. To bridge the gap between simulations and observations, we generate mock JWST-like images and extract non-parametric morphological parameters. Employing a k-nearest neighbours (KNN) classifier in a five-dimensional space (four morphological parameters and redshift), we identify mergers in the mock-observed dataset. Our analysis reveals a statistically significant enhancement of AGN activity in merging systems, particularly at lower redshifts (0.5 < z < 0.9), where central gas reservoirs are more depleted. This supports the view that mergers contribute more significantly to AGN triggering in environments with low internal gas reservoirs, while their impact may be less pronounced in gas-rich systems. However, when relying solely on morphological classifications from mock observations, the observed AGN-merger connection weakens, especially at higher redshifts. This underscores the challenges in detecting merger-induced AGN activity observationally and highlights the importance of combining simulations with realistic mock observations to fully understand the AGN-merger relationship.

ABSTRACT The enigmatic Little Red Dots (LRDs) discovered by the James Webb Space Telescope (JWST) exhibit properties challenging their interpretation as common galaxies or active galactic nuclei (AGNs). Understanding their nature is key to placing them within our picture of early galaxy and massive black hole (MBH) evolution. To this aim, we build a realistic comparison between LRD observations with photometric properties of galaxies and AGN simulated by the L-GalaxiesBH model in a NIRCam mock sky. We model stellar continua and emission lines, the MBH emission from accretion disc, infrared radiation from dusty torus, and lines from narrow and broad-line regions, accounting for dust attenuation and obscuration. Using realistic photometric cuts, we select a population of LRDs including both AGN and galaxies. The LRD fraction peaks at 40 per cent ($\sim 10^{-4}\rm Mpc^{-3}$) at $z\sim 4$. Our LRDs are central galaxies spanning $M_\star =10^8{\!-\!}10^{10.5}\rm M_\odot$. A population of galaxies with $M_\star \lt 10^9\rm M_\odot$ appear as LRDs due to older stellar populations. At higher masses, LRDs dominate the halo and stellar mass functions ($M_{\rm vir} \gt 10^{11.5}\rm M_\odot$, $M_\star \gt 10^{9.5}\rm M_\odot$), and the interplay between AGN and galaxy emission drives the LRD selection. AGN dominate rest-frame UV-optical emission, while dust obscuration is secondary. LRDs host lighter MBHs ($\sim 10^{6.5}\rm M_\odot$) than non-LRDs ($\sim 10^{7.5}\rm M_\odot$), with fainter emission unable to balance their hosts Balmer breaks. We find no evidence for dominant heavy-seed origin of MBHs. LRD Galaxies (97 per cent hosting MBHs) and LRD AGNs are disc-dominated, with LRD AGNs showing larger bulges formed mainly via disc instabilities.

Abstract The Evershed flow (EF), since its discovery in 1909, represents a longstanding issue in solar physics, with its three-dimensional velocity fields and underlying physical mechanisms still not fully understood. We present a comprehensive analysis of the velocity fields for 20 sunspots, using the full-disk spectroscopic observations from the Chinese H α Solar Explorer (CHASE). The height-dependent velocity fields are derived from the bisector analysis of the Si i 6560.58 Å, Fe i 6569.21 Å, and H α 6562.81 Å spectral lines, which are formed from the deep photosphere (∼70 km) to the upper chromosphere (∼1340 km). The statistical results based on CHASE observations reveal that the photospheric EF manifests as a nearly horizontal outflow with the maximum radial flow velocities of 2.5–2.9 km s −1 at the outer penumbral boundaries, and the inverse EF (IEF) at the chromosphere shows predominant inward flows of 1.6–3.5 km s −1 , with substantial downward flows. EF-like flows above the penumbra are found in all sunspots in the H α far wings, with a velocity reversal shown by the bisector analysis at H α ±0.65 Å (corresponding to a normalized line depth intensity of ∼0.6) in the limb-side region, and a small reversal in the center-side region, suggesting the coexistence of the EF outflows and IEF downflows at the flow reversal layers. We finally construct a three-dimensional schematic of the EF and IEF above sunspots, depicting the height-dependent flows from the deep photosphere to the upper chromosphere.

ABSTRACT We investigate the accretion dynamics of the black hole X-ray binary Swift J1727.8−1613 during its $2023\!-\!2024$ discovery outburst that lasted for $\sim 10$ months. Insight-HXMT monitored the rising phase of the outburst of Swift J1727.8−1613 roughly continuously from 2023 August 25 to 2023 October 5. Strong signatures of type-C quasi-periodic oscillations (QPOs) are observed during this phase of the outburst. In our recent paper, nature of the QPOs are studied with the propagating oscillatory shock (POS) model. In this paper, we report on the observation of both positive (or hard) and negative (or soft) time-lags in the 4–10 keV (LE), 10–30 keV (ME), and 30–150 keV (HE) bands, computed with respect to the 2–4 keV reference band. We detect a clear transition from hard to soft lags as the outburst evolves. We show the evolution of QPOs and associated time-lags between different X-ray energy bands, correlated with changes in the QPO frequency, spectral state, and the size of the Comptonizing region. Our analysis reveals strong anticorrelations between the time-lags and both QPO frequency and photon index, and a strong positive correlation with the shock location. These evolving lag characteristics and their correlations provide crucial insights into the changing accretion geometry and the interplay of radiative processes, further supporting dynamic models like the POS in explaining the coupled spectro-temporal evolution in black hole X-ray binaries.