G. G. Pooley

MERLIN observations of relativistic ejections from GRS 1915+105

We present high resolution MERLIN radio images of multiple relativistic ejections from GRS 1915+105 in 1997 October / November. The observations were made at a time of complex radio behaviour, corresponding to multiple optically-thin outbursts and several days of rapid radio flux oscillations. This activity followed � 20 days of a plateau state of inverted-spectrum radio emission and hard, quasi-stable X-ray emission. The radio imaging resolved four major ejection events from the system. As previously reported from earlier VLA observations of the source, we observe apparent superluminal motions resulting from intrinsically relativistic motions of the ejecta. However, our measured proper motions are significantly greater than those observed on larger angular scales with the VLA. Under the assumption of an intrinsically symmetric ejection, we can place an upper limit on the distance to GRS 1915+105 of 11.2 ± 0.8 kpc. Solutions for the velocities unambiguously require a higher intrinsic speed by about 0.1c than that derived from the earlier VLA observations, whilst the angle to the line-of-sight is not found to be significantly different. At a distance of 11 kpc, we obtain solutions of v = 0.98 +0.02 0.05c and � = 66±2 degrees. The jet also appears to be curved on a scale which corresponds to a period of around 7 days. We observe significant evolution of the linear polarisation of the approaching component, with large rotations in position angle and a general decrease in fractional polarisation. This may be due to increasing randomisation of the magnetic field within the ejected component. We do not at any time detect significant linear polarisation from the core, including periods when the flux density from this region is dominated by radio oscillations. The power input into the formation of the jet is very large, � 10 38 erg s 1 at 11 kpc for a pair plasma. If the plasma contains a cold proton for each electron, then the mass outflow rate, � 10 18 g s 1 is comparable to inflow rates

Rapid infrared flares in GRS 1915+105: evidence for infrared synchrotron emission

We report imaging photometry of the radio-jet black hole candidate source GRS 1915+105 in the infrared K band. The observations reveal rapid infrared flare events on timescales of less than an hour. These events are strikingly similar to those regularly observed in radio monitoring at 15 GHz. Furthermore, when dereddened, the infrared events have comparable amplitudes to the radio oscillations, and observations at 15 GHz made approx. 8 hr after our infrared observations reveal that the source was indeed displaying radio oscillations at this time. We suggest that we have observed infrared synchrotron emission from this source. We estimate the equipartition magnetic field and power required to accelerate the particles for the repeated radio events, and find both to be orders of magnitude greater than those estimated for any other X-ray binary.

Long term variability of Cygnus X-1. IV, Spectral evolution 1999–2004

Continuing the observational campaign initiated by our group, we present the long term spectral evolution of the Galactic black hole candidate Cygnus X-1 in the X-rays and at 15 GHz. We present ∼200 pointed observations taken between early 1999 and late 2004 with the Rossi X-ray Timing Explorer and the Ryle radio telescope. The X-ray spectra are remarkably well described by a simple broken power law spectrum with an exponential cutoff. Physically motivated Comptonization models, e.g., by Titarchuk (1994, ApJ, 434, 570, compTT) and by Coppi (1999, in High Energy Processes in Accreting Black Holes, ed. J. Poutanen, & R. Svensson (San Francisco: ASP), ASP Conf. Ser., 161, 375, eqpair), can reproduce this simplicity; however, the success of the phenomenological broken power law models cautions against “overparameterizing” the more physical models. Broken power law models reveal a significant linear correlation between the photon index of the lower energy power law and the hardening of the power law at ∼10 keV. This phenomenological soft/hard power law correlation is partly attributable to correlations of broad band continuum components, rather than being dominated by the weak hardness/reflection fraction correlation present in the Comptonization model. Specifically, the Comptonization models show that the bolometric flux of a soft excess (e.g., disk component) is strongly correlated with the compactness ratio of the Comptonizing medium, with L disk ∝( h / s ) −0.19 . Over the course of our campaign, Cyg X-1 transited several times into the soft state, and exhibited a large number of “failed state transitions”. The fraction of the time spent in such low radio emission/soft X-ray spectral states has increased from ∼10% in 1996–2000 to ∼34% since early 2000. We find that radio flares typically occur during state transitions and failed state transitions (at h / s ∼ 3), and that there is a strong correlation between the 10–50 keV X-ray flux and the radio luminosity of the source. We demonstrate that rather than there being distinctly separated states, in contrast to the timing properties the spectrum of Cyg X-1 shows variations between extremes of properties, with clear cut examples of spectra at every intermediate point in the observed spectral correlations.

