Session 3
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Carla Buemi1,2
cbuemi@ira.noto.cnr.it
1Instituto di Radioastronomia - Bologna, Italy
2Istituto di Astronomia, Universitá di Catania, Italy
A multi-wavelength observing campaign on the binary system AR Lac, planned for the next autumn, which involves radio and X-rays observations, will allow us to investigate stellar coronae at radio wavelengths.
The X-ray observations of close binary systems pointed out the coesistence in the coronal layers of regions with different temperature, pressure and size. The observed X-ray spectra of this kind of systems have often been fitted using thermal plasma models with two-temperature plasma components. The existence of such a structured corona is supported by previous multi-frequency VLA observations of close binary systems, which showed as the observed variability and the changes in the radio spectrum are consistent with a core-halo morphology of the radio corona, where the physical conditions vary due to the magnetic activity. Still is not clear if the radio emitting core and halo can be identified with the coronal structures observed in X-ray.
The simultaneous radio and X-ray observations of AR Lac will allow to further constrain the physical parameters of the radio emitting region and to check the validity of co-spatial model for the X-ray and radio emitting source in solar-like magnetic structures.
Artem M. Dzura
adzura@socrat.asc.rssi.ru
Astro Space Center of Lebedev's Physical Institute, Moscow, Russia
We present here the maps of two sources situated in star-forming regions - NGC 7538 and W75 S3. The sources were observed in the methanol 80-71A+ line at 95 GHz and in the CS (2-1) line at 98 GHz with the Onsala 20-m radiotelescope. This methanol rotational transition (80-71A+) gives rise to strong collision-pumped masers (belonging to Class I methanol masers). When the gas density is very high (about 107 cm-1), the maser transition become thermalized. We can suppose that at intermediate densities there should be observable a quasi-maser emission, when the transition remains inverted, but maser brightness is lower and angular size of a maser spot is greater. In order to detect this effect we made maps both in the methanol and CS lines (CS can be used as an indicator of high density cores).
Deconvolution of images was carried out with the maximum entropy method. Reconstructed maps show two brightness maximums in the both lines. As a result of density thermalisation, the position of the brighter maximum in CS coinsides with the position of the less brighter maximum in methanol.
Fabrice Herpin
herpin@observ.u-bordeaux.fr
Observatoire de Bordeaux, BP 89, 33270 Floirac, France
We have undertaken toward 30 Mira or semi-regular variables and one OH/IR object highly sensitive observations of the v = 1, J = 2 -> 1 and 3 -> 2 transitions of SiO simultaneously with observations of the J = 1 -> 0 transition of CO during three observing sessions in the period 1995 to 1996. As in our previous observations made in 1994, we confirm that for several stars the SiO profiles exhibit unusually broad wings, reaching, and sometimes exceeding, the terminal velocity of the expanding circumstellar envelope traced by the thermal CO emission. We have discovered a clear dependence of the activity in the line wings with the optical phase. These wings are probably due to peculiar gas motions in relation with the stellar pulsation. However, we cannot exclude other mechanisms contributing to the observed wings. In particular, SiO turbulent motions for the semi-regular variables or the asymmetric mass loss mechanism may play a role. It is likely that the bulk of the SiO emission and the line wings are not located at the same place.
Bryan Jacoby (jacoby@astro.uni.torun.pl), Wojciech Lewandowski (boe@astro.uni.torun.pl) and Malgorzata Redmerska (mare@class1.phys.uni.torun.pl)
Torun Center for Astronomy, Poland
We present the preliminary results of the Torun Centre for Astronomy pulsar observing program. This campaign uses the Penn State Pulsar Machine II, a 64 channel filterbank with 192 MHz bandwidth and 10 micros time resolution. This instrument was optimized for 1.4 GHz observations with the TCfA 32-m radiotelescope. We have compiled timing databases over a year in length for PSR B0329+54, PSR B0950+08, PSR B1133+16, PSR B1749-28, PSR B1933+16, PSR B2021+28, and PSR B2021+51, and three months in length for about thirty additional objects. We also present dynamic spectra and weak interstellar scintillation analyses for PSR B0329+54, PSR B0950+08, and PSR B1133+16. The dynamic spectra have about one minute time resolution and 3 MHz frequency resolution.
Tatyana Kaltman
kti@saoran.spb.su
St.-Petersburg branch of Special Astrophysical Observatory, Russia
The slowly varying component (S-component) of the solar radio emission includes the sources of different types. The radio emission of some typical sources have been modeled by taking into account the thermal and the nonthermal mechanisms.
The emission of a bipolar plage with a filament calculated by using the radio telescope RATAN-600 observations is presented as an example of such 3-dimensional modelling. The radio brightness distribution was modeled as a combined action of the bremsstrahlung and the nonthermal gyrosynchrotron radio emission. The source structure was considered as the coronal magnetic loops of the magnetosphere of the active region filled both with a thermal plasma and with a few account of subrelativistic electrons. A good agreement between the calculations and the observations as well as the considerable stability of the phenomenon (more than two solar rotations) indicates that there are the nonthermal processes which acting continuously in the solar corona in the period of the deep minimum of the solar activity.
