Astrophysics and Space Science最新文献

{"title":"Electrically charged white dwarfs in 4D Einstein-Gauss-Bonnet gravity","authors":"Krishna Pada Das,&nbsp;Ujjal Debnath","doi":"10.1007/s10509-025-04412-z","DOIUrl":"10.1007/s10509-025-04412-z","url":null,"abstract":"<div><p>As compact relativistic objects, white dwarfs are in different classes than neutron stars. Because white dwarfs are comparatively less compact than neutron stars are, the equation of state of a white dwarf is comparatively more certain. In this work, we investigated the basic properties of nonrotating white dwarfs composed of charged perfect fluid in the context of 4D Einstein-Gauss-Bonnet gravity. For example, we derived the mass, radius, energy density, pressure, charge distribution, and electric field of white dwarfs and demonstrated their dependency on the Gauss-Bonnet coupling constant <span>(alpha )</span> in terms of the effect of charge. The structural solutions of white dwarfs are obtained by adopting Chandrasekhar’s equation of state and a significant relationship between charged density and energy density. In this context, we solve the TOV equation with the addition of the charge profile numerically by considering appropriate boundary conditions at the center of the star. By adjusting different parameters, we present a detailed graphical discussion of several characteristics of white dwarfs. We emphasize the mass-radius relationship of our proposed white dwarfs and compare the results with the Chandrasekhar mass limit for viable white dwarf structures. Moreover, the nature of the sound speed profile and adiabatic index in the internal structure of white dwarfs are discussed. As a result, we obtain a physically viable charged white dwarf structure with a mass near the Chandrasekhar mass limit in the context of the Einstein-Gauss-Bonnet gravity.</p></div>","PeriodicalId":8644,"journal":{"name":"Astrophysics and Space Science","volume":"370 2","pages":""},"PeriodicalIF":1.8,"publicationDate":"2025-02-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143521581","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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":"Some musings on erythrogigantoacoustics","authors":"Douglas Gough","doi":"10.1007/s10509-025-04409-8","DOIUrl":"10.1007/s10509-025-04409-8","url":null,"abstract":"<div><p>Observations of stars other than the Sun are sensitive to oscillations of only low degree. Many are high-order acoustic modes. Acoustic frequencies of main-sequence stars, for example, satisfy a well-known pattern, which some astronomers have adopted even for red-giant stars. That is not wise, because the internal structures of these stars can be quite different from those on the Main Sequence, which is populated by stars whose structure is regular. Here I report on pondering this matter, and point out two fundamental deviations from the commonly adopted relation. There are aspects of the regular relation that are connected in a simple way to gross properties of the star, such as the dependence of the eigenfrequencies on the linear combination <span>(n+textstyle {frac{1}{2}}l)</span> of the order <span>(n)</span> and degree <span>(l)</span>, which is characteristic of a regular spherical acoustic cavity. That is not a feature of red-giant frequencies, because, as experienced by the waves, red-giant stars appear to have (phantom) singular centres, which substantially modify the propagation of waves. That requires a generalization of the eigenfrequency relation, which I present here. When fitted to the observed frequencies of the Sun, the outcome is consistent with the Sun being round, with no singularity in the core. That is hardly novel, but at least it provides some assurance that our understanding of stellar acoustic wave dynamics is on a sound footing.</p></div>","PeriodicalId":8644,"journal":{"name":"Astrophysics and Space Science","volume":"370 2","pages":""},"PeriodicalIF":1.8,"publicationDate":"2025-02-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s10509-025-04409-8.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143513328","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"A decade of sub-arcsecond imaging with the International LOFAR Telescope","authors":"Leah K. Morabito,&nbsp;Neal Jackson,&nbsp;Jurjen de Jong,&nbsp;Emmy Escott,&nbsp;Christian Groeneveld,&nbsp;Vijay Mahatma,&nbsp;James Petley,&nbsp;Frits Sweijen,&nbsp;Roland Timmerman,&nbsp;Reinout J. van Weeren","doi":"10.1007/s10509-025-04406-x","DOIUrl":"10.1007/s10509-025-04406-x","url":null,"abstract":"<div><p>The International LOFAR Telescope (ILT) is a pan-European radio interferometer with baselines up to 2000 km. This provides sub-arcsecond resolution at frequencies of &lt;200 MHz. Since starting science operations in 2012, the ILT has carried out observations for the state-of-the-art LOFAR Two-metre Sky Survey, which has 6 arcsec resolution at 144 MHz. Wide-area surveys at low frequencies, while scientifically productive, have to compromise on resolution. Sub-arcsecond imaging with the ILT has become more accessible over the last decade, thanks to efforts to build a publicly available pipeline using LOFAR-specific tools, which has resulted in a dramatic increase in the number of publications. The ILT’s combination of resolution, field of view, and low observing frequency make it a unique instrument for a wide range of scientific applications, and it will remain unparalleled even in the era of the Square Kilometre Array Observatory. Here we provide an overview of the technical considerations and calibration methods sub-arcsecond imaging with the ILT. This is followed by a review of the unique capabilities unlocked by sub-arcsecond imaging with the ILT, using examples from the literature for demonstration. Finally we describe ongoing work including: surveying large areas of the sky at high resolution, going deeper in fields with excellent ancillary information, producing images of polarisation, and extending to lower frequencies (&lt;100 MHz).