arXiv subjects

J. C. A. Miller-Jones

Publications and source records attributed to J. C. A. Miller-Jones.

At least 181 records · Page 10Linked to original sources

arXiv2011

A deep radio survey of hard state and quiescent black hole X-ray binaries

We have conducted a deep radio survey of a sample of black hole X-ray binaries in the hard and quiescent states, to determine whether any systems were sufficiently bright for astrometric follow-up with high-sensitivity very long baseline interferometric (VLBI) arrays. The one hard-state system, Swift J1753.5-0127, was detected at a level of 0.5 mJy/beam. All eleven quiescent systems were not detected. In the three cases with the highest predicted quiescent radio brightnesses (GRO J0422+32, XTE J1118+480, and GRO J1655-40), the new capabilities of the Expanded Very Large Array were used to reach noise levels as low as 2.6 microJy/beam. None of the three sources were detected, to 3-sigma upper limits of 8.3, 7.8, and 14.2 microJy/beam, respectively. These observations represent the most stringent constraints to date on quiescent radio emission from black hole X-ray binaries. The uncertainties in the source distances, quiescent X-ray luminosities at the times of the observations, and in the power-law index of the empirical correlation between radio and X-ray luminosities, make it impossible to determine whether these three sources are significantly less luminous in the radio band than expected. Thus it is not clear whether that correlation holds all the way down to quiescence for all black hole X-ray binaries.

astro-ph.HE↗

arXiv2011

Observing pulsars and fast transients with LOFAR

Low frequency radio waves, while challenging to observe, are a rich source of information about pulsars. The LOw Frequency ARray (LOFAR) is a new radio interferometer operating in the lowest 4 octaves of the ionospheric "radio window": 10-240MHz, that will greatly facilitate observing pulsars at low radio frequencies. Through the huge collecting area, long baselines, and flexible digital hardware, it is expected that LOFAR will revolutionize radio astronomy at the lowest frequencies visible from Earth. LOFAR is a next-generation radio telescope and a pathfinder to the Square Kilometre Array (SKA), in that it incorporates advanced multi-beaming techniques between thousands of individual elements. We discuss the motivation for low-frequency pulsar observations in general and the potential of LOFAR in addressing these science goals. We present LOFAR as it is designed to perform high-time-resolution observations of pulsars and other fast transients, and outline the various relevant observing modes and data reduction pipelines that are already or will soon be implemented to facilitate these observations. A number of results obtained from commissioning observations are presented to demonstrate the exciting potential of the telescope. This paper outlines the case for low frequency pulsar observations and is also intended to serve as a reference for upcoming pulsar/fast transient science papers with LOFAR.

astro-ph.IM↗

arXiv2011

The influence of spin on jet power in neutron star X-ray binaries

We investigate the role of the compact object in the production of jets from neutron star X-ray binaries. The goal is to quantify the effect of the neutron star spin, if any, in powering the jet. We compile all the available measures or estimates of the neutron star spin frequency in jet-detected neutron star X-ray binaries. We use as an estimate of the ranking jet power for each source, the normalisation of the power law which fits the X-ray/radio and X-ray/infrared luminosity correlations L_(radio/IR) proportional to L_(X)^(Gamma) (using infrared data for which there is evidence for jet emission). We find a possible relation between spin frequency and jet power (Spearman rank 97%), when fitting the X-ray/radio luminosity correlation using a power law with slope 1.4; Gamma=1.4 is observed in 4U 1728-34 and is predicted for a radiatively efficient disc and a total jet power proportional to the mass accretion rate. If we use a slope of 0.6, as observed in Aql X-1, no significant relation is found. An indication for a similar positive correlation is also found for accreting millisecond X-ray pulsars (Spearman rank 92%), if we fit the X-ray/infrared luminosity correlation using a power law with slope 1.4. While our use of the normalisation of the luminosity correlations as a measure of the ranking jet power is subject to large uncertainties, no better proxy for the jet power is available. However, we urge caution in over-interpreting the spin-jet power correlations, particularly given the strong dependence of our result on the (highly uncertain) assumed power law index of the luminosity correlations. We discuss the results in the framework of current models for jet formation in black holes and young stellar objects and speculate on possible different jet production mechanisms for neutron stars depending on the accretion mode.

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arXiv2011

An accurate position for the black hole candidate XTE J1752-223: re-interpretation of the VLBI data

Using high-precision astrometric optical observations from the Walter Baade Magellan Telescope in conjunction with high-resolution very long baseline interferometric (VLBI) radio imaging with the Very Long Baseline Array (VLBA), we have located the core of the X-ray binary system XTE J1752-223. Compact radio emission from the core was detected following the state transition from the soft to the hard X-ray state. Its position to the south-east of all previously-detected jet components mandated a re-analysis of the existing VLBI data. Our analysis suggests that the outburst comprised at least two ejection events prior to 2010 February 26. No radio-emitting components were detected to the south-east of the core at any epoch, suggesting that the receding jets were Doppler-deboosted below our sensitivity limit. From the ratio of the brightness of the detected components to the measured upper limits for the receding ejecta, we constrain the jet speed to be greater than 0.66c and the inclination angle to the line of sight to be less than 49 degrees. Assuming that the initial ejection event occurred at the transition from the hard intermediate state to the soft intermediate state, an initial period of ballistic motion followed by a Sedov phase (i.e. self-similar adiabatic expansion) appears to fit the motion of the ejecta better than a uniform deceleration model. The accurate core location can provide a long time baseline for a future proper motion determination should the system show a second outburst, providing insights into the formation mechanism of the compact object.

