arXiv subjects

J. Miller-Jones

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

10 recordsLinked to original sources

arXiv2021

NuSTAR reveals the hidden nature of SS433

SS433 is the only Galactic binary system known to persistently accrete at highly super-critical (or hyper-critical) rates, similar to those in tidal disruption events, and likely needed to explain the rapid growth of those very high redshift quasars containing massive SMBHs. Probing the inner regions of SS433 in the X-rays is crucial to understanding this system, and super-critical accretion in general, but is highly challenging due to obscuration by the surrounding wind, driven from the accretion flow. NuSTAR observed SS433 in the hard X-ray band across multiple phases of its 162 day super-orbital precession period. Spectral-timing tools allow us to infer that the hard X-ray emission from the inner regions is likely being scattered towards us by the walls of the wind-cone. By comparing to numerical models, we determine an intrinsic X-ray luminosity of $\ge$ 2$\times$10$^{37}$ erg/s and that, if viewed face on, we would infer an apparent luminosity of $>$ 1$\times$10$^{39}$ erg/s, confirming SS433's long-suspected nature as an ultraluminous X-ray source (ULX). We present the discovery of a narrow, $\sim 100$s lag due to atomic processes occurring in outflowing material travelling at least 0.14-0.29c, which matches absorption lines seen in ULXs and -- in the future -- will allow us to map a super-critical outflow for the first time.

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arXiv2017

ThunderKAT: The MeerKAT Large Survey Project for Image-Plane Radio Transients

ThunderKAT is the image-plane transients programme for MeerKAT. The goal as outlined in 2010, and still today, is to find, identify and understand high-energy astrophysical processes via their radio emission (often in concert with observations at other wavelengths). Through a comprehensive and complementary programme of surveying and monitoring Galactic synchrotron transients (across a range of compact accretors and a range of other explosive phenomena) and exploring distinct populations of extragalactic synchrotron transients (microquasars, supernovae and possibly yet unknown transient phenomena) - both from direct surveys and commensal observations - we will revolutionise our understanding of the dynamic and explosive transient radio sky. As well as performing targeted programmes of our own, we have made agreements with the other MeerKAT large survey projects (LSPs) that we will also search their data for transients. This commensal use of the other surveys, which remains one of our key programme goals in 2016, means that the combined MeerKAT LSPs will produce by far the largest GHz-frequency radio transient programme to date.

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arXiv2017

Paving the way to simultaneous multi-wavelength astronomy

Whilst astronomy as a science is historically founded on observations at optical wavelengths, studying the Universe in other bands has yielded remarkable discoveries, from pulsars in the radio, signatures of the Big Bang at submm wavelengths, through to high energy emission from accreting, gravitationally-compact objects and the discovery of gamma-ray bursts. Unsurprisingly, the result of combining multiple wavebands leads to an enormous increase in diagnostic power, but powerful insights can be lost when the sources studied vary on timescales shorter than the temporal separation between observations in different bands. In July 2015, the workshop "Paving the way to simultaneous multi-wavelength astronomy" was held as a concerted effort to address this at the Lorentz Center, Leiden. It was attended by 50 astronomers from diverse fields as well as the directors and staff of observatories and spaced-based missions. This community white paper has been written with the goal of disseminating the findings of that workshop by providing a concise review of the field of multi-wavelength astronomy covering a wide range of important source classes, the problems associated with their study and the solutions we believe need to be implemented for the future of observational astronomy. We hope that this paper will both stimulate further discussion and raise overall awareness within the community of the issues faced in a developing, important field.

