New Astronomy Reviews最新文献

In antiquity, all of the enduring celestial bodies that were seen to move relative to the background sky of stars were considered planets. During the Copernican revolution, this definition was altered to objects orbiting around the Sun, removing the Sun and Moon but adding the Earth to the list of known planets. The concept of planet is thus not simply a question of nature, origin, composition, mass or size, but historically a concept related to the motion of one body around the other, in a hierarchical configuration.

After discussion within the IAU Commission F2 “Exoplanets and the Solar System”, the criterion of the star-planet mass ratio has been introduced in the definition of the term “exoplanet”, thereby requiring the hierarchical structure seen in our Solar System for an object to be referred to as an exoplanet. Additionally, the planetary mass objects orbiting brown dwarfs, provided they follow the mass ratio criterion, are now considered as exoplanets. Therefore, the current working definition of an exoplanet, as amended in August 2018 by IAU Commission F2 “Exoplanets and the Solar System”, reads as follows:

Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars, brown dwarfs or stellar remnants and that have a mass ratio with the central object below the $L_4$/$L_5$ instability ($M/M_{\mathrm{central}}<2/(25+\sqrt{621})\approx 1/25$) are “planets”, no matter how they formed.

The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System, which is a mass sufficient both for self-gravity to overcome rigid body forces and for clearing the neighborhood around the object’s orbit.

Here we discuss the history and the rationale behind this definition.

The first stars (Pop III stars) and Black Holes (BHs) formed in galaxies at Cosmic Dawn (CD) have not been observed and remain poorly constrained. Theoretical models predict that indirect insights of those Pop III stars and BHs could be imprinted as an absorption signal in the 21cm line of the atomic hydrogen (HI) in the cold Intergalactic Medium (IGM), against the Cosmic Microwave Background (CMB), when the Universe was less than 200 million years old. The first tentative observation of an HI absorption in the 21cm line at redshifts z > 15 by the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) has stimulated a great deal of research. To explain the additional large amplitude of that absorption signal a plethora of models based on exotic physics and on astrophysical sources have been proposed. Among the latter are models that propose the existence of an additional synchrotron Cosmic Radio Background (CRB) from BH-jet sources of comparable intensity to that of the CMB that boosts the HI absorption signal at CD. The discovery of radio loud supermassive black holes (SMBHs) of ~10⁹ M_⊙ in high-z quasars of up to z ~7.5 suggests the existence of a CRB component from growing BHs at z > 15, of unknown intensity.

To match the onset of the EDGES signal a CRB of comparable intensity to that of the CMB would be required. With no judgment on whether the EDGES signal is of cosmic origin or not, here we provide approximate calculations to analyze highly redshifted HI absorption signals taking that of EDGES as an example to explore what could be learned on BHs at CD. Assuming a BH mass to radio luminosity ratio as observed in radio-loud Supermassive BHs (SMBHs) of ∼10⁹ M_⊙ in quasars at redshifts z = 6 – 7, by simple calculations we find that rapidly growing radio luminous BHs of Intermediate Mass (IMBHs) , in their way to become SMBHs, are the only type of astrophysical radio sources of a CRB that can explain the onset of the EDGES absorption at z = 18 – 20. At those redshifts the EDGES signal would imply that the global mass density of IMBHs must be dominant over that of stars, more than 70% of the maximum of Stellar Mass Density (SMD) expected at those high redshifts. This suggests that those IMBHs are formed before, and growing faster than the bulk of stars, with no need of a large mass contribution from stellar-mass BH remnants of typical Pop III stars. The highly redshifted signals from these IMBHs at cosmic dawn may be detected at long radio wavelengths with the next generation of ultrasensitive interferometers such as the Square Kilometer Array (SKA), in the infrared with the James Webb Space Telescope (JWST), and in the X-rays with future space missions.

INTEGRAL is an ESA mission in fundamental astrophysics that was launched in October 2002. It has been in orbit for over 18 years, during which it has been observing the high-energy sky with a set of instruments specifically designed to probe the emission from hard X-ray and soft $\gamma$-ray sources. This paper is devoted to the subject of black hole binaries, which are among the most important sources that populate the high-energy sky. We present a review of the scientific literature based on INTEGRAL data, which has significantly advanced our knowledge in the field of relativistic astrophysics. We briefly summarise the state-of-the-art of the study of black hole binaries, with a particular focus on the topics closer to the INTEGRAL science. We then give an overview of the results obtained by INTEGRAL and by other observatories on a number of sources of importance in the field. Finally, we review the main results obtained over the past 18 years on all the black hole binaries that INTEGRAL has observed. We conclude with a summary of the main contributions of INTEGRAL to the field, and on the future perspectives.

