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

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 P 1 ) but reveal that at apastron the emission is also affected by a second periodicity, P 2 . Strong radio outbursts also occur at apastron, which are affected by both periodicities (i.e. P 1 and P 2 ), and radio observations show that P 2 is the precession of the radio jet. Consistently, a long-term modulation, equal to the beating of P 1 and P 2 , 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 P 2 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 P 2 . 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 × 10 6 r s and a velocity β ∼ 0.006, whereas the jet at apastron has a length of 6.3 × 10 7 r 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 P 1 , times the Doppler factor, a function of β cos( f (P 2 )). At periastron, the Doppler factor is scarcely influenced by P 2 because of the low β. At apastron the larger β gives rise to a significant Doppler factor with noticeable variations induced by jet precession.

Radio QPO in the γ-ray-loud X-ray binary LS I +61°303

“…4 in Massi & Torricelli-Ciamponi 2016 and references therein), Be star variations last 2-3 cycles only and are of different lengths (Rivinius et al 2013). Timing analysis of 37 years of radio data performed by reveals that LS I +61 • 303 does not show merely 2-3 cycles which are of different lengths but a repetition of 8 full cycles of an identical length of 1628 days, well in agreement with the scenario of a microquasar with a precessing jet (Massi & Torricelli-Ciamponi 2014;Jaron et al 2016). In addition, as discussed in Jaron et al (2016), the orbital shift in the equivalent width of the Hα emission line (Paredes-Fortuny et al 2015), points to variations caused by a precessing jet.…”

Review on the multiwavelength emission of the gamma-ray binary LS I +61 303

“…Furthermore, through frequency radio studies it has been suggested that there are actually two close periods of ≈ 26.5 and ≈ 26.9 d modulating the emission of LS I +61 303, and the superorbital modulation is just the beating period between them [57,58,59]. In this case, we would assume the presence of a precessing jet with a period close to the orbital period of the system, naturally explaining the regularity of the long-term emission [60,61].…”

Relativistic Outflows From Compact Galactic Sources

“…Accretion-based models and pulsar-wind interaction scenarios keep being discussed as the powering engine of GBs (see e.g. Dubus 2006;Bosch-Ramon et al 2006b;Romero et al 2007;Torres et al 2012;Jaron et al 2016). An up-to-date summary accounting for some of the key properties of currently known GBs (not including 4FGL J1405.1-6119) is provided in (Paredes & Bordas 2019a) and Paredes & Bordas 2019b.…”