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

Monthly Notices of the Royal Astronomical Society: Letters

et al. 2017

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LS I +61• 303 is a γ-ray emitting X-ray binary with periodic radio outbursts with time scales of one month. Previous observations have revealed microflares superimposed on these large outbursts with periods ranging from a few minutes to hours. This makes LS I +61• 303, along with Cyg X-1, the only TeV emitting X-ray binary exhibiting radio microflares. To further investigate these microflaring activity in LS I +61• 303 we observed the source with the 100-m Effelsberg radio telescope at 4. 85, 8.35, and 10.45 GHz and performed timing analysis on the obtained data. Radio oscillations of 15 hours time scales are detected at all three frequencies. We also compare the spectral index evolution of radio data to that of the photon index of GeV data observed by Fermi-LAT. We conclude that the observed QPO could result from multiple shocks in a jet.

Section: Resultssupporting

confidence: 84%

“…In simultaneous radio and X-ray observations of GRS 1915+105, Klein-Wolt et al (2002) showed that X-ray dips were associated with radio peaks. Oscillations with time scales of days are known to occur in Cyg X-3 (Zimmermann et al 2015). These longer period QPO are present in both total flux density and radio spectral index ( Fig.…”

Section: Introductionmentioning

confidence: 96%

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

Jaron

Sharma

Monthly Notices of the Royal Astronomical Society: Letters

et al. 2017

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“…• 303 was already proven in the past (Gregory et al 1979;Massi & Kaufman Bernadó 2009) and was recently measured over the entire cm radio band (up to 9 mm) by Zimmermann et al (2015).…”

Section: Interferometric Images Of Ls I +61mentioning

confidence: 86%

“…• 303 as in other microquasars (Massi & Torricelli-Ciamponi 2014;Zimmermann et al 2015) A92, page 8 of 11 F. Jaron et al: Understanding the periodicities in radio and GeV emission from LS I +61…”

Section: Influence Of the θ Interval On Pmentioning

confidence: 99%

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

Jaron

Torricelli‐Ciamponi

2016

A&A

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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.

“…it dominates at low frequencies. At 8 GHz the second peak is often a minor peak, hardly traceable; instead, simultaneous observations at 2 GHz show a strong second peak, even larger at some epochs than the first peak (Massi & Kaufman Bernadó 2009;Massi 2014;Zimmermann et al 2015). We included data at 5 GHz to be able to cover the first cycles.…”

mentioning

confidence: 99%

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

Torricelli‐Ciamponi

2016

A&A

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Context. One of the most unusual aspects of the X-ray binary LS I +61 • 303 is that at each orbit (P 1 = 26.4960 ± 0.0028 d) one radio outburst occurs whose amplitude is modulated with P long , a long-term period of more than 4 yr. It is still not clear whether the compact object of the system or the companion Be star is responsible for the long-term modulation. Aims. We study here the stability of P long . Such a stability is expected if P long is due to periodic (P 2 ) Doppler boosting of periodic (P 1 ) ejections from the accreting compact object of the system. On the contrary it is not expected if P long is related to variations in the mass loss of the companion Be star. Methods. We built a database of 36.8 yr of radio observations of LS I +61 • 303 covering more than 8 long-term cycles. We performed timing and correlation analysis. We also compared the results of the analyses with the theoretical predictions for a synchrotron emitting precessing (P 2 ) jet periodically (P 1 ) refilled with relativistic electrons.Results. In addition to the two dominant features at P 1 and P 2 , the timing analysis gives a feature at P long = 1628 ± 48 days. The determined value of P long agrees with the beat of the two dominant features, i.e. P beat = 1/(ν 1 − ν 2 ) = 1626 ± 68 d. Lomb-Scargle results of radio data and model data compare very well. The correlation coefficient of the radio data oscillates at multiples of P beat , as does the correlation coefficient of the model data. Conclusions. Cycles in varying Be stars change in length and disappear after 2-3 cycles following the well-studied case of the binary system ζ Tau. On the contrary, in LS I +61 • 303 the long-term period is quite stable and repeats itself over the available 8 cycles. The long-term modulation in LS I +61 • 303 accurately reflects the beat of periodical Doppler boosting (induced by precession) with the periodicity of the ejecta. The peak of the long-term modulation occurs at the coincidence of the maximum number of ejected particles with the maximum Doppler boosting of their emission; this coincidence occurs every 1 ν 1 −ν 2 and creates the long-term modulation observed in LS I +61 • 303.