Direct imaging of exoplanetary systems is a powerful technique that can reveal Jupiter-like planets in wide orbits, can enable detailed characterization of planetary atmospheres, and is a key step toward imaging Earth-like planets. Imaging detections are challenging because of the combined effect of small angular separation and large luminosity contrast between a planet and its host star. High-contrast observations with the Keck and Gemini telescopes have revealed three planets orbiting the star HR 8799, with projected separations of 24, 38, and 68 astronomical units. Multi-epoch data show counter clockwise orbital motion for all three imaged planets. The low luminosity of the companions and the estimated age of the system imply planetary masses between 5 and 13 times that of Jupiter. This system resembles a scaled-up version of the outer portion of our solar system.

https://science.sciencemag.org/content/sci/322/5906/1348.full.pdf

Direct imaging of multiple planets orbiting the star HR 8799.

The grand challenges of contemporary fundamental physics---dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem---all involve gravity as a key component. And of all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some of the most remarkable predictions of General Relativity: event horizons, singularities and ergoregions. The hitherto invisible landscape of the gravitational Universe is being unveiled before our eyes: the historical direct detection of gravitational waves by the LIGO-Virgo collaboration marks the dawn of a new era of scientific exploration. Gravitational-wave astronomy will allow us to test models of black hole formation, growth and evolution, as well as models of gravitational-wave generation and propagation. It will provide evidence for event horizons and ergoregions, test the theory of General Relativity itself, and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to radically reshape our understanding of the cosmos and of the laws of Nature. The purpose of this work is to present a concise, yet comprehensive overview of the state of the art in the relevant fields of research, summarize important open problems, and lay out a roadmap for future progress.

/pdf/black-holes-gravitational-waves-and-fundamental-physics-a-zducc5s9cb.pdf

Black holes, gravitational waves and fundamental physics: a roadmap.

We have conducted a high-resolution imaging study of the Taurus-Auriga star-forming region in order to characterize the primordial outcome of multiple star formation and the extent of the brown dwarf desert. Our survey identified 16 new binary companions to primary stars with masses of 0.25-2.5 M_☉, raising the total number of binary pairs (including components of high-order multiples) with separations of 3-5000 AU to 90. We find that ~2/3-3/4 of all Taurus members are multiple systems of two or more stars, while the other ~1/4-1/3 appear to have formed as single stars; the distribution of high-order multiplicity suggests that fragmentation into a wide binary has no impact on the subsequent probability that either component will fragment again. The separation distribution for solar-type stars (0.7-2.5 M_☉) is nearly log-flat over separations of 3-5000 AU, but lower-mass stars (0.25-0.7 M_☉) show a paucity of binary companions with separations of ≳200 AU. Across this full mass range, companion masses are well described with a linear-flat function; all system mass ratios (q = M_(B) /M_(A)) are equally probable, apparently including substellar companions. Our results are broadly consistent with the two expected modes of binary formation (free-fall fragmentation on large scales and disk fragmentation on small scales), but the distributions provide some clues as to the epochs at which the companions are likely to form.

https://iopscience.iop.org/article/10.1088/0004-637X/731/1/8/pdf

Mapping the shores of the brown dwarf desert. ii. multiple star formation in taurus-auriga

After initial claims and a long hiatus, it is now established that several binary stars emit high- (0.1–100 GeV) and very high-energy (>100 GeV) gamma rays. A new class has emerged called “gamma-ray binaries”, since most of their radiated power is emitted beyond 1 MeV. Accreting X-ray binaries, novae and a colliding wind binary (η Car) have also been detected—“related systems” that confirm the ubiquity of particle acceleration in astrophysical sources. Do these systems have anything in common? What drives their high-energy emission? How do the processes involved compare to those in other sources of gamma rays: pulsars, active galactic nuclei, supernova remnants? I review the wealth of observational and theoretical work that have followed these detections, with an emphasis on gamma-ray binaries. I present the current evidence that gamma-ray binaries are driven by rotation-powered pulsars. Binaries are laboratories giving access to different vantage points or physical conditions on a regular timescale as the components revolve on their orbit. I explain the basic ingredients that models of gamma-ray binaries use, the challenges that they currently face, and how they can bring insights into the physics of pulsars. I discuss how gamma-ray emission from microquasars provides a window into the connection between accretion–ejection and acceleration, while η Car and novae raise new questions on the physics of these objects—or on the theory of diffusive shock acceleration. Indeed, explaining the gamma-ray emission from binaries strains our theories of high-energy astrophysical processes, by testing them on scales and in environments that were generally not foreseen, and this is how these detections are most valuable.

