E. B. Waltman

A One-sided Highly Relativistic Jet from Cygnus X-3

Very Long Baseline Array images of the X-ray binary Cygnus X-3 were obtained 2, 4, and 7 days after the peak of a 10 Jy flare on 1997 February 4. The first two images show a curved one-sided jet, and the third shows a scatter-broadened disk, presumably at the position of the core. The jet curvature changes from the first to the second epoch, which strongly suggests a precessing jet. The ratio of the flux density in the approaching jet to that in the (undetected) receding jet is 330; if this asymmetry is due to Doppler boosting, the implied jet speed is 0.81c. Precessing-jet model fits, together with the assumption that the jet is intrinsically symmetric and was ejected during or after the major flare, yield the following constraints: the jet inclination to the line of sight must be 14?, the cone opening angle must be 12?, and the precession period must be 60 days.

Light Curves and Radio Structure of the 1999 September Transient Event in V4641 Sagittarii (=XTE J1819-254=SAX J1819.3-2525)

We report on radio observations of the 1999 September event of the X-ray transient V4641 Sgr (=XTE J1819-254=SAX J1819.3-2525). This event was extremely rapid in its rise and decay across radio, optical, and X-ray wavelengths; the X-rays rose to 12 crab within 8 hr and faded to below 0.1 crab in less than 2 hr. Radio observations were made with seven telescopes during the first day following the onset of the strong X-ray event, revealing a strong radio source that was detected for 3 further weeks by the more sensitive telescopes. The radio source was resolved even in the first Very Large Array (VLA) images (September 16.027 UT), being ~025 long with an axis ratio of at least 10 : 1. The total flux density decayed by a factor of ~4 over the first day, and by September 17.94 UT the radio emission was confined to a slowly decaying, marginally resolved remnant located at one side of the early elongated emission. The H I absorption spectrum gives a minimum kinematic distance of about 400 pc; various other arguments suggest that the true distance is not much greater than this. The inferred proper motions for the early extended emission (04-11 day-1) correspond to v/c ~ 1.0-3.2 (d/0.5 kpc), and this together with the radio morphology argues that this is a relativistic jet source like GRS 1915+105 and GRO J1655-40. The proper motion of the late-time remnant is at least 100 times smaller. One simple interpretation posits the ejection of a single short-lived jet segment, followed by a more slowly decaying, optically thin jet segment ejection. These two components can explain both the multifrequency radio light curves and the radio images. The most likely parameters for the fast-jet system with net-averaged proper motion of ~04 day-1, assuming d = 0.5 kpc, are v ~ 0.85c and i ~ 63°, where i is the inclination to the line of sight. The corresponding apparent velocities are 1.4c and 0.6c for the approaching and receding jets, making V4641 Sgr the closest superluminal jet source known.

Simultaneous X-Ray and Radio Monitoring of the Unusual Binary LS I +61?303: Measurements of the Light Curve and High-Energy Spectrum

The binary system, LS I +61°303, is unusual both because of the dramatic, periodic, radio outbursts, and because of its possible association with the 100 MeV gamma-ray source, 2CG 135+01. We have performed simultaneous radio and Rossi X-Ray Timing Explorer X-ray observations at 11 intervals over the 26.5 day orbit, and in addition searched for variability on timescales ranging from milliseconds to hours. We confirm the modulation of the X-ray emission on orbital timescales originally reported by Taylor et al., and in addition we find a significant offset between the peak of the X-ray and radio flux. We argue that based on these results, the most likely X-ray emission mechanism is inverse Compton scattering of stellar photons off of electrons accelerated at the shock boundary between the relativistic wind of a young pulsar and the Be star wind. In these observations we also detected 2-150 keV flux from the nearby low-redshift quasar QSO 0241+622. Comparing these measurements to previous hard X-ray and gamma-ray observations of the region containing both LS I +61°303 and QSO 0241+622, it is clear that emission from the QSO dominates.

Radio and X-Ray Variability of the Galactic Superluminal Source GRS 1915+105

We report results of radio and hard X-ray monitoring observations of the Galactic superluminal X-ray source GRS 1915+105 carried out with the Green Bank Interferometer and the Burst and Transient Source Experiment during the period 1994 September through 1996 March. Both the radio and the hard X-ray light curves show a complex transient behavior. The radio emission monitored at 2.25 and 8.3 GHz is correlated with episodes of enhanced hard X-ray emission. A phenomenological classification of the radio emission indicates two distinct emission modes: plateau and flaring. Plateau radio emission is in general optically thick, with a flat-topped light curve showing a rapid onset and decrease of the flux density. The radio flaring state shows large radio flares that can increase in flux density by 2 orders of magnitude in less than 18 hr, followed by an optically thin exponential decay. These observed large radio flares are consistent with external propagation of plasmoids emitting synchrotron radiation. We comment on the significance of these results, and suggest a scenario for modeling the multiwavelength behavior of GRS 1915+105.

