Searcharxiv⌕ Search

SEARCH · Searcharxiv

Results for “astro-ph”

Search indexed arXiv papers on artificial intelligence, large language models, computer vision and robotics. Read source abstracts and follow links to arXiv.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 415 records · Page 23Linked to original sources
arXiv2009

Emergence of a Cyclic Universe from the Hagedorn Soup

One of the challenges of constructing a successful cyclic universe scenario is to be able to incorporate the second law of thermodynamics which typically leads to Tolman's problem of ever shrinking cycles. In this paper we construct a non-singular toy model where as the cycles shrink in the past they also spend more and more time in the entropy conserving Hagedorn phase. Thus in such a scenario the entropy asymptotes to a finite non-zero constant in the infinite past. The universe ``emerges'' from a small (string size) geodesically complete quasi-periodic space-time. This paradigm also naturally addresses some of the classic puzzles of Big Bang cosmology, such as the largeness, horizon and flatness problems.

hep-th↗
arXiv2008

An RMHD study of transition between prompt and afterglow GRB phases

We study the afterglow phases of a GRB through relativistic magnetohydrodynamic simulations. The evolution of a relativistic shell propagating into a homogeneous external medium is followed. We focus on the effect of the magnetization of the ejecta on the initial phases of the ejecta-external medium interaction. In particular we are studying the condition for the existence of a reverse shock into the ejecta, the timescale for the transfer of the energy from the shell to the shocked medium and the resulting multiwavelength light curves. To this end, we have developed a novel scheme to include non-thermal processeses which is coupled to the relativistic magnetohydrodynamic code MRGENESIS in order to compute the non-thermal synchrotron radiation.

astro-ph↗
arXiv2008

Senenmut: An Ancient Egyptian Astronomer

The celestial phenomenon have always been a source of wonder and interest to people, even as long ago as the ancient Egyptians. While the ancient Egyptians did not know all the things about astronomy that we do now, they had a good understanding of the some celestial phenomenon. The achievements in astronomy of ancient Egyptians are relatively well known, but we know very little about the people who made these achievements. The goal of this paper is to bring some light on the life of Senenmut, the chief architect and astronomer during the reign of Queen Hatshepsut.

astro-ph↗
arXiv2008

Cosmic ray acceleration at relativistic shocks, shear layers, .

A review of theoretical results on cosmic ray first-order Fermi acceleration at relativistic shock waves is presented, with recent results substantially changing the existing knowledge on these processes. In particular one can not expect such shocks to form particle distributions extending to very high energies. Instead, distributions with the shock compressed injected component followed by a more or less extended high energy tail are usually created. Increasing the shock Lorentz factor leads to steepening of the energetic tail. An observational check of real electron spectra in the Cyg A hot spots provides results clearly deviating from the standard expectations. The spectrum consist of a very flat low energy part (sigma ~1.5)up to electron energies ~1 GeV, and a much steeper part (sigma > 3)at higher energies. We conclude with remarks on the Fermi second-order processes acting in relativistic plasmas, possibly the main accelerating agent for very high energy cosmic rays.

astro-ph↗
arXiv2008

Spontaneous and Stimulated Star Formation in Galaxies

We present recent results from several on-going studies: The first addresses the question of gas-density thresholds for star formation, as probed by the outer disks of normal nearby galaxies. The second concerns the observational evidence for the existence of gravitating non-luminous (GNL) galaxies, as predicted by most recent simulations of galaxy formation in Lambda-CDM cosmologies. We find that (1) If star formation is traced by far-ultraviolet light, then there is no evidence for a threshold to star formation at any gas density so far probed, and (2) there is no evidence for GNL galaxies gravitationally interacting with known optical systems based on the observations (a) that there are no ring galaxies without plausible optically visible intruders, (b) all peculiar galaxies in the Arp Atlas that are bodily distorted have nearby plausibly interacting companions, and (c) there are no convincingly distorted/peculiar galaxies within Karachentsev's sample of more than 1,000 apparently/optically isolated galaxies.

astro-ph↗
arXiv2008

Einstein-aether gravity: a status report

This paper reviews the theory, phenomenology, and observational constraints on the coupling parameters of Einstein-aether gravity, i.e. General Relativity coupled to a dynamical unit timelike vector field. A discussion of open questions concerning both phenomenology and fundamental issues is included.