Gamma-ray binaries and related systems

The grand challenges of contemporary fundamental physics-dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem-all involve gravity as a key component. And of all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some of the most remarkable predictions of General Relativity: event horizons, singularities and ergoregions. The hitherto invisible landscape of the gravitational Universe is being unveiled before our eyes: the historical direct detection of gravitational waves by the LIGO-Virgo collaboration marks the dawn of a new era of scientific exploration. Gravitational-wave astronomy will allow us to test models of black hole formation, growth and evolution, as well as models of gravitational-wave generation and propagation. It will provide evidence for event horizons and ergoregions, test the theory of General Relativity itself, and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to radically reshape our understanding of the cosmos and of the laws of Nature. The purpose of this work is to present a concise, yet comprehensive overview of the state of the art in the relevant fields of research, summarize important open problems, and lay out a roadmap for future progress. This write-up is an initiative taken within the framework of the European Action on 'Black holes, Gravitational waves and Fundamental Physics'. © 2019 IOP Publishing Ltd.

/pdf/black-holes-gravitational-waves-and-fundamental-physics-a-2xpderzol2.pdf

Black holes, gravitational waves and fundamental physics: a roadmap

Context. Very high magnetic fields at the surface of neutron stars or in the accretion disk of black holes inhibit the production of jets. Aims. We quantify here the magnetic field strength for jet formation. Methods. By using the Alfven Radius, RA, we study what we call the basic condition, RA/R∗ = 1o rRA/RLSO = 1 (LSO, last stable orbit), in its dependency on the magnetic field strength and the mass accretion rate, and we analyse these results in 3-D and 2-D plots in the case of neutron star and black hole accretor systems, respectively. For this purpose, we did a systematic search of all available observational data for magnetic field strength and the mass accretion rate. Results. The association of a classical X-ray pulsar (i.e. B ∼ 10 12 G) with jets is excluded even if accreting at the Eddington critical rate. Z-sources may develop jets for B < 10 8.2 G, whereas Atoll-sources are potential sources of jets if B < 10 7.7 G. It is not ruled out that a millisecond X-ray pulsar could develop jets, at least for those sources where B < 10 7.5 G. In this case the millisecond X-ray pulsar could switch to a microquasar phase during its maximum accretion rate. For stellar-mass black hole X-ray binaries, the condition is that B < 1.35× 10 8 Ga ndB < 5× 10 8 G at the last stable orbit for a Schwarzschild and a Kerr black hole, respectively. For active galactic nuclei (AGNs), it reaches B < 10 5.9 G for each kind of black hole. These theoretical results are in complete agreement with available observational data.

/pdf/magnetic-field-upper-limits-for-jet-formation-3h8pva6r5w.pdf

Magnetic field upper limits for jet formation

Context. LS 5039 is one of the few TeV emitting X-ray binaries detected so far. The powering source of its multiwavelength emission can be accretion in a microquasar scenario or wind interaction in a young nonaccreting pulsar scenario. 
Aims. To present new high-resolution radio images and compare them with the expected behavior in the different scenarios. 
Methods. We analyze Very Long Baseline Array (VLBA) radio observations that provide morphological and astrometric information at milliarcsecond scales. 
Results. We detect a changing morphology between two images obtained five days apart. In both runs there is a core component with a constant flux density, and an elongated emission with a position angle (PA) that changes by 12+/-3 degrees between both runs. The source is nearly symmetric in the first run and asymmetric in the second one. The astrometric results are not conclusive. 
Conclusions. A simple and shockless microquasar scenario cannot easily explain the observed changes in morphology. An interpretation within the young nonaccreting pulsar scenario requires the inclination of the binary system to be very close to the upper limit imposed by the absence of X-ray eclipses.

