Astronomy & Astrophysics最新文献

{"title":"Coronal hole picoflare jets are progenitors of both fast and Alfvénic slow solar wind","authors":"L. P. Chitta, Z. Huang, R. D’Amicis, D. Calchetti, A. N. Zhukov, E. Kraaikamp, C. Verbeeck, R. Aznar Cuadrado, J. Hirzberger, D. Berghmans, T. S. Horbury, S. K. Solanki, C. J. Owen, L. Harra, H. Peter, U. Schühle, L. Teriaca, P. Louarn, S. Livi, A. S. Giunta, D. M. Hassler, Y.-M. Wang","doi":"10.1051/0004-6361/202452737","DOIUrl":"https://doi.org/10.1051/0004-6361/202452737","url":null,"abstract":"Solar wind, classified by its bulk speed and the Alfvénic nature of its fluctuations, generates the heliosphere. The elusive physical processes responsible for the generation of the different types of this wind are a topic of active debate. Recent observations reveal intermittent jets, with kinetic energy in the picoflare range, emerging from dark areas of a polar coronal hole threaded by open magnetic field lines. These could substantially contribute to solar wind. However, their ubiquity and direct links to solar wind have not been established. Here, we report a unique set of remote-sensing and in situ observations from the Solar Orbiter spacecraft that establish a unified picture of fast and Alfvénic slow wind, connected to the similar widespread picoflare jet activity in two coronal holes. Radial expansion of coronal holes ultimately regulates the speed of the emerging wind.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"61 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-02-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143191979","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"X-ray activity of nearby G-, K-, and M-type stars and implications for planet habitability around M stars","authors":"E. Zhu, T. Preibisch","doi":"10.1051/0004-6361/202452057","DOIUrl":"https://doi.org/10.1051/0004-6361/202452057","url":null,"abstract":"<i>Context.<i/> The intense X-ray and UV emission of some active M stars has raised questions about the habitability of planets around M-type stars.<i>Aims.<i/> We aim to determine the unbiased distribution of X-ray luminosities in complete, volume-limited samples of nearby M dwarfs, and compare them to those of K and G dwarfs.<i>Methods.<i/> We constructed volume-complete samples of 205 M stars with a spectral type ≤ M6 within 10 pc of the Sun, 129 K stars within 16 pc, and 107 G stars within 20 pc. We used X-ray data from <i>Chandra<i/>, <i>XMM-Newton<i/>, eROSITA, and ROSAT to obtain the X-ray luminosities of the stars.<i>Results.<i/> Our samples reach an X-ray detection completeness of 85%, 86%, and 80% for M, K, and G stars, respectively. The fractional X-ray luminosities relative to the bolometric luminosities, log(<i>L<i/><sub>X<sub/>/<i>L<i/><sub>bol<sub/>), of the M stars show a bimodal distribution, with one peak at around −5, mostly contributed by early M stars (M0–M4), and another peak around −3.5, contributed mainly by M4–M6 stars. The comparison of the different spectral classes shows that 63% of all M stars in our sample (80% of the M stars with a spectral type < M4) have <i>L<i/><sub>X<sub/>/<i>L<i/><sub>bol<sub/> values that are within the central 80% quantile of the distribution function for G stars. In addition, 55% of all M stars in our sample (and 72% of the M stars with a spectral type < M4) have <i>L<i/><sub>X<sub/>/<i>L<i/><sub>bol<sub/> less than 10 times the solar value.<i>Conclusions.<i/> The X-ray activity levels of the majority (≳60%) of nearby M dwarfs no later than M6 are actually not higher than the typical (80% quantile) levels for G-type stars. The X-ray irradiation of habitable-zone planets around these stars should therefore not present a specific problem for their habitability.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"11 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-02-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143192060","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Dimming GRS 1915+105 observed with NICER and Insight–HXMT","authors":"M. Zhou, V. Grinberg, A. Santangelo, C. Bambi, Q. Bu, C. M. Diez, L. Kong, J. F. Steiner, Y. Tuo","doi":"10.1051/0004-6361/202451558","DOIUrl":"https://doi.org/10.1051/0004-6361/202451558","url":null,"abstract":"The black hole X-ray binary GRS 1915+105 was bright for 26 years since its discovery and is well known for its disk instabilities, quasi-periodic oscillations, and disk wind signatures. We report a long-term spectral-timing tracing of this source from mid-2017 until the onset of the so-called obscured state based on the complete data from the Neutron Star Interior Composition Explorer (NICER) and the Insight–Hard X-ray Modulation Telescope (HXMT), whose