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arXiv2011

e-VLBI observations of Circinus X-1: monitoring of the quiescent and flaring radio emission on AU scales

A recent detection of the peculiar neutron star X-ray binary Circinus X-1 with electronic very long baseline interferometry (e-VLBI) prompted the suggestion that compact, non-variable radio emission persists through the entire 16.6-day orbit of the binary system. We present the results of a high angular resolution monitoring campaign conducted with the Australian Long Baseline Array in real-time e-VLBI mode. e-VLBI observations of Circinus X-1 were made on alternate days over a period of 20 days covering the full binary orbit. A compact radio source associated with Circinus X-1 was clearly detected at orbital phases following periastron passage but no compact radio emission was detected at any other orbital phase, ruling out the presence of a persistent, compact emitting region at our sensitivity levels. The jet was not resolved at any epoch of our 1.4-GHz monitoring campaign, suggesting that the ultrarelativistic flow previously inferred to exist in this source is likely to be dark. We discuss these findings within the context of previous radio monitoring of Circinus X-1.

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arXiv2011

An automated archival VLA transients survey

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 hours. 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 to rho less than or equal to 0.032 deg^-2 that have typical timescales 4.3 to 45.3 days. 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 Log N - Log S distribution 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.

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arXiv2011

Radiatively efficient accreting black holes in the hard state: the case study of H1743-322

In recent years, much effort has been devoted to unraveling 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 L_{Radio} \propto L_{X}^{b}). 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.

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arXiv2010

Radio observations of candidate magnetic O stars

Context: Some O stars are suspected to have to have (weak) magnetic fields because of the observed cyclical variability in their UV wind-lines. However, direct detections of these magnetic fields using optical spectropolarimetry have proven to be very difficult. Aims: Non-thermal radio emission in these objects would most likely be due to synchrotron radiation. As a magnetic field is required for the production of synchrotron radiation, this would be strong evidence for the presence of a magnetic field. Such non-thermal emission has already been observed from the strongly magnetic Ap/Bp stars. Methods: We have performed 6 & 21 cm observations using the WSRT and use these, in combination with archival VLA data at 3.6 cm and results from the literature, to study the radio emission of 5 selected candidate magnetic O stars. Results: Out of our five targets, we have detected three: $ξ$ Per, which shows a non-thermal radio spectrum, and $α$ Cam and $λ$ Cep, which show no evidence of a non-thermal spectrum. In general we find that the observed free-free (thermal) flux of the stellar wind is lower than expected. This is in agreement with recent findings that the mass-loss rates from O stars as derived from the H$α$ line are overestimated because of clumping in the inner part of the stellar wind.

astro-ph.SR↗

arXiv2010

Chandra localisation and optical/NIR follow-up of Galactic X-ray sources

We investigate a sample of eleven 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, IGRJ 16479-4514, IGR J16500-3307 and IGR J19308+530, the Chandra position confirms an existing association with an optical/NIR object, while for two sources (XTE J1716-389 and 18490-0000) 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 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.

astro-ph.HE↗

arXiv2010

Evolution of the radio - X-ray coupling throughout an entire outburst of Aquila X-1

The 2009 November outburst of the neutron star X-ray binary Aquila 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 disc-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 about 10% of the Eddington luminosity. We also present the first detections of Aql X-1 with Very Long Baseline Interferometry (VLBI), 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.

astro-ph.HE↗

arXiv2010

Investigating the disc-jet coupling in accreting compact objects using the black hole candidate Swift J1753.5-0127

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 J1753.5-0127, via multiwavelength coordinated observations over a period of ~4 years. 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 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 normalisation of the correlations, listing some possible parameters that broadband 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 about the nature of the compact object in the system, since its mass has not been dynamically measured.

astro-ph.HE↗

arXiv2009

The complete spectrum of the neutron star X-ray binary 4U0614+091

We observed the neutron star (NS) ultra-compact X-ray binary 4U0614+091 quasi-simultaneously in the radio band (VLA), mid-IR/IR (Spitzer/MIPS and IRAC), near-IR/optical (SMARTS), optical-UV (Swift/UVOT), soft and hard X-rays (Swift/XRT and RXTE). The source was steadily in its `hard state'. We detected the source in the whole range, for the first time in the radio band at 4.86 and 8.46 GHz and in the mid-IR at 24 um, up to 100 keV. The optically thick synchrotron spectrum of the jet is consistent with being flat from the radio to the mid-IR band. The flat jet spectrum breaks in the range (1-4)x10^(13) Hz to an optically-thin power-law synchrotron spectrum with spectral index ~-0.5. These observations allow us to estimate a lower limit on the jet radiative power of ~3x10^(32) erg/s and a total jet power Lj~10^(34) u_(0.05)^(-1) Ec^(0.53) erg/s (where Ec is the high-energy cutoff of the synchrotron spectrum in eV and u_(0.05) is the radiative efficiency in units of 0.05). The contemporaneous detection of the optically thin part of the compact jet and the X-ray tail above 30 keV allows us to assess the contribution of the jet to the hard X-ray tail by synchrotron self-Compton (SSC) processes. We conclude that, for realistic jet size, boosting, viewing angle and energy partition, the SSC emission alone, from the post-shock, accelerated, non-thermal population in the jet, is not a viable mechanism to explain the observed hard X-ray tail of the neutron star 4U0614+091.

