Understanding the periodicities in radio and GeV emission from LS I +61°303

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: fjaron@mpifr-bonn.mpg.de; mmassi@mpifr-bonn.mpg.de
² INAF−Osservatorio Astrofisico di Arcetri, L.go E. Fermi 5, 50125 Firenze, Italy
e-mail: torricel@arcetri.astro.it

Received: 18 March 2016
Accepted: 3 August 2016

Abstract

Context. One possible scenario to explain the emission from the stellar binary system LS I + 61°303 is that the observed flux is emitted by precessing jets powered by accretion. Accretion models predict two ejections along the eccentric orbit of LS I + 61°303: one major ejection at periastron and a second, lower ejection towards apastron. Our GeV gamma-ray observations show two peaks along the orbit (orbital period P₁) but reveal that at apastron the emission is also affected by a second periodicity, P₂. Strong radio outbursts also occur at apastron, which are affected by both periodicities (i.e. P₁ and P₂), and radio observations show that P₂ is the precession of the radio jet. Consistently, a long-term modulation, equal to the beating of P₁ and P₂, affects both radio and gamma-ray emission at apastron but it does not affect gamma-ray emission at periastron.

Aims. If there are two ejections, why does the one at periastron not produce a radio outburst there? Is the lack of a periastron radio outburst somehow related to the lack of P₂ from the periastron gamma-ray emission?

Methods. We develop a physical model in which relativistic electrons are ejected twice along the orbit. The ejecta form a conical jet that is precessing with P₂. The jet radiates in the radio band by the synchrotron process and the jet radiates in the GeV energy band by the external inverse Compton and synchrotron self-Compton processes. We compare the output fluxes of our physical model with two available large archives: Owens Valley Radio Observatory (OVRO) radio and Fermi Large Area Telescope (LAT) GeV observations, the two databases overlapping for five years.

Results. The larger ejection around periastron passage results in a slower jet, and severe inverse Compton losses result in the jet also being short. While large gamma-ray emission is produced, there is only negligible radio emission. Our results are that the periastron jet has a length of 3.0 × 10⁶r_s and a velocity β ~ 0.006, whereas the jet at apastron has a length of 6.3 × 10⁷r_s and β ~ 0.5.

Conclusions. In the accretion scenario the observed periodicities can be explained if the observed flux is the intrinsic flux, which is a function of P₁, times the Doppler factor, a function of βcos(f(P₂)). At periastron, the Doppler factor is scarcely influenced by P₂ because of the low β. At apastron the larger β gives rise to a significant Doppler factor with noticeable variations induced by jet precession.