Jets are ubiquitous in the Universe and are seen from a large number of astrophysical objects including active galactic nuclei, gamma ray bursters, micro-quasars, proto-planetary nebulae, young stars and even brown dwarfs. In every case they seem to be accompanied by an accretion disk and, while the detailed physics may change, it has been suggested that the same basic mechanism is responsible for generating the jet. Although we do not understand what that mechanism is, or even if it is universal, it is thought to involve the centrifugal ejection of matter from the disk along magnetic field lines. For a number of reasons, in particular their proximity and the abundant range of diagnostics to determine their characteristics, jets from young stars and their associated outflows may offer us the best opportunity to discover how jets are generated and the nature of the link between outflows and their accretion disks. Recently it has become clear that jets may be fundamental to the star formation process in removing angular momentum from the surrounding protoplanetary disk thereby allowing accretion to proceed. Moreover, with the realization that planetary formation begins much earlier than previously thought, jets may also help forge planets by determining initial environmental characteristics. This seems to be particularly true within the so-called terrestrial planet forming zone. Here we review observations of jets from young stars which have greatly benefitted from new facilities such as ALMA, space observatories like Spitzer, Herschel and HST, and radio facilities like LOFAR and the VLA. Interferometers such as CHARA and GRAVITY are starting to make inroads into resolving how they are launched, and we can look forward to a bright future in our understanding of this phenomenon when JWST and the SKA come on stream. In addition, we examine the various magnetohydrodynamic models for how jets from young stars are thought to be generated and how observations may help us select between these various options.

The European Space Agency’s INTErnational Gamma-Ray Astrophysics Laboratory (ESA/INTEGRAL) was launched aboard a Proton-DM2 rocket on 17 October 2002 at 06:41 CEST, from Baikonur in Kazakhstan. Since then, INTEGRAL has been providing long, uninterrupted observations (up to about 47  h, or 170  ksec, per satellite orbit of 2.7 days) with a large field-of-view (FOV, fully coded: 100 deg${}^2$), millisecond time resolution, keV energy resolution, polarization measurements, as well as additional wavelength coverage at optical wavelengths. This is realized by two main instruments in the 15  keV to 10  MeV energy range, the spectrometer SPI (spectral resolution 3 keV at 1.8  MeV) and the imager IBIS (angular resolution: 12 arcmin FWHM), complemented by X-ray (JEM-X; 3–35  keV) and optical (OMC; Johnson V-band) monitor instruments. All instruments are co-aligned to simultaneously observe the target region. A particle radiation monitor (IREM) measures charged particle fluxes near the spacecraft. The Anti-coincidence subsystems of the main instruments, built to reduce the background, are also very efficient all-sky $\gamma$-ray detectors, which provide virtually omni-directional monitoring above $\sim$75  keV. Besides the long, scheduled observations, INTEGRAL can rapidly (within a couple of hours) re-point and conduct Target of Opportunity (ToO) observations on a large variety of sources.

INTEGRAL observations and their scientific results have been building an impressive legacy: The discovery of currently more than 600 new high-energy sources; the first-ever direct detection of ⁵⁶Ni and ⁵⁶Co radio-active decay lines from a Type Ia supernova; spectroscopy of isotopes from galactic nucleo-synthesis sources; new insights on enigmatic positron annihilation in the Galactic bulge and disk; and pioneering gamma-ray polarization studies. INTEGRAL is also a successful actor in the new multi-messenger astronomy introduced by non-electromagnetic signals from gravitational waves and from neutrinos: INTEGRAL found the first prompt electromagnetic radiation in coincidence with a binary neutron star merger.

Up to now more than 1750 scientific papers based on INTEGRAL data have been published in refereed journals. In this paper, we will give a comprehensive update of the satellite status after more than 18 years of operations in a harsh space environment, and an account of the successful Ground Segment.

Within the central few hundred parsecs of the Milky Way, extending from longitude l = −1° to 1.5°, lies the Central Molecular Zone of the Galactic Centre. This extraordinary region is defined by a diverse variety of ISM features in numerous stages of evolution. Molecular cloud H₂ volume densities range from 10^3-8 cm⁻³ with an average of 10⁴ cm⁻³, two orders of magnitude above that of the galactic disk. The CMZ contains ∼3-5 × 10⁷ M_☉ of molecular gas, corresponding to around 5-10% of the content of the entire galaxy, and a similar fraction of its infrared luminosity. Gas temperatures, pressures and turbulent mach numbers are also significantly raised here, providing one of the more extreme environments for star formation within our observational reach.

