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Physics Astrophysics Solar and Stellar Astrophysics

astro-ph.SR Watch Solar and Stellar Astrophysics

Tuesday 8 September

Solar and Stellar Astrophysics · Cross-listed from astro-ph.IM

A Missing Tool for Calculating Auto/Cross-correlation Function under Nonuniform Sampling Observations

Chen-Ran Hu, Yong-Feng Huang, Jin-Jun Geng et al. (+10 more)

Nonuniform sampling presents a long-standing challenge in astrophysical time-domain analysis, invalidating the standard autocorrelation and cross-correlation functions and forcing researchers to adopt ad-hoc methods like interpolation or binning, which introduce unquantified biases and lack rigorous error estimation. Here we introduce a new method for calculating the nonuniform autocorrelation function (NUACF) and nonuniform cross-correlation function (NUCCF) for irregularly sampled time series. Instead of relying on interpolation, it naturally evaluates the correlation function by incorporating time-interval weights and misalignment penalties. Monte Carlo simulations provide confidence bands for significance assessment and a complete error budget for the time delays that accounts for both flux uncertainties and sampling irregularity (essential but generally absent from existing methods). Through extensive simulations, we demonstrate our method outperforms traditional methods across various conditions, from strictly periodic to complex repeating variability patterns (e.g., intermittent but aperiodic). Its effectiveness is demonstrated via various real astrophysical data sets, revealing repetitive variability in stellar light curves, measuring time delays for multi-band disc reverberation in the AGN Fairall 9, and providing model-independent validation of time delays for the gravitationally lensed quasar HE 0435-1223. The method provides a rigorous and general solution to the ubiquitous problem of nonuniform sampling, positioning it as a useful tool for large-scale time-domain survey data analysis. The framework is also directly applicable to emerging time-domain phenomena such as fast radio bursts (FRBs), enabling, e.g., the study of correlations between persistent radio source luminosity and repeating FRB activity, or among the multi-parameter variability curves of FRB emission itself.
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All authors Chen-Ran Hu, Yong-Feng Huang, Jin-Jun Geng, Orkash Amat, Ze-Cheng Zou, Chen Deng, Fan Xu, Xiao-Fei Dong, Chen Du, Nurimangul Nurmamat, Pei Wang, Lang Cui, Cheng-Ming Li

Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP

Radial velocity follow-up of Gaia astrometric substellar companions: six confirmed brown dwarfs and 13 impostor binaries

Marcus L. Marcussen, Simon H. Albrecht, Kamil K. Kalinowski et al. (+7 more)

With microarcsecond precision, Gaia has made the astrometric discovery of substellar companions feasible. However, follow-up observations are needed to check on orbital solutions and rule out astrophysical false positives. We validate and characterise a sample of 20 Gaia Data Release 3 (DR3) astrometric candidates for substellar companions, with the dual goal of identifying false positives and deriving robust physical parameters for genuine companions. We performed high-resolution spectroscopy using FIES, NIRPS, and NEID at the Nordic Optical Telescope, the ESO 3.6m telescope, and the WIYN Telescope, respectively. Double-lined spectroscopic binaries were identified as false positives, and surviving systems were characterised through joint Gaia astrometry and radial velocity modelling. Thirteen of the candidates proved to be near-twin binary stars for which the astrometric motion of the center of light is small enough to mimic that of a single star in response to a substellar companion. We confirm that six companions are brown dwarfs with masses in the range ${\sim}26$–$68\,M_\mathrm{Jup}$, two of them for the first time. In contrast to earlier studies of lower-mass Gaia candidates, we find that the astrometric orbital solutions are generally reliable to within the stated uncertainties.
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All authors Marcus L. Marcussen, Simon H. Albrecht, Kamil K. Kalinowski, Joshua N. Winn, Gu{\dh}mundur Stefánsson, Jens Reersted Larsen, Julie Gadeberg, Evan Fitzmaurice, Kevin Schlaufman, Suvrath Mahadevan

Solar and Stellar Astrophysics · Cross-listed from hep-ph

A distance-independent constraint on the axion-electron coupling from RGB stars

Thomas Levasseur, Oscar Straniero and Andrea Caputo

Interest in axions and axion-like particles has resurged, driven by their role in solving the strong CP problem and their appeal as dark matter candidates. The luminosity of the tip of the red giant branch (TRGB) offers a key observable for probing these particle properties. This work aims to improve existing bounds on the axion-electron coupling by adopting a differential observable that is less sensitive to distance, interstellar extinction, the zero-point of bolometric corrections (BCs), and other systematic uncertainties. We use the bolometric magnitude difference between the TRGB and the RGB bump (RGBB) as a distance-independent constraint, applied to three globular clusters of intermediate-to-high metallicity: NGC 104 (47 Tuc), NGC 362, and NGC 5904 (M5). Using photometric catalogs of RGB stars, we determine V- and I-band magnitudes of both features and convert them to bolometric values. After validating that our stellar models reproduce the observed RGBB luminosity, we perform a maximum likelihood analysis with Monte Carlo simulations to propagate uncertainties and derive new bounds on the coupling. A combined analysis of the three clusters yields a maximum likelihood at $g_{13} = g_{ae}/10^{-13} = 0.8$ and a 95% C.L. upper limit of 1.49. Although slightly less stringent than recent bounds from larger multi-cluster samples, this limit is substantially more robust. Under reasonable mass-loss assumptions, $g_{13} \gtrsim 7.5$ is ruled out, as it predicts the disappearance of the HB and AGB phases routinely observed in globular clusters, a limit more than an order of magnitude stronger than current direct experimental bounds such as XENONnT. We demonstrate the effectiveness of this differential, distance-independent method for constraining physics beyond the Standard Model. Applying it to a wider sample of globular clusters would further refine the constraint on $g_{ae}$.
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Solar and Stellar Astrophysics

Magnetic Signatures in Merger Products

Nicolas Muntean, Lisa Bugnet, Lynn Buchele et al. (+5 more)