30 years of multi-wavelength observations of 3C 273

We present a wide multi-wavelength database of most observations of the quasar 3C 273 obtained during the last 30 years. This database is the most complete set of observations available for an active galactic nucleus (AGN). It contains nearly 20 000 observations grouped together into 70 light curves covering 16 orders of magnitude in frequency from the radio to the γ -ray domain. The database is constituted of many previously unpublished observations and of most publicly available data gathered in the literature and on the World Wide Web (WWW). It is complete to the best of our knowledge, except in the optical (UBV ) domain where we chose not to add all observations from the literature. In addition to the photometric data, we present the spectra of 3C 273 obtained by the International Ultraviolet Explorer (IUE) satellite. In the X-ray domain, we used the spectral fit parameters from the literature to construct the light curves. Apart from describing the data, we show the most representative light curves and the average spectrum of 3C 273. The database is available on the WWW in a homogeneous and clear form and we wish to update it regularly by adding new observations.

Radio Emission and the Timing Properties of the Hard X-Ray State of GRS 1915+105

We combine a complete sample of 113 pointed observations taken with the Rossi X-Ray Timing Explorer between 1996 and 1999, monitoring observations taken with the Ryle Telescope and the Green Bank Interferometer, and selected observations with the Very Large Array to study the radio and X-ray properties of GRS 1915+105 when its X-ray emission is hard and steady. We establish that radio emission always accompanies the hard-steady state of GRS 1915+105 but that the radio flux density at 15.2 GHz and the X-ray flux between 2 and 200 keV are not correlated. Therefore, we study the X-ray spectral and timing properties of GRS 1915+105 using three approaches: first by describing in detail the properties of three characteristic observations, then by displaying the time evolution of the timing properties during periods of both faint and bright radio emission, and lastly by plotting the timing properties as a function of the radio flux density. We find that as the radio emission becomes brighter and more optically thick, (1) the frequency of a ubiquitous 0.5-10 Hz quasi-periodic oscillation (QPO) decreases, (2) the Fourier phase lags between hard (11.5-60 keV) and soft (2-4.3 keV) in the frequency range of 0.01-10 Hz change sign from negative to positive, (3) the coherence between hard and soft photons at low frequencies decreases, and (4) the relative amount of low-frequency power in hard photons compared to soft photons decreases. We discuss how these results reflect upon basic models from the literature describing the accretion flow around black holes and the possible connection between Comptonizing electrons and compact radio jets.

Discovery of Correlated Behavior Between the Hard X-Ray and the Radio Bands in Cygnus X-3

Using Compton Gamma Ray Observatory BATSE hard X-ray (HXR) data and GHz radio monitoring data from the Green Bank Interferometer, we have performed a long-term study (~1800 days) of the unusual X-ray binary Cyg X-3, resulting in the discovery of a remarkable relationship between these two wavelength bands. We find that during quiescent radio states, the radio flux is strongly anticorrelated with the intensity of the HXR emission. The relationship switches to a correlation with the onset of major radio flaring activity. During major radio flaring activity, the HXR drops to a very low intensity during quenching in the radio and recovers during the radio flare. Injection of plasma into the radio jets of Cyg X-3 occurs during changes in the HXR emission and suggests that disk-related and jet-related components are responsible for the high energy emission.

Radio Emission from Algol. I. Coronal Geometry and Emission Mechanisms Determined from VLBA and Green Bank Interferometer Observations

