Green Bank Interferometer Research Articles

Classification of multiwavelength transients with machine learning

ABSTRACT With the advent of powerful telescopes such as the Square Kilometer Array and the Vera C. Rubin Observatory, we are entering an era of multiwavelength transient astronomy that will lead to a dramatic increase in data volume. Machine learning techniques are well suited to address this data challenge and rapidly classify newly detected transients. We present a multiwavelength classification algorithm consisting of three steps: (1) interpolation and augmentation of the data using Gaussian processes; (2) feature extraction using wavelets; and (3) classification with random forests. Augmentation provides improved performance at test time by balancing the classes and adding diversity into the training set. In the first application of machine learning to the classification of real radio transient data, we apply our technique to the Green Bank Interferometer and other radio light curves. We find we are able to accurately classify most of the 11 classes of radio variables and transients after just eight hours of observations, achieving an overall test accuracy of 78 per cent. We fully investigate the impact of the small sample size of 82 publicly available light curves and use data augmentation techniques to mitigate the effect. We also show that on a significantly larger simulated representative training set that the algorithm achieves an overall accuracy of 97 per cent, illustrating that the method is likely to provide excellent performance on future surveys. Finally, we demonstrate the effectiveness of simultaneous multiwavelength observations by showing how incorporating just one optical data point into the analysis improves the accuracy of the worst performing class by 19 per cent.

Robert Lamme Brown

Radio astronomer Robert Lamme Brown died suddenly of a heart attack on 20 December 2014 in Florence, South Carolina, while traveling to Disney World to visit his grandchildren, to whom he was devoted.Robert Lamme BrownNRAO/AUI/NSFPPT|High resolutionBorn on 17 August 1943 in Los Angeles, Brown was raised in Inglewood, California. While attending the University of California, Berkeley, he worked in Kinsey Anderson’s cosmic-ray research group. He received his BA in 1965. He then went to the University of California, San Diego, where he worked under Robert Gould and was awarded a PhD in 1969. His thesis, “Interstellar absorption of cosmic x rays,” was published in Physical Review D in 1970 and was one of the 100 most cited papers in astronomy and physics during the 1970s.Brown joined the National Radio Astronomy Observatory (NRAO) as a postdoctoral fellow. Although trained as a theorist, he quickly became a skilled observer. A deft manager, he served as the site director of the NRAO’s Green Bank and Tucson facilities, head of its Millimeter Array (MMA) project, and NRAO deputy director.One of Brown’s most significant discoveries, made with Bruce Balick, was the compact radio source at the center of the galaxy, which he later named Sagittarius A*. On 13 and 15 February 1974, the NRAO Green Bank interferometer and a 45-foot antenna in Huntersville, West Virginia, combined to provide enough resolution and sensitivity to detect the faint, compact source. It has since been shown to be coincident with the center of the Milky Way and driven by a massive black hole. The discovery was a fundamental advance in our understanding of galactic nuclei. Brown was also an expert on radio recombination lines, and the 1978 review he wrote with Jay Lockman and Gillian Knapp remains the standard.Brown’s interest in extragalactic astronomy centered on spectral lines in high-redshift objects. He and Morton Roberts were the first to detect neutral hydrogen (HI) in absorption against a quasar, 3C286. Observing HI absorption led Brown to search for carbon monoxide emission from the hyperluminous IR galaxy F10214+4724. He and I found the CO(3→2) rotational line, which extended by a factor of more than 10 the redshift of CO emission detections in IR-rich galaxies. The discovery was a major motivation for the development of the MMA, one of three precursors that merged to become the Atacama Large Millimeter/Submillimeter Array (ALMA).His aptitude for doing several things simultaneously served Brown admirably at the NRAO. He could write effortlessly, and as deputy director he was responsible for drafting the many program plans, long-range plans, and reports required by NSF. A patient person, Brown was well suited to help during negotiations, such as of the agreements with the Soviet Union and Japan for space-based very long baseline interferometry; the merger of the MMA, the Large Millimeter/Submillimeter Array of Japan, and the European Large Southern Array into ALMA; and the frequently contentious process to determine the NRAO’s yearly investment in new instrumentation. His perseverance was essential during the years it took to secure funding to keep the MMA project alive and to join forces with Europe and Japan to form ALMA.A lover of adventure and travel, Brown was the first NRAO member to visit the Atacama Desert of northern Chile as a possible location for the MMA. He immediately recognized its potential and initiated site testing. The location eventually became the site for ALMA, and it has been critical to the array’s stunning success. Indeed, ALMA’s success owes as much to Brown’s efforts as to anyone’s.Brown was enthusiastic about working with young people. He supervised NRAO summer students, mentored postdoctoral fellows, and supervised five graduate students at the University of Virginia.In 2003 Brown left the NRAO to become the director of the National Astronomy and Ionosphere Center (NAIC) and its principal facility, the Arecibo Observatory. Commuting between Cornell University, which was NSF’s contractor for the NAIC, and Arecibo in Puerto Rico, Brown made many improvements to the observatory’s operation.To channel his energy after he retired in 2008, Brown and his wife, Karen, opened an ice cream parlor in Corolla, on the Outer Banks of North Carolina. He defined his shop role as “chief financial officer and head of janitorial services.”Friendly, outgoing, and well liked, Brown had an infectious laugh that resonated in the halls of the NRAO. He is missed by his family, friends, and colleagues.© 2015 American Institute of Physics.