</p></div>","PeriodicalId":8644,"journal":{"name":"Astrophysics and Space Science","volume":"370 2","pages":""},"PeriodicalIF":1.8,"publicationDate":"2025-02-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s10509-025-04406-x.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143481239","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Ionospheric disturbances in the African low-latitude region during the space weather event of September 2017","authors":"Teshome Dugassa,&nbsp;Valence Habyarimana","doi":"10.1007/s10509-025-04407-w","DOIUrl":"10.1007/s10509-025-04407-w","url":null,"abstract":"<div><p>The space weather event from September 5-11, 2017, was marked by high activity, with multiple solar flares and a geomagnetic storm. This study investigates the impact of solar flares and the associated geomagnetic storm on the equatorial and low-latitude region of Africa, utilizing data from the Global Navigation Satellite System (GNSS), in-situ electron density observations from the SWARM-A satellite, and ground-based magnetometer data from Mbour (<b>mbo</b>, 14.39^∘S, 16.96^∘W), Dakar, as well as the real-time prompt penetration equatorial electric field model (PPEFM). The analysis of ionospheric total electron content (TEC) disturbances involves comparing storm-time TEC with the mean of the quiet days of the month. The rate of change of TEC index (<b>ROTI</b>) and the rate of plasma density irregularity index (<b>RODI</b>) are employed to examine ionospheric irregularities on the equatorial and low-latitude African longitude. In order to analyze the TEC changes in the ionosphere due to solar flares, difference between the TEC value before the flare and the peak TEC value during the flare were used. Results of the study show that while the X2.2 solar flare did not significantly increase TEC, the X9.3 flare caused a notable enhancement, with TEC increase of 2.47 and 1.66 TECU in the East and West African sectors, respectively. While the X1.3 solar flare caused TEC increase of 1.03 and 0.44 TECU in the East and West African sectors, respectively. Sometimes, reduction in ionospheric TEC were also observed. The ionospheric TEC response during the first stage of the storm’s main phase was minimal, but significant variations were noted during the second stage of the storm’s main phase, in both Eastern and Western African sectors. Ionospheric irregularities during the first stage of the storm’s main phase were suppressed/enhanced in the Eastern/Western African sectors. On the other hand, during the second stage of the storm’s main phase, the occurrence of ionospheric irregularities were inhibited in both African sectors. This may be likely due to the decrease in the pre-reversal enhancement of the ionospheric zonal electric field. Lastly, the study also examines disturbances in ionospheric currents inferred from ground-based data, extracted from magnetometer located in the West African sector. Unfortunately, there is no magnetometer located in the East African sector.</p></div>","PeriodicalId":8644,"journal":{"name":"Astrophysics and Space Science","volume":"370 2","pages":""},"PeriodicalIF":1.8,"publicationDate":"2025-02-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143466024","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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":"Long-term Fermi observations of Mrk 421: clues for different non-stationary processes","authors":"B. Kapanadze,&nbsp;A. Gurchumelia,&nbsp;M. Aller","doi":"10.1007/s10509-025-04411-0","DOIUrl":"10.1007/s10509-025-04411-0","url":null,"abstract":"<div><p>This paper presents the gamma-ray spectral and timing results from the long-term regular observations of Mrk 421 with the Large Area Telescope (LAT) onboard <i>Fermi</i> during 2008 August–2023 August. We discerned six periods of the relatively stronger 0.3–300 GeV activity compared to other time intervals. The baseline brightness level varied on timescales from several months to years during these periods, which was superimposed by shorter-term flares of the different asymmetry. The latter are explained by various interplay between the light-crossing, particle acceleration and cooling timescales. The source also frequently exhibited two-peak flares, to be triggered by the propagation of forward and reverse shocks after collision between the “shells” of high-energy plasma, moving with different speeds down the jet. The strongest long-term flaring activity was recorded during 2012 June–2013 October and 2017 October–2018 March when the source was mostly brighter than <span>(10^{-7}text{ ph},text{cm}^{-2},text{s}^{-1})</span> in the 0.3–300 GeV energy range and robustly detectable even on intraday timescales. We detected 25 instances of intraday