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Hassall T.E.,University of Manchester | Stappers B.W.,University of Manchester | Hessels J.W.T.,Netherlands Institute for Radio Astronomy | Hessels J.W.T.,University of Amsterdam | And 105 more authors.
Astronomy and Astrophysics | Year: 2012

Dispersion in the interstellar medium is a well known phenomenon that follows a simple relationship, which has been used to predict the time delay of dispersed radio pulses since the late 1960s. We performed wide-band simultaneous observations of four pulsars with LOFAR (at 40-190 MHz), the 76-m Lovell Telescope (at 1400 MHz) and the Effelsberg 100-m Telescope (at 8000 MHz) to test the accuracy of the dispersion law over a broad frequency range. In this paper we present the results of these observations which show that the dispersion law is accurate to better than 1 part in 105 across our observing band. We use this fact to constrain some of the properties of the interstellar medium along the line-of-sight and use the lack of any aberration or retardation effects to determine upper limits on emission heights in the pulsar magnetosphere. We also discuss the effect of pulse profile evolution on our observations, and the implications that it could have for precision pulsar timing projects such as the detection of gravitational waves with pulsar timing arrays. © 2012 ESO.

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..

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.

Jonker P.G.,SRON Netherlands Institute for Space Research | Jonker P.G.,Harvard - Smithsonian Center for Astrophysics | Miller-Jones J.,NRAO Headquarters | Homan J.,Massachusetts Institute of Technology | And 11 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2010

In this paper, we report on radio (Very Large Array and Austrialian Telescope Compact Array) and X-ray (RXTE, Chandra and Swift) observations of the outburst decay of the transient black hole candidate H 1743-322 in early 2008. We find that the X-ray light curve followed an exponential decay, levelling off towards its quiescent level. The exponential decay time-scale is ≈4 days and the quiescent flux corresponds to a luminosity of erg s-1. This together with the relation between quiescent X-ray luminosity and orbital period reported in the literature suggests that H 1743-322 has an orbital period longer than ≈10 h. Both the radio and X-ray light curve show evidence for flares. The radio-X-ray correlation can be well described by a power-law with index ≈0.18. This is much lower than the index of ≈0.6-0.7 found for the decay of several black hole transients before. The radio spectral index measured during one of the radio flares while the source is in the low-hard state is -0.5 ± 0.15, which indicates that the radio emission is optically thin. This is unlike what has been found before in black hole sources in the low-hard state. We attribute the radio flares and the low index for the radio-X-ray correlation to the presence of shocks downstream the jet flow, triggered by ejection events earlier in the outburst. We find no evidence for a change in X-ray power-law spectral index during the decay, although the relatively high extinction of NH ≈ 2.3 × 1022 cm-2 limits the detected number of soft photons and thus the accuracy of the spectral fits. © 2009 RAS.

Coriat M.,University Paris Diderot | Coriat M.,University of Southampton | Corbel S.,University Paris Diderot | Prat L.,University Paris Diderot | And 7 more authors.
Proceedings of the International Astronomical Union | Year: 2010

In recent years, numerous efforts have been devoted to unravel the connection between accretion flow and jets in accreting compact objects. Here we report new constraints on these issues, through the long term study of the radio and X-ray behaviour of the black hole candidate H 1743-322. This source is known to be one of the "outliers" of the universal radio/X-ray correlation, i.e. a group of stellar mass accreting black holes displaying fainter radio emission for a given X-ray luminosity, than expected from the correlation. In this work we find, at high X-ray luminosity in the hard state, a tight radio/X-ray correlation with an unusual steep slope of b = 1.38 ± 0.03. This correlation then breaks below ∼ 5 × 10-3L Edd (M/10M)-1 in X-rays and becomes shallower. When compared with radio/X-ray data from other black hole X-ray binaries, we see that the deviant points of H 1743-322 join the universal correlation and seem to follow it at low luminosity. Based on these results, we investigate several hypotheses that could explain both the b ∼ 1.4 slope and the transition toward the universal correlation. © International Astronomical Union 2011.

