VLBA images of the precessing jet of LSI+61303

Context: In 2004, changes in the radio morphology of the Be/X-ray binary system LSI+61303 suggested that it is a precessing microquasar. In 2006, a set of VLBA observations performed throughout the entire orbit of the system were not used to study its precession because the changes in radio morphology could tentatively be explained by the alternative pulsar model. However, a recent radio spectral index data analysis has confirmed the predictions of the two-peak microquasar model, which therefore does apply in LSI+61303. Aims: We revisit the set of VLBA observations performed throughout the orbit to determine the precession period and improve our understanding of the physical mechanism behind the precession. Methods: By reanalyzing the VLBA data set, we improve the dynamic range of images by a factor of four, using self-calibration. Different fitting techniques are used and compared to determine the peak positions in phase-referenced maps. Results: The improved dynamic range shows that in addition to the images with a one-sided structure, there are several images with a double-sided structure. The astrometry indicates that the peak in consecutive images for the whole set of observations describes a well-defined ellipse, 6-7 times larger than the orbit, with a period of about 28 d. Conclusions: A double-sided structure is not expected to be formed from the expanding shocked wind predicted in the pulsar scenario. In contrast, a precessing microquasar model can explain the double- and one-sided structures in terms of variable Doppler boosting. The ellipse defined by the astrometry could be the cross-section of the precession cone, at the distance of the 8.4 GHz-core of the steady jet, and 28d the precession period.Comment: 7 pages, 5 figures, Accepted for publication in Astronomy and Astrophysics, added references for sect.

Discovery of Solar Rieger Periodicities in Another Star

The Rieger periods are solar cycles with a time scale of months, which are present in both flaring activity and sunspot occurrence. These short-term periodicities, tentatively explained by equatorially trapped Rossby-type waves modulating the emergence of magnetic flux at the surface, are considered a peculiar and not yet fully understood solar phenomenon. We chose a stellar system with solar characteristics, UX Arietis, and performed a timing analysis of two 9-year datasets of radio and optical observations. The analysis reveals a 294-day cycle. When the two 9-year datasets are folded with this period, a synchronization of the peak of the optical light curve (i.e., the minimum spot coverage) with the minimum radio flaring activity is observed. This close relationship between two completly independent curves makes it very likely that the 294-day cycle is real. We conclude that the process invoked for the Sun of a periodical emergence of magnetic flux may also be applied to UX Arietis and can explain the cyclic flaring activity triggered by interactions between successive cyclic emergences of magnetic flux.Comment: 4 Pages, 1 table, 3 figures (quality of Fig. 1 degraded to match the requested size), needs aa.cls, accepted to be published as a letter in Astronomy & Astrophysic

Investigation of magnetic loop structures in the corona of UX Arietis

Most of the emission coming from the solar corona is confined into closed magnetic structures in the form of arcs (loops). Very little is known about the structure of stellar coronae. The magnetic topology, however, can be inferred by studying the radio emission coming from electrons trapped in the magnetic loops. Evident morphological changes are produced in fact by stellar rotation. We have performed 4 VLBA+Effelsberg runs distributed in time so as to cover well the rotational period of 6.44 days of the active star UX Arietis. We present here some preliminary results, from those observations.Comment: Proceedings of the 6th European VLBI Network Symposium, Ros E., Porcas R.W., Lobanov A.P., & Zensus J.A. (eds.), MPIfR, Bonn, Germany, pp. 275-276 (2002). 2 pages, 1 figure, needs evn2002.cl

Interacting coronae of two T Tauri stars: first observational evidence for solar-like helmet streamers