Asteroseismic estimates of the magnetic field strength in the radiative interior of red giant stars depend strongly on the internal stellar structure derived from models. Since red giant branch merger products have been shown to be able to possess a different core structure than single stars of the same mass, we investigate how a mass-gain history influences our estimates of an internal magnetic field strength. We construct stellar models with and without a mass-gain event after the onset of the RGB evolutionary phase with masses of $1.1\,M_\odot \le M \le 2\,M_\odot$. First, by assuming a weak magnetic field, we investigate the influence of a mass-gain event on the global sensitivity of the oscillation frequencies to the magnetic field. We find that mass-gain models can be several times more sensitive to the field than single-star models of identical total mass at masses above $1.6\,M_\odot$. Therefore, considering a mass-gain evolutionary history for merger candidates allows a significant correction to the magnetic field strength. In the presence of strong magnetic fields, we also show that the critical field strength needed to suppress mixed dipole modes is significantly lower if a mass-gain event has occurred (for stars with masses $M\gtrsim1.6\,M_\odot$). The massive end of the suppressed stars' distribution is therefore strongly favored by a merger origin. We conclude that properly constraining the stellar evolutionary history is essential when aiming to constrain internal field strength estimates from asteroseismic observations.
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All authors Nicolas Muntean, Lisa Bugnet, Lynn Buchele, Debasish Dutta, Lukas Einramhof, Paul G. Beck, Lucas Barrault, Bastien Liagre

Solar and Stellar Astrophysics · Cross-listed from astro-ph.GA

Series of core collapse numerical simulations (SOLANUM) I: Modelling the early and late infall in a sun-like protostar

D. Navarro-Almaida, B. Commerçon, U. Lebreuilly et al. (+5 more)

Magnetohydrodynamical simulations and interferometric observations toward young stellar objects reveal the accretion of gas that may alter the physical properties and chemical composition of protostellar disks. Our goal is to constrain the physical history and properties of late infalling gas, and assess its potential to change the physical and chemical properties of the disk. We carried out a core collapse simulation of 5 M$_{\odot}$ of gas endowed with tracer and sink particles during $\sim 3.6\times 10^{5}$ yr. We analyzed the properties of the rotationally supported disk, and the origin and physical history of the gas from which it is initially formed. We also selected the tracer particles that describe the late infall to analyze their origin and physical history, and compare them to the information obtained from the disk. The final mass of the sink reaches 1.02 M$_\odot$, while the mass of the disk stays fairly constant around 0.02 M$_\odot$. Throughout its evolution, the disk radius increases up to $\sim 40$ au and its density is reduced by a factor of $\sim 8$. Filaments that channel gas from the envelope to the disk and sink appear as a result of the magnetic interchange instability. Tracer particles can identify late infalling gas, whose physical properties resemble that of observed streamers and are only accreted in the final snapshots of the simulation. The origin and physical history of the late infalling gas are different from those of the gas that initially forms the disk. Core collapse simulations predict the appearance of filaments that connect the envelope to the disk. The late accretion of gas through these channels has properties similar to those of streamers and brings gas to the disk with a different physical history that could translate into a different chemical composition.
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All authors D. Navarro-Almaida, B. Commerçon, U. Lebreuilly, P. Hennebelle, A. Ahmad, A. Fuente, P. Rivière-Marichalar, J. E. Pineda

Solar and Stellar Astrophysics · Cross-listed from astro-ph.HE

SN 2025fhm: A central-engine powered Ic-BL supernova associated with X-ray transient EP250304a

Cui-Ying Song, Nan Jiang, Xiaofeng Wang et al. (+32 more)

We present X-ray, optical, and radio follow-up observations of EP250304a, an extragalactic fast X-ray transient (EFXT) discovered by the Einstein Probe. Its X-ray light curve exhibits two broad pulses with comparable peak fluxes within the first $\sim$1~ks, a feature rarely seen among low-luminosity gamma-ray bursts or EFXTs. Optical follow-up observations were carried out using the Korea Microlensing Telescope Network, the Thai Robotic Telescope, the Las Cumbres Observatory 1~m global network, the Gemini Multi-Object Spectrograph on Gemini south telescope, and the Global Supernova Network. The fast-cooling phase (within 3 days) of optical data can be well fitted by a shocked cocoon model. However, during the supernova phase (SN 2025fhm, from 3 to 88 days), the late-time light curve cannot be explained solely by radioactive $^{56}$Ni decay, as demonstrated by a grid of simulations using the one-dimensional Lagrangian radiation hydrodynamics code SNEC, which reveals a significant energy excess at late epochs. To account for this excess, a central engine like a rapidly spinning, highly magnetized neutron star is needed to provide additional energy injection. This model yields a best-fit spin period of $\sim$12.60~ms and magnetic field strength of $\sim 3.52\times10^{15} \rm G$, and it successfully explains both the late-time bolometric light curve and the early X-ray pulse structures. Our results indicate that EP250304a/SN 2025fhm is likely powered by a central magnetar rather than by radioactive decay alone, offering new insights into the energy budget and physical origin of EFXTs and their associated supernovae.
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All authors Cui-Ying Song, Nan Jiang, Xiaofeng Wang, Yi-Han Iris Yin, Lingzhi Wang, Wenxiong Li, Shengyu Yan, Dae-Sik Moon, Tao An, Aleksandar Cikota, Samaporn Tinyanont, Liang-Duan Liu, Cui-Yuan Dai, Christopher D. Matzner, Bin-Bin Zhang, Lixin Yu, Qinyu Wu, Hong Soo Park, Sang Chul Kim, Youngdae Lee, Yu-Hao Zhang, Haowei Peng, Franz E. Bauer, Joseph R. Farah, Moira Andrews, Kathryn Wynn, Yuan Qi Ni, D. Andrew Howell, Curtis McCully, Ning-Chen Sun, Danfeng Xiang, Yuan Liu, Wenxin Wang, Yijia Zhang, Wei Chen

Solar and Stellar Astrophysics

Detection of resonant nodes in a pore chromosphere

T. Felipe, H. Kumar and E. G. Broock

Active region atmospheres host many oscillatory phenomena. The chromosphere is delimited by steep temperature gradients at the photosphere and transition region, where magnetoacoustic waves are trapped and can form standing oscillations within this resonant cavity. We aim to detect the signature of the resonant nodes of standing waves, which are expected to produce sudden jumps in the oscillatory phase and power dips. Spectroscopic temporal series of H$\alpha$ in a pore were acquired with the Swedish Solar Telescope. The velocity and temperature fluctuations at multiple atmospheric heights were inferred from the analysis of the intensity at many spectral positions along the line wings. Wavelet analysis was employed to characterize the phase differences and the power at different heights. The phase shift between velocity and temperature shows a $\pm90^{\circ}$ value, which is consistent with standing oscillations. Robust evidence of the presence of a nodal layer in the temperature at around the height probed by the intensity at H$\alpha\pm0.30$ Å\ is found, such as the detection of 180$^{\circ}$ jumps in the phase of the temperature oscillations and remarkable power dips at the same atmospheric layer. The exact height of the resonant nodes depends on the spatial location and time. We generally find a mixture of standing and propagating waves. This is consistent with a leaky resonator where waves are partially reflected at the transition region, while some of them are able to propagate into the corona. For the first time, we report the detection and characterization of resonant nodes in the solar chromosphere. This result provides strong observational support for the chromospheric resonant cavity model and paves the way for the development of new seismological techniques to investigate the structure of active region chromospheres.
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Solar and Stellar Astrophysics