Volodymyr V. Koshovy, A. B. Lozynsky, I. M. Romanyshyn,
Y. V. Romanchev
koshovy@ah.ipm.lviv.ua
Physico-Mechanical Institute of National Academy of Science Ukraine, Ukraine
A new multicircle method of radio image restoration of compact radio sources, based on measured by radiointerferometric method's visibility function, is proposing. The algorithm of determination the number of the circles, their angular diameter and flux density, based on Fourier-Bessel series, is showing. The radio image of CasA was restored on data, obtained from the measurements of visibility function of this source. These measurements were performed on short-base (up to 2000 wavelength) interferometer SBI URAN-3 in 1994, 1995 on 13 East-West baselines at 25 MHz with equivalent bandwidth 20 kHz, mostly in the night. Comparing the data, that were obtained with known high frequency results it has been discovered a presence a large-scale object. The absence of phase measurements cannot tell us if this object a halo or a jet. In the same time the central part of the large-scale model structure, obtained on decameter wavelengths, is inconsistent with the results, deducted from the measurements on the meter waveband.
Pushchino Radio Astronomy Observatory of P. N. Lebedev Physical Institute, Pushchino, Russia
Geminga was discovered more than twenty years ago as a strong gamma-ray source at the constellation of Gemini. Its nature, as a rotating neutron star, became apparent, when X-ray pulsations with a period of 237 ms were found by Halpern & Holt. Pulsed gamma-radiation was subsequently detected in older archived data. Thus Geminga showed all the attributes of a multi-frequency isolated neutron star like the Crab pulsar, however, with the unique difference that no radio emission was traceable. All attempts to detect either a continuum or pulsed radio source were unsuccessful so far. We report here the detection of pulsed radio emission from Geminga at a frequency of 102.5 MHz. Its flux density changes from 5 to 500 mJy, its pulse width changes between 10 and 80 ms and its dispersion measure is DM = 2.9±0.5 pc cm-3, confirming the proximity of Geminga and suggesting that this source is a radio pulsar which exhibits the weakest radio luminosity known. This result is very important for pulsar emission mechanisms, which must explain why Geminga exhibits so strong gamma-ray emission and such weak radio radiation.Janusz A. Gil1, Olaf Maron1,2,
George I. Melikidze1,2
olaf@astro.ca.wsp.zgora.pl
1Astronomical Centre, Zielona Gora, Poland
2Abastumani Astrophysical Observatory, Tbilisi, Georgia
The famous neutron star Geminga used to remain the only pulsar unvisible in radio regime, though detected as a strong pulsating gamma-, X-ray and optical source (the latter is though not pulsating). Recently three independent groups from the Pushchino Radio Astronomy Observatory (Moscow, Russia) reported the detection of pulsed radio emission from Geminga at a frequency of 102.5 MHz, i.e. the first detection of radio pulsar PSR J0633+1746. This pulsar exhibits the weakest radio luminosity known, and also its average pulse profile appeared to be very wide. Here we present our model explaining the peculiarities of this pulsar, based on the assumption that Geminga is almost aligned rotator. As a consequence, radio waves generated in the inner magnetosphere of this pulsar should reach the region with a weak magnetic field (on the distances of a few light cylinder radii but still inside the light cylinder surface) where they will be damped strongly due to the cyclotron resonance with particles of the magnetospheric electron-positron plasma. Finally we discuss plasma mechanisms of the high frequency cut-off for other pulsars as well, as it follows from our model.
Vera G. Medar
vera@saoran.spb.su
Main (Pulkovo) Astronomical Observatory, St.-Petersburg, Russia
On the basis of solar observations made with RATAN-600 radio telescope at microwaves (2-32 cm) during 1982-1990 it was shown that in period of the low solar activity the coronal holes (CHs) are clearly seen on the one-dimensional solar scans as "dark" regions against the background of the quiet Sun at wavelengths longer than 4 cm. At short centimeter wavelengths CHs are not detectable against the quiet Sun.
The results of CH observations on the descending phase and in the period of current minimum of the solar activity cycle (1992-1996) using the new panoramic spectral analyzer in the range of 1.8-31.6 cm were analyzed. It is shown that on the descending phase (1992-1993) the CHs which are clearly seen on the X-ray solar images are not detectable against the quiet Sun in the microwave range. The CHs registered in the current minimum have the same radio characteristics as in the previous minimum (the contrast of the brightness temperature of the CHs and the quiet Sun is 0.9-0.95 at 5-6 cm and increases up to 0.5-0.6 at 20 cm). The low polarized signal associated with one coronal hole was registered. It may be interpreted as the radio emission of thermal electrons in weak magnetic field of coronal hole.
