Research Center for Atomic Physics and Astrophysics

RO, Romania

Research Center for Atomic Physics and Astrophysics

RO, Romania
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Russell D.M.,University of Amsterdam | Lewis F.,University of Cardiff | Lewis F.,Open University Milton Keynes | Lewis F.,University of South Wales | And 10 more authors.
Proceedings of the International Astronomical Union | Year: 2010

Most accretion-powered relativistic jet sources in our Galaxy are transient X-ray binaries (XBs). Efforts to coordinate multiwavelength observations of these objects have improved dramatically over the last decade. Now the challenge is to interpret broadband spectral energy distributions (SEDs) of XBs that are well sampled in both wavelength and time. Here we focus on the evolution of the jet in their broadband spectra. Some of the most densely sampled broadband SEDs of a neutron star transient (IGR J00291+5934) are used to constrain the optically thick-thin break in the jet spectrum. For the black hole transient XTE J1550-564, infrared - X-ray correlations, evolution of broadband spectra and timing signatures indicate that synchrotron emission from the jet likely dominates the X-ray power law at low luminosities (∼ (2 × 10 -4 - 2 × 10-3 ) LEdd ) during the hard state outburst decline. © International Astronomical Union 2011.

McKean J.P.,ASTRON | Berciano Alba A.,ASTRON | Volino F.,Argelander Institute For Astronomie | Tudose V.,ASTRON | And 9 more authors.
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2011

The bright submillimetre (sub-mm) galaxy MM18423+5938 at redshift 3.9296 has been predicted from mid-infrared (MIR) and millimetre photometry to have an exceptionally large total IR luminosity. We present new radio imaging at 1.4 GHz with the Westerbork Synthesis Radio Telescope that is used to determine a radio-derived total IR luminosity for MM18423+5938 via the well-established radio-far-IR correlation. The flux density is found to be S 1.4GHz = 217 ± 37 μJy, which corresponds to a rest-frame luminosity density of L 1.4GHz = 2.32 ±0.40×10 25 μ -1 W Hz -1, where μ is the magnification from a probable gravitational lens. The radio-derived total IR luminosity and star formation rate are L 8-1000 μm = 5.6 +4.1 -2.4 × 10 13 μ -1 L⊙ and SFR = 9.4 +7.4 -4.9 × 10 3 μ -1M⊙ yr -1, respectively, which are ~9 times smaller than those previously reported. These differences are attributed to the IR spectral energy distribution of MM18423+5938 being poorly constrained by the limited number of reliable photometric data that are currently available, and from a previous misidentification of the object at 70 μm. Using the radio derived total IR luminosity as a constraint, the temperature of the cold dust component is found to be T d ~ 24 +7 -5 K for a dust emissivity of β = 1.5 ± 0.5. The radio-derived properties of this galaxy are still large given the low excitation temperature implied by the CO emission lines and the temperature of the cold dust. Therefore, we conclude that MM18423+5938 is probably gravitationally lensed. © 2011 The Authors Monthly Notices of the Royal Astronomical Society. © 2011 RAS.

Miller-Jones J.C.A.,NRAO Headquarters | Sivakoff G.R.,University of Virginia | Altamirano D.,University of Amsterdam | Tudose V.,Netherlands Institute for Radio Astronomy | And 24 more authors.
Astrophysical Journal Letters | Year: 2010

The 2009 November outburst of the neutron star X-ray binary Aquila X-1 (Aql X-1) was observed with unprecedented radio coverage and simultaneous pointed X-ray observations, tracing the radio emission around the full X-ray hysteresis loop of the outburst for the first time. We use these data to discuss the disk-jet coupling, finding the radio emission to be consistent with being triggered at state transitions, both from the hard to the soft spectral state and vice versa. Our data appear to confirm previous suggestions of radio quenching in the soft state above a threshold X-ray luminosity of ∼10% of the Eddington luminosity. We also present the first detections of Aql X-1 with very long baseline interferometry, showing that any extended emission is relatively diffuse and consistent with steady jets rather than arising from discrete, compact knots. In all cases where multi-frequency data were available, the source radio spectrum is consistent with being flat or slightly inverted, suggesting that the internal shock mechanism that is believed to produce optically thin transient radio ejecta in black hole X-ray binaries is not active in Aql X-1. © 2010. The American Astronomical Society. All rights reserved..