Multi-Height Spectropolarimetric Signatures of Magnetoacoustic Waves in Solar Plage

Taylor Sitterson, João M. da Silva Santos, Joshua Bentley et al. (+5 more)

We analyze high-cadence spectropolarimetric observations of solar plage obtained with the Visible Spectro-Polarimeter (ViSP) on the Daniel K. Inouye Solar Telescope (DKIST) to investigate low-chromospheric waves sampled by the Na I D1 5896 Angstrom and Ca II 8542 Angstrom lines. Observations were taken at an oblique viewing angle, providing sensitivity to transverse motions. Magnetic oscillations within plage elements exhibit significant line-of-sight (LOS) RMS amplitudes of approximately 9 G on average in the Na I D1 line, whereas the corresponding Ca II oscillations are only marginally above the noise level. Adaptive profile tracking provided line-core intensities and Doppler velocities, the weak-field approximation provided LOS magnetic fields, and non-LTE inversions provided mass densities and formation heights. Cross-spectral analysis shows velocity-intensity phase distributions coupled at -90 degrees, consistent with compressive oscillations. The velocity phases in Na I-Ca II indicate upward propagation and the phase speeds are predominantly sub-Alfvenic, consistent with slow magnetoacoustic waves. Although the slit geometry prevents a definitive identification of the tube eigenmode, the differing magnetic phase relationships suggest a height-dependent sausage-like behavior in the coupling between the magnetic and compressive perturbations. Additionally, our inferred wave fluxes are insufficient to heat active-region plage.
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All authors Taylor Sitterson, João M. da Silva Santos, Joshua Bentley, Tom Schad, Momchil Molnar, Bart De Pontieu, Juan Martinez-Sykora, Alexander G. M. Pietrow

Solar and Stellar Astrophysics

Investigating the Light Curves of 5 Long Period Variable Stars: A Study Cross-Harmonics using VStar, MESA, and GYRE

G. M. Sherren, A. Cross, N. Steenken et al. (+2 more)

Long Period Variables (LPVs) are Asymptotic Giant Branch stars with periods >100 days. Around one third to half of LPVs display a Long Secondary Period (LSP) that is 5-10 times the length of the main periods. The exact cause of LSPs are unknown, with one potential explanation lying in cross-harmonics caused by the interaction of two simultaneously occurring periods. In this paper, we use the Fourier analysis software VStar to examine the visual range photometric light curves obtained through the American Association of Variable Star Observers of a sample of 5 stars. We find that one star, V CVn, displays a secondary period over 20 times the length of its main period, inconsistent with traditional LSPs. To test if cross-harmonics can result in an LSP or VCVn's extra-long secondary period, we then construct evolutionary tracks and pulsation models of LPVs using Modules for Evolution in Stellar Astrophysics (MESA) and GYRE. After examining the interactions of the fundamental pulsation mode and the first 4 overtones with each other, we do not find evidence that LSPs or V CVn's extra-long secondary period could be caused by cross-harmonics.
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All authors G. M. Sherren, A. Cross, N. Steenken, R. Ignace, H. R. Neilson

Monday 7 September

Solar and Stellar Astrophysics

Tidally perturbed pulsations in OO Dra

Niveditha Parthasarathy, Mikhail Kovalev and Randa Asa'd

We perform a pulsation analysis of OO Dra using Light Curve data from TESS and present the first identification of six dipole modes as doublets from the primary star in OO Dra. The phases and amplitudes of these pulsation modes were analytically reconstructed. Among the six identified doublets, four are pulsating in $Y_{10x}$ mode with pulsation axis along the tidal axis, and two in $Y_{10y}$ with a pulsation axis orthogonal to tidal axis and the angular momentum axis. Our findings suggest that the primary star in a non-synchronized, post mass-transfer OO Dra could be a tidally tilted $\delta$ Scuti pulsator.
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Solar and Stellar Astrophysics

Investigating Potential Relationships Between Mass Transfer Indicators and $\delta$ Scuti Pulsations in oEA Systems

E. Çelik and F. Kahraman Aliçavuş

We explored potential associations between ongoing mass transfer and the pulsation properties of $\delta$ Scuti primaries in a sample of 17 oscillating eclipsing Algol systems. For this purpose, we combined binary modelling, updated O-C analyses, and frequency characterization based on TESS photometry. To ensure homogeneity, the $T_{\rm eff}$ of the components were derived using a consistent SED fitting approach. From the binary solutions and O-C diagrams, we determined the mass transfer indicators (Q, dP/dt, and dM/dt) and examined their relations with the dominant pulsation period and amplitude. For the full sample, the results indicate weak to moderate positive trends, with the strongest relation found between logdP/dt and logPpuls, while the relations involving dM/dt and pulsation amplitudes are weaker and statistically insignificant. After excluding the two prominent outliers, R CMa and BW Del, the correlations between logPpuls and the mass transfer indicators become statistically more significant, including relations with dP/dt, Q, and dM/dt under both conservative and non-conservative assumptions. These relationships may reflect the indirect effects of mass transfer on the structure of the accreting primary and on the orbital configuration, potentially reducing tidal constraints and allowing shifts toward longer pulsation periods. However, the large dispersion and limited sample size indicate that these relationships remain poorly constrained. Our results suggest that pulsation properties are primarily governed by intrinsic stellar parameters and the well established Porb-Ppuls relation, while ongoing mass transfer may contribute as a secondary factor. Although the present sample does not allow firm conclusions regarding the role of mass transfer, this study provides a homogeneous observational framework for future investigations based on larger samples and improved constraints.
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Solar and Stellar Astrophysics

The Association of Solar Radio Bursts with Eruptive and Confined Flares

Stephen M. White, Maria D. Kazachenko and Edward W. Cliver

We classify the metric radio emission associated with eruptive (CME-associated) and confined flares using the sample of events between 2010 and 2016 identified by Kazachenko (2023). We find striking differences in the occurrence of radio bursts between the two classes of flare: for soft X-ray flare sizes above M1.4, confined flares largely lack Type II (1% association rate) and Type IV (6%) emission. Approximately 15% of the sample of $\geq$M1.4 confined flares are associated with impulsive-phase Type III bursts. On the other hand, eruptive flares are associated with Type II, Type III (both impulsive and late phase), and Type IV bursts 45-60% of the time. The different types of radio burst are associated with different drivers (IIIs with electron beams, IIs with shocks, IVs with post-flare loops), so it is striking that the connection of all three burst types to eruptive flares is so pronounced. These results can be interpreted in terms of the reconnection topology of the principle candidate flare types, viz., reconnection between closed field lines for confined flares and X-point reconnection in a CSHKP model for eruptive flares, with interchange reconnection for jet-type flares and Type III bursts.
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Solar and Stellar Astrophysics · Cross-listed from astro-ph.GA

