Intrinsic physical properties and Doppler boosting effects in LSI+61303

Abstract

Our aim is to show how variable Doppler boosting of an intrinsically variable jet can explain the long-term modulation of 1667 \pm 8 days observed in the radio emission of LSI+61303. The physical scenario is that of a conical, magnetized plasma jet having a periodical (P1) increase of relativistic particles, Nrel, at a specific orbital phase, as predicted by accretion in the eccentric orbit of LSI+61303. Jet precession (P2) changes the angle, eta, between jet axis and line of sight, thereby inducing variable Doppler boosting. The problem is defined in spherical geometry, and the optical depth through the precessing jet is calculated by taking into account that the plasma is stratified along the jet axis. The synchrotron emission of such a jet was calculated and we fitted the resulting flux density Smodel(t) to the observed flux density obtained during a 6.5-year monitoring of LSI+61303 by the Green Bank radio interferometer. Our physical model for the system LSI+61303 is not only able to reproduce the long-term modulation in the radio emission, but it also reproduces all the other observed characteristics of the radio source, the orbital modulation of the outbursts, their orbital phase shift, and their spectral index properties. Moreover, a correspondence seems to exist between variations in the ejection angle induced by precession and the rapid rotation in position angle observed in VLBA images. We conclude that the peak of the long-term modulation occurs when the jet electron density is around its maximum and the approaching jet is forming the smallest possible angle with the line of sight. This coincidence of maximum number of emitting particles and maximum Doppler boosting of their emission occurs every 1667 days and creates the long-term modulation observed in LSI+61303.

12 pages, 8 figures, accepted for publication in A&A

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Cited by in corpus (23)

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• Discovery of a periodical apoastron GeV peak in LS I +61°303
• The broad-band radio spectrum of LSI+61303 in outburst
• Long-term OVRO monitoring of LSI+61303: confirmation of the two close periodicities
• Revisiting LS I +61 303 with VLBI astrometry
• Evidence for periodic accretion-ejection in LSI+61303
• Superorbital modulation at GeV energies in the gamma-ray binary LS I +61 303
• Radio QPO in the $\gamma$-ray-loud X-ray binary LS I +61${}^{\circ }$303
• Modelling the correlated keV/TeV light curves of Be/gamma-ray binaries
• The Multi-wavelength Characteristics of the TeV Binary LS I +61${}^{\circ }$ 303
• Simultaneous long-term monitoring of LS I +61°303 by OVRO and Fermi-LAT
• Understanding the periodicities in radio and GeV emission from LS I+61303
• Orbital and superorbital variability of LS I +61 303 at low radio frequencies with GMRT and LOFAR
• A Precessing Jet Scenario for the Multi-Wavelength Long-Term Modulation of LS I +61${}^{\circ }$303
• Temporal features of LS I +61${}^{\circ }$303 in hard X-rays from the Swift/BAT survey data
• Hour time-scale QPOs in the X-ray and radio emission of LS I +61${}^{\circ }$303
• Owens Valley Radio Observatory monitoring of LS I +61°303 completes three cycles of the super-orbital modulation
• A precessing stellar disk model for superorbital modulations of the gamma-ray binary LS I+61${}^{\circ }$ 303
• Opacity effect on core-shift and the spectral properties of jets
• Rapid X-ray variability of the gamma-ray binary LS I+61 303