Frank D. Ghigo

Overview of the coordinated ground-based observations of Titan during the Huygens mission

Coordinated ground-based observations of Titan were performed around or during the Huygens atmospheric probe mission at Titan on 14 January 2005, connecting the momentary in situ observations by the probe with the synoptic coverage provided by continuing ground-based programs. These observations consisted of three different categories: (1) radio telescope tracking of the Huygens signal at 2040 MHz, (2) observations of the atmosphere and surface of Titan, and (3) attempts to observe radiation emitted during the Huygens Probe entry into Titans atmosphere. The Probe radio signal was successfully acquired by a network of terrestrial telescopes, recovering a vertical profile of wind speed in Titans atmosphere from 140 km altitude down to the surface. Ground-based observations brought new information on atmosphere and surface properties of the largest Saturnian moon. No positive detection of phenomena associated with the Probe entry was reported. This paper reviews all these measurements and highlights the achieved results. The ground-based observations, both radio and optical, are of fundamental importance for the interpretation of results from the Huygens mission.

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.

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.

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.

Green Bank Telescope Measurement of the Systemic Velocity of the Double Pulsar Binary J0737–3039 and Implications for Its Formation

We report on the measurement at 820 and 1400 MHz of the orbital modulation of the diffractive scintillation timescale from pulsar A in the double-pulsar system J07373039 using the Green Bank Telescope. With fits to this modulation, we determine the systemic velocity in the plane of the sky to be km s 1 . The V 140.9 6.2 ISS parallel and perpendicular components of this velocity with respect to the line of nodes of the pulsar’s orbit are km s 1 and km s 1 , respectively. The large implies that pulsar B was V 96.0 3.7 V 103.1 7.7 V plane perp perp born with a kick speed of 100 km s 1 . Future Very Long Baseline Array determinations of the angular proper motion in conjunction with improved measurements should provide a precise distance to the system. Using VISS high-precision timing data and the model, we estimate a best-fit orbital inclination of . Vi p 88 .7 0 .9 ISS

A Dual-Frequency, Multiyear Monitoring Program of Compact Radio Sources

We present light curves for 149 sources monitored with the Green Bank Interferometer. The light curves are at two radio frequencies (approximately 2.5 and 8.2 GHz) and range from 3 to 15 yr in length, covering the interval 1979-1996, and have a typical sampling of one flux density measurement every 2 days. We have used these light curves to conduct various variability analysis (rms flux density variations and autoregressive, integrated, moving average modeling) of these sources. We find suggestive though not unambiguous evidence that these sources have a common, broadband mechanism for intrinsic variations, in agreement with previous studies of a subset of these sources. We also find that the sources generally display a short-term variability (~10 days) that arises from radio-wave scattering in an extended medium. These conclusions extend those of Fiedler et al., who used a subsample of these data. The primary motivation for this monitoring program was the identification of extreme scattering events (ESEs). In an effort to identify ESEs in a systematic manner, we have taken the wavelet transform of the light curves. We find 15 events in the light curves of 12 sources that we classify as probable ESEs. However, we also find that five ESEs previously identified from these data do not survive our wavelet selection criteria. Future identification of ESEs will probably continue to rely on both visual and systematic methods. Instructions for obtaining the data are also presented.

RADIOASTRON OBSERVATIONS OF THE QUASAR 3C273: A CHALLENGE TO THE BRIGHTNESS TEMPERATURE LIMIT

Inverse Compton cooling limits the brightness temperature of the radiating plasma to a maximum of