We report dual circular polarization VLBA observations of Algol made at orbital phases 0.22-0.30 using a differential phase referencing technique. The flux density of Algol varied from 10 to 20 mJy during the observations. The radio maps show a double-lobed source separated by 1.6 mas (1.4 times the K star diameter). Although the total emission is only weakly circularly polarized, the individual lobes are strongly circularly polarized and of opposite helicity. Snapshot VLBI maps made at 3 hour intervals show variations in the flux density of both components, but no significant motions of the centroids. We also analyze Green Bank Interferometer (GBI) synoptic observations of right- and left-circularly polarized (RCP and LCP) flux densities of Algol at 2.3 and 8.3 GHz several times a day from early 1995 to mid-1997. The resulting data set, which consists of more than 2500 observations over 2 years, is by far the most comprehensive available for any stellar system. In addition, we analyzed GBI observations of the very similar (but noneclipsing) binary system HR 1099 over the same time period in order to compare the two systems. We summarize the GBI observations using several statistical descriptions. We find no phase dependence of either the radio luminosity or circular polarization for either system. The luminosity histograms for the two systems are remarkably similar. The distribution functions are not well represented by exponentials as previously suggested, but can be represented by power laws truncated at low luminosity. The cutoff occurs at 20-30 mJy and may represent emission from a slowly varying basal level that is always detected. We confirm several previous results, including the strong dependence of spectral index on luminosity, the decrease of fractional circular polarization with luminosity, and the dependence of fractional circular polarization on orbital inclination angle. We suggest that the radio emission at 8.3 GHz is x-mode gyrosynchrotron emission from optically thin emission regions containing mildly relativistic electrons in a dipolar magnetic field. There is no evidence for highly circularly polarized coherent flares at 8.3 GHz, although it is possible that such flares occasionally occur at 2.25 GHz. The lack of orbital phase dependence in the GBI light curves, combined with the significant inclination of the VLBA structure with respect to the orbital plane, is inconsistent with previous models in which the radio lobes are located in the equatorial plane. The individual lobes seen in the VLBI maps may be associated with the polar regions, with the strong circular polarization resulting from the opposed mean magnetic field vector component along the observers line of sight in opposite hemispheres. Astrometric results from the VLBA observations are discussed in a companion paper.

Long-Term Flux Monitoring of LSI +61°303 at 2.25 and 8.3 GHz

LSI +61°303 is an exotic binary system consisting of a ~10 M☉ B star and a compact object that is probably a neutron star. The system is associated with the interesting radio source GT 0236+610, which exhibits bright radio outbursts with a period of 26.5 days. We report the results of continuous daily radio interferometric observations of GT 0236+610 at 2.25 and 8.3 GHz from 1994 January to 1996 February. The observations cover 25 complete (and three partial) cycles with multiple observations each day. We detect substantial cycle-to-cycle variability of the radio emission characterized by a rapid onset of the radio flares followed by a more gradual decrease of the emission. We detect a systematic change of the radio spectral index α (defined as Sν ∝ να), which typically becomes larger than zero at the onset of the radio outbursts. This behavior is suggestive of expansion of material initially optically thick to radio frequencies, indicating either that synchrotron or inverse Compton cooling are important or that the free-free optical depth to the source is rapidly changing. After two years of observations, we see only weak evidence for the proposed 4 year periodic modulation in the peak flux of the outbursts. We observe a secular trend in the outburst phases according the best published ephemeris. This trend indicates either orbital period evolution, or a drift in outburst orbital phase in response to some other change in the system.

Statistical Analysis of 5 Year Continuous Radio Flare Data from β Persei, V711 Tauri, δ Librae, and UX Arietis

We report on the longest-running continuous radio flare survey of two Algol-type systems (β Per and δ Lib) and two RS CVn systems (V711 Tau and UX Ari). All four systems have late-type components, and all were known to display radio flaring activity. The primary aim of the campaign was to determine the timescales for flaring activity in these systems. The radio continuum flux at 2.3 and 8.3 GHz was monitored with the NRAO-Green Bank Interferometer from 1995 January to 2000 October. The survey spanned 2096 days with interruptions during maintenance runs and temporary closings of the interferometer. Many strong flares were detected with continuum fluxes at 8.3 GHz as high as 1.17 Jy in β Per, 1.44 Jy in V711 Tau, and 0.82 Jy in UX Ari. Only two flares were detected from δ Lib during 1123 days of monitoring, and the continuum flux reached a maximum of only 0.034 Jy at 8.3 GHz. The independent techniques of Power Spectrum Analysis and Phase Dispersion Minimization were used to determine the periodicity of flaring activity in each binary. The strongest periodicities found were 48.9 ± 1.7 days for β Per, 120.7 ± 3.4 days for V711 Tau, and 141.4 ± 4.5 days for UX Ari, with other significant periodicities of 80.8 ± 2.5 days for V711 Tau and 52.6 ± 0.7 days for UX Ari. In the case of δ Lib, the strongest periodicities were related to the duration of the two monitoring cycles within the data set and are not real. The continuous survey has demonstrated that there are active and quiescent flaring cycles in V711 Tau and β Per. During both of these cycles, β Per had more flares than V711 Tau, but its strongest flares were typically weaker than those of V711 Tau.