| 1. |
- Balucinska-Church, M., et al.
(författare)
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Dipping in Cygnus X-2 in a multi-wavelength campaign due to absorption of extended ADC emission
- 2011
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 530
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Tidskriftsartikel (refereegranskat)abstract
- We report results of one-day simultaneous multiwavelength observations of Cygnus X-2 using XMM, Chandra, the European VLBI Network and the XMM Optical Monitor. During the observations, the source did not exhibit Z-track movement, but remained in the vicinity of the soft apex. It was in a radio quiescent/quiet state of 60 degrees these structures can still cover large parts of the extended ADC, without absorbing emission from the central neutral star.
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| 2. |
- Batejat, Fabien, 1979, et al.
(författare)
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Rapid variability of the compact radio sources in Arp220 Evidence for a population of microblazars?
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 542
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Tidskriftsartikel (refereegranskat)abstract
- Context. The two nuclei of the starburst galaxy Arp220 contain multiple compact radio sources previously identified as radio supernovae or supernova remnants. Aims. In order to search for an embedded radio AGN, or other possible exotic objects, we have carried out a programme of VLBI monitoring at 6 cm over three epochs each separated by four months. Methods. Combining the new data with existing data at 6 cm and 18 cm (spanning 4 and 12 years respectively) we are able to characterise source flux density variability on a range of timescales. Additionally we analyse the variability of sources in shape and position. Results. We detect rapid (<4 months) variability in three sources (W7, W26, and W29). These sources show possible superluminal motion (>4c) of jet-like features near rapidly varying almost stationary components. These enigmatic sources might be associated with an AGN or a highly beamed microquasar (i.e. microblazar). Other hypotheses include that the apparent variability is intrinsic and is produced by neutron star powered central components within a supernova remnant, by a sequence of several supernovae within super star clusters, or is extrinsic and is produced by Galactic interstellar scintillation of very compact non-varying objects. Conclusions. A microquasar/microblazar origin seems to be the best explanation for the nature of the variable sources in Arp220.
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| 3. |
- Rushton, Anthony, 1983, et al.
(författare)
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A weak compact jet in a soft state of Cygnus X-1
- 2012
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 419:4, s. 3194-3199
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Tidskriftsartikel (refereegranskat)abstract
- We present evidence for the presence of a weak compact jet during a soft X-ray state of Cygnus X-1. Very-high-resolution radio observations were taken with the VLBA, EVN and MERLIN during a hard-to-soft spectral state change, showing the hard state jet to be suppressed by a factor of about 35 in radio flux and unresolved to direct imaging observations (i.e. ?1 mas at 4 cm). High time-resolution X-ray observations with the RXTE-PCA were also taken during the radio monitoring period, showing the source to make the transition from the hard state to a softer state (via an intermediate state), although the source may never have reached the canonical soft state. Using astrometric very long baseline interferometry (VLBI) analysis and removing proper motion, parallax and orbital motion signatures, the residual positions show a scatter of similar to 0.2 mas (at 4 cm) and similar to 3 mas (at 13 cm) along the position angle of the known jet axis; these residuals suggest that there is a weak unresolved outflow, with varying size or opacity, during intermediate and soft X-ray states. Furthermore, no evidence was found for extended knots or shocks forming within the jet during the state transition, suggesting that the change in outflow rate may not be sufficiently high to produce superluminal knots.
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| 4. |
- Rushton, Anthony, 1983, et al.
(författare)
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Steady jets from radiatively efficient hard states in GRS 1915+105
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 524
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Tidskriftsartikel (refereegranskat)abstract
- Recent studies of different X-ray binaries (XRBs) have shown a clear correlation between the radio and X-ray emission. We present evidence of a close relationship found between the radio and X-ray emission at different epochs for GRS 1915+105, using observations from the Ryle Telescope and Rossi X-ray Timing Explorer satellite. The strongest correlation was found during the hard state (also known as the "plateau" state), where a steady AU-scale jet is known to exist. Both the radio and X-ray emission were found to decay from the start of most plateau states, with the radio emission decaying faster. An empirical relationship of S radio. S. X-ray was then fitted to data taken only during the plateau state, resulting in a power-law index of xi similar to 1.7 +/- 0.3, which is significantly higher than in other black hole XRBs in a similar state. An advection-flow model was then fitted to this relationship and compared to the universal XRB relationship as described by Gallo et al. (2003, MNRAS, 344, 60). We conclude that either (I) the accretion disk in this source is radiatively efficient, even during the continuous outflow of a compact jet, which could also suggest a universal turn-over from radiatively inefficient to efficient for all stellar-mass black holes at a critical mass accretion rate (m(c) approximate to 10(18.5) g/s); or (II) the X-rays in the plateau state are dominated by emission from the base of the jet and not the accretion disk (e. g. via inverse Compton scattering from the outflow).
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| 5. |
- Rushton, Anthony, 1983, et al.
(författare)
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VLBI constraints on the “Jet-line” of Cygnus X-1
- 2010
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Ingår i: Proceedings of Science. - 1824-8039. ; 125
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Konferensbidrag (refereegranskat)abstract
- Results are presented from recent VLBI observations of Cygnus X-1 during X-ray spectral state changes. Using the EVN in e-VLBI mode and the VLBA with disk recording, we observed the X-ray binary at very high angular resolution and studied changes in the compact jets as the source made transitions from hard X-ray states to softer states. The radio light curves show that these transitions were accompanied by radio flaring events followed by a quenching of the radio emission, as expected from the current paradigm for disc-jet coupling in X-ray binaries. While we see structural changes in the compact jets during these transitions, there was no evidence for the expected ejection of bright, relativistically-moving jet knots. However, we find strong evidence that the jet does not switch off completely in the soft X-ray state of Cygnus X-1, such that a weak, compact jet persists during this phase of radio quenching.
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