Origins of complex organic molecules in L1551 IRS 5: Interplay of thermal heating and accretion shocks

Ji-hyun Kang, Jeong-Eun Lee, Seokho Lee et al. (+1 more)

Complex organic molecules (COMs) are typically observed in the warm inner envelopes of Class 0 protostars. In more evolved Class I sources, COM emission is confined to the compact inner disk and obscured by high dust opacity, but the FU Orionis-type binary L1551 IRS 5 offers a valuable opportunity to probe disk chemistry, as its outburst luminosity thermally sublimates ices out to large radii. We sought to spatially resolve the origins of COMs in this system to distinguish between central radiative heating and shock-induced chemistry driven by heterogeneous accretion pathways. We analyzed high-angular-resolution ALMA archival data, focusing on COMs (e.g., CH3OH, CH3OCHO) and the shock tracer SO, and compared their spatial morphologies and kinematics to diagnose the excitation conditions associated with different accretion flows. We find that the COM emission in L1551 IRS 5 arises from both radiative heating and localized accretion shocks. In the northern source, the widespread low-excitation COM emission is primarily consistent with thermal heating by the recent outburst. This source also exhibits dual-mode accretion with distinct chemical signatures, with COMs along the spiral arm tracing steady equatorial circumbinary disk-to-circumstellar disk accretion and high-excitation COMs with bright SO along the eastern disk surface tracing a vertical envelope-to-disk streamer impact. In the southern source, methanol is confined to the southeastern streamer impact site, whereas SO forms a ring-like structure at the tidal truncation radius. We attribute this difference to differential chemical survival timescales. L1551 IRS 5 thus serves as an ideal laboratory to investigate how shock chemistry and binary-regulated accretion shape the chemical structure of protostellar disks.
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All authors Ji-hyun Kang, Jeong-Eun Lee, Seokho Lee, Shigehisa Takakuwa

Solar and Stellar Astrophysics

SN 2024afyu interpreted as a Pair Instability Supernova

P. J. Pessi, S. Barmentloo, Ł. Wyrzykowski et al. (+63 more)

Pair-instability supernovae (PISNe) are the predicted explosions of very massive stars triggered by electron-positron pair production. Numerous transients have been proposed as PISN candidates, yet none has provided unambiguous confirmation of this explosion mechanism. The predicted strengths of nebular emission lines offer a powerful means of testing the PISN scenario. We investigate the nature of SN 2024afyu, a nearby (z = 0.0085), long-lived (trise = 85 +- 11.7 days) SN with peculiar spectral evolution, with the aim of identifying its powering mechanism. We analyse multi-band photometry and optical and near-infrared spectroscopy from shortly after explosion to the nebular phase (around 500 days past peak). Besides early appearance of [Ca II] features, we identify a number of sulfur and silicon emission lines, for which we estimate electron temperatures and elemental masses. SN 2024afyu has an inferred 56Ni mass of around 0.4 to 1.0 solar masses and an inferred sulfur mass of the order of 3 solar masses, substantially larger than expected for conventional core-collapse explosions. SN 2024afyu is photometrically similar (although fainter, M_Peak(r) = -18.9 +- 0.04 mag) but spectroscopically distinct to other proposed PISNe. Yet, existing PISN models broadly reproduce several key characteristics, including the overall spectral appearance and broad photometric evolution. SN 2024afyu is a strong PISN candidate, since alternative scenarios would struggle to explain the combination of broad light curve, large intermediate-mass-element abundance, and general spectroscopic evolution. The discrepancies between the observations and currently available theoretical models highlight the need for new PISN calculations spanning a wider range of progenitor masses, metallicities, mixing prescriptions, and circumstellar environments.
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All authors P. J. Pessi, S. Barmentloo, Ł. Wyrzykowski, S. Schulze, J. Sollerman, A. Gangopadhyay, P. J. Mikołajczyk, S. Rose, K. Kotysz, C. Fremling, A. Jerkstrand, R. Lunnan, L. Yan, C. Fransson, J. L. Wise, M. Dubey, K. Misra, A. P. Cotter, D. Athanasopoulos, A. Bochenek, M. Bronikowski, U. Burgaz, W. Burzynski, J. M. Carrasco, E. Concepcion, C. Galdies, A. F. Gillan, A. Gkini, E. Gkiokas, E. Głowacki, F-J. Hambsch, J. Japelj, N. Karaman, P. King, S. Kurowski, C. Liu, M. Maskoliunas, Z. McGrath, M. Mihelčič, M. Nikolajuk, W. Ogloza, E. Pakštienė, U. Pylypenko, A. Pucek, N. Rehemtulla, R. M. Rich, A. O. Simon, A. Singh, C. Skoglund, E. Stonkute, Y. Stsefanenka, P. Szober, K. Tsalapatas, B. F. A. van Baal, C. Ventura, K. Vrontaki, A. Wozniak, J. Zdanavicius, S. Zola, E. C. Bellm, J. Castaneda Jaimes, M. J. Graham, G. Helou, F. J. Masci, J. N. Purdum, R. Riddle

Solar and Stellar Astrophysics · Cross-listed from gr-qc

Nonradial oscillations of realistic anisotropic neutron stars: Polar modes

L. M. Becerra, José F. Rodríguez-Ruiz, E.A. Becerra-Vergara et al. (+1 more)

In this work, we study the polar perturbations of static, spherically symmetric neutron stars with anisotropic pressure in full general relativity, including linear-order perturbations of both the metric and the fluid. We calculate the $f$-mode frequencies and the corresponding damping times using a consistent treatment of the perturbation of the radial vector $k^\alpha$. In particular, its Lagrangian perturbation $\Delta k^\alpha$ is determined by the metric perturbations and the fluid Lagrangian displacement and is constrained to the $(\tilde{u},\tilde{k})$ plane, where $\tilde{u}^\alpha$ is the normalized fluid four-velocity. This constraint introduces an additional dynamical degree of freedom into the perturbation equations. Considering three equations of state and the Horvat and Bowers-Liang prescriptions for pressure anisotropy, we find that the $f$-mode frequency increases with stellar mass, ranging from $1$ to $3$~kHz, while the damping time decreases, ranging from $0.5$ to $1.25$~s. Increasing anisotropy, in the sense of tangential pressure exceeding radial pressure, generally lowers the oscillation frequency, while its effect on the damping time depends on the anisotropy prescription: the damping time decreases with increasing anisotropy for the Horvat model but increases with increasing anisotropy for the Bowers-Liang model. We further find quasi-universal relations between the real and imaginary parts of the $f$-mode frequency, $M\omega_R$ and $M\omega_I$, and the stellar compactness $\mathcal{C}=M/R$, which are largely insensitive to the equation of state. Polynomial fits to these relations achieve an accuracy better than $10\%$, providing a simple phenomenological framework for constraining neutron-star pressure anisotropy through future asteroseismology observations.
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All authors L. M. Becerra, José F. Rodríguez-Ruiz, E.A. Becerra-Vergara, F. D. Lora-Clavijo