We have hence been provided with a unique laboratory for probing the effects of these environments on the interplay between the ISM and star formation, and high resolution observations of both individual features and the large-scale structure of the CMZ can improve our understanding of the formation and evolution of this region, which we can then apply to similar regions in nearby galaxies.

This review will address historical and recent advancements in our observational and theoretical interpretations of the morphologies, dynamics and processes occurring in the ISM and massive stellar populations in the central few hundred parsecs. It will demonstrate how, across various spatial scales, episodic cycles of star formation, matter transport and feedback can be identified and potentially linked to observed features. The evolutionary states of molecular clouds, star forming regions and stellar clusters can be linked to their positions along orbits spanning the CMZ, and may be regulated by episodic processes such as material inflow or feedback. The concentric series of expanding bubbles and fronts visible in various electromagnetic bands can be related to echoes of past activity in the central cluster and Sgr A*. The ensemble of stellar ages and populations in the highly inhospitable environment of the central few parsecs points towards a series of accretion and starburst events.

The range of timescales and spatial scales involved in the aforementioned processes raises the possibility of a nested series of episodic cycles occurring concurrently, in which shorter timescale cycles regulate longer ones. The resulting complex and highly time-variable picture can help to explain many of the currently observed characteristics of the Galactic Centre, such as its deficient star forming efficiency, and can be applied to our understanding of the evolution of the galaxy as a whole.

Dark sector, constituting about 95% of the Universe, remains the subject of numerous studies. There are lots of models dealing with the cause of the effects assigned to “dark matter” and “dark energy”. This brief review is devoted to the very recent theoretical advances in these areas: only to the advances achieved in the last few years. For example, in section devoted to particle dark matter we overview recent publications on sterile neutrinos, self-interacting dark matter, dibarions (hexaquarks), dark matter from primordial “bubbles”, primordial black holes as dark matter, axions escaping from neutron stars, and dark and usual matter interacting via the fifth dimension. We also overview the second flavor of hydrogen atoms: their existence was proven by analyzing atomic experiments and is also evidenced by the latest astrophysical observations of the 21 cm spectral line from the early Universe. While discussing non-particle models of the cause of dark matter effects, we refer to modified Newtonian dynamics and modifications of the strong equivalence principles. We also consider exotic compact objects, primordial black holes, and retardation effects. Finally, we review recent studies on the cause of “dark energy effects”. Specifically, we cover two disputes that arose in 2019 and 2020 on whether the observations of supernovas, previously interpreted as the proof of the existence of dark energy, could have alternative explanations. Besides, we note a study of 2021, where dark energy is substituted by a new hypothetical type of dark matter having a magnetic-type interaction. We also refer to the recent model of a system of nonrelativistic neutral gravitating particles providing an alternative explanation of the entire dynamics of the Universe expansion – without introducing dark energy or new gravitational degrees of freedom.

The numerous compact sources associated with neutron stars and white dwarfs discovered in recent decades are analyzed in terms of the Gravimagnetic Rotator model (GMR paradigm–Lipunov, 1987a, 1992). We offer the instrument for understanding of various observed features and evolutionary relationships of neutron stars and white dwarfs. We depict in a single diagram all objects from radio pulsars and dwarf novae to ultra luminous X-ray sources and a radio pulsating white dwarf. This diagram directly demonstrates the genetic link between different types of compact sources thereby making it possible to confirm and illustrate clearly the established evolutionary connections–such as that between bulge X-ray sources and millisecond pulsars. This approach allows us to understand the evolutionary status of Ultra Luminous X-ray sources. In addition, we propose an additional evolutionary branch of the formation of Magnetars. When our work was completed, an article by Kirsten et al.2021, was published, which reports the localization of FRB 20,200,120 in one of the globular clusters of the galaxy M81. This shows that the accretion-induced collapse scenario of the white dwarf (Lipunov and Postnov, 1985), considered in detail in this work, is a possible genealogical branch of Magnetar production.

ESA’s INTEGRAL space mission has achieved unique results for solar and terrestrial physics, although spacecraft operations nominally excluded the possibility to point at the Sun or the Earth. The Earth avoidance was, however, exceptionally relaxed for special occultation observations of the Cosmic X-ray Background (CXB), which on some occasions allowed the detection of strong X-ray auroral emission. In addition, the most intense solar flares can be bright enough to be detectable from outside the field of view of the main instruments. This article presents for the first time the auroral observations by INTEGRAL and reviews earlier studies of the most intense solar flares. We end by briefly summarising the studies of the Earth’s radiation belts, which can be considered as another topic of serendipitous science with INTEGRAL.