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Laguna de Negrillos, Spain

Mezcua M.,Institute Astroflsica Of Canarias Iac | Mezcua M.,University of La Laguna | Roberts T.P.,Durham University | Sutton A.D.,Durham University | Lobanov A.P.,Max Planck Institute for Radio Astronomy
Monthly Notices of the Royal Astronomical Society | Year: 2013

The most extreme ultraluminous X-ray sources (ULXs), with LX > 5 × 1040 erg s-1, are amongst the best candidates for hosting intermediate-mass black holes (IMBHs) in the haloes of galaxies. Jet radio emission is expected from a sub-Eddington accreting IMBH in the low/hard (radio bright) state. In a search for such IMBH jet radio emission, we have observed with the Very Large Array (VLA) at 5 GHz a sample of seven extreme ULXs whose X-ray properties indicate they are in the hard state. Assuming they remain in this state, the nondetection of radio emission for six of the target sources allows us to constrain their black hole mass to the IMBH regime, thus ruling out a supermassive black hole nature. For the extreme ULX in the galaxy NGC 2276, we detect extended radio emission formed by two lobes of total flux density 1.43 ± 0.22 mJy and size ̃650 pc. The X-ray counterpart is located between the two lobes, suggesting the presence of a black hole with jet radio emission. The radio luminosity allows us to constrain the black hole mass of this source to the IMBH regime; hence, the extreme ULX in NGC 2276 could be the first detection of extended jet radio emission from an IMBH. The radio emission could also possibly come from a radio nebula powered by the ULX with a minimum total energy of 5.9 × 1052 erg, thus constituting the most powerful and largest ULX radio nebula ever observed. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

Curran P.A.,Curtin University Australia | Coriat M.,University of Cape Town | Miller-Jones J.C.A.,Curtin University Australia | Armstrong R.P.,University of Cape Town | And 18 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2014

Swift J1745-26 is an X-ray binary towards the Galactic Centre that was detected when it went into outburst in 2012 September. This source is thought to be one of a growing number of sources that display 'failed outbursts', in which the self-absorbed radio jets of the transient source are never fully quenched and the thermal emission from the geometrically thin inner accretion disc never fully dominates the X-ray flux. We present multifrequency data from the Very Large Array, Australia Telescope Compact Array and Karoo Array Telescope (KAT- 7) radio arrays, spanning the entire period of the outburst. Our rich data set exposes radio emission that displays a high level of large-scale variability compared to the X-ray emission and deviations from the standard radio-X-ray correlation that are indicative of an unstable jet and confirm the outburst's transition from the canonical hard state to an intermediate state. We also observe steepening of the spectral index and an increase of the linear polarization to a large fraction (50 per cent) of the total flux, as well as a rotation of the electric vector position angle. These are consistent with a transformation from a self-absorbed compact jet to optically thin ejecta - the first time such a discrete ejection has been observed in a failed outburst - and may imply a complex magnetic field geometry. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

Russell D.M.,Institute Astroflsica Of Canarias Iac | Russell D.M.,University of La Laguna | Russell D.M.,Abu Dhabi University | Shahbaz T.,Institute Astroflsica Of Canarias Iac | Shahbaz T.,University of La Laguna
Monthly Notices of the Royal Astronomical Society | Year: 2014

Polarization measurements of the microquasar Cygnus X-1 exist at γ-ray, X-ray, ultraviolet, optical and radio frequencies. The γ-ray emission has been shown to be highly linearly polarized. Here, we present new infrared polarimetric data of Cygnus X-1 taken with the 10.4 m Gran Telescopio Canarias and the 4.2 mWilliam Herschel Telescope.We show that the broad-band, radio-to-γ-ray flux spectrum and polarization spectrum in the hard state are largely consistent with a simple phenomenological model of a strongly polarized synchrotron jet, an unpolarized Comptonized corona and a moderately polarized interstellar dust component. In this model, the origin of the γ-ray, X-ray and some of the infrared polarization is the optically thin synchrotron power law from the inner regions of the jet. The model requires the magnetic field in this region to be highly ordered and perpendicular to the axis of the resolved radio jet. This differs from studies of some other X-ray binaries, in which the magnetic field is turbulent, variable and aligned with the jet axis. The model is able to explain the approximate polarization strength and position angle at all wavelengths including the detected X-ray (3-5 keV) polarization, except the observed position angle of the γ-ray polarization, which differs from the model by ̃60°. Past numerical modelling has shown that a curved synchrotron spectrum can produce a shift in position angle by ̃60°, which may account for this. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

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