Solar and Stellar Astrophysics

Precise physical and kinematical parameters of massive eclipsing binaries in the Small Magellanic Cloud

M. Kaya, Ö. Çak{\i}rl{\i}, B. Hoyman et al. (+3 more)

The formation of massive stars serves an important field of study, requiring research into whether these stars need to form together within clusters of low-mass stars or if they are able to form independently. Currently, there has been an increase in the number of massive stars found far from clusters in the Milky Way (MW) and Small Magellanic Cloud (SMC). The existence of isolation and composition in binary systems is an uncommon phenomenon among massive stars, and their rare occurrence presents valuable insights that are regarded as evidence in this field. This research looks for ways to increase the catalogue of identified massive stars in eclipsing binaries, particularly those that show significant indications of isolation. For the definition of the first step, we are analysing the eight double-lined spectroscopic binaries in the SMC, as detailed in the OGLE variable star catalogues, with the goal of identifying their astrophysical properties. Later, we examine the expected locations and characteristics of binary stars thought to be part of a cluster, using absolute parameters and a cross-validation approach to determine the connection between kinematic analysis and these parameters. If the kinematic connection with the cluster and the physical parameters match precisely with the cluster's isochrone, such stars could act as reliable standard candles for determining the cluster's distance and age.
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All authors M. Kaya, Ö. Çak{\i}rl{\i}, B. Hoyman, O. Özdarcan, T. Yontan, R. Canbay

Solar and Stellar Astrophysics

An Approximate Bayesian Deep Learning Approach for Uncertainty-aware Differential Emission Measure Estimates in the Solar Corona from the SDO

N. Balodhi and R. J. Morton

Accurately estimating the temperature distribution of solar coronal plasma, known as the Differential Emission Measure (DEM), is vital for understanding the thermodynamics of the corona and associated heating. However, recovering the DEM from multispectral observations like those from the Atmospheric Imaging Assembly (AIA) on board NASA's Solar Dynamics Observatory (SDO) is a mathematically ill-posed, underdetermined problem, and traditional regularization-based inversion methods are computationally intensive and provide limited uncertainty quantification. We present a deep learning framework for DEM reconstruction that incorporates Monte Carlo Dropout to perform approximate Bayesian inference, yielding per-pixel empirical distributions over the DEM that characterize epistemic or systematic model uncertainty in the learned inversion. The network is trained with a dual-head architecture supervising both AIA image reconstruction and DEM fidelity, with non-negativity enforced by construction. The network is trained directly on real SDO/AIA observations along with the associated DEM solutions from regularized inversion. We validate performance against both synthetic thermal distributions and real coronal data, demonstrating accurate recovery of thermal structure across a range of plasma conditions, while maintaining significant computational efficiency over traditional inversion techniques. This method ensures physically legitimate, non-negative solutions and provides per-pixel uncertainties, making it a reliable, high-speed, and uncertainty-aware tool for large-scale solar data analysis.
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Solar and Stellar Astrophysics

Global 3D simulations of convection and dynamos in Red Giants

Carolina Ortiz-Rodríguez, Petri Käpylä, Juan Pablo Hidalgo et al. (+2 more)

Understanding the internal dynamics of red giants is essential for studying angular momentum transport and magnetic field generation during stellar evolution. We present three-dimensional magnetohydrodynamic simulations of a $1M_{\odot}$ red giant at the bottom of the red giant branch using a star-in-a-box setup with the Pencil Code, together with ongoing efforts to refine the numerical model. By varying the rotation rate, we explore different Rossby numbers and identify systematic changes in the convective dynamics and large-scale flows. As the rotation increases, convective motions become more anisotropic, showing clear signatures of rotational constraint and a shift in the dominant convective scales. These changes are accompanied by variations in the differential rotation profile, reflecting the redistribution of angular momentum within the convective envelope. The impact of rotation is also evident in the growth rate and saturation level of the magnetic field, indicating a strong dependence of dynamo efficiency on the underlying flow structure. Together, these results highlight the interplay between convection, rotation, and large-scale flows in shaping magnetic activity in red giant stars.
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All authors Carolina Ortiz-Rodríguez, Petri Käpylä, Juan Pablo Hidalgo, Dominik Schleicher, Robi Banerjee

Solar and Stellar Astrophysics

R-matrix calculations for opacities: V. Temperature-density dependence of photoabsorption cross sections and opacity spectra of oxygen ions O VI and O VII

Divya Chari, Sultana N Nahar and Anil K Pradhan

We present R-matrix photoabsorption cross sections for Li-like oxygen O VI and He-like oxygen O VII and examine how plasma broadening modifies them as a function of temperature and density in astrophysical and laboratory high-energy-density (HED) plasma sources. All atomic systems are subject to plasma environment effects, and the propagation of radiation depends on photoabsorption via bound-bound transitions as spectral lines and autoionizing resonances in bound-free photoionization cross sections. We identify and illustrate low and high temperature-density limits for the onset of plasma broadening in O VI and O VII, demonstrating general features and methodology applicable across a broad range of plasma conditions. Calculations are presented along two representative isotherms corresponding to the solar base of convection zone (BCZ) at $T = 1 \times 10^6$ and $2 \times 10^6$ K, and electron densities $N_e = 10^{18}$-$10^{23}$ cm$^{-3}$. Autoionizing resonances progressively dissolve into the continuum with increasing electron density at each isotherm, flattening and merging into the background cross sections at BCZ conditions, at much lower densities than bound-bound line features require. Illustrative examples are given for energy regions containing Rydberg resonance series, including large photoexcitation-of-core (PEC) resonances. Comparisons with previous Opacity Project results reveal that (i) the R-matrix photoionization cross sections cover a much higher energy range where a significantly richer spectrum of autoionizing resonances are present that are not included in OP, and (ii) the corresponding monochromatic opacities show significant quantitative differences. This work is generally applicable to modeling HED plasmas in astrophysics, and to the analysis of transmission spectra in laboratory experiments on inertial confinement fusion (ICF) devices.
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Solar and Stellar Astrophysics

