Pulsars, transients and relativistic astrophysics

Abstract:

Binary supermassive black hole (SMBH) systems result from galaxy mergers, and will eventually coalesce due to gravitational wave (GW) emission if the binary separation can be reduced to . 0.1 pc by other mechanisms. Here, we explore a gravitational self-lensing binary SMBH model for the sharp (duration ⇠ 1 hr), quasi-regular X-ray flares – dubbed quasiperiodic eruptions – recently observed from two low mass active galactic nuclei: GSN 069 and RX J1301.9+2747. In our model, the binary is observed ⇠edge-on, such that each SMBH gravitationally lenses light from the accretion disc surrounding the other SMBH twice per orbital period. The model can reproduce the flare spacings if the current eccentricity of RX J1301.9+2747 is n0 & 0.16, implying a merger within ⇠ 1000 yrs. However, we cannot reproduce the observed flare profiles with our current calculations. Model flares with the correct amplitude are ⇠ 2/5 the observed duration, and model flares with the correct duration are ⇠ 2/5 the observed amplitude. Our modelling yields three distinct behaviours of self-lensing binary systems that can be searched for in current and future X-ray and optical time-domain surveys: i) periodic lensing flares, ii) partial eclipses (caused by occultation of the background mini-disc by the foreground mini-disc), and iii) partial eclipses with a very sharp in-eclipse lensing flare. Discovery of such features would constitute very strong evidence for the presence of a supermassive binary, and monitoring of the flare spacings will provide a measurement of periastron precession.

Abstract:

© 2021 2020 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society. B[e] supergiants are a rare and unusual class of massive and luminous stars, characterized by opaque circumstellar envelopes. GG Carinae is a binary whose primary component is a B[e] supergiant and whose variability has remained unsatisfactorily explained. Using photometric data from ASAS, OMC, and ASAS-SN and spectroscopic data from the Global Jet Watch and FEROS to study visible emission lines, we focus on the variability of the system at its ∼31-d orbital period and constrain the stellar parameters of the primary. There is one photometric minimum per orbital period and, in the emission line spectroscopy, we find a correlation between the amplitude of radial velocity variations and the initial energy of the line species. The spectral behaviour is consistent with the emission lines forming in the primary's wind, with the variable amplitudes between line species being caused by the less energetic lines forming at larger radii on average. By modelling the atmosphere of the primary, we are able to model the radial velocity variations of the wind lines in order to constrain the orbit of the binary. We find that the binary is even more eccentric than previously believed (e = 0.5 ± 0.03). Using this orbital solution, the system is brightest at periastron and dimmest at apastron, and the shape of the photometric variations at the orbital period can be well described by the variable accretion by the secondary of the primary's wind. We suggest that the evolutionary history of GG Carinae may need to be re-evaluated in a binary context.