Expanding hot flow in the black hole binary SWIFT J1753.5-0127: evidence from optical timing

We describe the evolution of optical and X-ray temporal characteristics during the outburst decline of the black hole X-ray binary SWIFT J1753.5-0127. The optical/ X-ray crosscorrelation function demonstrates a single positive correlation at the outburst peak, then it has multiple dips and peaks during the decline stage, which are then replaced by the precognition dip plus peak structure in the outburst tail. Power spectral densities and phase lags show a complex evolution, revealing the presence of intrinsically connected optical and X-ray quasi-periodic oscillations. For the first time, we quantitatively explain the evolution of these timing properties during the entire outburst within one model, the essence of which is the expansion of the hot accretion flow towards the tail of the outburst. The pivoting of the spectrum produced by synchrotron Comptonization in the hot flow is responsible for the appearance of the anticorrelation with the X-rays and for the optical quasi-periodic oscillations. Our model reproduces well the cross-correlation and phase lag spectrum during the decline stage, which could not be understood with any model proposed before

An apparently normal gamma-ray burst with an unusually low luminosity

Much of progress in gamma-ray bursts has come from the studies of distant events (redshift z~1). The brightest GRBs are the most collimated events and seen across the Universe due to their brilliance. It has long been suspected that nearest (and most common) events have been missed because they are not so collimated or under-energetic or both. Here we report soft gamma-ray observations of GRB 031203, the nearest event to date (z=0.106). This event with a duration of 40 s and peak energy of >190 keV appears to be a typical long duration GRB. However, the isotropic gamma-ray energy <~10^50 erg, about three orders of magnitude smaller than the cosmological population. This event as well as the other nearby but somewhat controversial event GRB 980425 are clear outliers for the much discussed isotropic-energy peak-energy relation and luminosity spectral-lag relations. Radio calorimetry shows that both these events are under-energetic explosions. We conclude that there does indeed exist a large population of under-energetic events.Comment: 11 pages, 3 figure