Amplification of active region equatorial dipole

Ismo Tähtinen

The effect of active region parameters on the evolution of solar axial dipole is well understood. However, their effect on the solar equatorial dipole has not been studied in detail. Understanding the development of the equatorial dipole is important as it drives the interplanetary magnetic field on intracyclic timescales (~ 1yr). We study how the latitude, tilt angle, and the polarity separation affect the evolution of solar equatorial dipole. We use dipole flux transport (DFT), a matrix implementation of surface flux transport (SFT) model, to simulate the dipole evolution of synthetic bipolar magnetic regions (BMRs) with a wide range of tilt angles and latitudes. We also study the evolution of active regions in HMI SHARP database and their associated BMRs. We quantify the dipole evolution by means of the maximum and mean amplification and the growth and decay times of the equatorial dipole. Tilt angle controls the amplification of the equatorial dipole, with larger tilt angles leading to stronger equatorial dipole. Latitude controls the timescale at which the equatorial dipole grows and decays. Low-latitude regions produce the longest living equatorial dipole. We find that only 31% of anti-Joy regions experience amplification of equatorial dipole, compared with 96 of regularly tilted active regions. Latitudes and tilt angles of active regions can have considerable effect on the solar equatorial dipole. Systematic changes in their properties could increase/decrease the strength of the equatorial dipole relative to the solar activity, which could lead to similar relative change in geomagnetic activity. We suggest that large low-latitude coronal holes and relatively high geomagnetic activity in the declining phase of solar cycle 20, could be related to emergence of unusually low-latitude active regions that produced strong and persistent equatorial dipole.
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Solar and Stellar Astrophysics

An Eccentric Massive Protobinary Assembled via a Core-merger Parabolic Encounter

Yao Wang, Yichen Zhang, Rubén Fedriani et al. (+14 more)

Most massive stars form in binary systems, which profoundly influence their subsequent evolution. However, how such systems form remains poorly understood, with several competing scenarios proposed, including disk fragmentation, core fragmentation and capture. Determining the orbital architectures of massive binaries, particularly during their earliest embedded phases, is therefore crucial for distinguishing among these formation pathways, but direct measurements of their three-dimensional motions have remained exceptionally challenging. Here we present high-resolution, multi-epoch sub-millimeter-to-centimeter ALMA and JVLA observations of the massive protobinary IRAS 07299$-$1651, complemented by JWST and VLT infrared imaging. We detect orbital proper motion of the binary components, enabling a full three-dimensional orbital reconstruction. Combining orbital fitting, multi-wavelength continuum modelling, hydrogen recombination line kinematics and jet observations, we find that the preferred orbital solutions are highly eccentric and close to parabolic, while both circumstellar disks are strongly misaligned with the orbital plane. These properties are naturally explained by a “core-merger” scenario in which the two protostars originated independently from initially unbound cores that recently underwent a near-parabolic encounter, producing an eccentric binary with a current separation of about 200 au. These findings suggest that the core-merger process may represent an important pathway for forming eccentric massive binaries.
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All authors Yao Wang, Yichen Zhang, Rubén Fedriani, Kei E. I. Tanaka, Viviana Rosero, Kai Yang, Morten Andersen, Maria T. Beltrán, Mélisse Bonfand, Yu Cheng, James M. De Buizer, Yihuan Di, Guido Garay, Prasanta Gorai, Zhi-Yun Li, Yao-Lun Yang, Jonathan C. Tan

Solar and Stellar Astrophysics

Statistical structure and physical interpretation of the fillout factor distribution in contact binary stars

A. Poro, K. Li, E. Paki et al. (+2 more)

Contact binary systems exhibit complex evolutionary behavior governed by mass transfer, angular momentum loss (AML), and Roche geometry. The fillout factor ($f$) is widely used as a descriptor of the degree of overcontact, yet its physical and statistical role remains nonuniformly defined across the literature. We used a compiled observational sample of W UMa-type contact binaries to construct a data-driven classification framework based on $f$ and its relation to fundamental system parameters. Unsupervised clustering applied to the one-dimensional $f$ distribution reveals a robust three-class structure corresponding to shallow-, medium-, and deep-contact configurations. The statistically determined boundaries are located at $f \simeq 0.257$ and $f \simeq 0.561$, with a stable silhouette score of $S \simeq 0.613$. The stability of this classification is confirmed through perturbation tests in $f$ and parameter-conditioned samples. A random forest regression model shows that 60.7% of the variance in $f$ can be explained by the considered physical and geometric system parameters, with geometric parameters providing the largest predictive contribution. Among the considered parameters, the mass ratio emerges as the dominant predictor. The identified subclasses therefore appear to reflect a continuous geometric sequence rather than discrete physical states, governed by the progressive evolution of the Roche equipotential configuration under mass transfer and AML. Notably, the second boundary separating medium- and deep-contact systems shows lower stability than the first boundary, suggesting that the transition toward deep contact may be more sensitive to the physical composition of the analyzed sample.
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All authors A. Poro, K. Li, E. Paki, F. Alicavus, N. Alan

Solar and Stellar Astrophysics · Cross-listed from astro-ph.IM

Filter and baffle design for the short-wavelength camera on SOLAR-C EUVST

James McKevitt, Duncan Rust, Berend Winter et al. (+10 more)

SOLAR-C, which is scheduled to launch in the late 2020s with the Extreme Ultraviolet High-Throughput Spectroscopic Telescope (EUVST), will deliver EUV spectroscopy of the solar atmosphere with unprecedented temperature coverage and resolution. We present the optical design and mechanical impacts for the short-wavelength camera, and how our design choices affect scientific performance using complete forward modelling. We discuss the design and optical impact of the thin-film aluminium filter and supporting mesh, including diffraction and shadowing effects in spectral and spatial dimensions. We analyse this using our own Fourier optics method and analytical modelling, and quantify the scientific impact using forward modelling driven by a simulated solar atmosphere. We address the vulnerability of thin-film filters during launch depressurisation using vacuum chamber testing. We describe the stray-light baffle design and low-scatter coatings which suppress visible stray light.
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All authors James McKevitt, Duncan Rust, Berend Winter, David Walton, Sarah Matthews, Ted Pyne, Yukio Katsukawa, Ryohko Ishikawa, Alexey Shitvov, Chloe Wishart, Barry Whiteside, Jacob Parker, Diagarajen Carpanen

