Professor Rob Fender

Abstract:

Relativistic plasma jets are observed in many systems that host accreting black holes. According to theory, coiled magnetic fields close to the black hole accelerate and collimate the plasma, leading to a jet being launched 1-3 . Isolating emission from this acceleration and collimation zone is key to measuring its size and understanding jet formation physics. But this is challenging because emission from the jet base cannot easily be disentangled from other accreting components. Here, we show that rapid optical flux variations from an accreting Galactic black-hole binary are delayed with respect to X-rays radiated from close to the black hole by about 0.1 seconds, and that this delayed signal appears together with a brightening radio jet. The origin of these subsecond optical variations has hitherto been controversial 4-8 . Not only does our work strongly support a jet origin for the optical variations but it also sets a characteristic elevation of 10 3 Schwarzschild radii for the main inner optical emission zone above the black hole 9 , constraining both internal shock 10 and magnetohydrodynamic 11 models. Similarities with blazars 12,13 suggest that jet structure and launching physics could potentially be unified under mass-invariant models. Two of the best-studied jetted black-hole binaries show very similar optical lags 8,14,15 , so this size scale may be a defining feature of such systems.

Abstract:

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 ∼1.7s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg(2) 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 ∼40Mpc) 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.