Discovery of a periodical apoastron GeV peak in LS I +61°303

Aims. The aim of this paper is to analyse the previously discovered discontinuity of the periodicity of the GeV $\gamma$-ray emission of the radio-loud X-ray binary LS I +61{\deg}303 and to determine its physical origin. Methods. We used wavelet analysis to explore the temporal development of periodic signals. The wavelet analysis was first applied to the whole data set of available Fermi-LAT data and then to the two subsets of orbital phase intervals $\Phi = 0.0 - 0.5$ and $\Phi = 0.5 - 1.0$. We also performed a Lomb-Scargle timing Analysis. We investigated the similarities between GeV $\gamma$-ray emission and radio emission by comparing the folded curves of the Fermi-LAT data and the Green Bank Interferometer radio data. Results. During the epochs when the timing analysis fails to determine the orbital periodicity, the periodicity is present in the two orbital phase intervals $\Phi = 0.0 - 0.5$ and $\Phi = 0.5 - 1.0$. That is, there are two periodical signals, one towards periastron (i.e., $\Phi = 0.0 - 0.5$) and another one towards apoastron ($\Phi = 0.5 - 1.0$). The apoastron peak seems to be affected by the same orbital shift as the radio outbursts and, in addition, reveals the same two periods $P_1$ and $P_2$ that are present in the radio data. Conclusions. The $\gamma$-ray emission of the apoastron peak normally just broadens the emission of the peak around periastron. Only when it appears at $\Phi \approx 0.8 - 1.0$, because of the orbital shift, it is enough detached from the first peak to become recognisable as a second orbital peak, which is the reason why the timing analysis fails. Two $\gamma$-ray peaks along the orbit are predicted by the two-peak accretion model for an eccentric orbit, that was proposed by several authors for LS I +61{\deg}303.

Long-term periodicity in LS I +61°303 as beat frequency between orbital and precessional rate

Context: In the binary system LSI+61303 the peak flux density of the radio outburst, which is related to the orbital period of 26.4960 +/- 0.0028d, exibits a modulation of 1667 +/- 8 d. The radio emission at high spatial resolution appears structured in a precessing jet with a precessional period of 27-28 d. Aims: How close is the precessional period of the radio jet to the orbital period? Any periodicity in the radio emission should be revealed by timing analysis. The aim of this work is to establish the accurate value of the precessional period. Methods: We analyzed 6.7 years of the Green Bank Interferometer database at 2.2 GHz and 8.3 GHz with the Lomb-Scargle and phase dispersion minimization (PDM) methods and performed simulations. Results: The periodograms show two periodicities, P1 = 26.49 +/- 0.07 d (\nu1=0.03775 d^{-1}) and P2 = 26.92 +/- 0.07 d (\nu2 = 0.03715 d^{-1}). Whereas radio outbursts have been known to have nearly orbital occurrence P1 with timing residuals exhibiting a puzzling sawtooth pattern, we probe in this paper that they are actually periodical outbursts and that their period is Paverage= (2/(\nu1 + \nu2)= 26.70 +/- 0.05 d. The period Paverage as well as the long-term modulation Pbeat=1/(\nu1 - \nu2)=1667 +/- 393 d result from the beat of the two close periods, the orbital P1 and the precessional P2 periods. Conclusions: The precessional period, indicated by the astrometry to be of 27--28 d, is P2=26.92 d. The system \lsi seems to be one more case in astronomy of beat, i.e., a phenomenon occurring when two physical processes create stable variations of nearly equal frequencies. The very small difference in frequency creates a long-term variation of period 1/(\nu1-\nu2). The long-term modulation of 1667 d results from the beat of the two close orbital and precessional rates.