Spectral evidence for a powerful compact jet from XTE J1118+480

ABSTRA C T We present observations of the X-ray transient XTE J11181480 during its low/hard X-ray state outburst in 2000, at radio and submillimetre wavelengths with the VLA, Ryle Telescope, MERLIN and JCMT. The high-resolution MERLIN observations reveal all the radio emission (at 5 GHz) to come from a compact core with physical dimensions smaller than 65d (kpc) au. The combined radio data reveal a persistent and inverted radio spectrum, with spectral index ,10:5: The source is also detected at 350 GHz, on an extrapolation of the radio spectrum. Flat or inverted radio spectra are now known to be typical of the low/ hard X-ray state, and are believed to arise in synchrotron emission from a partially selfabsorbed jet. Comparison of the radio and submillimetre data with reported near-infrared observations suggest that the synchrotron emission from the jet extends to the near-infrared, or possibly even optical regimes. In this case the ratio of jet power to total X-ray luminosity is likely to be PJ=LX @ 0:01; depending on the radiative efficiency and relativistic Doppler factor of the jet. Based on these arguments we conclude that during the period of our observations XTE J11181480 was producing a powerful outflow which extracted a large fraction of the total accretion power.

Orbital modulation and longer term variability in the radio emission from Cygnus␣X‐1

20 months of observations of the radio emission at 15 GHz from Cygnus X-l, starting in 1996 October, show variations at the binary period of 5.6 d, but with a phase offset from those at Xray wavelengths. There are also longer term variations on a time-scale of 150 d, which are only loosely related to the soft X-ray flux. The source was in the hard/low X-ray state throughout this period. The mean 15-GHz flux density is 13 mJy, the radio spectrum is flat and the semiamplitude of the orbital modulation is about 2 mJy. We discuss the possible origins of the modulation and the relationship to the soft X-ray emission.

The radio source counts at 15 GHz and their implications for cm-wave CMB imaging

A B ST R A C T W e presentthe prelim inary results ofa new survey ofradio sourcesusing the Ryle Telescopeat15.2 GHz.Thisisthehighestfrequency atwhich a survey hasbeen done thatisrelevantto theissueofradio sourcecontam ination in CM B experim ents.The dierentialsourcecountofthe 66 sourcesfound in 63 deg 2 is80(S=Jy) 2:0 Jy 1 sr 1 , from � 20 m Jy to � 500 m Jy.Extrapolating thisto 34 GHz (where m any cm -wave CM B experim ents operate) gives an estim ated tem perature contribution ofsources �T conf = 9�K in a CM B im age,with a beam corresponding to m ultipole l � 500.A m eansofsourcesubtraction isevidently necessary,otherwisethesignal-to-noiseratio in CM B im ages willbe lim ited to 4 or 5,becom ing worse at higher resolution.W e com pare the population ofsourcesobserved in thisnew survey to thatpredicted by extrapolation from lowerfrequency surveys,�nding thatsourceuxes,and indeed the existenceofm any sources,cannotbedeterm ined by extrapolation. K ey w ords: cosm icm icrowavebackground { surveys{ radio continuum :galaxies

Long term variability of Cygnus X-1 - III. Radio–X-ray correlations

Long time scale radio-X-ray correlations in black holes during the hard state have been found in many sources and there seems to emerge a universal underlying relationship which quantitatively describes this behavior. Although it would appear only natural to detect short term emission patterns in the X-ray and - with a certain time lag - in the radio, there has been little evidence for this up to now. The most prominent source for radio-X-ray correlations on short time scales (minutes) so far remains GRS 1915+105 where a single mass ejection could be detected successively in X-ray, IR, and radio wavebands. We analyze a database of more than 4 years of simultaneous radio-X-ray data for Cygnus X-1 from the Ryle Telescope and RXTE PCA/HEXTE. We confirm the existence of a radio-X-ray correlation on long time scales, especially at hard energies. We show that apparent correlations on short time scales in the lightcurves of Cygnus X-1 are most likely the coincidental outcome of white noise statistics. Interpreting this result as a breakdown of radio-X-ray correlations on shorter time scales, this sets a limit to the speed of the jet.