Tudose V.,Netherlands Institute for Radio Astronomy | Tudose V.,University of Amsterdam | Tudose V.,Astronomical Institute of the Romanian Academy | Tudose V.,Research Center for Atomic Physics and Astrophysics | And 15 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2010

In order to test the recently proposed classification of the radio/X-ray states of the X-ray binary Cygnus X-3 (Cyg X-3), we present an analysis of the radio data available for the system at much higher spatial resolutions than used for defining the states. The radio data set consists of archival Very Long Baseline Array data at 5 or 15 GHz and new electronic European Very Long Baseline Interferometry Network data at 5 GHz. We also present 5-GHz Multi-Element Radio Linked Interferometer Network observations of an outburst of Cyg X-3. In the X-ray regime, we use quasi-simultaneous with radio, monitoring and pointed Rossi X-ray Timing Explorer observations. We find that when the radio emission from both jet and core is globally considered, the behaviour of Cyg X-3 at mas scales is consistent with that described at arcsec-scales. However, when the radio emission is disentangled in a core component and a jet component, the situation changes. It becomes clear that in active states the radio emission from the jet is dominating that from the core. This shows that in these states the overall radio flux cannot be used as a direct tracer of the accretion state. © 2009 RAS.

Lewis F.,University of Cardiff | Lewis F.,Open University Milton Keynes | Lewis F.,University of South Wales | Russell D.M.,University of Amsterdam | And 31 more authors.
Astronomy and Astrophysics | Year: 2010

Context. In August 2008, the accreting milli-second X-ray pulsar (AMXP), IGR J00291+5934, underwent an outburst lasting ∼100 days, the first since its discovery in 2004. Aims. We present data from the 2008 double-peaked outburst of IGR J00291+5934 from Faulkes Telescope North, the Isaac Newton Telescope, the Keck Telescope, PAIRITEL, the Westerbork Synthesis Radio Telescope and the Swift, XMM-Newton and RXTE X-ray missions. We study the outburst's evolution at various wavelengths, allowing us to probe accretion physics in this AMXP. Methods. We study the light curve morphology, presenting the first radio-X-ray spectral energy Distributions (SEDs) for this source and the most detailed UV-IR SEDs for any outbursting AMXP. We show simple models that attempt to identify the emission mechanisms responsible for the SEDs. We analyse short-timescale optical variability, and compare a medium resolution optical spectrum with those from 2004. Results. The outburst morphology is unusual for an AMXP, comprising two peaks, the second containing a "plateau" of ∼10 days at maximum brightness within 30 days of the initial activity. This has implications on duty cycles of short-period X-ray transients. The X-ray spectrum can be fitted by a single, hard power-law. We detect optical variability of ∼0.05 mag, on timescales of minutes, but find no periodic modulation. In the optical, the SEDs contain a blue component, indicative of an irradiated disc, and a transient near-infrared (NIR) excess. This excess is consistent with a simple model of an optically thick synchrotron jet (as seen in other outbursting AMXPs), however we discuss other potential origins. The optical spectrum shows a double-peaked Hα profile, a diagnostic of an accretion disc, but we do not clearly see other lines (e.g. He I, II) that were reported in 2004. Conclusions. Optical/IR observations of AMXPs appear to be excellent for studying the evolution of both the outer accretion disc and the inner jet, and may eventually provide us with tight constraints to model disc-jet coupling in accreting neutron stars. © 2010 ESO.

Sell P.H.,University of Wisconsin - Madison | Heinz S.,University of Wisconsin - Madison | Calvelo D.E.,University of Southampton | Tudose V.,Netherlands Institute for Radio Astronomy | And 13 more authors.
Astrophysical Journal Letters | Year: 2010

We report the discovery of multi-scale X-ray jets from the accreting neutron star X-ray binary, Circinus X-1. The bipolar outflows show wide opening angles and are spatially coincident with the radio jets seen in new highresolution radio images of the region. The morphology of the emission regions suggests that the jets from Circinus X-1 are running into a terminal shock with the interstellar medium, as is seen in powerful radio galaxies. This and other observations indicate that the jets have a wide opening angle, suggesting that the jets are either not very well collimated or precessing. We interpret the spectra from the shocks as cooled synchrotron emission and derive a cooling age of ∼1600 yr. This allows us to constrain the jet power to be 3×1035 erg s-1 ≲ Pjet ≲ 2×1037 erg s-1, making this one of a few microquasars with a direct measurement of its jet power and the only known microquasar that exhibits stationary large-scale X-ray emission. © 2010. The American Astronomical Society. All rights reserved.