variability and a large number of the flux doubling/halving instances, allowing to constrain the upper limit to the emission zone size to be in the range of <span>(1.3times 10^{16}text{ cm}text{--}1.1times 10^{18}text{ cm})</span>. The source generally showed a lognormal variability in the LAT energy range, explained as an imprinting of the disc nonstationary processes on the jet, proton-initiated hadronic cascades or random fluctuations in the particle acceleration rate. Most of the 0.3–300 GeV spectra were well-fit with a simple power-law model and showed a very broad range of the photon-index from <span>(Gamma sim 2.8)</span> down to <span>(Gamma sim 1.2)</span>, with the mean values <span>(Gamma _{mathrm{mean}}=1.75text{--}1.84)</span> and distribution peaks <span>(Gamma _{mathrm{p}}=1.73text{--}1.82)</span> during the periods of strong LAT-band activity. Our spectral study also revealed the features of inverse-Compton upscatter of X-ray photons in the Klein-Nishina regime, relativistic magnetic reconnection, first-order Fermi mechanism within the magnetic field of different confinement efficiencies and stochastic acceleration.</p></div>","PeriodicalId":8644,"journal":{"name":"Astrophysics and Space Science","volume":"370 2","pages":""},"PeriodicalIF":1.8,"publicationDate":"2025-02-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143466025","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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":"A coronal mass ejection from AR10715 on December 30, 2004","authors":"Mahalakshmi K","doi":"10.1007/s10509-025-04408-9","DOIUrl":"10.1007/s10509-025-04408-9","url":null,"abstract":"<div><p>The initiation of the Coronal Mass Ejection (CME) on 30 December 2004 at 10:57 UT, with an initial speed of 1250 km/s, has been studied using multiwavelength data. EUV and radio H-alpha images play a significant role in identifying multiple reconnections. The reconnection occurring in the northern and southern parts of Active Region (AR10715) led to C1.3 and M2.2 flares. The southern arcade system expands and reconnects with the overlying field lines, resulting in a fast, wide CME. Hence, our multiwavelength analysis provides evidence that internal reconnection, followed by external reconnection, causes large-scale eruption at 10:50 UT. Radio signatures also support this scenario.</p></div>","PeriodicalId":8644,"journal":{"name":"Astrophysics and Space Science","volume":"370 2","pages":""},"PeriodicalIF":1.8,"publicationDate":"2025-02-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s10509-025-04408-9.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143446374","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Spectral study of sample of GeV emission gamma-ray bursts with quiescent episodes","authors":"Maryam Imran","doi":"10.1007/s10509-025-04403-0","DOIUrl":"10.1007/s10509-025-04403-0","url":null,"abstract":"<div><p>Gamma-ray bursts, or GRBs, are the most energetic phenomena in the universe that may occur as a result of mergers between compact objects and supernova explosions. While many GRBs are characterized by having a single emission peak, a few have been found to occur with multiple peaks having potential emission gaps dubbed “quiescent episodes.” While the occurrence of such intervals remains in debate, the spectral study of these GRBs can provide an insight about the evolution of parameters within different peaks of the same GRB. We have performed the time-resolved analysis of a sample of four GRBs having emission in both the GBM and LAT range. These GRBs were selected based on a particular criteria. We have also studied the evolution of spectral parameters and tested each interval for the Amati Correlation through the E_iso and E_i,peak relations. We have found the individual peaks to fit with simple models. The low-energy index was typically found to evolve with a hard-to-soft spectrum. The flux indicates a decay in time, while peak energy evolution may reveal different trends. One of the GRBs in our sample with a potential precursor emission seems to evolve differently than the rest of the GRBs in the sample.</p></div>","PeriodicalId":8644,"journal":{"name":"Astrophysics and Space Science","volume":"370 2","pages":""},"PeriodicalIF":1.8,"publicationDate":"2025-02-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143388865","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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":"Towards cosmological inference on unlabeled out-of-distribution HI observational data","authors":"Sambatra Andrianomena,&nbsp;Sultan Hassan","doi":"10.1007/s10509-025-04405-y","DOIUrl":"10.1007/s10509-025-04405-y","url":null,"abstract":"<div><p>We present an approach that can be utilized in order to account for the covariate shift between two datasets of the same observable with different distributions. This helps improve the generalizability of a neural network model trained on in-distribution samples (IDs) when inferring cosmology at the field level on out-of-distribution samples (OODs) of <i>unknown labels</i>. We make use of HI maps from the two simulation suites in CAMELS, IllustrisTNG and SIMBA. We consider two different techniques, namely adversarial approach and optimal transport, to adapt a target network whose initial weights are those of a source network pre-trained on a labeled dataset. Results show that after