Feasibility study of Lense-Thirring precession in LS I +61°303

2012

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Context. The TeV emitting X-ray binary LS I +61• 303 has two radio periodicities that correspond to a large periodic outburst with the same period as the orbit, 26.5 days (phase Φ), and a second periodicity of 1667 days (phase Θ), which modulates the orbital phase and amplitude of the large outburst. Analyses of the radio spectral index revealed in LS I +61• 303 the presence of the critical transition typical for microquasars from optically thick emission (related to a steady jet) to an optically thin outburst (related to a transient jet), and found that it occurs at Φ crit , which is modulated by Θ: Φ crit = f (Θ). Aims. We examine the possible implications of averaging high energy data over large Θ and Φ intervals in the light of puzzling published INTEGRAL results, which differ for different averaging of the data. Methods. In microquasars, a simultaneous transition between two X-ray states occurs at the switch from optically thick radio emission to an optically thin radio outburst, from the low/hard to the steep power-law state. Assuming that the same transition occurs in LS I +61• 303 at Φ crit , we can show qualitatively the effect of averaging high energy data on Θ, by analysing the effects of averaging radio spectral index data across the same Θ interval. We then model the two X-ray states, low/hard and steep power-law state, and show quantitatively how their mixing can affect the results. Results. When folded over too large a Θ interval, spectral data from INTEGRAL can yield a false picture of the emission behaviour of the source along the orbit because it may be mixing two spectral states. Furthermore, averaging the data along the orbit may result in a dominant low/hard spectral state, which, for insufficiently extended sampling, might appear without a cut-off. Conclusions. The INTEGRAL results can be interpreted as two X-ray states that alternate with each other along the orbit. The implications of this analysis for HE/VHE data from LS I +61• 303 and a possible connection between HE/VHE emission and the steep power-law state are discussed.

Section: Conclusion and Discussionsupporting

confidence: 90%

Section: Introductionmentioning

confidence: 93%

Implications of the radio spectral index transition in LS I +61°303 for its INTEGRAL data analysis

Zimmermann

2012

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“…An accurate determination of the period is mandatory to establish the physical process responsible for precession. Massi & Zimmermann (2010) computed the precessional period for the accretion disk in LS I +61 • 303 under tidal forces of the Be star (P tidal−forces ) and under the effect of frame dragging produced by the rotation of the compact object (P Lense−Thirring ). By using those equations, we find that P tidal−forces of 28 d would require the unrealistic value for the size of the accretion disk of R out = 0.5−0.8 × 10 13 cm, i.e, nearly the semimajor axis, and can therefore be ruled out.…”

Section: Conclusion and Discussionmentioning

confidence: 99%

“…In known precessing X-ray binaries, the timescale for tidally forced precession of the accretion disk around the compact object, induced by the companion star, lies within the range 8-22 times the orbital period (Larwood 1998;Massi & Zimmermann 2010). In this context, the peculiarity of the variations in LS I +61 • 303 is their short timescale with respect to the orbital period of 26.496 d. Massi et al (2004) found that MERLIN images revealed a surprising variation of 60 • in position angle in only one day.…”

Section: Introductionmentioning

confidence: 99%

VLBA images of the precessing jet of LS I +61°303

Ros

Zimmermann

2012

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Context. In 2004, changes in the radio morphology of the Be/X-ray binary system LS I +61 • 303 suggested that it is a precessing microquasar. In 2006, a set of VLBA observations performed throughout the entire orbit of the system were not used to study its precession because the changes in radio morphology could tentatively be explained by the alternative pulsar model. However, a recent radio spectral index data analysis has confirmed the predictions of the two-peak microquasar model, which therefore does apply in LS I +61 • 303. Aims. We revisit the set of VLBA observations performed throughout the orbit to determine the precession period and improve our understanding of the physical mechanism behind the precession. Methods. By reanalyzing the VLBA data set, we improve the dynamic range of images by a factor of four, using self-calibration. Different fitting techniques are used and compared to determine the peak positions in phase-referenced maps. Results. The improved dynamic range shows that in addition to the images with a one-sided structure, there are several images with a double-sided structure. The astrometry indicates that the peak in consecutive images for the whole set of observations describes a well-defined ellipse, 6−7 times larger than the orbit, with a period of about 28 d. Conclusions. A double-sided structure is not expected to be formed from the expanding shocked wind predicted in the pulsar scenario. In contrast, a precessing microquasar model can explain the double-and one-sided structures in terms of variable Doppler boosting. The ellipse defined by the astrometry could be the cross-section of the precession cone, at the distance of the 8.4 GHz-core of the steady jet, and 28 d the precession period.

“…As a result the compact object could be tilted (Fragile et al 2007). In this case either the accretion disk is coplanar with the compact object and, therefore, subject to the gravitational torque of the Be star or, instead, the accretion disk is coplanar with the orbit but tilted with respect to the compact object which induces, in the context of general relativity, Lense-Thirring precession if the compact object rotates (Massi & Zimmermann 2010). A deep investigation of these or other mechanisms of precession requires the knowledge of the precession parameters, such as the period of precession and the angle of the precession cone.…”

Section: Introductionmentioning

confidence: 99%

Long-term periodicity in LS I +61°303 as beat frequency between orbital and precessional rate

Jaron

2013

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Context. In the binary system LS I +61• 303 the peak flux density of the radio outburst, which is related to the orbital period of 26.4960 ± 0.0028 d, exibits a modulation of 1667 ± 8 d. The radio emission at high spatial resolution appears structured in a precessing jet with a precessional period of 27−28 d. Aims. How close is the precessional period of the radio jet to the orbital period? Any periodicity in the radio emission should be revealed by timing analysis. The aim of this work is to establish the accurate value of the precessional period. Methods. We analyzed 6.7 years of the Green Bank Interferometer database at 2.2 GHz and 8.3 GHz with the Lomb-Scargle and phase dispersion minimization methods and performed simulations. Results. The periodograms show two periodicities, P 1 = 26.49 ± 0.07 d (ν 1 = 0.03775 d −1 ) and P 2 = 26.92 ± 0.07 d (ν 2 = 0.03715 d −1 ). Whereas radio outbursts have been known to have nearly orbital occurrence P 1 with timing residuals exhibiting a puzzling sawtooth pattern, we probe in this paper that they are actually periodical outbursts and that their period is P average = 2 ν 1 +ν 2 = 26.70 ± 0.05 d. The period P average as well as the long-term modulation P beat = 1 ν 1 −ν 2 = 1667 ± 393 d result from the beat of the two close periods, the orbital P 1 and the precessional P 2 periods. Conclusions. The precessional period, indicated by the astrometry to be of 27-28 d, is P 2 = 26.92 d. The system LS I +61• 303 seems to be one more case in astronomy of beat, i.e., a phenomenon occurring when two physical processes create stable variations of nearly equal frequencies. The very small difference in frequency creates a long-term variation of period 1/(ν 1 −ν 2 ). The long-term modulation of 1667 d results from the beat of the two close orbital and precessional rates.