RADIO SPECTRAL INDEX ANALYSIS AND CLASSES OF EJECTION IN LS I +61°303

LS I +61303 is a gamma-ray binary with periodic radio outbursts coincident with the orbital period of P=26.5 d. The origin of the radio emission is unclear,it could be due either to a jet, as in microquasars, or to the shock boundary between the Be star and a possible pulsar wind. We here analyze the radio spectral index over 6.7 yr from Green Bank Interferometer data at 2.2 GHz and 8.3 GHz. We find two new characteristics in the radio emission. The first characteristic is that the periodic outbursts indeed consist of two consecutive outbursts; the first outburst is optically thick, whereas the second outburst is optically thin. The spectrum of LS I +61 303 is well reproduced by the shock-in-jet model commonly used in the context of microquasars and AGNs: the optically thin spectrum is due to shocks caused by relativistic plasma ("transient jet") traveling through a pre-existing much slower steady flow ("steady jet"). This steady flow is responsible for the preceding optically thick spectrum. The second characteristic we find is that the observed spectral evolution, from optically thick to optically thin emission, occurs twice during the orbital period. We observed this occurrence at the orbital phase of the main 26.5 d outburst and also at an earlier phase, shifted by $\Delta \Phi \sim$ 0.3 (i.e almost 8 days before). We show that this result qualitatively and quantitatively agrees with the two-peak accretion/ejection model proposed in the past for LS I +61303. We conclude that the radio emission in LS I +61303 originates from a jet and suggest that the variable TeV emission comes from the usual Compton losses expected as an important by-product in the shock-in-jet theory.

Interstellar Scintillation Observations of 146 Extragalactic Radio Sources

From 1979--1996 the Green Bank Interferometer was used by the Naval Research Laboratory to monitor the flux density from 146 compact radio sources at frequencies near 2 and 8 GHz. We filter the ``light curves'' to separate intrinsic variations on times of a year or more from more rapid interstellar scintilation (ISS) on times of 5--50 d. Whereas the intrinsic variation at 2 GHz is similar to that at 8 GHz (though diminished in amplitude), the ISS variation is much stronger at 2 than at 8 GHz. We characterize the ISS variation by an rms amplitude and a timescale and examine the statistics of these parameters for the 121 sources with significant ISS at 2 GHz. We model the scintillations using the NE2001 Galactic electron model assuming the sources are brightness-limited. We find the observed rms amplitude to be in general agreement with the model, provided that the compact components of the sources have about 50% of their flux density in a component with maximum brightness temperatures $10^{11}$--$10^{12}$K. Thus our results are consistent with cm-wavelength VLBI studies of compact AGNs, in that the maximum brightness temperatures found are consistent with the inverse synchrotron limit at $3 \times 10^{11}$ K, boosted in jet configurations by Doppler factors up to about 20. The average of the observed 2 GHz ISS timescales is in reasonable agreement with the model at Galactic latitudes above about 10\de. At lower latitudes the observed timescales are too fast, suggesting that the transverse plasma velocity increases more than expected beyond about 1 kpc.

Periodicities of Radio Flares in RS CVn and Algol Binaries

Periodicities of radio flaring activity from the Algol systems β Per and δ Lib and the RS CVn systems V711 Tau and UX Ari were determined from a continuous 5-year survey. The radio continuum fluxes at 2.3 GHz and 8.3 GHz were monitored with the NRAO—Green Bank Interferometer for 2096 days from 1995 January to 2000 October. The maximum detected flare strengths at 8.3 GHz were 1.17 Jy for β Per, 0.034 Jy for δ Lib, 1.44 Jy for V711 Tau, and 0.82 Jy for UX Ari. 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 days for β Per, 120.7 days for V711 Tau, and 141.4 days for UX Ari. No significant periodicities were found for δ Lib. The continuous survey has demonstrated that there are active and quiescent flaring cycles in V711 Tau and β Per.

Correlated Radio:X‐Ray Emission in the Hard States of Galactic Microquasars

We present results of our study of correlated radio and X-ray emission in two black hole candidates and Galactic microquasars GRS 1915+105 and Cyg X-1 in their steady long-term hard states, along with Cyg X-3 (using data obtained from the Rossi X-Ray Timing Explorer all-sky monitor [RXTE-ASM], Compton Gamma Ray Observatory Burst and Transient Source Experiment [CGRO-BATSE], and Green Bank Interferometer [GBI]). We detect a pivotal behavior in the X-ray spectrum of GRS 1915+105, correlated to the radio emission. Similar to the results obtained for Cyg X-3, the flux of X-rays softer than the pivoting point correlates with the radio emission, while the corresponding harder X-ray flux anticorrelates with both the radio and the softer X-ray emission, in this state. We examine all the previously reported correlations of X-ray properties with the radio emission in Galactic microquasars and argue that these are consistent with a general picture where a spectral pivoting is a common feature in these sources with the shape of the spectrum determining the flux of radio emission, during the hard states. We also detect a general monotonic increase in the radio emission of these sources with the soft X-ray emission spanning about 5 orders of magnitude. We qualitatively explain these findings with a two-component advective flow model where the location of a boundary layer between the thin disk and the Comptonizing region determines the spectral shape and also the amount of outflow.

