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Register a new userSynthesis of radioactive elements in novae and supernovae and their use as a diagnostic tool
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Novae and supernovae play a key role in many fields of Astrophysics and Cosmology. Despite their importance, an accurate description of which objects explode and why and how they explode is still lacking. One of the main characteristics of such explosions is that they are the main suppliers of newly synthesized chemical elements in the Galaxy. Since some of these isotopes are radioactive, it is possible to use the corresponding gamma-rays as a diagnostic tool of the explosion thanks to their independence on the thermal state of the debris. The drawback is the poor sensitivity of detectors in the MeV energy domain. As a consequence, the radioactive lines have only been detected in one core collapse supernova (SN 1987A), one Type Ia supernova (SN 2014J), and one supernova remnant (Cas A). Nevertheless these observations have provided and are providing important information about the explosion mechanisms. Unfortunately, novae are still eluding detection. These results emphasize the necessity to place as soon as possible a new instrument in orbit with enough sensitivity to noticeably enlarge the sample of detected events
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We put constraints on the properties of the progenitors of peculiar calcium-rich transients using the distribution of locations within their host galaxies. We confirm that this class of transients do not follow the galaxy stellar mass profile and are more likely to be found in remote locations of their apparent hosts. We test the hypothesis that these transients are from low metallicity progenitors by comparing their spatial distributions with the predictions of self-consistent cosmological simulations that include star formation and chemical enrichment. We find that while metal-poor stars and our transient sample show a consistent preference for large offsets, metallicity alone cannot explain the extreme cases. Invoking a lower age limit on the progenitor helps to improve the match, indicating these events may result from a very old metal-poor population. We also investigate the radial distribution of globular cluster systems, and show that they too are consistent with the class of calcium-rich transients. Because photometric upper limits exist for globular clusters for some members of the class, a production mechanism related to the dense environment of globular clusters is not favoured for the calcium-rich events. However the methods developed in this paper may be used in the future to constrain the effects of low metallicity on radially distant core-collapse events or help establish a correlation with globular clusters for other classes of peculiar explosions.
We report on the X-ray emission from the radio jet of 3C 17 from Chandra observations and compare the X-ray emission with radio maps from the VLA archive and with the optical-IR archival images from the Hubble Space Telescope. X-ray detections of two knots in the 3C 17 jet are found and both of these features have optical counterparts. We derive the spectral energy distribution for the knots in the jet and give source parameters required for the various X-ray emission models, finding that both IC/CMB and synchrotron are viable to explain the high energy emission. A curious optical feature (with no radio or X-ray counterparts) possibly associated with the 3C 17 jet is described. We also discuss the use of curved jets for the problem of identifying inverse Compton X-ray emission via scattering on CMB photons.
The origin of ultra-wide massive binaries (orbital separations $10^3-2times 10^5$~AU) and their properties are not well characterized nor understood. Here we use the second Gaia data release to search for wide astrometric companions to Galactic O-B5 stars which share similar parallax and proper motion with the primaries. Using the data we characterize the frequency and properties of such binaries. We find an ultra-wide multiplicity fraction of $4.4pm0.5$ per cent, to our completeness limit (up to $approx 17$~mag; down to G-stars at distances of 0.3-2~kpc, excluding stars in clusters). The secondary mass-function is generally consistent with a Kroupa initial stellar function; if extrapolated to lower mass companion stars we then might expect a wide-binary fraction of $sim 27pm5%$. In addition we use these data as a verification sample to test the existence of ultra-wide binaries among neutron stars (NSs) and black holes (BHs). We propose that the discovery of such binary can provide unique constraints on the weakest natal kicks possible for NSs/BHs. If a compact object is formed in an ultra-wide binary and receives a very-low natal kick, such a binary should survive as a common proper motion pair. We therefore use Gaia data to search for ultra-wide companions to pulsars (normal and millisecond ones) and X-ray binaries. We find no reliable pairs. Future data could potentially provide stringent constraints through this method.
Classical novae are the most common astrophysical thermonuclear explosions, occurring on the surfaces of white dwarf stars accreting gas from companions in binary star systems. Novae typically expel ~10^(-4) solar masses of material at velocities exceeding 1,000 kilometres per second. However, the mechanism of mass ejection in novae is poorly understood, and could be dominated by the impulsive flash of thermonuclear energy, prolonged optically thick winds, or binary interaction with the nova envelope. Classical novae are now routinely detected in gigaelectronvolt gamma-ray wavelengths, suggesting that relativistic particles are accelerated by strong shocks in the ejecta. Here we report high-resolution radio imaging of the gamma-ray-emitting nova V959 Mon. We find that its ejecta were shaped by the motion of the binary system: some gas was expelled rapidly along the poles as a wind from the white dwarf, while denser material drifted out along the equatorial plane, propelled by orbital motion. At the interface between the equatorial and polar regions, we observe synchrotron emission indicative of shocks and relativistic particle acceleration, thereby pinpointing the location of gamma-ray production. Binary shaping of the nova ejecta and associated internal shocks are expected to be widespread among novae, explaining why many novae are gamma-ray emitters.
It is now clear that a subset of supernovae display evidence for jets and are observed as gamma-ray bursts. The angular momentum distribution of massive stellar endpoints provides a rare means of constraining the nature of the central engine in core-collapse explosions. Unlike supermassive black holes, the spin of stellar-mass black holes in X-ray binary systems is little affected by accretion, and accurately reflects the spin set at birth. A modest number of stellar-mass black hole angular momenta have now been measured using two independent X-ray spectroscopic techniques. In contrast, rotation-powered pulsars spin-down over time, via magnetic braking, but a modest number of natal spin periods have now been estimated. For both canonical and extreme neutron star parameters, statistical tests strongly suggest that the angular momentum distributions of black holes and neutron stars are markedly different. Within the context of prevalent models for core-collapse supernovae, the angular momentum distributions are consistent with black holes typically being produced in GRB-like supernovae with jets, and with neutron stars typically being produced in supernovae with too little angular momentum to produce jets via magnetohydrodynamic processes. It is possible that neutron stars are imbued with high spin initially, and rapidly spun-down shortly after the supernova event, but the available mechanisms may be inconsistent with some observed pulsar properties.
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