adaptation, salient features that are extracted by source and target encoders are well aligned in the embedding space. This indicates that the target encoder has learned the representations of the target domain via the adversarial training and optimal transport. Furthermore, in all scenarios considered in our analyses, the target encoder, which does not have access to any labels (<span>(Omega _{mathrm{m}})</span>) during adaptation phase, is able to retrieve the underlying <span>(Omega _{mathrm{m}})</span> from out-of-distribution maps to a great accuracy of <span>(R^{2})</span> score ≥ 0.9, comparable to the performance of the source encoder trained in a supervised learning setup. We further test the viability of the techniques when only a few out-of-distribution instances are available for training and find that the target encoder still reasonably recovers the matter density. Our approach is critical in extracting information from upcoming large scale surveys.</p></div>","PeriodicalId":8644,"journal":{"name":"Astrophysics and Space Science","volume":"370 2","pages":""},"PeriodicalIF":1.8,"publicationDate":"2025-02-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143373255","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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":"Spectroscopic diagnosis of a B-class flare and an associated filament eruption","authors":"B. Suresh Babu,&nbsp;Pradeep Kayshap,&nbsp;Sharad C. Tripathi","doi":"10.1007/s10509-025-04404-z","DOIUrl":"10.1007/s10509-025-04404-z","url":null,"abstract":"<div><p>The flare ribbon and an associated filament eruption are diagnosed using O <span>iv</span> 1401.16 Å, Si <span>iv</span> 1402.77 Å, and Mg <span>ii</span> k 2796.35 Å spectral lines provided by Interface Region Imaging Spectrograph (IRIS). The flare ribbons have downflow (redshifts) in all these lines, and this redshift decreases from the transition region to the chromosphere. While the overlapping region (flare-ribbon+filament rise/eruption) is dominated by upflows (blueshifts) in all three spectral lines. We found an extremely blueshifted Si <span>iv</span> profile (i.e., blueshift around −180 km/s) in the overlapping region. The mean non-thermal velocity (v_<i>nt</i>) in the flare ribbons is higher in O <span>iv</span> than Si <span>iv</span>. While, in the overlapping region, O <span>iv</span> have lower v_<i>nt</i> than Si <span>iv</span>. Note that very high v_<i>nt</i> around 80 km/s (in Si <span>iv</span>) exists in this weak B-class flare. The Mg <span>ii</span> k line widths are almost the same in the flare ribbon and overlapping region but, they are extremely broad than previously reported. We found double peak profiles of Si <span>iv</span> and O <span>iv</span> in the overlapping region. Most probably, one peak is due to downflow (flare ribbon) and another due to upflow (filament rise/eruption). We report a high redshift of more than 150 km/s in the weak B-class flare. In some cases, both peaks show upflows which might be the result of the superposition of two different sources, i.e., overlapping of two different velocity distributions in the line of sight.</p></div>","PeriodicalId":8644,"journal":{"name":"Astrophysics and Space Science","volume":"370 2","pages":""},"PeriodicalIF":1.8,"publicationDate":"2025-02-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143373341","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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":"Solar orbiter: a short review of the mission and early science results","authors":"Louise Harra,&nbsp;Daniel Müller","doi":"10.1007/s10509-025-04400-3","DOIUrl":"10.1007/s10509-025-04400-3","url":null,"abstract":"<div><p>On 9 February 2020 at 11:03 pm EST, an Atlas V 411 rocket launched the ESA/ NASA Solar Orbiter mission. This mission was the culmination of decades of work across many countries to achieve the goal of getting close to the Sun and measuring how the Sun creates and maintains the heliosphere. The mission’s goal is to understand how the inner heliosphere works and how solar activity impacts it. The spacecraft achieves this with a specially designed highly elliptical orbit that gets close to the Sun twice a year. It reaches as close as 0.28 au requiring a novel heat shield to protect the instruments from the intense heat (the front side of the heat shield reaches around 500 ^∘C at this location). There are ten scientific instruments onboard: Six remote-sensing instruments observe solar activity across the electromagnetic spectrum on small and large scales, including imaging the source regions of the solar wind. They are accompanied by four in-situ instruments to probe the properties of the solar wind as it flows past the spacecraft. This review paper describes a selection of results from Solar Orbiter during its cruise phase and the beginning of its nominal scientific operations phase, and looks towards the next phases of the mission, when the spacecraft leaves the ecliptic plane to observe the solar poles for the first time.</p></div>","PeriodicalId":8644,"journal":{"name":"Astrophysics and Space Science","volume":"370 2","pages":""},"PeriodicalIF":1.8,"publicationDate":"2025-02-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s10509-025-04400-3.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143184792","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}