On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ∼ 1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40 − 8 + 8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 M ⊙ . An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ∼ 40 Mpc ) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ∼ 9 and ∼ 16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.

The optical/near‐infrared (OIR) region of the spectra of low‐mass X‐ray binaries (XBs) appears to lie at the intersection of a variety of different emission processes. In this paper we present quasi‐simultaneous OIR–X‐ray observations of 33 XBs in an attempt to estimate the contributions of various emission processes in these sources, as a function of X‐ray state and luminosity. A global correlation is found between OIR and X‐ray luminosity for low‐mass black hole candidate XBs (BHXBs) in the hard X‐ray state, of the form LOIR∝L0.6X. This correlation holds over eight orders of magnitude in LX and includes data from BHXBs in quiescence and at large distances [Large Magellanic Cloud (LMC) and M31]. A similar correlation is found in low‐mass neutron star XBs (NSXBs) in the hard state. For BHXBs in the soft state, all the near‐infrared (NIR) and some of the optical emissions are suppressed below the correlation, a behaviour indicative of the jet switching off/on in transition to/from the soft state. We compare these relations to theoretical models of a number of emission processes. We find that X‐ray reprocessing in the disc and emission from the jets both predict a slope close to 0.6 for BHXBs, and both contribute to the OIR in BHXBs in the hard state, the jets producing ∼90 per cent of the NIR emission at high luminosities. X‐ray reprocessing dominates the OIR in NSXBs in the hard state, with possible contributions from the jets (only at high luminosity) and the viscously heated disc. We also show that the optically thick jet spectrum of BHXBs extends to near the K band. OIR spectral energy distributions of 15 BHXBs help us to confirm these interpretations. We present a prediction of the LOIR–LX behaviour of a BHXB outburst that enters the soft state, where the peak LOIR in the hard state rise is greater than in the hard state decline (the well‐known hysteretical behaviour). In addition, it is possible to estimate the X‐ray, OIR and radio luminosity and the mass accretion rate in the hard state quasi‐simultaneously, from observations of just one of these wavebands, since they are all linked through correlations. Finally, we have discovered that the nature of the compact object, the mass of the companion and the distance/reddening can be constrained by quasi‐simultaneous OIR and X‐ray luminosities.

White dwarfs are compact stars, similar in size to Earth but approximately 200,000 times more massive. Isolated white dwarfs emit most of their power from ultraviolet to near-infrared wavelengths, but when in close orbits with less dense stars, white dwarfs can strip material from their companions and the resulting mass transfer can generate atomic line and X-ray emission, as well as near- and mid-infrared radiation if the white dwarf is magnetic. However, even in binaries, white dwarfs are rarely detected at far-infrared or radio frequencies. Here we report the discovery of a white dwarf/cool star binary that emits from X-ray to radio wavelengths. The star, AR Scorpii (henceforth AR Sco), was classified in the early 1970s as a δ-Scuti star, a common variety of periodic variable star. Our observations reveal instead a 3.56-hour period close binary, pulsing in brightness on a period of 1.97 minutes. The pulses are so intense that AR Sco's optical flux can increase by a factor of four within 30 seconds, and they are also detectable at radio frequencies. They reflect the spin of a magnetic white dwarf, which we find to be slowing down on a 10-year timescale. The spin-down power is an order of magnitude larger than that seen in electromagnetic radiation, which, together with an absence of obvious signs of accretion, suggests that AR Sco is primarily spin-powered. Although the pulsations are driven by the white dwarf's spin, they mainly originate from the cool star. AR Sco's broadband spectrum is characteristic of synchrotron radiation, requiring relativistic electrons. These must either originate from near the white dwarf or be generated in situ at the M star through direct interaction with the white dwarf's magnetosphere.

The binary neutron star merger event GW170817 was detected through both electromagnetic radiation and gravitational waves. Its afterglow emission may have been produced by either a narrow relativistic jet or an isotropic outflow. High-spatial-resolution measurements of the source size and displacement can discriminate between these scenarios. We present very-long-baseline interferometry observations, performed 207.4 days after the merger by using a global network of 32 radio telescopes. The apparent source size is constrained to be smaller than 2.5 milli–arc seconds at the 90% confidence level. This excludes the isotropic outflow scenario, which would have produced a larger apparent size, indicating that GW170817 produced a structured relativistic jet. Our rate calculations show that at least 10% of neutron star mergers produce such a jet.