gr-qc↗
arXiv2008

Nuclear constraints on the momenta of inertia of neutron stars

Properties and structure of neutron stars are determined by the equation of state (EOS) of neutron-rich stellar matter. While the collective flow and particle production in relativistic heavy-ion collisions have constrained tightly the EOS of symmetric nuclear matter up to about five times the normal nuclear matter density, the more recent experimental data on isospin-diffusion and isoscaling in heavy-ion collisions at intermediate energies have constrained considerably the density dependence of the nuclear symmetry energy at subsaturation densities. Although there are still many uncertainties and challenges to pin down completely the EOS of neutron-rich nuclear matter, the heavy-ion reaction experiments in terrestrial laboratories have limited the EOS of neutron-rich nuclear matter in a range much narrower than that spanned by various EOSs currently used in astrophysical studies in the literature. These nuclear physics constraints could thus provide more reliable information about properties of neutron stars. Within well established formalisms using the nuclear constrained EOSs we study the momenta of inertia of neutron stars. We put the special emphasis on the component A of the extremely relativistic double neutron star system PSR J0737-3039. Its moment of inertia is found to be between 1.30 and 1.63 $(\times10^{45}g$ $cm^2)$. Moreover, the transition density at the crust-core boundary is shown to be in the narrow range of $ρ_t=[0.091-0.093](fm^{-3})$.

astro-ph↗
arXiv2008

On the time-modulation of the K-shell electron capture decay of H-like 140Pr58+ ions produced by neutrino-flavour mixing

According to recent experimental data at GSI, the rate of the number of daughter ions 140Ce58+, produced by the nuclear K-shell electron capture (EC) decay of the H-like ion 140Pr58+, is modulated in time with a period T_d = 7.06(8) sec and an amplitude a_(EC) = 0.20(2). We show that this phenomenon can be explained by neutrino-flavour mixing and derive a value for the difference of squared masses Delta m^2_(21) = m^2_2 - m^2_1 = 2.22(3)x10^(-4) eV^2.

nucl-th↗
arXiv2008

Spin-Gravity Coupling

Mathisson's spin-gravity coupling and its Larmor-equivalent interaction, namely, the spin-rotation coupling are discussed. The study of the latter leads to a critical examination of the basic role of locality in relativistic physics. The nonlocal theory of accelerated systems is outlined and some of its implications are described.

gr-qc↗
arXiv2008

The GALEX UV emission in shell galaxies

Shell galaxies are widely considered the debris of recent accretion/merging episodes. Their high frequency in low density environment suggests that such episodes could be among the driver of the early-type galaxy secular evolution. We present far and near UV (FUV and NUV respectively hereafter) GALEX photometric properties of a sample of shell galaxies.

astro-ph↗
arXiv2008

Transient Emissions from Radio-Active Stars: Implications for Wide-Field Radio Surveys

Variability is a common characteristic of magnetically active stars. Flaring variability is usually interpreted as the observable consequence of transient magnetic reconnection processes happening in the stellar outer atmosphere. Stellar flares have been observed now across 11 decades in wavelength/frequency/energy; such a large span implies that a range of physical processes takes place during such events. Despite the fact that stellar radio flares have long been recognized and studied, key unanswered questions remain. I will highlight what, in my opinion, are some of these questions. I will also describe recent results on stellar flare emissions at radio wavelengths, discussing the nature of coherent and incoherent emissions and the prospects of wide-field radio imaging telescopes for studying such events.

astro-ph↗
arXiv2008

Detection of new variable stars in the SMC cluster NGC 121

New candidate variable stars have been identified in the Small Magellanic Cloud cluster NGC121, by applying both the image subtraction technique (ISIS, Alard 2000) and the Welch & Stetson (1993) detection method to HST WFPC2 archive and ACS proprietary images of the cluster. The new candidate variable stars are located from the cluster's Main Sequence up to Red Giant Branch. Twenty-seven of them fall on the cluster Horizontal Branch and are very likely RR Lyrae stars. They include the few RR Lyrae stars already discussed by Walker & Mack (1988). We also detected 20 Dwarf Cepheid candidates in the central region of NGC121. Our results confirm the "true" globular cluster nature of NGC121, a cluster that is at the young end of the Galactic globulars' age range.

astro-ph↗
arXiv2008

Paschen is Partially Back

We present a discussion of the partial Paschen-Back (PB) effect in magnetic Ap stars. An overview of the theory is illustrated with examples of how splittings deviate non-linearly from the simple Zeeman picture; normally forbidden ``ghost lines'' appear in strong fields. Resulting asymmetric stellar Stokes profiles for a dipolar magnetic geometry are shown for the FeII 6149 line and it is established that PB lines may be subject to wavelength shifts. Modelling of Stokes profiles in the PB regime opens exciting new diagnostics.