Context {The young binary system V773 Tau A exhibits a persistent radio flaring activity that gradually increases from a level of a few mJy at apoastron to more than 100 mJy at periastron. Interbinary collisions between very large (> 15 R) magnetic structures anchored on the two rotating stars of the system have been proposed to be the origin of these periodic radio flares. Magnetic structures extended over tens of stellar radii, that can also account for the observed fast decay of the radio flares, seem to correspond to the typical solar semi-open quite extended magnetic configurations called helmet streamers.} Aims {We aim to find direct observational evidence for the postulated, solar-like, coronal topologies.} Methods {We performed seven-consecutive-day VLBI observations at 8.4 GHz using an array consisting of the VLBA and the 100-m Effelsberg telescope.} Results {Two distintive structures appear in the radio images here presented. They happen to be associated with the primary and secondary stars of the V773 Tau A system. In one image (Fig.2-B) the two features are extended up to 18 R each and are nearly parallel revealing the presence of two interacting helmet streamers. One image (Fig.2-E) taken a few hours after a flare monitored by the 100-m Effelsberg telescope shows one elongated fading structure substantially rotated with respect to those seen in the B run. The same decay scenario is seen in Fig.2-G for the helmet streamer associated with the other star.} Conclusions {This is the very first direct evidence revealing that even if the flare origin is magnetic reconnection due to interbinary collision, both stars independently emit in the radio range with structures of their own. These structures are helmet streamers, observed for the first time in stars other than the Sun.}Comment: 7 pages, 3 figures, A&A in pres

Probing the Origins of Increased Activity of the E22Q “Dutch” Mutant Alzheimer's β-Amyloid Peptide

AbstractThe amyloid peptide congener Aβ(10–35)-NH2 is simulated in an aqueous environment in both the wild type (WT) and E22Q “Dutch” mutant forms. The origin of the noted increase in deposition activity resulting from the Dutch mutation is investigated. Multiple nanosecond time scale molecular dynamics trajectories were performed and analyzed using a variety of measures of the peptide's average structure, hydration, conformational fluctuations, and dynamics. The results of the study support the conclusions that 1) the E22Q mutant and WT peptide are both stable in “collapsed coil” conformations consistent with the WT structure of Zhang et al. (2000, J. Struct. Biol. 130:130–141); 2) the E22Q peptide is more flexible in solution, supporting early claims that its equilibrium structural fluctuations are larger than those of the WT peptide; and 3) the local E22Q mutation leads to a change in the first solvation layer in the region of the peptide's “hydrophobic patch,” resulting in a more hydrophobic solvation of the mutant peptide. The simulation results support the view that the noted increase in activity due to the Dutch mutation results from an enhancement of the desolvation process that is an essential step in the aggregation of the peptide

EVN and MERLIN observations of microquasar candidates at low galactic latitudes

In an attempt to increase the number of known microquasars, Paredes et al. (2002) have presented a long-term project focused on the search for new objects of this type. They performed a cross-identification between X-ray and radio catalogs under very restrictive selection criteria for sources with |b|<5 degrees, and obtained a sample of 13 radio-emitting X-ray sources. Follow-up observations of 6 of these sources with the VLA provided accurate coordinates, which were used to discover optical counterparts for all of them. We have observed these six sources with the EVN and MERLIN at 5 GHz. Five of the six objects have been detected and imaged, presenting different morphologies: one source has a two-sided jet, three sources have one-sided jets, and one source is compact. With all the presently available information, we conclude that two of the sources are promising microquasar candidates in our Galaxy.Comment: 9 pages, 5 figures. Published in A&A, see http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002A%26A...394..983

Acoustic energy transfer by friction induced vibrations

Friction-induced vibrations are often investigated for their unwanted effects, such as surface wear and dynamic instabilities. This article focuses on the exploitation of friction-induced vibrations to transfer the energy between different acoustic fields by an interface under frictional contact. One of the main possible applications is the use of the generated acoustic field for passive structural health monitoring (SHM). A mechanical device (secondary acoustic source, SAS), able to perform the energy transfer, is here tested on a simplified benchmark. The energy transfer is obtained between two vibrational fields: a primary field, which is the ambient acoustic field on the structure and is generated by a known source, and a secondary field with a different frequency content produced with the developed device by friction-induced vibrations. The test bench analyzed in this work is composed by a main structure, which is excited by the primary (ambient) acoustic field, and the SAS, able to absorb part of the acoustic energy of the primary field and radiate it within the secondary acoustic field. The device is composed by a main resonator, excited by the primary acoustic field, in frictional contact with a secondary resonator to provide a broadband secondary acoustic field. The objective of the article is to analyze and estimate the power flows from the main structure to the SAS and vice versa, within the two acoustic fields