arXiv subjects

C. Brocksopp

Publications and source records attributed to C. Brocksopp.

41 records · Page 3Linked to original sources

arXiv1999

The very flat radio - millimetre spectrum of Cygnus X-1

We present almost-simultaneous detections of Cygnus X-1 in the radio and mm regimes, obtained during the low/hard X-ray state. The source displays a flat spectrum between 2 and 220 GHz, with a spectral index flatter than 0.15 (3sigma). There is no evidence for either a low- or high-frequency cut-off, but in the mid-infrared (~30 microns) thermal emission from the OB-type companion star becomes dominant. The integrated luminosity of this flat-spectrum emission in quiescence is > 2 x 10^{31} erg/s (2 x 10^{24} W). Assuming the emission originates in a jet for which non-radiative (e.g. adiabatic expansion) losses dominate, this is a very conservative lower limit on the power required to maintain the jet. A comparison with Cyg X-3 and GRS 1915+105, the other X-ray binaries for which a flat spectrum at shorter than cm wavelengths has been observed, shows that the jet in Cyg X-1 is significantly less luminous and less variable, and is probably our best example to date of a continuous, steady, outflow from an X-ray binary. The emissive mechanism reponsible for such a flat spectral component remains uncertain. Specifically, we note that the radio-mm spectra observed from these X-ray binaries are much flatter than those of the `flat-spectrum' AGN, and that existing models of synchrotron emission from partially self-absorbed radio cores, which predict a high-frequency cut-off in the mm regime, are not directly applicable.

astro-ph↗

arXiv1999

Orbital, precessional and flaring variability of Cygnus X-1

We present the results of a 2.5-year multiwavelength monitoring programme of Cygnus X-1, making use of hard and soft X-ray data, optical spectroscopy, UBVJHK photometry and radio data. In particular we confirm that the 5.6-day orbital period is apparent in all wavebands and note the existence of a wavelength-dependence to the modulation, in the sense that higher energies reach minimum first. We also find a strong modulation at a period of 142 +/- 7 days, which we suggest is due to precession and/or radiative warping of the accretion disc. Strong modulation of the hard and soft X-ray flux at this long period may not be compatible with simple models of an optically thin accretion flow and corona in the low state. We present the basic components required for more detailed future modelling of the system - including a partially optically thick jet, quasi-continuous in the low state, the base of which acts as the Comptonising corona. In addition, we find that there are a number of flares which appear to be correlated in at least two wavebands and generally in more. We choose two of these flares to study in further detail and find that the hard and soft X-rays are well-correlated in the first and that the soft X-rays and radio are correlated in the second. In general, the optical and infrared show similar behaviour to each other but are not correlated with the X-rays or radio.

astro-ph↗

arXiv1998

An Improved Orbital Ephemeris for Cygnus X-1

By accumulating all available radial velocity data and combining it with our own spectroscopy we present a new and improved ephemeris for Cygnus X-1 (V1357 Cygni/HDE226868). We initially calculate an orbital period of 5.599847+/-0.000018.days and then, by including the more recent radial velocities of LaSala et al. (1998) and Sowers et al.(1998) we refine this to 5.599829+/-0.000016 days. Using 27 years' worth of photometry, we also calculate a photometric period of 5.599836+/-0.000024 - remarkably similar to the spectroscopic one.

astro-ph↗

arXiv1998

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-1, starting in 1996 October, show variations at the binary period of 5.6 days, but with a phase offset from those at X-ray wavelengths. There are also longer-term variations on a time-scale of 150 days 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 semi-amplitude of the orbital modulation about 2 mJy. We discuss the possible origins of the modulation and the relationship to the soft X-ray emission.

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arXiv1997

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.

astro-ph↗