Yuri Ilyasov1, Sergei Kopeikin2, and Alexander
Rodin1
rodin@prao.psn.ru
1Pushchino Radio Astronomy Observatory of P.N.Lebedev
Physical Institute, Pushchino, Russia
2Friedrich-Schiller-Universität Jena, Theoretisch-Physikalisches Institut,
Jena, Germany
A close pulsar binary system is proposed as a reference frequency - time standard. Orbital period of binary pulsar can be used for a generation of a new fundamental astronomical ephemeris time scale, when the orbital motion being assumed to be well-defined. The new standard is expected to generate very precise and stable time intervals as a number of binary orbital period during long time intervals about 10-100 years. Such a high stability of the orbital period and consequently the accurate measurements of the time intervals are necessary, in particular, for searching for a background stochastic gravitational waves in 10-9 -10-12 Hz frequency range and studying an upper limit of the accuracy of tests of general relativity by using binary pulsars. A theoretical analysis of a real stability of the new time scale is investigated under assumption that some stochastic time variations are presented in time-of-arrival (TOA) of pulses. It is suggested that the time variations consist of components of a white and a red noise. An energy spectrum of the components has a power law dependence of frequency f of form f -s whith s = 0, 1, 2, ..., 6.
It is proved that the dynamical time scale based on the pulsar's orbital motion under certain orbital parameters is more stable on long time intervals compared with the kinematic pulsar time scale based on the process of pulsar's proper rotation.
Carmen Sánchez Contreras
sanchez@oan.es
Observatorio Astronómico Nacional (IGN), Alcalá de Henares, Spain
We have carried out maps of microwave lines of 8 different molecules (12CO, 13CO, SiO, HCO+, SO2, CS, HCN and HNC) in OH231.8+4.2, a bipolar protoplanetary nebula. Such nebulae are the intermadiate state between the Asymptotic Giant Branch (AGB) and the planetary nebula phase. The obtained spectra (except for HCO+) show similar emission features: an intense component in the central velocity that comes from the source center, and weaker wing emission originating in the nebula lobes. Such lobes appear fragmented in several gas components axially flowing at high velocity. We find a practically constant velocity gradient, ~ 6 km s-1 arcsec-1, in the direction of the polar axis. We have calculated the mass, momentum and molecular abundances in the different components of the nebula. The total molecular mass in the envelope is 0.5-1 MSun, and at least 0.2 MSun are axially flowing at high velocity. It is argued that this material corresponds to a large fraction of the envelope ejected in the previous AGB phase, after being accelerated by interaction with the fast post-AGB jets. The abundance of most molecules is found to be practically constant across the nebula. HCO+ is however much more abundant in the axial flow than in the central component of the nebula, suggesting that this molecule is efficiently formed in the lobes by shock-induced reactions.
Alastair McD. Stirling1, R. E.
Spencer1 and M. A. Garrett2
ams@jb.man.ac.uk
1NRAL, Jodrell Bank, University of Manchester, U.K.
2JIVE, NFRA, Netherlands
The classic black hole candidate Cygnus X-1 exhibits many similarities to galactic jet sources (micro-quasars) in soft gamma and hard X-ray rays. This has fuelled suspicions that radio jets also may be present in Cygnus X-1. The lack of radio flaring in Cygnus X-1 has been associated with the presence of continuous jets rather than multiple plasmon ejection however no evidence for this has been observed with the VLA, or with MERLIN. Recent VLBA observations at 15 GHz have detected extended emission and further observations at lower frequencies are required to confirm the presence of jets on a milli-arcsecond level.
NRAL, Jodrell Bank, University of Manchester, Macclesfield, United Kingdom
Symbiotic novae have the red cool continuum and superimposed strong emission lines characteristic of all symbiotic stars in their spectra and have been seen to undergo an optical outburst, or nova-type event. New 6-cm MERLIN observations of V1016 Cygni show small-scale structure. The resolution of ~10 milliarcsec shows changes in shape and intensity within the central 300 milliarcsec of the source over a five year period.J. Zweigle1, R. Neri1, R.
Bachiller2, V. Bujarrabal2 and M. Grewing1
zweigle@iram.fr
1Institut de Radio Astronomie Millimétrique (IRAM), 300 Rue de la
Piscine, F-38406 Saint Martin d'Heres, France
2Observatorio Astronómico Nacional (I.G.N), Apartado 1143, E-28800 Alcalá de
Henares, Madrid, Spain
We have mapped the 12CO J = 1-0 emission in the bipolar planetary nebula M 2-9 using the IRAM interferometer. From the maps we were able to investigate in detail the morphology and the kinematics of the molecular gas. The data are best explained by assuming that the molecular gas is concentrated in an expanding, clumpy torus.
The torus, which surrounds the nucleus of M 2-9, has a mean diameter of about 6 arcseconds. Its symmetry axis is tilted by 17 degrees with respect to the plane of the sky. The de-projected expansion velocity is 7 km s-1, and its kinematical age is about 2100 years assuming a distance of 1 kpc. The lower limit for the total mass of the molecular gas is estimated to be 9 10-3 solar masses, i.e. at least comparable to the ionized mass in the nebula.