Solar and Stellar Astrophysics · Cross-listed from astro-ph.GA

Catching Up with the Fastest Star in the Galaxy: A Two-Passage GRAVITY+ Campaign to Detect S301's Schwarzschild Precession

Joseph Catanzarite

S301 is the fastest known star in the Galaxy. It orbits Sgr A* every 8.68 yr on an e = 0.9832 orbit that carries it within 136 Schwarzschild radii at 25,600 km/s (GRAVITY Collaboration 2026). An orbit like that must precess, by the same Schwarzschild effect that turns Mercury's perihelion, whatever the spin of the black hole. We ask whether an astrometry-only GRAVITY+ campaign covering the 2031.8 and 2040.5 periapsis passages can catch it. We validate a relativistic light-curve model, design a 131-epoch Fisher-optimal cadence with a periapsis floor, fit a seven-parameter orbit including the light-travel-time delay, and build a systematic error budget. With white noise at the 207 microarcseconds achieved on S301, the fit recovers the injected rate, 0.2307 deg/yr, as 0.2324 +/- 0.0023 deg/yr (+/-0.0012 at the forecast 100 microarcseconds). Real astrometric noise is not white. We add per-run calibration offsets and a wandering Sgr A* photocentre at GRAVITY's published amplitudes, marginalize over the reference zero point, mass and distance, add the bounded systematics in quadrature, and inflate by the excess scatter of real GRAVITY orbit fits. What survives is 0.0074 deg/yr, a 31 sigma detection. The Sun hides the first periapsis for 153 d, at a cost to the period and periapsis epoch but not to the precession rate. Radial velocities are out of reach at S301's magnitude; photometry only validates the light-curve model. The spin of Sgr A* could shift the rate by 5.17-5.74 per cent and cannot be bounded, so the campaign is a spin-agnostic test of general relativity, and a forecast until the star comes round.
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Sunday 6 September

Solar and Stellar Astrophysics · Cross-listed from astro-ph.EP

The Effect of X-Ray and Accretion Variability on Mid-Infrared Lines in the Young Disk-Bearing Binary DQ Tau

Bayron Portilla-Revelo, Konstantin V. Getman, Ágnes Kóspál et al. (+5 more)

A star's radiation field is the main energy source of a planet-forming disk. Stellar photons—from X-rays to optical—are reprocessed in the inner protoplanetary disk enabling mid-IR diagnostics of the disk's physical conditions. Most pre-main-sequence stars accrete at moderate rates and emit X-rays efficiently. Such activity is expected to drive stochastic variability in the stellar spectrum. It is not clear to what extent such variability affects the thermochemical structure of protoplanetary disks. We aim to examine the effect of routine accretion variability and X-ray flares on the thermochemical structure and mid-IR line emission spectra of protoplanetary disks. We combine multiepoch, contemporaneous X-ray and NUV/optical observations of the DQ Tau binary with JWST/MIRI spectra of its circumbinary disk. We interpret the data and assess the effect of a varying stellar spectrum on the disk using a thermochemical model illustrative of the system. Synthetic fluxes of CO, CO2, HCN, and H2O are systematically stronger at periastron than at apastron due to an enhanced accretion luminosity. Variations in the integrated fluxes lie within a factor of two between epochs, in agreement with the JWST/MIRI spectra of DQ Tau. Routine stellar variability induces only modest changes in the disk's structure, yet it can still shift the snowline outwards by 17 percent relative to its apastron position. Because the JWST observations did not coincide with X-ray flares, our flare results rely on simulations: moderate-intensity flares leave most mid-IR molecular emission unchanged; in contrast, X-ray flares strongly enhance the abundances and mid-IR luminosities of H I, Ar II, and Ne II in the disk surface. Our results imply that the Spitzer/JWST line variability observed in non-outbursting systems is consistent with moderately variable stellar spectra driven by accretion and magnetospheric activity.
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All authors Bayron Portilla-Revelo, Konstantin V. Getman, Ágnes Kóspál, Dmitry Semenov, Vitaly Akimkin, Lis Zwicky, Jan Forbrich, Sierk van Terwisga

Solar and Stellar Astrophysics · Cross-listed from astro-ph.IM

Multimessenger Spectroscopy

Kristen Lackeos

A multimessenger method is developed for scheduling optical spectroscopy of ultracompact double white dwarf binaries from Laser Interferometer Space Antenna (LISA) data. The orbital parameters inferred from the gravitational wave (GW) signal are used to predict the argument of latitude $u(t)$, which sets the phase dependence of the radial velocity. With $u(t)$ known in advance, spectra can be taken at selected orbital phases. The GW and spectroscopic descriptions are linked by $u(t)=\Phi_{\rm GW}(t)/2+\delta_{e,\omega}(t)+\mathcal{O}(e^2)$, where $\Phi_{\rm GW}$ is the GW phase, $e$ the eccentricity, and $\delta_{e,\omega}$ the equation of the centre to $\mathcal{O}(e)$, which vanishes for a circular orbit. Radial velocities are regressed on $\cos u$; for double-lined binaries, the two slopes determine the semi-amplitudes $K_1$ and $K_2$. With the inclination and orbital period measured by LISA, both component masses follow from the semi-amplitudes through Kepler's third law, and the resulting chirp mass with the GW amplitude gives the source distance. We derive waveforms to first order in the Laplace–Lagrange parameters; the circular quadrupole waveform is recovered as $e\to0$. Using simulated LISA data for one circular and one eccentric system, we forecast how long $u(t)$ remains accurate enough for phase-resolved spectroscopy. Masses derived from the velocity amplitudes are free of the biases that tides, mass transfer, or acceleration along the line of sight introduce when $\dot f_{\rm GW}$ alone is used. For a four-year LISA observation, scheduling at quadrature remains viable for roughly a decade after the observation ends. Four years after the observation midpoint, the ephemeris predicts the orbital phase to $0.3$~s for the eccentric system and $0.9$~s for the circular one, compared with $1$–$30$~s for optical ephemerides propagated over the same baseline.
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Solar and Stellar Astrophysics

Elusive helium stars in the gap between subdwarfs and Wolf-Rayet stars III. Formation of the Galactic population of stripped helium stars