No abstract

We investigated the reported distances of Galactic black hole (BH) and neutron star low-mass X-ray binaries (LMXBs). Comparing the distances derived for the neutron stars Cyg X-2 and XTE J2123-058 using the observed Eddington limited photospheric radius expansion bursts with the distances derived using the observed radius and effective temperature of the companion star, we find that the latter are smaller by approximately a factor of 1.5-2. The latter method is often employed to determine the distance to BH LMXBs. A possible explanation for this discrepancy is that the stellar absorption lines in fast rotating companion stars are different from those in the slowly rotating template stars as was found before for early-type stars. This could lead to a systematic mis-classification of the spectral type of the companion star, which in turn would yield a systematic error in the distance. Further, we derive a distance of 4.0 +2.0 −1.2 kpc for V404 Cyg, using parameters available in the literature. The interstellar extinction seems to have been overestimated for XTE J1550-564 and possibly for two other BH sources (H 1705-25 and GS 2000+25) as well. As a result of this, the distance to XTE J1550-564 may have been underestimated by as much as a factor three. We find that, using the new distances for XTE J1550-564 and V404 Cyg, the maximum outburst luminosity for at least five, but perhaps even seven, of the 15 BH soft X-ray transients exceed the Eddington luminosity for a 10-M BH -showing that these systems would be classified as ultra-luminous X-ray sources had we observed them in other Galaxies. This renders support for the idea that many ultra-luminous X-ray sources are stellar-mass rather than intermediate-mass BHs. We find that the rms-value of the distance to the Galactic plane for BHs is consistent with that of neutron star LMXBs. This suggests that BHs could also receive a kick-velocity during their formation, although this has to be investigated in more detail. We find that the Galactic neutron star and BH l-and b-distributions are consistent with being the same. The neutron star and BH distribution is asymmetric in l with an excess of systems between −30 • < l < 0 • over systems with 0 • < l < 30 • .

Deep observations with the Very Large Array of A0620–00, performed in 2005 August, resulted in the first detection of radio emission from a black hole binary at X‐ray luminosities as low as 10−8.5 times the Eddington limit. The measured radio flux density, of 51 ± 7 μJy at 8.5 GHz, is the lowest reported for an X‐ray binary system so far, and is interpreted in terms of partially self‐absorbed synchrotron emission from outflowing plasma. Making use of the estimated outer accretion rate of A0620−00 in quiescence, we demonstrate that the outflow kinetic power must be energetically comparable to the total accretion power associated with such rate, if it was to reach the black hole with the standard radiative efficiency of 10 per cent. This favours a model for quiescence in which a radiatively inefficient outflow accounts for a sizable fraction of the missing energy, and, in turn, substantially affects the overall dynamics of the accretion flow. Simultaneous observations in the X‐ray band, with Chandra, confirm the validity of a non‐linear radio/X‐ray correlation for hard state black hole binaries down to low quiescent luminosities, thereby contradicting some theoretical expectations. Taking the mass term into account, the A0620−00 data lie on the extrapolation of the so‐called Fundamental Plane of black hole activity, which has thus been extended by more than two orders of magnitude in radio and X‐ray luminosity. With the addition of the A0620−00 point, the plane relation provides an empirical proof for the scale invariance of the jet‐accretion coupling in accreting black holes over the entire parameter space observable with current instrumentation.

We demonstrate that at relatively low mass accretion rates, black hole candidate (BHC) X-ray binaries (XRBs) should enter 'jet-dominated' states, in which the majority of the liberated accretion power is in the form of a (radiatively inefficient) jet and not dissipated as X-rays in the accretion flow. This result follows from the empirically established non-linear relation between radio and X-ray power from low/hard state BHC XRBs, which we assume also to hold for neutron star (NS) XRBs. Conservative estimates of the jet power indicate that all BHC XRBs in 'quiescence' should be in this jet-dominated regime. In combination with an additional empirical result, namely that BHC XRBs are more 'radio-loud' than NS XRBs, we find that in quiescence NS XRBs should be up to two orders of magnitude more luminous in X-rays than BHC XRBs, without requiring any significant advection of energy into a black hole. This ratio is as observed, and such observations should therefore no longer be considered as direct evidence for the existence of black hole event horizons. Furthermore, even if BHCs do contain black holes with event horizons, this work demonstrates that there is no requirement for the advection of significant amounts of accretion energy across the horizon.