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.

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.

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 observer's line of sight in opposite hemispheres. Astrometric results from the VLBA observations are discussed in a companion paper.

Discovery of Correlated Behavior between the Hard X‐Ray and the Radio Bands in Cygnus X‐3

Using CGRO/BATSE hard X-ray data and GHz radio monitoring data from the Green Bank Interferometer (GBI), we have performed a long term study (approximately 1800 days) of the unusual X-ray binary Cygnus X-3 resulting in the discovery of a remarkable relationship between the two wavelength bands. We find that quiescent radio states are strongly anticorrelated with the intensity of the hard X-ray emission. With the onset of a flaring state the relationship switches to a correlation. This is most easily seen in the larger (> or = 1 Jy) flares where the preflare quenched radio emission is accompanied by a very low hard X-ray intensity, and recovery of the hard X-ray flux during the radio flare. The injection of plasma into the radio jets, associated with Cyg X-3, can be directly related to changes in the hard X-ray emission, and suggests the possibility of accretion-related and jet-related components of the high energy emission.

EGRET Observations of the Gamma‐Ray Source 2CG 135+01

The COS B source 2CG 135+01 has been observed by the EGRET instrument on 10 different occasions during the first ~52 months of the Compton Gamma Ray Observatory mission. The source is detected in all but one of the observations. For that one, the exposure was inadequate. The only likely source that is spatially coincident with the gamma-ray position is the radio source GT 0236+610/LS I +61°303. However, there is no compelling evidence for time variations in the gamma-ray emission associated with the radio outbursts from GT 0236+610. Spectral determinations on a timescale of a few days also give no strong evidence for a spectral variation associated with the radio emission of GT 0236+610. Such fluctuations might be expected based on models involving a compact object in an elliptical binary orbit about a massive star. The search for correlations simultaneous with the 8.4 GHz radio outbursts were supported by coordinated observations with the Madrid Deep Space Network during one of the exposures and by Green Bank Interferometer observations on two others. Although there is some possible variability in the gamma-ray flux, it is not clear that it is related to the radio phasing.

The evolution of outbursts in Cygnus X-3

Quenching of the X-ray binary Cygnus X-3 at 2.25 GHz prior to outbursts during 1990–1993 is compared with published multi-frequency results. Flux density measurements obtained at 15 GHz with the Ryle Telescope during 1994–1995 are compared with flux densities at 2.25 GHz obtained with the NRL Green Bank Interferometer. Quenched quiescent emission prior to outbursts and partially quenched emission during small flaring episodes may be explained by disk instabilities during super-Eddington and near-Eddington accretion in X-ray binaries.

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.

Radio Observations of the γ-Ray Blazar 0528+134

We report multifrequency observations of the γ-ray blazar 0528+134 with the Effelsberg 100-m telescope, the IRAM 30-m telescope at Pico Veleta and the NRL Green Bank Interferometer. The observing methods are described elsewhere (Reich et al., 1993; Pohl et al., 1995). The radio lightcurves are given in Fig.1 in comparison to the status of 0528+134 in the EGRET energy range. The uncertainties in the flux densities quoted there are less than 5% at 10.55 GHz and lower frequencies, while slightly exceeding this value at 32 GHz and 86 GHz.

