Searching for low mass objects around nearby dMe radio stars

Nearby M-dwarfs are best suited for searches of low mass companions. VLBI phase-referencing observations with sensitive telescopes are able to detect radio star flux-densities of tenths of mJy as well as to position the star on the sky with submilliarcsecond precision. We have initiated a long-term observational program, using EVN telescopes in combination with NASA DSN dishes, to revisit the kinematics of nearby, single M dwarfs. The precision of the astrometry allows us to search for possible companions with masses down to 1 Jupiter mass. In this contribution we report preliminary results of the first observation epochs, in which we could detect some of the radio stars included in our program.Comment: Proceedings of the 6th European VLBI Network Symposium, Ros E., Porcas R.W., Lobanov A.P., & Zensus J.A. (eds.), MPIfR, Bonn, Germany, p. 255-258 (2002). 4 pages, 3 figures, needs evn2002.cl

Compact Structure in FIRST Survey Sources

We present preliminary results from a statistical survey of compact structure in faint radio sources. Around 1000 sources from the VLA FIRST survey (flux densities larger than 1 mJy at 1.4 GHz) have been observed with the single-baseline interferometer Effelsberg-Arecibo. We observed each source, selected from a narrow strip of sky at declination 28 deg, for just one minute. The baseline sensitivity at 1.4 GHz, using 512 Mb/s recording, is such that any FIRST source, selected at random, would be detected if most of its flux density is in compact structure. We discuss the detection-rate statistics from one epoch of these observations.Comment: 4 pages. 12 figures. Proceedings of the 7th European VLBI Network Symposium held in Toledo, Spain on October 12-15, 2004. Editors: R. Bachiller, F. Colomer, J.-F. Desmurs, P. de Vicente (Observatorio Astronomico Nacional), p. 31-34. Needs evn2004.cl

Evidence for Nonlinear X-ray Variability from the Broad-line Radio Galaxy 3C 390.3

We present analysis of the light curve from the ROSAT HRI monitoring observations of the broad-line radio galaxy 3C 390.3. Observed every three days for about 9 months, this is the first well sampled X-ray light curve on these time scales. The flares and quiescent periods in the light curve suggest that the variability is nonlinear, and a statistical test yields a detection with >6 sigma confidence. The structure function has a steep slope ~0.7, while the periodogram is much steeper with a slope ~2.6, with the difference partially due to a linear trend in the data. The non-stationary character of the light curve could be evidence that the variability power spectrum has not turned over to low frequencies, or it could be an essential part of the nonlinear process. Evidence for X-ray reprocessing suggests that the X-ray emission is not from the compact radio jet, and the reduced variability before and after flares suggests there cannot be two components contributing to the X-ray short term variability. Thus, these results cannot be explained easily by simple models for AGN variability, including shot noise which may be associated with flares in disk-corona models or active regions on a rotating disk, because in those models the events are independent and the variability is therefore linear. The character of the variability is similar to that seen in Cygnus X-1, which has been explained by a reservoir or self-organized criticality model. Inherently nonlinear, this model can reproduce the reduced variability before and after large flares and the steep PDS seen generally from AGN. The 3C 390.3 light curve presented here is the first support for such models to explain AGN variability on intermediate time scales from a few days to months.Comment: 10 pages using (AASTeX) aaspp4.sty and 3 Postscript figures. Astrophysical Journal Letters, in pres

DiFX2: A more flexible, efficient, robust and powerful software correlator

Software correlation, where a correlation algorithm written in a high-level language such as C++ is run on commodity computer hardware, has become increasingly attractive for small to medium sized and/or bandwidth constrained radio interferometers. In particular, many long baseline arrays (which typically have fewer than 20 elements and are restricted in observing bandwidth by costly recording hardware and media) have utilized software correlators for rapid, cost-effective correlator upgrades to allow compatibility with new, wider bandwidth recording systems and improve correlator flexibility. The DiFX correlator, made publicly available in 2007, has been a popular choice in such upgrades and is now used for production correlation by a number of observatories and research groups worldwide. Here we describe the evolution in the capabilities of the DiFX correlator over the past three years, including a number of new capabilities, substantial performance improvements, and a large amount of supporting infrastructure to ease use of the code. New capabilities include the ability to correlate a large number of phase centers in a single correlation pass, the extraction of phase calibration tones, correlation of disparate but overlapping sub-bands, the production of rapidly sampled filterbank and kurtosis data at minimal cost, and many more. The latest version of the code is at least 15% faster than the original, and in certain situations many times this value. Finally, we also present detailed test results validating the correctness of the new code.Comment: 28 pages, 9 figures, accepted for publication in PAS