L.R. Yungelson, A.G. Kuranov and A.V. Mishakina

Population synthesis based on the grid of pre-computed evolutionary sequences for close binaries is applied to the problem of formation of the Galactic population of stripped helium stars, the remnants of $5 \lessapprox M/M_\odot \lessapprox 24$ primary components of close binaries after RLOF. Stellar rotation is taken into account. For the current star formation rate in the Galaxy $2\,M_\odot yr^{-1}$, the estimate of the number of 1 to $7\,M_\odot$ objects with $T_{eff} \gtrapprox $25000K ranges from 14700 to 28500, while for the number of objects with masses from $2\,M_\odot$ to $7\,M_\odot$ it is 3200 to 5500, depending on the assumptions about formation of common envelopes and their outcomes. Other factors influencing the estimate are adopted star formation rate, distributions of close binaries over initial masses of primary components, mass ratios of components, orbital periods. Observational selection may severely limit the number of detectable objects. Angular velocities of rotation of companions to stripped helium stars are 60 to 80 per cent of the critical one. Using the models of initially homogeneous helium stars as approximations for the remnants after mass loss results in the underestimate of pre-supernovae masses.
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Solar and Stellar Astrophysics

PySeshat: A Validated Python Pipeline for Binary-Star Orbit Determination and Al-Wardat Stellar Atmosphere Modeling

R. I. El-Kholy and Z. M. Hayman

Precise stellar masses and physical parameters for binary and multiple star systems require, respectively, well-constrained orbital solutions and independent stellar-atmosphere modeling. Existing open-source software addresses these separately, and hierarchical triple systems additionally require simultaneous fitting of their inner and outer orbits, a treatment few existing tools provide. We present PySeshat, an open-source Python package unifying binary and hierarchical-triple orbit determination with stellar SED fitting in a single, validated pipeline. The package fits visual, spectroscopic, and combined binary orbits by least-squares, and hierarchical-triple orbits through two parallel formalisms, one of which fits both inner and outer orbits simultaneously rather than sequentially. Both share a common Bayesian posterior-sampling layer. A second module fits two-component synthetic spectra to a binary's combined, unresolved photometry to recover each component's temperature and radius. The software was validated against twelve real, published benchmark systems, cross-checked against three independent orbit-fitting packages and real space-telescope archive data. All twelve benchmark systems reach fit quality consistent with their published solutions. Comparing the simultaneous and sequential triple-fitting formalisms on the same real systems shows the simultaneous approach is not merely preferable but necessary: one system's orbital configuration cannot be represented by the sequential formalism at all. Fitting four independently published systems' photometry reveals a degeneracy in recovering individual temperatures for near-twin-temperature stellar components from unresolved light, independent of how much photometric data is available. The package provides a validated, open, and extensible pipeline for binary and multiple star system analysis, publicly released and freely installable.
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Solar and Stellar Astrophysics

Stage-dependent superhump waveform evolution and non-stationary positive-superhump timing in the near-period-gap dwarf nova YZ Cancri

Zhibin Dai, Xuefei Chen and Zhanwen Han

We present a photometric and timing study of the near-period-gap SU~UMa-type dwarf nova YZ~Cancri, based on nearly continuous Transiting Exoplanet Survey Satellite (TESS) photometry and long-baseline ground-based observations. Our main observational result is that the positive-superhump (SH) waveform follows a repeatable, stage-dependent sequence during superoutbursts (SOs). In the two well-covered TESS SOs, and consistently in the long-baseline ground-based SO sample, the plateau waveform evolves from an early saw-tooth profile (ST), through a double-humped profile with unequal maxima (DHd), to a more symmetric double-humped profile with nearly similar maxima (DHs). The global and time-resolved Lomb–Scargle periodograms show that power near the positive-SH time scale and its harmonics is concentrated during SOs, whereas quiescent and normal-outburst intervals lack a comparably persistent SH-band signal. The dense TESS maxima–minima timing sequence shows different clock stability in different waveform stages: precursor modulations have slightly longer local periods, the middle-to-late DHs plateau is the most regular timing interval, and the post-plateau evolution is affected by phase switching and possible secondary/late-SH contamination. The TESS data also reveal profile-clock coupling, with SH amplitude and rise/decay durations evolving together with the timing residuals. As a secondary timing constraint, we obtain a common TESS-timing-based mean positive-SH period of $P'_{\rm sh}=0.09043(27)$~d, corresponding to a SH excess of $4.03(31)\%$ and an approximate mass ratio of $q=0.175(11)$. The repeatable ST–DHd–DHs sequence makes YZ~Cnc a useful system near the lower edge of the period gap, and may trace the growth, redistribution, stabilization, and decay of the light source associated with an eccentric, precessing accretion disk during SOs.
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Solar and Stellar Astrophysics

IPHAS J190812.63+045728.1: A deeply eclipsing, X-ray bright cataclysmic variable with frequent outbursts

Raimundo Lopes de Oliveira, Albert Bruch, Koji Mukai et al. (+3 more)

We present a multi-wavelength characterization of IPHAS J190812.63+045728.1. Independently discovered by us after being serendipitously observed by XMM-Newton, followed up with optical photometry with SPARC4/OPD and spectroscopy with GMOS/Gemini South, and exploring archival TESS and ZTF observations, we reveal the system to be an eclipsing accreting white dwarf (WD) system with an orbital period of 5.073 h. The deep optical and flat-bottomed X-ray eclipses completely occult the WD and the accretion disk, providing a model-independent, uncontaminated view of the donor star which we classify as an M2.3 dwarf with a mass of $\sim 0.50\,M_\odot$. The system is a modestly luminous (L$_{x;0.3-10 keV} \sim$ 1.7 $\times$ 10$^{32}$ erg s$^{-1}$) hard thermal (consistent with a shock-heated plasma cooling down from $k$T $\sim$ 26 keV) X-ray source. While these X-ray properties are typical of low luminosity intermediate polars (IPs), long-term archival photometry reveals frequent, large-amplitude outbursts ($\Delta m \sim 2$ mag in the ZTF $zr$ band) that are the typical characteristics of dwarf novae, and would be unusual for IPs. The weakness of the HeII $\lambda$4686 emission line does not allow us to decide between the IP and the dwarf nova scenarios. On the other hand, the optical and X-ray light curves do not exhibit periodic variations caused by the WD rotation. If the system is indeed an IP, the accretion geometry is likely unusual while the dwarf nova-like outbursts indicating disk instabilities would make IPHAS J190812.63+045728.1 a benchmark for testing disk instability models in the presence of magnetically truncated accretion disks. Otherwise, the system is a dwarf nova with X-ray properties at the extreme end of its class.
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All authors Raimundo Lopes de Oliveira, Albert Bruch, Koji Mukai, A. S. de Araujo, Guilherme P. Guimarães, Ted Leandro de Almeida