Gamma-ray astronomy

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 present preliminary results on a global study of X-ray binaries using 14 Ms of data from the Rossi X-ray Timing Explorer RXTE satellite. Our initial study on GX 339-4 is recapped as an introduction to the methods used. We use a consistent analysis scheme for all objects, with three different spectral models to fit the powerlaw and disc components. We also take into account the possibility of a line being present in the data. The resulting almost 4000 observations allow the tracking of the spectral properties of the binaries as they evolve through an outburst. Our investigations concentrate on the disc and line properties of the binaries when in outburst. We also show the Disc-Fraction Luminosity diagram for the population of X-ray binaries studied which will enable us to further links with AGN. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.

Abstract:

Compact object displays drastic spectral and timing changing from the beginning to the end of an outburst, showing the different efficiencies of accretion processes. Black hole binaries hence exhibit schematically two different states in X-ray spectra: The first dominated by a thermal component and the second by a hard powerlaw shape like. Whereas the hard component is often attributed to the emission of a radiatively inefficient corona, the thermal component is interpreted as the emission of the optically thick accretion disc. The commonly accepted picture suggests that the observed transition between hard and soft states is associated by a drop in the accretion efficiency of the thermal component by a recession of the internal disc radius in hard states. However, recent studies based on relativistically broadened iron line and the thermal component strength analysis would tend to show the presence of the disc in the vicinity of the horizon. By a reanalysis of archive spectra where thermal emission is present, we tracked the values of the disc radii during outbursts among several sources. Indeed, whereas a constant inner radius would imply that the disc luminosity should monotonically depends on the temperature, we show that this relationship seems to deviate at the lowest luminosities. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.

Abstract:

High energy astrophysics has made good use of combined high energy (X-ray, g-ray) and radio observations to uncover connections between outbursts, accretion, particle acceleration and kinetic feedback to the local ambient medium. In the field of microquasars the connections have been particularly important. However, radio astronomy has been relying on essentially the same facilities for the past ∼ 25 years, whereas high-energy astrophysics, in particular space-based research, has had a series of newer and more powerful missions. In the next fifteen years this imbalance is set to be redressed, with a whole familiy of new radio facilities under development en route to the Square Kilometre Array (SKA) in the 2020s. In this brief review I will summarize these future prospects for radio astronomy, and focus on possibly the most exciting of the new facilities to be built in the next decade, the Low Frequency Array LOFAR, and its uses in high energy astrophysics. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial- ShareAlike Licence.