Studies of GRB host galaxies are crucial to understanding GRBs. However, since they are identified by the superposition in the plane of the sky of a GRB afterglow and a galaxy there is always a possibility that an association represents a chance alig
nment, rather than a physical connection. We examine a uniform sample of 72 GRB fields to explore the probability of chance superpositions. There is typically a ~1% chance that an optical afterglow will coincide with a galaxy by chance. While spurious host galaxy detections will, therefore, be rare, the possibility must be considered when examining individual GRB/host galaxy examples. It is also tempting to use the large and uniform collection of X-ray afterglow positions to search for GRB-associated galaxies. However, we find that approximately half of the 14 superpositions in our sample are likely to occur by chance, so in the case of GRBs localized only by an X-ray afterglow, even statistical studies are suspect.
We present optical and near-IR (OIR) observations of the major outbursts of the neutron star soft X-ray transient binary system Aquila X-1, from summer 1998 -- fall 2007. The major outbursts of the source over the observed timespan seem to exhibit tw
o main types of light curve morphologies, (a) the classical Fast-Rise and Exponential-Decay (FRED) type outburst seen in many soft X-ray transients and (b) the Low-Intensity State (LIS) where the optical-to-soft-X-ray flux ratio is much higher than that seen during a FRED. Thus there is no single correlation between the optical (R-band) and soft X-ray (1.5-12 keV, as seen by the ASM onboard RXTE) fluxes even within the hard state for Aquila X-1, suggesting that LISs and FREDs have fundamentally different accretion flow properties. Time evolution of the OIR fluxes during the major LIS and FRED outbursts is compatible with thermal heating of the irradiated outer accretion disk. No signature of X-ray spectral state changes or any compact jet are seen in the OIR, showing that the OIR color-magnitude diagram (CMD) can be used as a diagnostic tool to separate thermal and non-thermal radiation from X-ray binaries where orbital and physical parameters of the system are reasonably well known. We suggest that the LIS may be caused by truncation of the inner disk in a relatively high mass accretion state, possibly due to matter being diverted into a weak outflow.
Radial velocity studies of accreting binary stars commonly use accretion disk emission lines to determine the radial velocity of the primary star and therefore the mass ratio. These emission line radial velocity curves are often shifted in phase from
the expected motion of the primary. These phase shifts cast doubt on the use of disk emission lines in the determination of mass ratios. We present a systematic study of phase shifts, using data from the literature to distinguish between possible explanations of the phase shift. We find that one widely adopted class of models is contradicted by observations (section 2). We present a generalized form of another class of models, which we call measurement offset models. We show that these models are quantitatively consistent with existing data (figures 2 and 3, and the discussion in section 4.4). We consider the implications of adopting measurement offset models, for both disk structure and determination of binary parameters. Specifically, we describe in section 6 how measurement offset models may be used improve determinations of the mass ratio based on disk emission lines. This could be a valuable new tool in determining masses of important astrophysical objects such as accreting neutron stars and black holes.
