arXiv subjects

Peter Duffy

Publications and source records attributed to Peter Duffy.

21 records · Page 2Linked to original sources

arXiv2004

Jet evolution, flux ratios and light-travel time effects

Studies of the knotty jets in both quasars and microquasars frequently make use of the ratio of the intensities of corresponding knots on opposite sides of the nucleus in order to infer the product of the intrinsic jet speed (beta) and the cosine of the inclination angle of the jet-axis (cos{theta}), via the formalism I_{a}/I_{r} = ((1+beta cos{theta})/(1-beta cos{theta}))^{3+alpha}, where alpha relates the intensity I_{nu} as a function of frequency nu as I_{nu} propto nu^{-alpha}. Where beta cos{theta} is determined independently, the intensity ratio of a given pair of jet to counter-jet knots is over-predicted by the above formalism compared with the intensity ratio actually measured from radio images. As an example in the case of Cygnus X-3 the original formalism predicts an intensity ratio of about 185, whereas the observed intensity ratio at one single epoch is about 3. Mirabel and Rodriguez (1999) have refined the original formalism, and suggested measuring the intensity ratio of knots when they are at equal angular separations from the nucleus. This method is only applicable where there is sufficient time-sampling with sufficient physical resolution to interpolate the intensities of the knots at equal distances from the nucleus, and can therefore be difficult to apply to microquasars and is impossible to apply to quasars. Accounting for both the light-travel time between the knots and the simple evolution of the knots themselves reconciles this over-prediction and renders the original formalism obsolete.

astro-ph↗

arXiv2003

Time-sequenced Multi-Radio-Frequency Observations of Cygnus X-3 in Flare

Multifrequency observations from the VLA, VLBA and OVRO Millimeter Array of a major radio outburst of Cygnus X-3 in 2001 September are presented, measuring the evolution of the spectrum of the source over three decades in frequency, over a period of six days. Following the peak of the flare, as the intensity declines the high-frequency spectrum at frequency nu steepens from nu^{-0.4} to nu^{-0.6}, after which the spectral index remains at this latter terminal value; a trend previously observed but hitherto not satisfactorily explained. VLBA observations, for the first time, track over several days the expansion of a sequence of knots whose initial diameters are approximately 8 milliarcseconds. The light-crossing time within these plasmons is of the same order as the time-scale over which the spectrum is observed to evolve. We contend that properly accounting for light-travel time effects in and between plasmons which are initially optically thick, but which after expansion become optically thin, explains the key features of the spectral evolution, for example the observed timescale. Using the VLBA images, we have directly measured for the first time the proper motions of individual knots, analysis of which shows a two-sided jet whose axis is precessing. The best-fit jet speed is roughly beta = 0.63 and the precession period is about 5 days, significantly lower than fitted for a previous flare. Extrapolation of the positions of the knots measured by the VLBA back to zero-separation shows this to occur approximately 2.5 days after the detection of the rise in flux density of Cygnus X-3.

astro-ph↗

arXiv1999

On "box" models of shock acceleration and electron synchrotron spectra

The recent detection of high energy gamma-rays coming from supernova remnants and active galactic nuclei has revived interest in the diffusive shock acceleration of electrons. In the present paper we examine the basis of the so-called ``box'' model for particle acceleration and present a more physical version of it. Using this we determine simple criteria for the conditions under which ``pile-ups'' can occur in shock accelerated electron spectra subject to synchrotron or inverse Compton losses (the latter in the Thompson limit). An extension to include nonlinear effects is proposed.

astro-ph↗