Hintze Centre for Astrophysical Surveys

Abstract:

We present Galaxy Evolution Explorer far-ultraviolet (FUV) and near-ultraviolet (NUV) imaging of 34 nearby early-type galaxies from the SAURON representative sample of 48 E/S0 galaxies, all of which have ground-based optical imaging from the MDM Observatory. The surface brightness profiles of nine galaxies (≈26 per cent) show regions with blue UV-optical colours suggesting RSF. Five of these (≈15 per cent) show blue integrated UV-optical colours that set them aside in the NUV integrated colour-magnitude relation. These are objects with either exceptionally intense and localized NUV fluxes or blue UV-optical colours throughout. They also have other properties confirming they have had RSF, in particular Hβ absorption higher than expected for a quiescent population and a higher CO detection rate. This suggests that residual star formation is more common in early-type galaxies than we are used to believe. NUV blue galaxies are generally drawn from the lower stellar velocity dispersion (σe < 200 km s-1) and thus lower dynamical mass part of the sample. We have also constructed the first UV Fundamental Planes and show that NUV blue galaxies bias the slopes and increase the scatters. If they are eliminated, the fits get closer to expectations from the virial theorem. Although our analysis is based on a limited sample, it seems that a dominant fraction of the tilt and scatter of the UV Fundamental Planes is due to the presence of young stars in preferentially low-mass early-type galaxies. Interestingly, the UV-optical radial colour profiles reveal a variety of behaviours, with many galaxies showing signs of RSF, a central UV-upturn phenomenon, smooth but large-scale age and metallicity gradients and in many cases a combination of these. In addition, FUV-NUV and FUV-V colours even bluer than those normally associated with UV-upturn galaxies are observed at the centre of some quiescent galaxies. Four out of the five UV-upturn galaxies are slow rotators. These objects should thus pose interesting challenges to stellar evolutionary models of the UV upturn. © 2009 RAS.

Abstract:

We study the accretionejection processes (i.e. discjet coupling) in the neutron star X-ray binary Aquila X-1 via a multiwavelength approach. We use in the radio band the publicly available Very Large Array archive containing observations of the object between 1986 and 2005, in the X-ray band the archival Rossi X-ray Timing Explorer data (Proportional Counter Array and High Energy X-ray Timing Experiment) between 1997 and 2008, and in optical (R band) observations with the Small and Moderate Aperture Research Telescope System recorded between 1998 and 2007. In the combined data set, we find three outbursts for which quasi-simultaneous radio, optical (R band) and X-ray data exist and focus on them to some extent. We provide evidence that the discjet coupling in Aquila X-1 is similar to what has been observed in black hole X-ray binaries, at least from the point of view of the behaviour in the hardness-intensity diagrams (the hysteresis effect included), when the phenomenology of the jet is taken into account. Although based on a very small number of observations, a radioX-ray correlation seems to exist for this system, with a slope of α = 0.40 ± 0.07 (Fradio ∝ F αX), which is different than the slope of α = 1.40 ± 0.25 found for another atoll source, 4U 1728-34, but interestingly enough is relatively close to the values obtained for several black hole X-ray binaries. No significant correlation is found between the radio and optical (R-band) emissions. We also report a significant drop in the radio flux from Aquila X-1 above an X-ray flux of ∼5 × 10-9 erg cm -2 s-1. This behaviour, also reported in the neutron star X-ray binary 4U 1728-34, may be analogous to the suppression of radio emission in black hole X-ray binaries in bright, soft X-ray states. It suggests that from this point of view neutron star X-ray binaries can mimic the behaviour of black hole X-ray binaries in suppressing the jet in softdisc-dominated X-ray states. © 2009 RAS.

Abstract:

We present a multiwavelength study of the black hole X-ray binary V404 Cyg in quiescence, focusing upon the spectral energy distribution (SED). Radio, optical, ultraviolet (UV) and X-ray coverage is simultaneous. We supplement the SED with additional non-simultaneous data in the optical through infrared where necessary. The compiled SED is the most complete available for this, the X-ray and radio brightest quiescent black hole system. We find no need for a substantial contribution from accretion light from the near-UV to the near-IR, and in particular the weak UV emission constrains published spectral models for V404 Cyg. We confirm that no plausible companion spectrum and interstellar extinction can fully explain the mid-IR, however, and an infrared (IR) excess from a jet or cool disc appears to be required. The X-ray spectrum is consistent with a F ∼ 2 power law as found by all other studies to date. There is no evidence for any variation in the hardness over a range of a factor of 10 in luminosity. The radio flux is consistent with a flat spectrum (in f ν). The break frequency between a flat and optically thin spectrum most likely occurs in the mid or far-IR, but is not strongly constrained by these data. We find the radio to be substantially variable but with no clear correlation with X-ray variability. © 2009 RAS.

Abstract:

We propose a physically motivated and self-consistent prescription for the modeling of transient neutron star low-mass X-ray binary (LMXB) properties, such as duty cycle (DC), outburst duration, and recurrence time. We apply this prescription to the population synthesis models of field LMXBs presented by Fragos etal., and compare the transient LMXB population to the Chandra X-ray survey of the two elliptical galaxies NGC3379 and NGC4278, which revealed several transient sources. We are able to exclude models with a constant DC for all transient systems, while models with a variable DC based on the properties of each system are consistent with the observed transient populations. We predict that the majority of the observed transient sources in these two galaxies are LMXBs with red giant donors. Finally, our comparison suggests that transient LMXBs are very rare in globular clusters (GCs), and thus the number of identified transient LMXBs may be used as a tracer of the relative contribution of field and GC LMXB populations. © 2009. The American Astronomical Society.