hard coverage decisively informs the modeling at lower energies. In the soft state predating 2018, we observed highly ionized winds. However, in the hard state shortly before transitioning into the obscured state on May 14, 2019 (MJD 58617), the winds exhibited a discernible reduction in ionization degree (log <i>ξ<i/>), which decreased from above 4 to approximately 3. Our analysis involves the measurement of the frequencies of the quasi-periodic oscillations and the estimation of the properties of the ionized winds and the intensities of different spectral components through spectroscopy during the decay phase. We studied the origin of these infrequently observed warm outflows in the hard state. The launching radius of the winds in the hard decay phase is similar to that in the soft state, which indicates that the launching mechanism of these winds likely is the same in both states. The presence of the ionized winds is preferentially dependent on the periphery of the accretion disk, but it is not directly related to the corona activities in the center of the binary system.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"42 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-02-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143192311","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Understanding observational characteristics of solar flare current sheets","authors":"Zining Ren, Yulei Wang, Xin Cheng, Mingde Ding","doi":"10.1051/0004-6361/202451158","DOIUrl":"https://doi.org/10.1051/0004-6361/202451158","url":null,"abstract":"<i>Context.<i/> The elongated bright structures above solar flare loops are suggested to be current sheets, where magnetic reconnection takes place. Observations have revealed various characteristics of the current sheet; however, their physical origin remains to be ascertained.<i>Aims.<i/> In this study we aim to reveal the relations of observational characteristics of current sheets with the fundamental processes of magnetic reconnection.<i>Methods.<i/> Using high-resolution 3D magnetohydrodynamic simulations of turbulent magnetic reconnection within a solar flare current sheet, we synthesized the remote-sensing observations of the current sheet and determined their physical properties.<i>Results.<i/> Turbulent magnetic reconnection can significantly broaden the apparent width of the current sheet, which is much larger than the realistic physical width because of the superposition effect. The differential emission measures of the current sheet have two peaks; the high-temperature component is spatially related to confirmed small-scale reconnection sites, showing that the current sheet is directly heated by reconnection. Moreover, we demonstrate that strong turbulence can cause the nonthermal broadening of spectral lines at both the current sheet and flare loop-top regions. A strong correlation between them in time is also observed.<i>Conclusions.<i/> Our 3D turbulent magnetic reconnection flare model can be used to interpret primary observational characteristics of the elongated bright current sheets of solar flares.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"207 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-02-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143192055","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Importance of modelling the nebular continuum in galaxy spectra","authors":"Henrique Miranda, Ciro Pappalardo, José Afonso, Polychronis Papaderos, Catarina Lobo, Ana Paulino-Afonso, Rodrigo Carvajal, Israel Matute, Patricio Lagos, Davi Barbosa","doi":"10.1051/0004-6361/202451648","DOIUrl":"https://doi.org/10.1051/0004-6361/202451648","url":null,"abstract":"&lt;i&gt;Context.&lt;i/&gt; Neglecting to model stellar and nebular emission has been shown to have a significant impact on the derived physical properties of galaxies experiencing high levels of star formation. This impact has been seen at low redshifts for galaxies in a period of extremely significant star formation, the so-called extreme emission-line galaxies. It has also been suggested as a more general phenomenon among star-forming galaxies at high-redshifts. Even though various studies have approached the issue, a clear limit for the relevant effect of nebular contribution to the total optical emission has not been established.&lt;i&gt;Aims.&lt;i/&gt; We aim to correlate the nebular contribution in the optical regime with different tracers and to define a threshold, in terms of the nebular contribution, above which there is a significant impact on the estimation of physical properties of galaxies. Additionally, we want to investigate the implication of the results for high-redshift galaxies.