870 micron observations of nearby 3CRR radio galaxies

We present submillimeter continuum observations at 870 microns of the cores of low redshift 3CRR radio galaxies, observed at the Heinrich Hertz Submillimeter Telescope. The cores are nearly flat spectrum between the radio and submillimeter which implies that the submillimeter continuum is likely to be synchrotron emission and not thermal emission from dust. The emitted power from nuclei detected at optical wavelengths and in the X-rays is similar in the submillimeter, optical and X-rays. The submillimeter to optical and X-ray power ratios suggest that most of these sources resemble misdirected BL Lac type objects with synchrotron emission peaking at low energies. However we find three exceptions, the FR I galaxy 3C264 and the FR II galaxies 3C390.3 and 3C338 with high X-ray to submillimeter luminosity ratios. These three objects are candidate high or intermediate energy peaked BL Lac type objects. With additional infrared observations and from archival data, we compile spectral energy distributions (SEDs) for a subset of these objects. The steep dips observed near the optical wavelengths in many of these objects suggest that extinction inhibits the detection and reduces the flux of optical continuum core counterparts. High resolution near or mid-infrared imaging may provide better measurements of the underlying synchrotron emission peak.Comment: accepted for publication in A

Optical Monitoring of 3C 390.3 from 1995 to 2004 and Possible Periodicities in the Historical Light Curve

We report V, R, and I band CCD photometry of the radio galaxy 3C 390.3 obtained with the 1.56-m telescope of the Shanghai Astronomical Observatory from March 1995 to August 2004. Combining these data with data from the literature, we have constructed a historical light curve from 1894 to 2004 and searched for periodicities using the CLEANest program. We find possible periods of 8.30+-1.17, 5.37+-0.49, 3.51+-0.21, and 2.13+-0.08 years.Comment: Accepted by AJ, 34 pages, 11 figure

EVN observations of H2O masers towards high-mass star forming regions

We have conducted Very Long Baseline Interferometry (VLBI) observations of the 22.2 GHz water masers towards three massive star forming regions. With three observing epochs, few distinct maser features were detected for each source. For features persistent in time, accurate values of the proper motions are derived. A preliminary analysis indicates that the kinematics traced by water masers is consistent with expanding motions, possibly driven by wide-angle winds

EVN observations of water masers towards the high-mass young stellar object in AFGL 5142

We have conducted multi-epoch EVN observations of the 22.2 GHz water masers towards the high-mass young stellar object in AFGL 5142. With four observing epochs, spanning a time of ~1 year, 12 distinct maser features have been detected, 7 out of these detected in more than one epoch. The positions and the velocities of the VLBI features agree well with those of the emission centers previously identified by means of VLA observations. For a few features, persistent over three or four epochs, accurate values of the proper motions are derived. On the basis of their spatial distribution, the observed maser features can be divided into two groups. A model fit to the positions and velocities of the maser features of Group I, detected in the same region (within ~500 mas) where the massive YSO should be located, demonstrates that these might arise on the surface of a nearly edge-on Keplerian disk, rotating around a massive young stellar object. The maser features of Group II, found at large distances from the YSO (> 1 arcsec), have positions and line-of-sight velocities in agreement with the blue-shifted lobe of a large scale molecular outflow (traced by the HCO+ and SiO emission), and might result from the interaction between the gas flowing away from the young stellar object and the ambient gas of the progenitor molecular core.Comment: 9 pages, 3 figures, to be published in A&

VLBA Polarimetry of Three Powerful Radio Galaxy Cores

We present sensitive, high dynamic range, Very Long Baseline Array (VLBA) polarimetric observations of the cores of three powerful radio galaxies: 3C166, 3C236 and 3C390.3. Significant polarization is detected in one source (3C166) allowing us to map out the Faraday Rotation Measure (RM) distribution and projected magnetic field direction. The inner jet of 3C166 is found to have a rest frame RM of -2300 rad/m**2, similar to those found in quasars cores. No polarized flux was detected from the other two sources, but in both counterjets are seen. The counterjet in 3C236 was previously known, but the detection in 3C390.3 is a new discovery. We suggest that the low fractional polarization in radio galaxy cores is the result of Faraday depolarization by ionized gas associated with the accretion disk. The lower polarization of radio galaxy cores compared to quasars is then naturally explained by unified models as a result of the viewing angle.Comment: Accepted to ApJ, 19 pages including 5 figure

VLBI Observations of a Complete Sample of Radio Galaxies. 10 Years Later

A complete sample of 27 radio galaxies was selected from the B2 and 3CR catalogs, in order to study their properties on the milliarcsecond scale. In the Appendix of this paper we present new radio images for 12 of them. Thanks to the present data, all the sources in this sample have been imaged at mas resolution. We discuss the general results. In particular we stress the evidence for high velocity jets in low power radio galaxies, we compare high and low power sources, and discuss the source properties in the light of the unified scheme models. We derive that the properties of parsec scale jets are similar in sources with different total radio power and kpc scale morphology. From the core - total radio power correlation, we estimate that relativistic jets with Lorentz factor $\gamma$ in the range 3 - 10 are present in high and low power radio sources. We discuss also the possible existence of a two velocity structure in the jets (fast spine and lower velocity external shear layer).Comment: 38 pages, 18 figures with an Appendix; ApJ in press (vol. 551, Apr. 20, 2001 issue