New Astronomy Reviews最新文献

We carry out analyses of some parameters of the galactic spiral arms, in the currently available samples.

We present a catalog of the observed pitch angle for each spiral arm in the Milky Way disk. For each long spiral arm in the Milky Way, we investigate for each individual arm its pitch angle, as measured through different methods (parallaxes, twin-tangent arm, kinematical, etc), and assess their answers.

Second, we catalog recent advances in the cartography of the Galaxy (global mean arm pitch, arm number, arm shape, interarm distance at the Sun). We statistically compare the results over a long time frame, from 1980 to 2017. Histograms of about 90 individual results published in recent years (since mid-2015) are compared to 66 earlier results (from 1980 to 2005), showing the ratio of primary to secondary peaks to have increased by almost a factor of 3. Similarly, many earlier discrepancies (expressed in r.m.s.) have been reduced by almost a factor 3.

This review arose from the European Radio Astronomy Technical Forum (ERATec) meeting held in Firenze, October 2015, and aims to highlight the breadth and depth of the high-impact science that will be aided and assisted by the use of simultaneous mm-wavelength receivers.

Recent results and opportunities are presented and discussed from the fields of: continuum VLBI (observations of weak sources, astrometry, observations of AGN cores in spectral index and Faraday rotation), spectral line VLBI (observations of evolved stars and massive star-forming regions) and time domain observations of the flux variations arising in the compact jets of X-ray binaries.

Our survey brings together a large range of important science applications, which will greatly benefit from simultaneous observing at mm-wavelengths. Such facilities are essential to allow these applications to become more efficient, more sensitive and more scientifically robust. In some cases without simultaneous receivers the science goals are simply unachievable. Similar benefits would exist in many other high frequency astronomical fields of research.

Neutrino-dominated accretion flows (NDAFs) around rotating stellar-mass black holes (BHs) are plausible candidates for the central engines of gamma-ray bursts (GRBs). NDAFs are hyperaccretion disks with accretion rates in the range of around 0.001–10 $M_{\odot }{s}^{-1},$ which have high density and temperature and therefore are extremely optically thick and geometrically slim or even thick. We review the theoretical progresses in studying the properties of NDAFs as well as their applications to the GRB phenomenology. The topics include: the steady radial and vertical structure of NDAFs and the implications for calculating neutrino luminosity and annihilation luminosity, jet power due to neutrino-antineutrino annihilation and Blandford–Znajek mechanism and their dependences on parameters such as BH mass, spin, and accretion rate, time evolution of NDAFs, effect of magnetic fields, applications of NDAF theories to the GRB phenomenology such as lightcurve variability, extended emission, X-ray flares, kilonovae, etc., as well as probing NDAFs using multi-messenger signals such as MeV neutrinos and gravitational waves.

Recent results in the field of high energy active galactic nuclei (AGN) astrophysics, benefiting from improvements to gamma-ray instruments and observational strategies, have revealed a surprising wealth of unexpected phenomena. These developments have been brought about both through observational efforts to discover new very high energy gamma-ray emitters, as well as from further in-depth observations of previously detected and well studied objects. I here focus specifically on the discovery of repeated temporal structures observed in AGN lightcurves, and new hard spectral components within the spectral energy distributions of other AGN systems. The challenges that these new features place on the modeling of the sources are highlighted, along with some reflections on what these results tell us about the underlying nature of the emission processes at play.

It is believed that stellar black holes (BHs) can be formed in two different ways: Either a massive star collapses directly into a BH without a supernova (SN) explosion, or an explosion occurs in a proto-neutron star, but the energy is too low to completely unbind the stellar envelope, and a large fraction of it falls back onto the short-lived neutron star (NS), leading to the delayed formation of a BH. Theoretical models set progenitor masses for BH formation by implosion, namely, by complete or almost complete collapse, but observational evidences have been elusive. Here are reviewed the observational insights on BHs formed by implosion without large natal kicks from: (1) the kinematics in three dimensions of space of five Galactic BH X-ray binaries (BH-XRBs), (2) the diversity of optical and infrared observations of massive stars that collapse in the dark, with no luminous SN explosions, possibly leading to the formation of BHs, and (3) the sources of gravitational waves (GWs) produced by mergers of stellar BHs so far detected with LIGO. Multiple indications of BH formation without ejection of a significant amount of matter and with no natal kicks obtained from these different areas of observational astrophysics, and the recent observational confirmation of the expected dependence of BH formation on metallicity and redshift, are qualitatively consistent with the high merger rates of binary black holes (BBHs) inferred from the first detections with LIGO.

