Gamma-ray astronomy

Abstract:

We present a 30-day monitoring campaign of the optical counterpart of the bright X-ray transient Swift J1745-26, starting only 19min after the discovery of the source.We observe the system peaking at i~17.6 on day six (MJD 561 92) to then decay at a rate of ~0.04 mag d-1. We show that the optical peak occurs at least 3 d later than the hard X-ray (15-50 keV) flux peak. Our measurements result in an outburst amplitude greater than 4.3 mag, which favours an orbital period ≲21 h and a companion star with a spectral type later than ~A0. Spectroscopic observations taken with the Gran Telescopio de Canarias 10.4 m telescope reveal a broad (full width at half-maximum ~1100 km s-1), double-peaked Ha emission line from which we constrain the radial velocity semi-amplitude of the donor to be K2 > 250 km s-1. The breadth of the line and the observed optical and X-ray fluxes suggest that Swift J1745-26 is a new black hole candidate located closer than ~7 kpc. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Abstract:

The black hole candidate, XTE J1752-223, was discovered in 2009 October when it entered an outburst. We obtained radio data from the Australia Telescope Compact Array for the duration of the ~9 month event. The light curves show that the radio emission from the compact jet persisted for the duration of an extended hard state and through the transition to the intermediate state. The flux then rose rapidly by a factor of 10 and the radio source entered a series of at least seven maxima, the first of which was likely to be emission associated with the compact jet. The subsequent six flares were accompanied by variable behaviour in terms of radio spectrum, degree of linear polarization, morphology and associated X-ray behaviour. They were, however, remarkably similar in terms of the estimated minimum power required to launch such an ejection event. We compare the timing of radio peaks with the location of the ejecta, imaged by contemporaneous Very Long Baseline Interferometry experiments. We then discuss the mechanism behind the events, in terms of whether discrete ejections are the most likely description of the behaviour. One ejection, at least, appears to be travelling with apparent superluminal motion. The range of properties, however, suggests that multiple mechanisms may be relevant and that at least some of the emission is coming from shocked interactions amongst the ejecta and between the ejecta and the interstellar medium. We also compare the radio flux density with the X-ray source during the hard state and conclude that XTE J1752-223 is a radio-weak/X-ray-bright outlier on the universal correlation for black hole transient sources. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.