Soleri P.,University of Amsterdam | Soleri P.,University of Groningen | Fender R.,University of Amsterdam | Fender R.,University of Southampton | And 21 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2010

In studies of accreting black holes in binary systems, empirical relations have been proposed to quantify the coupling between accretion processes and ejection mechanisms. These processes are probed, respectively, by means of X-ray and radio/optical-infrared observations. The relations predict, given certain accretion conditions, the expected energy output in the form of a jet. We investigated this coupling by studying the black hole candidate Swift J. 1753.5-0127, via multiwavelength-coordinated observations over a period of ∼4 yr. We present the results of our campaign showing that, all along the outburst, the source features a jet that is fainter than expected from the empirical correlation between the radio and the X-ray luminosities in a hard spectral state. Because the jet is so weak in this system the near-infrared emission is, unusually for this state and luminosity, dominated by thermal emission from the accretion disc. We briefly discuss the importance and the implications of a precise determination of both the slope and the normalization of the correlations, listing some possible parameters that broad-band jet models should take into account to explain the population of sources characterized by a dim jet. We also investigate whether our data can give any hint on the nature of the compact object in the system, since its mass has not been dynamically measured. © 2010 The Authors. Journal compilation © 2010 RAS.

Jones S.,University of Southampton | Mchardy I.,University of Southampton | Moss D.,University of Southampton | Seymour N.,University of Southampton | And 8 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2011

We present intensive quasi-simultaneous X-ray and radio monitoring of the narrow line Seyfert 1 galaxy NGC 4051, over a 16-month period in 2000-01. The X-ray observations were made with the Proportional Counter Array on the Rossi Timing X-ray Explorer (RXTE) and radio observations were made at 8.4 and 4.8 GHz with the Very Large Array (VLA). In the X-ray band NGC 4051 behaves very much like the analogue of a Galactic black hole binary (GBH) system in a 'soft state'. In such systems, there has so far been no firm evidence for an active, radio-emitting jet like those found in 'hard-state' GBHs. VLBI observations of NGC 4051 show three well-separated compact components almost in a line. This structure resembles the core and outer hot spots seen in powerful, jet-dominated, extragalactic radio sources and, although no jet is visible in NGC 4051, suggests that a weak jet may exist. However it has not previously been clear whether the nucleus is currently active in the radio band and whether there is any link between the radio and X-ray emission processes. Radio monitoring of the core of NGC 4051 is complicated by the presence of surrounding extended emission and by the changing array configurations of the VLA. Only in the A configuration is the core reasonably resolved. We have carefully removed the differing contaminations of the core by extended emission in the various arrays. The resulting lightcurve shows no sign of large amplitude variability (i.e. factor 50 per cent) over the 16-month period and is consistent with being constant. Within the 6 A configuration observations where we have greatest sensitivity we see marginal evidence for radio core variability of ∼25 per cent (∼0.12 mJy at 8.4 GHz) on a 2-week time-scale, correlated with X-ray variations. These percentage variations are similar to those of the Seyfert galaxy NGC 5548, which is 10 times brighter. Even if the radio variations in NGC 4051 are real, the percentage variability is much less than in the X-ray band. Within the B configuration observations, although sensitivity is somewhat reduced, there is no sign of correlated X-ray/radio variability. NGC 4051 is one decade lower in radio luminosity than the radio/X-ray/mass fundamental plane for jet-dominated hard-state black holes, although the scatter around the plane is of the same order. The lack of radio variability commonly seen in hard-state GBHs may be explained by orientation effects. Another possibility, consistent with the lack of radio variability, is that the radio emission arises from the X-ray corona although, in that case, the linear structure of the compact radio components is hard to explain. A combination of corona and jet may explain the observations. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Rushton A.,European Southern Observatory | Rushton A.,Onsala Space Observatory | Miller-Jones J.C.A.,Curtin University Australia | Campana R.,Istituto di Astrofisica Spaziale e Fisica Cosmica | And 11 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

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 3-5 in radio flux and unresolved to direct imaging observations (i.e. ≲1 mas at 4cm). 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 ∼0.2 mas (at 4cm) and ∼3 mas (at 13cm) 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. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Marti-Vidal I.,Max Planck Institute for Radio Astronomy | Tudose V.,Netherlands Institute for Radio Astronomy | Tudose V.,Astronomical Institute of the Romanian Academy | Tudose V.,Research Center for Atomic Physics and Astrophysics | And 14 more authors.
Astronomy and Astrophysics | Year: 2011

We report on the VLBI detection of supernova SN2011dh at 22 GHz using a subset of the EVN array. The observations took place 14 days after the discovery of the supernova, thus resulting in a VLBI image of the youngest radio-loud supernova ever. We provide revised coordinates for the supernova with milli-arcsecond precision, linked to the ICRF. The recovered flux density is a factor ~2 below the EVLA flux density reported by other authors at the same frequency and epoch of our observations. This discrepancy could be due to extended emission detected with the EVLA or to calibration problems in the VLBI and/or EVLA observations. © ESO, 2011.

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