New Astronomy Reviews最新文献

The INTEGRAL hard X-ray surveys have proven to be of fundamental importance. INTEGRAL has mapped the Galactic plane with its large field of view and excellent sensitivity. Such hard X-ray snapshots of the whole Milky Way on a time scale of a year are beyond the capabilities of past and current narrow-FOV grazing incidence X-ray telescopes. By expanding the INTEGRAL X-ray survey into shorter timescales, a productive search for transient X-ray emitters was made possible. In more than fifteen years of operation, the INTEGRAL observatory has given us a sharper view of the hard X-ray sky, and provided the triggers for many follow-up campaigns from radio frequencies to gamma-rays. In addition to conducting a census of hard X-ray sources across the entire sky, INTEGRAL has carried out, through Earth occultation manoeuvres, unique observations of the large-scale cosmic X-ray background, which will without question be included in the annals of X-ray astronomy as one of the mission’s most salient contribution to our understanding of the hard X-ray sky.

In the last 25 years a new generation of X-ray satellites imparted a significant leap forward in our knowledge of X-ray pulsars. The discovery of accreting and transitional millisecond pulsars proved that disk accretion can spin up a neutron star to a very high rotation speed. The detection of MeV-GeV pulsed emission from a few hundreds of rotation-powered pulsars probed particle acceleration in the outer magnetosphere, or even beyond. Also, a population of two dozens of magnetars has emerged. INTEGRAL played a central role to achieve these results by providing instruments with high temporal resolution up to the hard X-ray/soft, γ-ray band and a large field of view imager with good angular resolution to spot hard X-ray transients. In this article we review the main contributions by INTEGRAL to our understanding of the pulsating hard X-ray sky, such as the discovery and characterization of several accreting and transitional millisecond pulsars, the generation of the first catalog of hard X-ray/soft γ-ray rotation-powered pulsars, the detection of polarization in the hard X-ray emission from the Crab pulsar, and the discovery of persistent hard X-ray emission from several magnetars.

Accreting white dwarfs (WDs) constitute a significant fraction of the hard X-ray sources detected by the INTEGRAL observatory. Most of them are magnetic Cataclysmic Variables (CVs) of the intermediate polar (IP) and polar types, but the contribution of the Nova-likes systems and the systems with optically thin boundary layers, Dwarf Novae (DNs) and Symbiotic Binaries (or Symbiotic Stars, SySs) in quiescence is also not negligible. Here we present a short review of the results obtained from the observations of cataclysmic variables and symbiotic binaries by INTEGRAL. The highlight results include the significant increase of the known IP population, determination of the WD mass for a significant fraction of IPs, the establishment of the luminosity function of magnetic CVs, and uncovering origin of the Galactic ridge X-ray emission which appears to largely be associated with hard emission from magnetic CVs.

OB associations are unbound groups of young stars made prominent by their bright OB members, and have long been thought to be the expanded remnants of dense star clusters. They have been important in astrophysics for over a century thanks to their luminous massive stars, though their low-mass members have not been well studied until the last couple of decades. This has changed thanks to data from X-ray observations, spectroscopic surveys and astrometry from Gaia that allows their full stellar content to be identified and their dynamics to be studied, which in turn is leading to changes in our understanding of these systems and their origins, with the old picture of Blaauw (1964a) now being superseded. It is clear now that OB associations have considerably more substructure than once envisioned, both spatially, kinematically and temporally. These changes have implications for the star formation process, the formation and evolution of planetary systems, and the build-up of stellar populations across galaxies.

This review summarizes INTEGRAL results on two topics: the electron-positron annihilation line and X-ray & Gamma-ray diffuse emission of the Milky Way.

The electron-positron annihilation line at 511 keV is the most prominent spectral feature in the gamma-ray spectrum of the Milky Way. From the observational perspective, INTEGRAL has already provided constraints on the total flux, morphology of the annihilation line distribution, the spectral shape of the line and the strength of the three-photon annihilation continuum. In particular, the most salient morphological feature in the all-sky map of the annihilation emission based on INTEGRAL data is the so-called ”Bulge” component, with the characteristic size of $\sim 6-{10}^{\circ }$ and the positrons’ annihilation rate of $\sim {10}^{43}{s}^{-1}$. A more extended ”Disc” component is also present, although its total luminosity is model dependent. The brightness of the Bulge component compared to the Disc is in contrast with other multi-wavelength images of the Milky Way. The annihilation spectrum consists of a line centered at 511 keV and the ortho-positronium continuum. The strength of the latter indicates that the majority of annihilations go via the positronium formation channel. The shape of the annihilation spectrum is consistent with the assumption that most of the positrons annihilate in a warm and partly ionized medium, although more complicated scenarios are also possible. From the theoretical point of view, a successful model should answer three main questions: (i) physical mechanism(s) responsible for production of positrons, (ii) positrons spatial migration (if any) from the production sites, and (iii) physics of annihilation. Remarkably, despite significant progress provided by INTEGRAL in the characterization of the Milky Way annihilation emission, the origin of positrons remains an open question. The essence of the problem is the abundance of positron production channels and the uncertainty in the distance positrons can travel before annihilation.

The spectral-imaging mapping of the Milky Way by INTEGRAL provides important constraints on the nature of the Galactic diffuse continuum hard X-rays and soft gamma-rays in the 20 keV – 2  MeV band. Below  ~ 60 keV, numerous unresolved objects (accreting white dwarfs) dominate the flux, but their contribution fades away at higher energies. Models of cosmic-ray induced emission suggest that the dominant diffuse component above  ~ 60 keV (excluding annihilation emission) is inverse Compton scattering from GeV electrons on interstellar radiation fields. Non-thermal bremsstrahlung contributes at a lo