&lt;i&gt;Methods.&lt;i/&gt; We selected a sample of galaxies from SDSS-DR7 with a wide range of star-forming activity levels and analysed their spectra with two conceptually distinct spectral fitting tools: one that self-consistently models stellar and nebular emission and ensures that the best-fitting star formation and chemical enrichment history obtained reproduces the observed nebular characteristics of a galaxy (FADO), and another that lacks such a self-consistency concept (STARLIGHT) and was applied using a purely stellar base. We estimated the nebular contribution and correlate it with different tracers. Then, we compared the stellar properties estimated by the two spectral fitting tools for different degrees of optical nebular contribution. Additionally, we estimated the stellar properties using FADO in pure-stellar mode to further strengthen the robustness of our results.&lt;i&gt;Results.&lt;i/&gt; The rest-frame H&lt;i&gt;α&lt;i/&gt; and H&lt;i&gt;β&lt;i/&gt; equivalent widths (EWs) show a strong linear correlation with the optical nebular contribution and are suitable tracers. We find that for an optical nebular contribution above 8%, which corresponds to EW(H&lt;i&gt;α&lt;i/&gt;) ≃ 500 Å and EW(H&lt;i&gt;β&lt;i/&gt;) ≃ 110 Å, there is a significant impact on the estimated physical properties and underlying stellar populations of a galaxy. Given the different definition of FADO for the continuum, this threshold actually corresponds to EW(H&lt;i&gt;α&lt;i/&gt;) ≃ 375 Å for works considering a pseudo-continuum, which is more commonly used in the literature. These findings were corroborated when considering the results from the application of FADO in pure-stellar mode. Considering the observed redshift evolution of EW(H&lt;i&gt;α&lt;i/&gt;), galaxies in the stellar mass range between &lt;i&gt;M&lt;i/&gt;&lt;sub&gt;*&lt;sub/&gt; = 10&lt;sup&gt;7&lt;sup/&gt; and 10&lt;sup&gt;11&lt;sup/&gt; M&lt;sub&gt;⊙&lt;sub/&gt; will reach, on average, this threshold in the &lt;i&gt;z&lt;i/&gt; ∼ 2–6 interval and the optical nebular contribution cannot be neglected.&lt;i&gt;Conclusions.&lt;i/&gt; Our results highlight the importance of taking into account both ","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"21 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-02-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143192056","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"ZF-UDS-7329: A relic galaxy in the early Universe","authors":"Eduardo A. Hartmann, Ignacio Martín-Navarro, Marc Huertas-Company, João P. V. Benedetti, Patricia Iglesias-Navarro, Alexandre Vazdekis, Mireia Montes","doi":"10.1051/0004-6361/202452781","DOIUrl":"https://doi.org/10.1051/0004-6361/202452781","url":null,"abstract":"The formation time scales of quiescent galaxies can be estimated in two different ways: by their star formation history and by their chemistry. Previously, the methods yielded conflicting results, especially when considering <i>α<i/>-enhanced objects. This is primarily due to the time resolution limitations of very old stellar populations, which prevent accurately constraining their star formation histories. We analysed the JWST observations of the extremely massive galaxy ZF-UDS-7329 at <i>z<i/> ∼ 3.2 and show that the higher time resolution necessary to match the chemical formation time scales using stellar population synthesis can be achieved by studying galaxies at high redshift. We compare the massive galaxy to the well-known relic galaxy NGC 1277, arguing that ZF-UDS-7329 is an early Universe example of the cores of present-day massive elliptical galaxies or, if left untouched, a relic galaxy.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"55 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-02-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143191981","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Protostellar Outflows at the EarliesT Stages (POETS) VI. Evidence of disk-wind in G11.92-0.61 MM1","authors":"O. S. Bayandina, L. Moscadelli, R. Cesaroni, M. T. Beltrán, A. Sanna, C. Goddi","doi":"10.1051/0004-6361/202452843","DOIUrl":"https://doi.org/10.1051/0004-6361/202452843","url":null,"abstract":"<i>Context<i/>. Magnetohydrodynamic disk-winds are thought to play a key role in the formation of massive stars by providing the fine-tuning between accretion and ejection, where excess angular momentum is redirected away from the disk, allowing further mass growth of a young protostar. However, only a limited number of disk-wind sources have been detected to date. To better constrain the exact mechanism of this phenomenon, expanding the sample is critical.