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arXiv2015

Pulsar polarisation below 200 MHz: Average profiles and propagation effects

We present the highest-quality polarisation profiles to date of 16 non-recycled pulsars and four millisecond pulsars, observed below 200 MHz with the LOFAR high-band antennas. Based on the observed profiles, we perform an initial investigation of expected observational effects resulting from the propagation of polarised emission in the pulsar magnetosphere and the interstellar medium. The predictions of magnetospheric birefringence in pulsars have been tested using spectra of the pulse width and fractional polarisation from multifrequency data. The derived spectra offer only partial support for the expected effects of birefringence on the polarisation properties, with only about half of our sample being consistent with the model's predictions. It is noted that for some pulsars these measurements are contaminated by the effects of interstellar scattering. For a number of pulsars in our sample, we have observed significant variations in the amount of Faraday rotation as a function of pulse phase, which is possibly an artefact of scattering. These variations are typically two orders of magnitude smaller than that observed at 1400 MHz by Noutsos et al. (2009), for a different sample of southern pulsars. In this paper we present a possible explanation for the difference in magnitude of this effect between the two frequencies, based on scattering. Finally, we have estimated the magnetospheric emission heights of low-frequency radiation from four pulsars, based on the phase lags between the flux-density and the PA profiles, and the theoretical framework of Blaskiewicz, Cordes & Wasserman (1991). These estimates yielded heights of a few hundred km; at least for PSR B1133+16, this is consistent with emission heights derived based on radius-to-frequency mapping, but is up to a few times larger than the recent upper limit based on pulsar timing.

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arXiv2015

The physics of accretion-ejection with LOFT

This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of the physics of accretion and ejection around compact objects. For a summary, we refer to the paper.

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arXiv2011

Rapid Intrinsic Variability of Sgr A* at Radio Wavelengths

Sgr A* exhibits flares in radio, millimeter and submm wavelengths with durations of $\sim 1$ hour. Using structure function, power spectrum and autocorrelation function analysis, we investigate the variability of Sgr A* on time scales ranging from a few seconds to several hours and find evidence for sub-minute time scale variability at radio wavelengths. These measurements suggest a strong case for continuous variability from sub-minute to hourly time scales. This short time scale variability constrains the size of the emitting region to be less than 0.1\,AU. Assuming that the minute time scale fluctuations of the emission at 7\,mm arise through the expansion of regions of optically thick synchrotron-emitting plasma, this suggests the presence of explosive, energetic expansion events at speeds close to $c$. The required rate of mass processing and energy loss of this component are estimated to be $\ga 6\times 10^{-10} \msol$ yr$^{-1}$ and 400\,$L_\odot$ respectively. The inferred scale length corresponding to one-minute light travel time is comparable to the time averaged spatially resolved 0.1AU scale observed at 1.3mm emission of Sgr A*. This steady component from Sgr A* is interpreted mainly as an ensemble average of numerous weak and overlapping flares that are detected on short time scales. The nature of such short time scale variable emission or quiescent variability is not understood but could result from fluctuations in the accretion flow of Sgr A* that feed the base of an outflow or jet.

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arXiv2010

Multi-Wavelength Study of Sgr A*: The Short Time Scale Variability

To understand the correlation and the radiation mechanism of flare emission in different wavelength bands, we have coordinated a number of telescopes to observe SgrA* simultaneously. We focus only on one aspect of the preliminary results of our multi-wavelength observing campaigns, namely, the short time scale variability of emission from SgrA* in near-IR, X-ray and radio wavelengths. The structure function analysis indicate most of the power spectral density is detected on hourly time scales in all wavelength bands. We also report minute time scale variability at 7 and 13mm placing a strong constraint on the nature of the variable emission. The hourly time scale variability can be explained in the context of a model in which the peak frequency of emission shifts toward lower frequencies as a self-absorbed synchrotron source expands adiabatically near the acceleration site. The short time scale variability, on the other hand, places a strong constraint on the size of the emitting region. Assuming that rapid minute time scale fluctuations of the emission is optically thick in radio wavelength, light travel arguments requires relativistic particle energy, thus suggesting the presence of outflow from SgrA*.

