Discovery of a periodical apoastron GeV peak in LS I +61°303

Abstract

Aims. The aim of this paper is to analyse the previously discovered discontinuity of the periodicity of the GeV γ-ray emission of the radio-loud X-ray binary LS I +61• 303 and to determine its physical origin. Methods. We used a wavelet analysis to explore the temporal development of periodic signals. The wavelet analysis was first applied to the whole data set of available Fermi-LAT data and then to the two subsets of orbital phase intervals Φ = 0.0−0.5 and Φ = 0.5−1.0. We also performed a Lomb-Scargle timing analysis. We investigated the similarities between GeV γ-ray emission and radio emission by comparing the folded curves of the Fermi-LAT data and the Green Bank Interferometer radio data. Results. During the epochs when the timing analysis fails to determine the orbital periodicity, the periodicity is present in the two orbital phase intervals Φ = 0.0−0.5 and Φ = 0.5−1.0. That is, there are two periodical signals, one towards periastron (i.e., Φ = 0.0−0.5) and another one towards apoastron (Φ = 0.5−1.0). The apoastron peak seems to be affected by the same orbital shift as the radio outbursts and, in addition, reveals the same two periods P 1 and P 2 that are present in the radio data. Conclusions. The γ-ray emission of the apoastron peak normally just broadens the emission of the peak around periastron. Only when it appears at Φ ≈ 0.8−1.0 because of the orbital shift, it is enough detached from the first peak to become recognisable as a second orbital peak, which is the reason why the timing analysis fails. Two γ-ray peaks along the orbit are predicted by the two-peak accretion model for an eccentric orbit that was proposed by several authors for LS I +61• 303.

The broad-band radio spectrum of LS I +61°303 in outburst

“…a smaller high energy outburst) and the relativistic electrons can propagate out of the orbital plane into a radio jet. Recent Fermi/LAT observations also confirmed this second, smaller high energy outburst (Jaron & Massi 2014). …”

Origin of the long-term modulation of radio emission of LS I +61°303

“…Finally, periods P 1 and P 2 have also been determined in the Lomb-Scargle spectrum of gamma-ray emission at apastron of Fermi-LAT data (Jaron & Massi 2014). Moreover, Paredes-Fortuny et al (2015) discovered that the orbital phase shift also affects the variations in the equivalent width (EW) of the Hα emission line from LS I +61 • 303.…”

“…The hypothesis that the compact object in LS I +61 • 303, which accretes material from the Be wind, undergoes a periodical (P 1 ) increase in the accretion rateṀ at a particular orbital phase along an eccentric orbit has been suggested and developed by several authors (Taylor et al 1992;Marti & Paredes 1995;Bosch-Ramon et al 2006;Romero et al 2007). Recently, the presence of P 1 and P 2 have been confirmed as stable features in more recent Fermi-LAT and OVRO monitorings of the GeV gamma-ray emission (Jaron & Massi 2014) and of the radio emission , respectively. The aim of the timing analysis was to obtain a better determination of the precessional period, but there was an additional and totally unexpected result: when combining ν 1 and ν 2 one obtains P average = 2 ν 1 +ν 2 = 26.70±0.05 d and P beat = 1 ν 1 −ν 2 = 1667±393 d .…”

Long-term OVRO monitoring of LS I +61°303: confirmation of the two close periodicities

“…The timing analysis in Jaron & Massi (2014) for the whole data set gave P 2 as significant (p < 1%) in the randomization tests even if it is a rather weak feature in the periodograms of their Fig. 3a-c.…”

“…Recently, a Lomb-Scargle analysis of Fermi-LAT data around apastron revealed the same periodicities P 1 γ = 26.48 ± 0.08 d and P 2 γ = 26.99 ± 0.08 d (Jaron & Massi 2014). The similar behaviours of the emission at high (GeV) and low (radio) energy would imply that the emission is caused by the same population of electrons in a precessing jet.…”