astro-ph↗
arXiv2008

The Helium and Heavy Elements Enrichment of the Galactic Disk

We present chemical evolution models for the Galactic disk. We also present a new determination of X, Y, and Z for M17 a Galactic metal-rich Hii region. We compare our models for the Galactic disk with the Galactic Hii regions abundances. The $ΔY/ΔO$ ratio predicted from the galactic chemical evolution model is in very good agreement with the $ΔY/ΔO$ value derived from M17 and the primordial helium abundance, Yp, taking into account the presence of temperature variations in this Hii region. From the M17 observations we obtain that $ΔY/ΔZ = 1.97 \pm 0.41$, in excellent agreement with two $ΔY/ΔZ$ determinations derived from K dwarf stars of the solar vicinity that amount to $2.1 \pm 0.4$ and $2.1 \pm 0.9$ respectively. We also compare our models with the solar abundances. The solar and Orion nebula O/H values are in good agreement with our chemical evolution model.

astro-ph↗
arXiv2008

Evolution of Gas in Galaxies

The SKA will be a unique instrument with which to study the evolution of the gas content of galaxies. A proposed deep (~8 Msec) 'pencil-beam' survey is simulated using recently updated specifications for SKA sensitivity and survey speed. Almost 10^7 galaxies could be detected in the redshifted 21cm line, most at redshifts in excess of two. This will enable confident statements to be made about the evolution of the cosmic HI density and the HI mass function to z=3, corresponding to a lookback time of 11 Gyr. However, galaxies or groups of galaxies with masses the same as the most HI-massive galaxies at z=0 will be detectable at redshifts of 6, if they exist. The ideal instrument for studying HI evolution would have an instantaneous sensitivity at least a factor of two higher than current specifications in the critical frequency range 200-500 MHz, or A/T > 2x10^4 m^2/K. The capabilities of the SKA will be highly complementary to ALMA which will be able to study the evolution of the molecular gas component over the same redshift range.

astro-ph↗
arXiv2008

The MOND paradigm

I review briefly different aspects of the MOND paradigm, with emphasis on phenomenology, epitomized here by many MOND laws of galactic motion--analogous to Kepler's laws of planetary motion. I then comment on the possible roots of MOND in cosmology, possibly the deepest and most far reaching aspect of MOND. This is followed by a succinct account of existing underlying theories. I also reflect on the implications of MOND's successes for the dark matter (DM) paradigm: MOND predictions imply that baryons alone accurately determine the full field of each and every individual galactic object. This conflicts with the expectations in the DM paradigm because of the haphazard formation and evolution of galactic objects and the very different influences that baryons and DM are subject to during the evolution, as evidenced, e.g., by the very small baryon-to-DM fraction in galaxies (compared with the cosmic value). All this should disabuse DM advocates of the thought that DM will someday be able to reproduce MOND: It is inconceivable that the modicum of baryons left over in galaxies can be made to determine everything if a much heavier DM component is present.

astro-ph↗
arXiv2008

Soft X-ray to Far Infrared luminosities ratio in star-forming galaxies

A good correlation has been found in star-forming galaxies between the soft X-ray and the far infrared or radio luminosities. In this work we analyze the relation between the soft X-ray and far infrared luminosities as predicted by evolutionary population synthesis models, aiming first to test the validity of the soft X-ray luminosity as a star formation rate estimator, using the already known calibration of the FIR luminosity as a proxy, and second proposing a calibration based on the predictions of evolutionary synthesis models. We have computed the soft X-ray and far infrared luminosities expected for a massive starburst as a function of evolutionary state, efficiency of the conversion of mechanical energy into soft X-ray luminosity, star formation regime (instantaneous or extended) and dust abundance, and have compared the predictions with observational values for a sample of 62 star-forming galaxies taken from the literature. The observational Lsoftx/Lfir ratios are consistent with the predictions by the models under realistic assumptions (young starbursts, efficiency in the re-processing of mechanical energy of few percent), confirming the direct relation of the diffuse soft X-ray emission with the star formation episode. The soft X-ray emission of the diffuse, extended gas surrounding massive star forming regions can be used as a Star Formation Rate estimator. The empirical calibrations presented in the literature are supported by the predictions of evolutionary synthesis models and by the analysis of a larger sample of star-forming galaxies, but it is biased towards galaxies dominated by relatively unevolved starbursts.

astro-ph↗