The mechanism responsible for the afterglow emission of Gamma Ray Bursts (GRBs) and its connection to the prompt γ-ray emission is still a debated issue. Relations between intrinsic properties of the prompt or afterglow emission can help to discriminate between plausible theoretical models of GRB production. Here we present an overview of the afterglow and prompt-afterglow two parameter relations, their physical interpretations, their use as redshift estimators and as possible cosmological tools. A similar task has already been correctly achieved for Supernovae (SNe) Ia by using the peak magnitude-stretch relation, known in the literature as the Phillips relation (Phillips 1993). The challenge today is to make GRBs, which are amongst the farthest objects ever observed, standardizable candles as the SNe Ia through well established and robust relations. Thus, the study of relations amongst the observable and physical properties of GRBs is highly relevant together with selection biases in their physical quantities.

Therefore, we describe the state of the art of the existing GRB relations, their possible and debated interpretations in view of the current theoretical models and how relations are corrected for selection biases. We conclude that only after an appropriate evaluation and correction for selection effects can GRB relations be used to discriminate among the theoretical models responsible for the prompt and afterglow emission and to estimate cosmological parameters.

The unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, 11.3 and 12.7  µ m are ubiquitously seen in a wide variety of astrophysical regions in the Milky Way and nearby galaxies as well as distant galaxies at redshifts z ≳ 4. The UIE features are characteristic of the stretching and bending vibrations of aromatic hydrocarbon materials. The 3.3  µ m feature which results from the C–H stretching vibration in aromatic species is often accompanied by a weaker feature at 3.4  µ m. The 3.4  µ m feature is often thought to result from the C–H stretch of aliphatic groups attached to the aromatic systems. The ratio of the observed intensity of the 3.3  µ m aromatic C–H feature (I_3.3) to that of the 3.4  µ m aliphatic C–H feature (I_3.4) allows one to estimate the aliphatic fraction (e.g., N_{C, aliph}/N_{C, arom}, the number of C atoms in aliphatic units to that in aromatic rings) of the carriers of the UIE features, provided that the intrinsic oscillator strengths (per chemical bond) of the 3.3  µ m aromatic C–H stretch (A_3.3) and the 3.4  µ m aliphatic C–H stretch (A_3.4) are known. In this review we summarize the computational results on A_3.3 and A_3.4 and their implications for the aromaticity and aliphaticity of the UIE carriers. We use density functional theory and second-order perturbation theory to derive A_3.3 and A_3.4 from the infrared vibrational spectra of seven polycyclic aromatic hydrocarbon (PAH) molecules with various aliphatic substituents (e.g., methyl-, dimethyl-, ethyl-, propyl-, butyl-PAHs, and PAHs with unsaturated alkyl chains). The mean band strengths of the aromatic (A_3.3) and aliphatic (A_3.4) C–H stretches are derived and then employed to estimate the aliphatic fraction of the carriers of the UIE features by comparing the ratio of the intrinsic band strength of the two stretches (A_3.4/A_3.3) with the ratio of the observed intensities (I_3.4/I_3.3). We conclude that the UIE emitters are predominantly aromatic, as revealed by the observationally-derived mean ratio of ⟨I_3.4/I_3.3⟩ ≈ 0.12 and the computationally-derived mean ratio of ⟨A_3.4/A_3.3⟩ ≈ 1.76 which suggest an upper limit of N_{C, aliph}/N_{C, arom} ≈ 0.02 for the aliphatic fraction of the UIE carriers.

[Evidence] of polarized γ-ray emission (> 50 keV) from Gamma-Ray Bursts (GRBs) has been accumulated in recent years. Measurements have been reported with levels in the range of 30–80%, typically with limited statistical significance. No clear picture has yet emerged with regards to the polarization properties of GRBs. Taken at face value, the data suggest that most GRBs have a relatively large level of polarization (typically, > 50%), which may suggest synchrotron emission associated with an ordered magnetic field structure within the GRB jet. But these results are far from conclusive. Here, we review the observations that have been made, concentrating especially on the instrumental issues and the lessons that might be learned from these data.

I review the influence jets and the bubbles they inflate might have on their ambient gas as they operate through a negative jet feedback mechanism (JFM). I discuss astrophysical systems where jets are observed to influence the ambient gas, in many cases by inflating large, hot, and low-density bubbles, and systems where the operation of the JFM is still a theoretical suggestion. The first group includes cooling flows in galaxies and clusters of galaxies, star-forming galaxies, young stellar objects, and bipolar planetary nebulae. The second group includes core collapse supernovae, the common envelope evolution, the grazing envelope evolution, and intermediate luminosity optical transients. The suggestion that the JFM operates in these four types of systems is based on the assumption that jets are much more common than what is inferred from objects where they are directly observed. Common to all eight types of systems reviewed here is the presence of a compact object inside an extended ambient gas. The ambient gas serves as a potential reservoir of mass to be accreted on to the compact object. If the compact object launches jets as it accretes mass, the jets might reduce the accretion rate as they deposit energy to the ambient gas, or even remove the entire ambient gas, hence closing a negative feedback cycle.