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.

Long-term periodicity in LSI+61303 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.Comment: 8 pages, 4 figures, accepted for publication in A&

Near- and Far-Infrared Counterparts of Millimeter Dust Cores in the Vela Molecular Ridge Cloud D

The aim of this paper is to identify the young protostellar counterparts associated to dust millimeter cores of the Vela Molecular Ridge Cloud D through new IR observations (H_2 narrow-band at 2.12 micron and N broad band at 10.4 micron) along with an investigation performed on the existing IR catalogues. The association of mm continuum emission with infrared sources from catalogues (IRAS, MSX, 2MASS), JHK data from the literature and new observations, has been established according to spatial coincidence, infrared colours and spectral energy distributions. Only 7 out of 29 resolved mm cores (and 16 out of the 26 unresolved ones) do not exhibit signposts of star formation activity. The other ones are clearly associated with: far-IR sources, H_2 jets or near-IR objects showing a high intrinsic colour excess. The distribution of the spectral indices pertaining to the associated sources is peaked at values typical of Class I objects, while three objects are signalled as candidates Class 0 sources. We remark the high detection rate (30%) of H_2 jets driven by sources located inside the mm-cores. They appear not driven by the most luminous objects in the field, but rather by less luminous objects in young clusters, testifying the co-existence of both low- and intermediate-mass star formation. The presented results reliably describe the young population of VMR-D. However, the statistical evaluation of activity vs inactivity of the investigated cores, even in good agreement with results found for other star forming regions, seems to reflect the limiting sensitivity of the available facilities rather than any property intrinsic to the mm-condensations.Comment: 38 pages. To be published to Astronomy & Astrophysic

Long-term OVRO monitoring of LSI+61303: confirmation of the two close periodicities

Context: The gamma-ray binary LSI+61303 shows multiple periodicities. The timing analysis of 6.7 yr of GBI radio data and of 6 yr of Fermi-LAT GeV gamma-ray data both have found two close periodicities P1(GBI) = 26.49 \pm 0.07 d, P2(GBI)=26.92 \pm 0.07 d and P1(gamma)=26.48 \pm 0.08 d, P2(gamma) = 26.99 \pm 0.08 d. Aims: The system LSI+61303 is the object of several continuous monitoring programs at low and high energies. The frequency difference between f1 and f2 of only 0.0006 d(-1) requires long-term monitoring because the frequency resolution in timing analysis is related to the inverse of the overall time interval. The Owens Valley Radio Observatory (OVRO) 40 m telescope has been monitoring the source at 15 GHz for five years and overlaps with Fermi-LAT monitoring. The aim of this work is to establish whether the two frequencies are also resolved in the OVRO monitoring. Methods: We analysed OVRO data with the Lomb-Scargle method. We also updated the timing analysis of Fermi-LAT observations. Results: The periodograms of OVRO data confirm the two periodicities P1(OVRO) = 26.5 \pm 0.1 d and P2(OVRO) = 26.9 \pm 0.1 d. Conclusions: The three indipendent measurements of P1 and P2 with GBI, OVRO, and Fermi-LAT observations confirm that the periodicities are permanent features of the system LSI+61303. The similar behaviours of the emission at high (GeV) and low (radio) energy when the compact object in LSI+61303 is toward apastron suggest that the emission is caused by the same periodically (P1) ejected population of electrons in a precessing (P2) jet.Comment: 4 pages, 7 figures, A&A Letters in pres