<i>Aims<i/>. We performed a detailed analysis of the disk-wind candidate G11.92-0.61 MM1 by estimating the physical parameters of the massive protostellar system and constraining the wind-launching mechanism.<i>Methods<i/>. Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 observations of G11.92-0.61 MM1 were conducted in September 2021 with ALMA’s longest baselines, which provided a synthesised beam of ~30 mas. We obtained high-resolution images of the CH<sub>3<sub/>CN (<i>υ<i/><sub>8<sub/>=1 and <i>υ<i/>=0), CH<sub>3<sub/>OH, SO<sub>2<sub/>, and SO molecular lines, as well as the 1.3 mm continuum.<i>Results<i/>. Our high-resolution molecular data allowed us to refine the parameters of the disk-outflow system in MM1. The rotating disk is resolved into two regions with distinct kinematics: the inner region (<300 au) is traced by high-velocity emission of high-excitation CH<sub>3<sub/>CN lines and shows a Keplerian rotation; the outer region (>300 au), traced by mid-velocity CH<sub>3<sub/>CN emission, rotates in a sub-Keplerian regime. The central source is estimated to be ~20 <i>M<i/><sub>⊙<sub/>, which is about half the mass estimated in previous lower-resolution studies. A strong collimated outflow is traced by SO and SO<sub>2<sub/> emission up to ~3400 au around MM1a. The SO and SO<sub>2<sub/> emissions show a rotation-dominated velocity pattern, a constant specific angular momentum, and a Keplerian profile that suggests a magneto-centrifugal disk-wind origin with launching radii of ~50–100 au.<i>Conclusions<i/>. G11.92-0.61 MM1 appears to be one of the clearest cases of molecular line-traced disk-winds detected around massive protostars.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"40 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-02-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143192052","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"PAStar: A model for stellar surface from the Sun to active stars","authors":"Antonino Petralia, Jesús Maldonado, Giuseppina Micela","doi":"10.1051/0004-6361/202450316","DOIUrl":"https://doi.org/10.1051/0004-6361/202450316","url":null,"abstract":"<i>Context.<i/> The characterisation of exoplanets requires a good description of the host star. Stellar activity acts as a source of noise, which can alter planet radii as derived from the transit depth or atmospheric characterisation.<i>Aims.<i/> Here, we propose PAStar, a model to describe photospheric activity in the form of spots and faculae, which could be applied to a wide range of stellar observations, from photometric to spectroscopic time series, making it possible to correctly extract planetary and stellar properties.<i>Methods.<i/> The adopted stellar atmosphere is a combination of three components: the quiet photosphere, spots, and faculae. The model takes into account the effects of star inclination and Doppler shifts due to stellar rotation and limb darkening, which is independent for each component. Several synthetic products have been presented to show the capabilities of the model.<i>Results.<i/> The model is able to retrieve the input surface-inhomogeneity configuration through photometric or spectroscopic observations. The model has been validated against optical solar data and compared to alternative stellar surface activity models; for example SOAP code. The Sun is a unique laboratory to test stellar models because of the possibility to unambiguously relate flux variations to surface inhomogeneities’ configuration. This validation has been done by analysing a photometric time series from the Variability of Solar Irradiance and Gravity Oscillations (VIRGO) photometer on-board Solar and Heliospheric Observatory (SOHO) mission. Results have been compared to real solar images from the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) to confirm the goodness of the results in terms of surface inhomogeneities’ positions and dimensions.<i>Conclusions.<i/> The description of stellar activity is a fundamental step in several astrophysical contexts and it is covered by the method we present. Our model offers a flexible and valuable tool to describe the activity of stars when it is dominated by spots and faculae.