Gamma-ray astronomy

Abstract:

We present the results of radio observations from the eMERLIN telescope combined with X-ray data from Swift for the short-duration gamma-ray burst (GRB) 200826A, located at a redshift of 0.71. The radio light curve shows evidence of a sharp rise, a peak around 4–5 d post-burst, followed by a relatively steep decline. We provide two possible interpretations based on the time at which the light curve reached its peak. (1) If the light curve peaks earlier, the peak is produced by the synchrotron self-absorption frequency moving through the radio band, resulting from the forward shock propagating into a wind medium and (2) if the light curve peaks later, the turnover in the light curve is caused by a jet break. In the former case we find a minimum equipartition energy of ∼3 × 1047 erg and bulk Lorentz factor of ∼5, while in the latter case we estimate the jet opening angle of ∼9–16°. Due to the lack of data, it is impossible to determine which is the correct interpretation, however due to its relative simplicity and consistency with other multiwavelength observations which hint at the possibility that GRB 200826A is in fact a long GRB, we prefer the scenario one over scenario two.

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.