The Evolution of Outbursts in Cygnus X-3 at 2.25 and 8.3 GHz

view Abstract Citations (60) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Evolution of Outbursts in Cygnus X-3 at 2.25 and 8.3 GHz Waltman, E. B. ; Ghigo, F. D. ; Johnston, K. J. ; Foster, R. S. ; Fiedler, R. L. ; Spencer, J. H. Abstract Detailed observations of a long outburst in the X-ray binary Cygnus X-3 at 2.25 and 8.3 GHz in early 1994 are presented. The source has been monitored at dual radio frequencies on a daily basis with the Green Bank Interferometer since 1982. This outburst was predicted based on the behavior of the quiescent flux densities prior to the major flare. The data show that the 1994 February 20 flare occurred just one day after 19 days of very low quiescent flux densities and exhibited higher opacity at 2 GHz than usual for Cygnus X-3 flares. Later flares in the 1994 February-March outburst sequence displayed progressively less opacity as determined from the observed ratios of peak flux densities at the two frequencies, the lags between the peaks, and the spectral index. Examination of the evolution of spectral index during the 1994 February outburst, compared to previous outbursts in the history of Cygnus X-3, shows that the trend toward optically thin flares begins with the onset of outburst activity immediately following the very low quiescent flux densities and may relate to the very low flux densities. This behavior can be interpreted either as evidence for a declining opacity from thermal gas in the binary system or for evolution of the exponential lateral expansion rate of the jet during the series of flares in the outburst. Publication: The Astronomical Journal Pub Date: July 1995 DOI: 10.1086/117518 Bibcode: 1995AJ....110..290W Keywords: BINARIES: CLOSE; X-RAYS: STARS full text sources ADS |

A summary of extreme scattering events and a descriptive model

view Abstract Citations (109) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Summary of Extreme Scattering Events and a Descriptive Model Fiedler, R. ; Dennison, B. ; Johnston, K. J. ; Waltman, E. B. ; Simon, R. S. Abstract An ongoing monitoring program of compact extragalactic radio sources over the past 11 years has revealed many instances of unusual variations that cannot be explained using the standard models for intrinsic variability or normal refractive interstellar scintillation. The observations included simultaneous daily measurements at 2.695 and 8.085 GHz of 40 to 150 sources using the Green Bank interferometer over the period 1979-1989. Approximately 330 sources were also monitored with the NRAO 300 foot radio telescope at 3.25 and 4.675 GHz over the 6 month period 1987 December to 1988 May. In total, 594 source-years of observations have been obtained, of which 4.8 yr constitutes the total time span of unusual variability. The unusual variations, denoted as extreme scattering events (ESEs), are summarized here. The most striking property of ESEs is their frequency dependence, in which departures from the quiescent flux density (by as much as 50% at 2.7 GHz) have no counterpart at 8.1 GHz. The single exception occurred in the 8.1 GHz light curve of 0954+658, which showed spiky variations during the 2.7 GHz event. To date, 10 ESEs have been tentatively identified in the 2.7 GHz light curves of nine quasars. These ESEs appear to occur preferentially near loops in the Galactic foreground brightness distribution. A simple model based on diffraction or refraction through Galactic regions of unusually high electron density turbulence is presented. Publication: The Astrophysical Journal Pub Date: August 1994 DOI: 10.1086/174432 Bibcode: 1994ApJ...430..581F Keywords: Electron Density (Concentration); Electron Scattering; Extragalactic Radio Sources; Galactic Structure; Interstellar Matter; Plasma Turbulence; Refraction; Astronomical Models; Brightness Distribution; Cosmology; Light Curve; Quasars; Astrophysics; ISM: STRUCTURE; RADIO CONTINUUM: GALAXIES; TURBULENCE full text sources ADS | data products SIMBAD (15) NED (9)

Daily observations of compact radio sources at 2.7 and 8.1 GHz - 1979-1987

2.7 and 8.1 GHz radio light curves are presented for 46 compact radio sources observed with the Green Bank interferometer during the period 1979-1987. Flux densities for the period 1979-1985 were reported in a previous paper. The flux densities reported here extend the daily monitoring an additional 2 years. The interferometer is currently dedicated to searching for extreme scattering events (ESEs). ESEs may be found in the radio light curves of 0300+470, 0333+321, 0954+658, 1502+106, 1611+343, 1821+107, and 2352+495 during the period 1979-1987, including three events observed in 0300+470 and 0333+321 during the additional 2 years reported here

Long-Term Variations of the Compact Radio Source Sgr A∗ at the Galactic Center

We investigate the long-term flux density variations of the compact radio source Sgr A∗ at the galactic center by combining recent VLA observations with previous Green Bank interferometer data. We present radio flux density light-curves for Sgr A∗ at 20, 11, 6 and 3.7 cm from 1974 to 1987. Long-term variability with a timescale of at least 5 years is seen at 20 cm and there is evidence for more rapid variations at the shorter wavelengths. The variability timescales at 20, 11 and 6 cm fit the λ2 scaling predicted by the theory of refractive scintillation suggesting that the variability could be due to this cause. However, the timescales are relatively short, implying an unusually high velocity in the scattering screen. The modulation index of the variability is large and relatively independent of wavelength.