Coriat M.,University Paris Diderot | Corbel S.,University Paris Diderot | Corbel S.,Institut Universitaire de France | Prat L.,University Paris Diderot | And 8 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2011

In recent years, much effort has been devoted to unravelling the connection between the accretion flow and the jets in accreting compact objects. In the present work, we report new constraints on these issues, through the long-term study of the radio and X-ray behaviour of the black hole candidate H1743-322. This source is known to be one of the 'outliers' of the universal radio/X-ray correlation, i.e. a group of accreting stellar-mass black holes displaying fainter radio emission for a given X-ray luminosity than expected from the correlation. Our study shows that the radio and X-ray emission of H1743-322 are strongly correlated at high luminosity in the hard spectral state. However, this correlation is unusually steep for a black hole X-ray binary: b~ 1.4 (with Lradio∝Lb X). Below a critical luminosity, the correlation becomes shallower until it rejoins the standard correlation with b~ 0.6. Based on these results, we first show that the steep correlation can be explained if the inner accretion flow is radiatively efficient during the hard state, in contrast to what is usually assumed for black hole X-ray binaries in this spectral state. The transition between the steep and the standard correlation would therefore reflect a change from a radiatively efficient to a radiatively inefficient accretion flow. Finally, we investigate the possibility that the discrepancy between 'outliers' and 'standard' black holes arises from the outflow properties rather than from the accretion flow. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Ratti E.M.,SRON Netherlands Institute for Space Research | Bassa C.G.,University of Manchester | Torres M.A.P.,Harvard - Smithsonian Center for Astrophysics | Kuiper L.,SRON Netherlands Institute for Space Research | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2010

We investigate a sample of 11 Galactic X-ray sources recently discovered with INTEGRAL or RXTE with the goal of identifying their optical and/or near-infrared (NIR) counterpart. For this purpose, new Chandra positions of nine objects are presented together with follow-up observations of all the targets in the optical and NIR. For the four sources IGR J16194-2810, IGR J16479-4514, IGR J16500-3307 and IGR J19308+0530, the Chandra position confirms an existing association with an optical/NIR object, while for two sources, XTE J1716-389 and IGR J18490-0000, it rules out previously proposed counterparts indicating new ones. In the case of IGR J17597-220, a counterpart is selected out of the several possibilities proposed in the literature, and we present the first association with an optical/NIR source for IGR J16293-4603 and XTE J1743-363. Moreover, optical/NIR observations are reported for XTE J1710-281 and IGR J17254-3257; we investigate the counterpart to the X-ray sources based on their XMM-Newton positions. We discuss the nature of each system considering its optical/NIR and X-ray properties. © 2010 The Authors. Journal compilation © 2010 RAS.

Bell M.E.,University of Southampton | Fender R.P.,University of Southampton | Swinbank J.,University of Amsterdam | Miller-Jones J.C.A.,NRAO Headquarters | And 12 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2011

In this paper we present the results of a survey for radio transients using data obtained from the Very Large Array archive. We have reduced, using a pipeline procedure, 5037 observations of the most common pointings - i.e. the calibrator fields. These fields typically contain a relatively bright point source and are used to calibrate 'target' observations: they are therefore rarely imaged themselves. The observations used span a time range ~1984-2008 and consist of eight different pointings, three different frequencies (8.4, 4.8 and 1.4 GHz) and have a total observing time of 435 h. We have searched for transient and variable radio sources within these observations using components from the prototype LOFAR transient detection system. In this paper we present the methodology for reducing large volumes of Very Large Array data; and we also present a brief overview of the prototype LOFAR transient detection algorithms. No radio transients were detected in this survey, therefore we place an upper limit on the snapshot rate of GHz frequency transients >8.0 mJy toρ≤0.032 deg-2 that have typical time-scales 4.3 to 45.3 d. We compare and contrast our upper limit with the snapshot rates - derived from either detections or non-detections of transient and variable radio sources - reported in the literature. When compared with the current LogN-LogSdistribution formed from previous surveys, we show that our upper limit is consistent with the observed population. Current and future radio transient surveys will hopefully further constrain these statistics, and potentially discover dominant transient source populations. In this paper we also briefly explore the current transient commissioning observations with LOFAR, and the impact they will make on the field. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

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