astro-ph.HE↗

arXiv2009

The first accurate parallax distance to a black hole

Using astrometric VLBI observations, we have determined the parallax of the black hole X-ray binary V404 Cyg to be 0.418 +/- 0.024 milliarcseconds, corresponding to a distance of 2.39 +/- 0.14 kpc, significantly lower than the previously accepted value. This model-independent estimate is the most accurate distance to a Galactic stellar-mass black hole measured to date. With this new distance, we confirm that the source was not super-Eddington during its 1989 outburst. The fitted distance and proper motion imply that the black hole in this system likely formed in a supernova, with the peculiar velocity being consistent with a recoil (Blaauw) kick. The size of the quiescent jets inferred to exist in this system is less than 1.4 AU at 22 GHz. Astrometric observations of a larger sample of such systems would provide useful insights into the formation and properties of accreting stellar-mass black holes.

astro-ph.HE↗

arXiv2009

Probing the behaviour of the X-ray binary Cygnus X-3 with very-long-baseline radio interferometry

In order to test the recently proposed classification of the radio/X-ray states of the X-ray binary 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 VLBA data at 5 or 15 GHz and new e-EVN data at 5 GHz. We also present 5 GHz MERLIN observations of an outburst of Cyg X-3. In the X-ray regime we use quasi-simultaneous with radio, monitoring and pointed RXTE observations. We find that when the radio emission from both jet and core is globally considered, the behaviour of Cyg X-3 at milliarcsecond scales is consistent with that described at arcsecond 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.

astro-ph.HE↗

arXiv2009

The proper motion and changing jet morphology of Cygnus X-3

We present analysis of 25 years' worth of archival VLA, VLBA and EVN observations of the X-ray binary Cygnus X-3. From this, we deduce the source proper motion, allowing us to predict the location of the central binary system at any given time. However, the line of sight is too scatter-broadened for us to measure a parallactic distance to the source. The measured proper motion allows us to constrain the three-dimensional space velocity of the system, implying a minimum peculiar velocity of 9 km/s. Reinterpreting VLBI images from the literature using accurate core positions shows the jet orientation to vary with time, implying that the jets are oriented close to the line of sight and are likely to be precessing.

astro-ph.GA↗

arXiv2009

A new perspective on GCRT J1745-3009

Two WSRT observations were performed and five archival VLA data were reduced in order to redetect the enigmatic radio transient GCRT J1745-3009. The source was not redetected. We were, however, able to extract important new information from the discovery dataset. Our reanalysis excludes models that predict symmetric bursts, but the transient white dwarf pulsar is favoured. Although we now have more contraints on the properties of this source, we are still unsure about its basic model.

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arXiv2008

The formation of the black hole in the X-ray binary system V404 Cyg

Using new and archival radio data, we have measured the proper motion of the black hole X-ray binary V404 Cyg to be 9.2+/-0.3 mas/yr. Combined with the systemic radial velocity from the literature, we derive the full three-dimensional heliocentric space velocity of the system, which we use to calculate a peculiar velocity in the range 47-102 km/s, with a best fitting value of 64 km/s. We consider possible explanations for the observed peculiar velocity, and find that the black hole cannot have formed via direct collapse. A natal supernova is required, in which either significant mass (approximately 11 solar masses) was lost, giving rise to a symmetric Blaauw kick of up to 65 km/s, or, more probably, asymmetries in the supernova led to an additional kick out of the orbital plane of the binary system. In the case of a purely symmetric kick, the black hole must have been formed with a mass of approximately 9 solar masses, since when it has accreted 0.5-1.5 solar masses from its companion.

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arXiv2008

Zooming in on a sleeping giant: milliarcsecond HSA imaging of the black hole binary V404 Cyg in quiescence

Observations of the black hole X-ray binary V404 Cyg with the very long baseline interferometer HSA (the High Sensitivity Array) have detected the source at a frequency of 8.4 GHz, providing a source position accurate to 0.3 mas relative to the calibrator source. The observations put an upper limit of 1.3 mas on the source size (5.2 AU at 4 kpc) and a lower limit of 7 x 10^6 K on its brightness temperature during the normal quiescent state, implying that the radio emission must be non-thermal, most probably synchrotron radiation, possibly from a jet. The radio lightcurves show a short flare, with a rise time of about 30 min, confirming that the source remains active in the quiescent state.

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