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"7 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-02-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143192100","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Stellar obliquities of eight close-in gas giant exoplanets","authors":"J. Zak, H. M. J. Boffin, E. Sedaghati, A. Bocchieri, Z. Balkoova, M. Skarka, P. Kabath","doi":"10.1051/0004-6361/202452171","DOIUrl":"https://doi.org/10.1051/0004-6361/202452171","url":null,"abstract":"The Rossiter–McLaughlin effect allows us to measure the projected stellar obliquity of exoplanets. From the spin-orbit alignment, planet formation and migration theories can be tested to improve our understanding of the currently observed exoplanetary population. Despite having the spin-orbit measurements for more than 200 planets, the stellar obliquity distribution is still not fully understood, warranting additional measurements to sample the full parameter space. We analyzed archival HARPS and HARPS-N spectroscopic transit time series of eight gas giant exoplanets on short orbits and derive their projected stellar obliquity <i>λ<i/>. We report a prograde, but misaligned orbit for HAT-P-50b (), possibly hinting at previous high-eccentricity migration given the presence of a close stellar companion. We measure sky-projected obliquities that are consistent with aligned orbits for the rest of the planets: WASP- 48b (<i>λ<i/> = −4° ± 4), WASP-59b (), WASP-140 Ab (<i>λ<i/> = −1° ± 3), WASP-173 Ab (<i>λ<i/> = 9° ± 5), TOI-2046b (<i>λ<i/> = 1° ± 6), HAT-P-41 Ab (), and Qatar-4b (). We measure the true stellar obliquity <i>ψ<i/> for four systems. We infer a prograde, but misaligned, orbit for TOI-2046b with deg. Additionally, for WASP-140 Ab, for WASP-173 Ab, and for Qatar-4b. The aligned orbits are consistent with slow disk migration, ruling out violent events that would excite the orbits over the history of these systems. Finally, we provide a new age estimate for TOI-2046 of at least 700 Myr and for Qatar-4 of at least 350–500 Myr, contradicting previous results.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"30 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-02-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143191983","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Examining the evolution of the supersoft X-ray source RX J0513.9 − 6951","authors":"A. Tavleev, V. F. Suleimanov, K. Werner, A. Santangelo","doi":"10.1051/0004-6361/202453039","DOIUrl":"https://doi.org/10.1051/0004-6361/202453039","url":null,"abstract":"<i>Context.<i/> Supersoft X-ray sources (SSSs) are thought to be accreting white dwarfs (WDs) in close binary systems, with thermonuclear burning on their surfaces. The SSS RX J0513.9 − 6951 in the Large Magellanic Cloud (LMC) exhibits cyclic variations between optical low and high states, which are anti-correlated with its X-ray flux. This behaviour is believed to be the result of the periodic expansion and contraction of the WD due to variations in the accretion rate in the system.<i>Aims.<i/> We analyse the eight high-resolution XMM and six grating <i>Chandra<i/> spectra of RX J0513.9 − 6951 with our grid of model atmosphere spectra of hot WDs computed under the assumption of local thermodynamic equilibrium. Our aim is to test a contraction model of the source variability by tracking the evolution of the WD properties.<i>Methods.<i/> We use a recently computed grid of hot WD model atmospheres, spanning a wide range of effective temperatures (<i>T<i/><sub>eff<sub/> = 100−1000 kK in steps of 25 kK) and eight values of surface gravity. The LMC chemical composition of the atmospheres was assumed.<i>Results.<i/> The obtained fitting parameters (effective temperature <i>T<i/><sub>eff<sub/>, surface gravity log <i>g<i/>, and bolometric luminosity <i>L<i/>) evolve on the <i>T<i/><sub>eff<sub/> − log <i>g<i/> and <i>T<i/><sub>eff<sub/> − <i>L<i/> planes. This evolution follows the model tracks of WDs with masses of 1.05 − 1.15 <i>M<i/><sub>⊙<sub/> and thermonuclear burning on the surface. We show that, when the source has a relatively small photospheric radius and is optically bright, it lies below the stable-burning strip with a relatively low bolometric luminosity. Conversely, the fainter optical states correspond to higher bolometric luminosity and larger photospheric radii of the hot WD. RXJ0513 lies within the stable-burning strip during this state. This means that the optical brightness of the system is lower when the WD is larger, more luminous, and illuminates the accretion disc more effectively. These results contradict the contraction model, which predicts the opposite behaviour of the source. We use a model that assumes that the far UV/soft X-ray flux is reprocessed into the optical band due to multiple scattering in the cloud system above the accretion disc. More significant illumination can lead to rarefying of the cloud slab, thereby reducing the reprocessing efficiency and making the source fainter in the optical band.","PeriodicalId":8571,"journal":{"name":"Astronomy & Astrophysics","volume":"40 1","pages":""},"PeriodicalIF":6.5,"publicationDate":"2025-02-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143124708","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}