Variable circular polarization associated with relativistic ejections from GRS 1915 + 105

We report the discovery of variable circularly polarized radio emission associated with relativistic ejections from GRS 1915 + 105, based on observations with the Australia Telescope Compact Array (ATCA) and the Multi-Element Radio-Linked Interferometer Network (MERLIN). Following a radio flare in 2001 January, significant and variable circular polarization, at a fractional level of 0.2–0.4 per cent, was measured with ATCA at four frequencies between 1 and 9 GHz. Following an additional outburst 65 d later in 2001 March, further ATCA observations measured a comparable sign and level of circular polarization at two frequencies. At this second epoch, contemporaneous MERLIN observations directly imaged a relativistic ejection event and allowed us confidently to associate both the circularly and the linearly polarized emission with the relativistic ejecta, allowing a detailed measurement of the full polarization properties in the optically thin phase. The fractional circular polarization spectrum appears to flatten at higher frequencies in 2001 January, when there is strong evidence for multiple components at different optical depths. While we cannot conclusively distinguish between synchrotron or propagation-induced conversion as the origin of the circularly polarized component, we do not consider that coherent or birefringent scintillation mechanisms are likely. The implication is therefore that the ejections from GRS 1915 + 105 are associated with a significant population of low-energy electrons, with associated consequences for the energetics of relativistic ejection events. We briefly compare the data for SS 433 and GRS 1915 + 105 with those for active galactic nuclei, and note that linear-to-circular polarization ratios 1 at the higher frequencies, illustrating the role of Faraday depolarization. In addition, the 2001 January ATCA observations reveal a linear polarization ‘rotator’ event, probing the (variable) orientation of the magnetic field structure in the outflow.

Long term variability of Cygnus X-1 - V. State definitions with all sky monitors

We present a scheme for determining the spectral state of the canonical black hole Cyg X-1 using data from previous and current X-ray all sky monitors (RXTE-ASM, Swift-BAT, MAXI, and Fermi-GBM). Determinations of the hard/intermediate and soft state agree to better than 10% between different monitors, facilitating the determination of the state and its context for any observation of the source, potentially over the lifetimes of different individual monitors. A separation of the hard and the intermediate states, which strongly differ in their spectral shape and short-term timing behavior, is only possible when data in the soft X-rays (<5 keV) are available. A statistical analysis of the states confirms the different activity patterns of the source (e.g., month- to year-long hard-state periods or phases during which numerous transitions occur). It also shows that the hard and soft states are stable, with the probability of Cyg X-1 remaining in a given state for at least one week to be larger than 85% in the hard state and larger than 75% in the soft state. Intermediate states are short lived, with a 50% probability that the source leaves the intermediate state within three days. Reliable detection of these potentially short-lived events is only possible with monitor data that have a time resolution better than 1 d.

Characterizing a new class of variability in GRS 1915+105 with simultaneous INTEGRAL/RXTE observations

We report on the analysis of 100 ks INTEGRAL observations of the Galactic microquasar GRS 1915+105. We focus on INTEGRAL Revolution number 48 when the source was found to exhibit a new type of variability as preliminarily reported in Hannikainen (2003, A&A, 411, L415). The variability pattern, which we name ξ, is characterized by a pulsing behaviour, consisting of a main pulse and a shorter, softer, and smaller amplitude precursor pulse, on a timescale of 5 min in the JEM-X 3-35 keV lightcurve. We also present simultaneous RXTE data. From a study of the individual RXTE/PCA pulse profiles we find that the rising phase is shorter and harder than the declining phase, which is opposite to what has been observed in other otherwise similar variability classes in this source. The position in the colour-colour diagram throughout the revolution corresponds to Stale A (Belloni et al. 2000, A&A, 355, 271) but not to any previously known variability class. We separated the INTEGRAL data into two subsets covering the maxima and minima of the pulses and fitted the resulting two broadband spectra with a hybrid thermal-non-thermal Comptonization model. The fits show the source to be in a soft state characterized by a strong disc component below ∼6 keV and Comptonization by both thermal and non-thermal electrons at higher energies.