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arXiv2009

Following the 2008 outburst decay of the black hole candidate H1743-322 in X-ray and radio

In this Paper we report on radio (VLA and ATCA) and X-ray (RXTE, Chandra and Swift) observations of the outburst decay of the transient black hole candidate H1743-322 in early 2008. We find that the X-ray light curve followed an exponential decay, leveling off towards its quiescent level. The exponential decay timescale is ~4 days and the quiescent flux corresponds to a luminosity of 3x10^32 (d/7.5 kpc)^2 erg/s. This together with the relation between quiescent X-ray luminosity and orbital period reported in the literature suggests that H1743-322 has an orbital period longer than ~10 hours. 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 N_H =2.3x10^22 cm^-2 limits the detected number of soft photons and thus the accuracy of the spectral fits.

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arXiv2007

The spectral energy distribution of quiescent black hole X-ray binaries: new constraints from Spitzer

(Abridged) Among the various issues that remain open in the field of accretion onto black hole X-ray binaries (BHBs) is the way the gas accretes at very low Eddington ratios, in the so-called quiescent regime. While there is general agreement that the X-rays are produced by a population of high-energy electrons near to the BH, the controversy comes about in modeling the contribution from inflowing vs. outflowing particles, and their relative energy budget. Recent Spitzer observations of three quiescent BHBs have shown evidence for excess emission with respect to the tail of the companion star between 8-24 micron. We suggest that synchrotron emission from a partially self-absorbed outflow might be responsible for the observed mid-IR excess, in place of, or in addition to, thermal emission from circumbinary material. If so, then the jet synchrotron luminosity exceeds the measured 2-10 keV luminosity by a factor of a few in these systems. In turn, the mechanical power stored in the jet exceeds the bolometric X-ray luminosity at least by 4 orders of magnitude. We then compile the broadband spectral energy distribution (SED) of A0620-00, the lowest Eddington-ratio stellar mass BH with a known radio counterpart, by means of simultaneous radio, optical and X-ray observations, and the archival Spitzer data. We are able to fit the SED of A0620-00 with a `maximally jet-dominated' model in which the radio through the soft X-rays are dominated by synchrotron emission, while the hard X-rays are dominated by inverse Compton at the jet base. The fitted parameters land in a range of values that is reminiscent of the Galactic Center super-massive BH Sgr A*. Most notably, the inferred ratio of the jet acceleration rate to local cooling rates is two orders of magnitude weaker with respect to higher luminosity, hard state sources.

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arXiv2006

A radio-emitting outflow in the quiescent state of A0620-00: implications for modelling low-luminosity black hole binaries

Deep observations with the Very Large Array of A0620-00, performed in 2005 August, resulted in the first detection of radio emission from a black hole binary at X-ray luminosities as low as 10^{-8.5} times the Eddington limit. The measured radio flux density, of 51 \pm 7 microJy at 8.5 GHz, is the lowest reported for an X-ray binary system so far, and is interpreted in terms of partially self-absorbed synchrotron emission from outflowing plasma. Making use of the estimated outer accretion rate of A0620-00 in quiescence, we demonstrate that the outflow kinetic power must be energetically comparable to the total accretion power associated with such rate, if it was to reach the black hole with the standard radiative efficiency of 10 per cent. This favours a model for quiescence in which a radiatively inefficient outflow accounts for a sizable fraction of the missing energy, and, in turn, substantially affects the overall dynamics of the accretion flow. Simultaneous observations in the X-ray band, with Chandra, confirm the validity of a non-linear radio/X-ray correlation for hard state black hole binaries down to low quiescent luminosities, thereby contradicting some theoretical expectations. Taking the mass term into account, the A0620-00 data lie on the extrapolation of the so called Fundamental Plane of black hole activity, which has thus been extended by more than 2 orders of magnitude in radio and X-ray luminosity. With the addition of the A0620-00 point, the plane relation provides an empirical proof for the scale-invariance of the jet-accretion coupling in accreting black holes over the entire parameter space observable with current instrumentation.

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