/pdf/the-changing-milliarcsecond-radio-morphology-of-the-gamma-28izdrprro.pdf

The changing milliarcsecond radio morphology of the gamma-ray binary LS 5039

Context. One of the most unusual aspects of the X-ray binary LS I +61◦303 is that at each orbit (P1 = 26.4960 ± 0.0028 d) one radio outburst occurs whose amplitude is modulated with Plong, a long-term period of more than 4 yr. It is still not clear whether the compact object of the system or the companion Be star is responsible for the long-term modulation. Aims. We study here the stability of Plong. Such a stability is expected if Plong is due to periodic (P2) Doppler boosting of periodic (P1) ejections from the accreting compact object of the system. On the contrary it is not expected if Plong is related to variations in the mass loss of the companion Be star. Methods. We built a database of 36.8 yr of radio observations of LS I +61◦303 covering more than 8 long-term cycles. We performed timing and correlation analysis. We also compared the results of the analyses with the theoretical predictions for a synchrotron emitting precessing (P2) jet periodically (P1) refilled with relativistic electrons. Results. In addition to the two dominant features at P1 and P2, the timing analysis gives a feature at Plong = 1628 ± 48 days. The determined value of Plong agrees with the beat of the two dominant features, i.e. Pbeat = 1/(ν1 − ν2) = 1626 ± 68 d. Lomb-Scargle results of radio data and model data compare very well. The correlation coefficient of the radio data oscillates at multiples of Pbeat, as does the correlation coefficient of the model data. Conclusions. Cycles in varying Be stars change in length and disappear after 2-3 cycles following the well-studied case of the binary system ζ Tau. On the contrary, in LS I +61◦303 the long-term period is quite stable and repeats itself over the available 8 cycles. The long-term modulation in LS I +61◦303 accurately reflects the beat of periodical Doppler boosting (induced by precession) with the periodicity of the ejecta. The peak of the long-term modulation occurs at the coincidence of the maximum number of ejected particles with the maximum Doppler boosting of their emission; this coincidence occurs every 1 ν1−ν2 and creates the long-term modulation observed in LS I +61◦303.

/pdf/origin-of-the-long-term-modulation-of-radio-emission-of-ls-i-1aezeolef3.pdf

Origin of the long-term modulation of radio emission of LS I +61°303

Context: Very high magnetic fields at the surface of neutron stars or in the accretion disk of black holes inhibit the production of jets. Aims: We quantify here the magnetic field strength for jet formation. Methods: By using the Alfven Radius, R_A, we study what we call {\it the basic condition}, R_A/R_*=1 or R_A/R_{LSO}=1 (LSO, last stable orbit), in its dependency on the magnetic field strength and the mass accretion rate, and we analyse these results in 3-D and 2-D plots in the case of neutron star and black hole accretor systems, respectively. For this purpose, we did a systematic search of all available observational data for magnetic field strength and the mass accretion rate. Results: The association of a classical X-ray pulsar (i.e. B ~10^{12} G) with jets is excluded even if accreting at the Eddington critical rate. Z-sources may develop jets for B \simlt 10^{8.2} G, whereas Atoll-sources are potential sources of jets if B \simlt 10^{7.7} G. It is not ruled out that a millisecond X-ray pulsar could develop jets, at least for those sources where B \simlt 10^{7.5} G. In this case the millisecond X-ray pulsar could switch to a microquasar phase during its maximum accretion rate. For stellar-mass black hole X-ray binaries, the condition is that B \simlt 1.35 x 10^8 G and B \simlt 5 x 10^8 G at the last stable orbit for a Schwarzschild and a Kerr black hole, respectively. For active galactic nuclei (AGNs), it reaches B \simlt 10^{5.9} G for each kind of black hole. These theoretical results are in complete agreement with available observational data.

Magnetic Field Upper Limits for Jet Formation

Context. MWC 656 is the recently discovered first binary system case composed of a Be-type star and an accreting black hole. Its low X-ray luminosity indicates that the system is in a quiescent X-ray state.Aims. The aim of our investigation is to establish if the MWC 656 system has detectable radio emission and if the radio characteristics are consistent with those of quiescent black hole systems.Methods. We used three archived VLA data sets, one hour each at 3 GHz, and seven new VLA observations, two hours each at 10 GHz, to produce very high sensitivity images down to ~1μ Jy.Results. We detected the source twice in the new observations: in the first VLA run, at apastron passage, with a flux density of 14.2 ± 2.9μ Jy and by combining all together the other six VLA runs, with a flux density of 3.7 ± 1.4μ Jy. The resulting combined map of the archived observations has the sensitivity of 1σ = 6.6 μ Jy, but no radio emission is detected there.Conclusions. The radio and X-ray luminosities agree with the behaviour of accreting binary black holes in the hard and quiescent state. In particular, MWC 656 in the L X , L R plane occupies the same region as A0620−00 and XTE J1118+480, the faintest known black holes up to now.

/pdf/radio-emission-from-the-be-black-hole-binary-mwc-656-3frfuglb5h.pdf

Radio emission from the Be/black hole binary MWC 656

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